Preparatory studies on diiodo-molecules for use in gas phase electron diffraction

Preparatory studies on

diiodo-molecules for use in gas phase electron diffraction

Untersuchung von Diiodomolekülen zur Nutzbarkeit in Elektronenbeugungsexperimenten in der Gasphase

von

Sarah Scheitz

im Studiengang Nanowissenschaften B. Sc.

Universität Hamburg

02. November 2017

1. Gutachter: Prof. Dr. Jochen Küpper 2. Gutachter: Dr. Ruth Livingstone

Abstract

In the framework of this thesis several candidate molecules were tested with regard to their feasibility for benchmark electron diffraction experiments on controlled gaseous samples.

Electron diffraction serves as a tool for structure determination of molecules in the gas phase without the need for crystalline samples. This is especially important for a variety of biomolecules which are difficult to crystallize and cannot be examined using conven- tional x-ray diffraction techniques. The diffraction signal, and therefore informational content of structure, can be increased when the molecules are prepared in a controlled fashion.

During this work 2,5-Diiodobenzonitrile, 1,2-Diiodoethane and 4,4’-Diiodoazobenzene were chosen as candidate molecules and their suitability for electron diffraction was ver- ified with the help of simulations and experimental studies. The simulated diffraction patterns were created using the program CMIdiffractand served as a first aid for de- ciding on which molecule would provide the most analysable data.

Two candidates, 2,5-Diiodobenzonitrile and 1,2-Diiodoethane, were selected and intro- duced into an experimental set-up that combined a molecular beam creating apparatus with an electron gun. Firstly, attempts were made to create stable molecular beams of the samples and determine their molecular composition by recording time-of-flight spectra.

Subsequently, control of molecules was tested by deflecting the beams with inhomoge- neous electrostatic fields which leads to a separation of the target molecules from seeding gas. Successful deflection served as a first preparatory step towards the actual electron diffraction experiment.

Mandatory properties of molecules for use in electron diffraction are discussed and the examined candidate molecules are assessed regarding their experimental feasibility. En- countered difficulties with the selected candidate molecules and possible solutions are proposed in the outlook of this thesis.

Zusammenfassung

Im Rahmen dieser Arbeit wurden ausgewählte Moleküle in Hinsicht auf ihre Eignung für neuartige Elektronenbeugungsexperimente in der Gasphase zur Strukturbestimmung untersucht. Elektronenbeugung wird eingesetzt um die Struktur von Molekülen zu un- tersuchen ohne die Notwendigkeit einer kristallinen Probe. Dies ist vor allem bei der Untersuchung von Biomolekülen von Interesse, da diese häufig kaum oder nur unzure- ichend kristallisiert werden können. Um das Beugungssignal und damit den Information- sgehalt über die Struktur einer gasförmigen Probe zu verstärken, können Eigenschaften der Moleküle wie Ausrichtung im Raum oder Quanten Zustand kontrolliert werden.

Innerhalb dieses Projekts wurden 2,5-Diiodobenzonitril, 1,2-Diiodoethan und 4,4’-Diiodo- azobenzen als Kandidatmoleküle ausgewählt und hinsichtlich ihrer Eignung für Elektro- nenbeugung mit Hilfe von Simulationen und Experimenten untersucht. Dabei wurden die erwarteten Beugungsbilder mit dem Programm CMIdiffract simuliert. Diese dienten als erste Auswahlhilfe für die weitere experimentelle Untersuchung der Stoffe.

Zwei Kandidatmoleküle, 2,5-Diiodobenzonitril und 1,2-Diiodoethan, wurden zu experi- mentellen Untersuchungen ausgewählt. Der experimentell genutzte Aufbau bestand dabei aus einem Molekülstrahl generierenden und kontrollierenden Teil, kombiniert mit einer Elektronenkanone für das eigentliche Beugungsexperiment. Erste Experimente beinhal- teten das Generieren von stabilen Molekülstrahlen und die Bestimmung der molekularen Zusammensetzung der Proben durch die Aufnahme von Flugzeitspektren. Darauffolgend wurden Versuche unternommen, die Zielmoleküle von dem Trägergas über elektrostatische Ablenkung zu trennen, indem ein angelegtes inhomogenes elektrisches Feld die Moleküle entsprechend ihres effektiven Dipolmoments unterschiedlich stark ablenkte. Eine erfolgre- iche Trennung der Zielmoleküle vom Trägergas und Verunreinigungen dient als wichtige Voraussetzung um genügend Beugungssignal des Moleküls im eigentlichen Elektronenbeu- gungsexperiment zu erhalten.

Das Potenzial der untersuchten Molekülkandidaten zur erfolgreichen Ablenkung aus dem Molekülstrahl wurde analysiert und daraus schließend eine Bewertung der Moleküle im Hinblick auf ihre Tauglichkeit für Elektronenbeugung gegeben. Auftretende Komplikatio- nen mit den ausgewählten Moleküle und Lösungsvorschläge für zukünftige Experimente werden im Ausblick dieser Arbeit diskutiert.

Contents

Abstract 3

Zusammenfassung 5

1 Introduction 9

1.1 Benefits of electron diffraction . . . 9

1.2 Outline . . . 11

2 Simulation of electron diffraction patterns 13 2.1 Theoretical background of the code . . . 13

2.2 Properties of suitable molecules for electron diffraction . . . 16

2.3 Simulations for candidate molecules . . . 19

3 Experimental validation of electron diffraction suitability 21 3.1 The experimental set-up . . . 21

3.2 Preparing candidates for electron diffraction . . . 26

3.2.1 Characterization of 2,5-Diiodobenzonitrile molecular beam . . . 26

3.2.2 Characterization of 1,2-Diiodoethane molecular beam . . . 31

3.2.3 Deflection of 1,2-Diiodoethane . . . 37

3.3 Feasibility of candidate molecules for electron diffraction . . . 40

4 Outlook and Conclusion 43 4.1 Challenges in electron deflection and outlook for the presented set-up . . . 43

4.2 Conclusion . . . 44

Bibliography 45

Acknowledgements 51

Eidesstattliche Versicherung 53

1 Introduction

1.1 Benefits of electron diffraction

Electron diffraction serves as a tool for imaging structures of molecules up to an atomic- level resolution [1, 2]. Since the structure of an object is directly correlated to its function it is most important to unravel the structure of a system in order to understand its func- tionality. Starting with light microscopy and determining first micro organisms with a resolution less than one micron [3, 4] we are nowadays capable of imaging the atomic positions in molecules with near-atomic resolution [5]. Those high resolutions are of great importance for scientist in all fields especially material science, chemistry, biochemistry and medicine. They all pursue the same aim of understanding the function of a target object and adapting those functions for novel applications. For over a century diffrac- tive imaging methods are established tools for investigating the structure of solid state samples. Especially x-ray crystallography is one of the most widespread methods which was first experimentally used by Bragg in 1913 [6]. This method has since been respon- sible for many ground breaking scientific discoveries like the confirmation of the helical structure of DNA by Franklin and Gosling [7]. Nowadays x-ray crystallography is used in particular by biochemists to determine the structure of protein crystals. However, many large biomolecules like proteins or viruses that are of interest in biology are very hard to crystallize or do not form crystals at all. Until 2011 less than 300 structures of membrane proteins could be examined via x-ray crystallography because of major issues in crys- tallizing them [8]. The success of x-ray crystallography hence relies heavily in the prior formation of sample crystals of sufficient size. The need for imaging methods that do not require a long range order like apparent in crystals is hence huge in the field of biology and chemistry. To image non periodic objects, however, diffractive imaging sources have to meet specific requirements. Without the periodicity of a crystal the diffracted signal is much weaker for non-crystalline samples [9]. This results in very poor signal-to-noise ratios because the samples exhibit no repetition of unit cells and hence no coherent ad- dition of diffraction signals [10]. To gain enough scattering signal from a single molecule extremely bright and coherent sources are necessary while ensuring atomic resolution.

However, these high energetic x-ray pulses lead to significant radiation damage and sub- sequent sample destruction which hinders the recording of diffraction data of the intact sample [11]. Collecting diffraction images of the undamaged sample was solved by us- ing ultra short femtosecond x-ray pulses with a duration of 25 fs [11] which created the

diffraction pattern faster than the occurring damaging processes. This method labelled as the so called "diffraction before destruction" concept was proposed by Neutze et al. [12]

and was first experimentally proven by Chapman et al. [11]. This methods requires ultra short and extremely intense x-ray pulses which are provided by novel x-ray free electron lasers (XFELs). With XFELs in the hard x-ray regime diffractive imaging with atomic resolutions on non-crystalline samples or single targets is promising for all biochemists.

First proof of principle experiments where performed by imaging the structure of a large biomolecule with an XFEL source [8]. Due to their high repetition rates and incident energies novel sources like the recently finished European XFEL facility [13] will push the boundaries of diffractive imaging resolutions and will enable the structure determination of single molecules. Furthermore, short laser pulses in the order of nowadays a few fem- toseconds [14] make it possible to follow molecular dynamics during chemical reactions.

Observing molecular dynamics was first carried out by Zewail et. al in the late 19ths century who determined transition states in several chemical reactions [15–17]. Using femtosecond spectroscopy it was possible to follow dynamics in chemical reactions that usually occur on a time scale from femtoseconds to several years depending on the type of reaction [18]. For his implementation of femtosecond spectroscopy to follow chemical reactions, Zewail was honored with a Nobel prize in 1999 [19]. Femtosecond chemistry combined with x-ray or electron diffraction lead to time resolved determination of struc- tural changes in molecules [5, 20]. Besides x-ray diffraction, electron diffraction has been equally used in the structural determination of gas phase molecules since already 1930 [21]. The advantage of electrons compared to x-rays is that the necessary parameters like high intensity, repetition rates and short pulse of sub-100 fs durations can be met in table top set-ups [22] and do not require a 3.4 km long tunnel like in the case of the European XFEL. Electron diffraction enabled the determination of gas phase intermediates struc- tures with an atomic-level resolution [23, 24]. With a 10⁴times greater elastic scattering intensity of electrons compared to X-rays, electrons can compensate for the low density of gas phase molecules [25]. A further advantage of electrons is that they interact both with nuclei and shell electrons in the sample due to Coulomb forces. X-rays only interact with electrons of the sample due to the electric fields affecting the electrons and hence only mapping of the electron distribution in the sample is possible. To enhance diffraction signals created by gaseous samples or single molecules it is possible to examine them in a controlled delivery approach. Thereby molecules are first prepared before undergoing electron diffraction. This preparation includes quantum state selection, spatial orienta- tion of the molecules and alignment [26]. Molecules controlled in that manner deliver diffraction patterns with enhanced informational content and ease the reconstruction of their structure. However, the even lower density of controlled gas phase molecules require sources of large cross-section particles with sufficient energy to obtain atomic resolution [27]. Electron sources provide all those necessary properties of an imaging technique for single molecules in table-top set-ups.

1.2 Outline

This thesis is concerned with the validation of three different solid state samples in their suitability for novel gas phase electron diffraction experiments with controlled single molecules. The validation included simulations of the experiment and subsequent ex- perimental trials with two of the chosen candidate molecules. In chapter 2 necessary properties of sample molecules for an electron diffraction experiment with controlled delivery approach of the samples are discussed. Three molecules are presented that were chosen based on these properties. The examined molecules in this thesis are 2,5- Diiodobenzonitrile, 1,2-Diiodoethane and 4,4’-Diiodoazobenzene. Furthermore, chapter 2 includes a brief explanation on theoretical assumptions used in the simulation program for electron diffraction. Finally, simulations of diffraction patterns of all three candidate molecules are presented which served as a support in down-selecting molecules for ac- tual experiments. The experimental validation was only performed on two of the three molecules due to time restrictions. In chapter 3 the presentation of the experimental set-up and all characterizing experiments that were carried out on 2,5-Diiodobenzonitrile and 1,2-Diiodoethane are given. Transition of the solid state samples into the gas phase via a pulsed valve, and hence creating a beam of the samples seeded in a carrier gas, is described. The molecular beams were characterized and optimised to obtain a maximum of target molecules in the interaction region of the apparatus. Furthermore, the separa- tion of different quantum states of the target molecules using electrostatic deflection was undertaken. The gained results are elaborated in chapter 4 with regard to the feasibility of all examined molecules for electron diffraction experiments. Apparent challenges of the experiment and the used set-up are discussed and necessary properties of candidate molecules are revised one more time with set-up specific challenges. In the end an outlook on possible continuation of this project is given while also pointing out the deficiencies in the used set-up and proposing possible improvements.

2 Simulation of electron diffraction patterns

In order to select suitable molecules as samples for electron diffraction the experiment was first simulated using the program CMIdiffract. This simulation represents the actual experiment and predicts the diffraction pattern for an isotropic sample. Concerning the signals in simulated diffraction patterns, chosen molecules can be evaluated as suitable starting samples for the given experimental set-up. CMIdiffract therefore serves as an aid in selecting molecules for the experiment and provides a specific idea on what to expect in the experimental results.

2.1 Theoretical background of the code

The code implemented in the program CMIdiffract is able to depict the diffraction pattern of a chosen sample as expected in the actual experiment. To simulate the total expected scattering intensity, both elastic and inelastic electron scattering were included in the code. In that way a realistic rendering of the experiment was provided which could serve as an accurate comparison with experimentally recorded data. In this section general assumptions used in the code are explained and verified. A detailed step-by-step derivation of the code is given in reference [27].

Throughout the code electrons are treated as plane waves with infinite spread regarding space and time. This is an assumption since it would be more realistic to treat them as wave packets [28]. However, in relation to the extent of the coulomb potential of an atom or molecule which scatters the electron, the electron wave packet is much larger and therefore can be assumed to be infinite. For calculation of the scattered electron wave function all atoms were considered independent scattering centres. This assumption simplifies the coulomb potentials of all atoms in a molecule to be spherical. Changes in electron density distributions due to chemical bindings in a molecule were therefore neglected. In order to get the scattered wave function Ψ⁰ of an electron that was scattered on a molecule, all scattered wave functions Ψ⁰n forN individual atoms are calculated and then added to form the total scattered wave function:

Ψ⁰ = ^XN

n=1

KAe^ik0R

R f_n(θ)e^i(k0−k0)rn (2.1)

The scattered wave function includes the constant K = 8π²m_ee²/h², the wave vector of the incoming (k0) and the scattered electrons (k⁰), the distance between interaction region and detector R and the atomic position vector rn. fn(θ) is the atomic scattering factor and specific for every atom. It is dependent on the angle θ between the incoming and scattered electron. How f_n is obtained for the different atoms will be explained in the further course of this section.

Since the independent atom model assumption used here disregards chemical binding effects and only approximates the structure of molecules, the simulation is not as accurate as the experiment. With the scattered electron wave function an equation to calculate the total scattering intensity can be obtained [27]:

I_t(s) = K²I₀ R²

N

X

n=1

(|f_n(s)|²+S_n) + K²I₀ R²

N

X

n=1 N

X

m6=nm=1

f_n(s)f_m^∗(s)sin(sr_nm)

srnm (2.2)

It shall be pointed out that the scattering intensity is now described as a function of s, which is the absolute value of the difference between incoming and scattered electron wave-vectors:

|s|=|k₀−k⁰|= 2k₀sinθ

2 (2.3)

s is directly equivalent to the number of pixels on the detector screen, which is illustrated in Figure 2.1. The relation between s and the detector radiusr

s = 4π λ sin(1

2arctan(r

R)) (2.4)

is used to transform I(s) to I(p).

Figure 2.1: Relation between the connecting vector s of incoming (k0) and scattered electron (k⁰) and the radius r of the corresponding recorded signal on the detector. The distance between interaction region and detector plane is depicted as R.

The total scattering intensity can be split into two parts

I_t(s) =I_b(s) +I_m(s) (2.5) regarding signal containing no information about the molecular structureI_b(s), considered background, and signal of which the molecular structure can be derived I_m(s).

I_b(s) = I_a^e(s) +I_aⁱ(s)

= K²I₀ R²

N

X

n=1

(|f_n(s)|²+S_n) (2.6) The first part represents the atomic elastic scattering I_a^e(s) = ^K_R²2^I0

PN

n=1|f_n(s)|². It also takes all inelastic scattering from the atoms I_aⁱ(s) into account by including the inelastic scattering factorSn. This part of the equation is independent of inter atomic distances and hence provides no information on the structure of the examined molecule. It summarizes atomic scattering intensities like all atoms in the molecule were considered isolated and is therefore considered backgroundI_b(s). The inelastic scattering factor S_n(s) was included in the code to predict a diffraction pattern as realistic as possible. However, it was only added to this part of the equation that is not depended on inter atomic distances and is assumed to be incoherent. Coherent effects of inelastic scattered electrons are considered to be highly unlikely and neglected in this code. Values for S_n(s) were derived from corresponding atomic x-ray inelastic scattering factors [27].

Information about inter atomic distances are encoded in the second part of the equation which represents the elastic molecular scattering intensity:

I_m(s) = K²I₀ R²

N

X

n=1 N

X

m6=nm=1

f_n(s)f_m^∗(s)sin(sr_nm)

sr_nm (2.7)

This intensity includes interference terms due to scattered electrons of neighbouring atoms. With the distance rnm = |rn−rm| between two atoms An and Am informa- tion about the molecular structure can be derived. Since the orientation of the examined samples in the context of this work is assumed to be isotropic, the equation for I_m(s) shown here is the integrated formula over different orientations of the molecule.

Equation 2.2 was implemented in the code to simulate the total scattering intensities for isotropic samples. To calculate the electron scattering intensities the atomic scattering amplitudes f_n(s) where determined using the first Born approximation:

fn(s) = Zn−Fn(s)

s² (2.8)

The atomic scattering amplitudes were derived from the corresponding x-ray scattering amplitudes F_n(s). Within the program CMIdiffract, Equation 2.8 was approximated by a sum of Gaussians [27] and calculated using tabulated values for included parameters [29]. It has to be noted that those parameters do not exist for all elements and it should be checked beforehand if the molecule of interest contains any unreferenced atoms.

The first Born approximation was chosen because it allows for parametrization of the scat- tering amplitudes. The disadvantage of this approximation is, that its accuracy decreases with higher Z and lower electron energies [28]. In the course of this thesis, Iodine was examined for which the first Born approximation might already fail because of its large atomic number of Z = 53. However, the high used electron energy of 15 keV compensates for that and the first Born approximation can be used. When using the code for simu- lation of molecules containing even heavier or uncommon atoms, it has to be considered that f_n(s) might not be calculated correctly. A cautious handling of the code is hence necessary for obtaining a realistic electron diffraction pattern.

The first Born approximation and the independent atom model included in the code limit the precise rendering of the actual diffraction pattern. However, the implemented code in CMIdiffract allows for a sufficient prediction of electron diffraction patterns which can be supportive in the selection of suitable molecules.

2.2 Properties of suitable molecules for electron diffraction

In the course of this work, several molecules functioning as possible candidates for the diffraction experiment were discussed. Chosen molecules had to meet certain requirements in order to aim for a successful experiment. The focus of possible diffraction candidates was on diiodo molecules. Iodine alongside bromine and fluorine is a common substituent used in modern chemistry and offers convenient properties for diffraction. With an atomic number of Z=53 iodine possesses a much larger scattering cross section which allows for stronger electron diffraction. This is of great importance for the diffraction experiment which is still considered prototypical. Hence a preferably strong diffraction signal is nec- essary to exceed the level of the still strong background signal [27] and to obtain evaluable diffraction patterns. Additionally, gaseous samples need to contain molecules with strong molecular scattering intensitiesIm(s) in order to compensate for the relatively low molec- ular density compared to solid state samples. For these reasons, molecules containing two or more iodine atoms are favourable for first diffraction experiments. Moreover, a high dipole-moment-to-mass ratio m/µ of the molecule is required to allow for electro- static deflection [30]. Deflecting the sample is necessary in the used set-up to separate the molecule of interest from carrier gas or other impurities. Further information on deflection are given in section 3.1. When working with solid-state samples the melt-

ing point temperature needs to be considered likewise. Transitioning the molecules into the gas phase requires heating up of the sample jet. A moderate temperature is always preferable to reduce the risk of damaging the sensitive valve which creates the molecu- lar beam. Finally, samples are to be preferred that are neither toxic nor carcinogenic to ensure a non-hazardous work. Considering these specifications several candidates for electron diffraction were selected. 2.5-Diiodobenzonitrile (DIBN), shown in Figure 2.2e, is one of the major candidates since it was already introduced into the set-up and used for first attempts on electron diffraction [27]. As further suitable molecules 1.2-Diiodoethane (DIE) and 4.4’-Diiodoazobenzene (DIAB) were contemplated. Important properties of the mentioned molecules for the diffraction experiment are summarized in Table 2.1.

Table 2.1: Properties of selected candidates for electron diffraction.

molecule dipole xxx

moment µ* mass m µ/m

[D·mol/g] melting

point hazard- ousness gauche-DIE [31] 2.29 D 281.86 g/mol 8.12·10⁻³ 80-82 °C irritant DIBN [32] 4.24 D 354.91 g/mol 11.95·10⁻³ 164-169 °C irritant cis-DIAB [33] 1.01 D 434.01 g/mol 2.33·10⁻³ no data no data

* µwere determined by ab-initio calculations (GAMESS-US MP2/6-311G**)[34].

Since the anti-configuration of DIE does not obtain a dipole moment due to its symmet- ric structure, this conformer cannot be deflected and is therefore not suitable for electron diffraction with the here used set-up. It was shown however, that in the gas phase equilib- rium 12 % of DIE molecules are being present in the gauche-conformation which exhibits a strong dipole moment [35]. This small percentage of gauche-DIE can efficiently be de- flected and is hence usable for electron diffraction. The molecular structures for both DIE conformers are depicted in Figure 2.2a and b.

Due to only 12 % of the total DIE sample being deflected, a much lower molecular density of the deflected beam is to be expected compared to a molecule that is deflected en- tirely like DIBN. Furthermore, DIBN shows the highest dipole-moment-to-mass ratio of all three candidates and will hence be deflected the easiest. Despite the low percentage of the actual examinable DIE molecules it has been chosen as a candidate for electron diffrac- tion. An advantage of this molecule is its moderate melting point of about 82^◦C which is preferable for the usage of the delicate valve. The expected well deflectable DIBN on the other hand, has a higher melting point of about 165^◦C. Though DIBN is presumably the favourite candidate because of its good deflection, the higher operating temperature could damage the valve, which should be considered when working with DIBN. DIAB was likewise chosen to be a candidate, though only the cis-configuration exhibits a net dipole moment and can be deflected. The anti-conformer does not have a dipole moment because of its structural symmetry (Figure 2.2c). For DIAB no ratio between the two conformers

has been experimentally examined yet. The dipole-moment-to-mass ratio of cis-DIAB is with 2.33·10⁻³ D·mol/g the weakest of all three candidates and proper deflection might be harder to achieve than with DIE or DIBN. All these properties have to be taken into account when choosing a molecule and should be considered additionally when evaluating the simulated diffraction signals.

a) anti-DIE

b) gauche-DIE

c) trans-DIAB

d) cis-DIAB

e) DIBN

Figure 2.2: Molecular structures of candidate molecules. Grey globes represent carbon atoms, violett globes iodine and blue globes nitrogen atoms.

2.3 Simulations for candidate molecules

To be able to compare the different candidates regarding their feasibility for the ex- periment, diffraction patterns of all previous discussed molecules were simulated using CMIdiffract. To ensure good agreement between experiment and simulation set-up specific parameters were implemented in the code. For instance, electron energies of 15 keV were determined in agreement with first benchmark experiments done with the set-up. Using the relativistic De-Broglie equation this corresponds to a wave length of about λ≈10 pm. Further parameters were set like the detector diameterd=42 mm and size of 480 x 480 pixels, the distance between detector and scattering centre ofR=180 mm and the beam block diameter of 6 mm. With the stated diameter of the detector the size of each pixel corresponds to about p = 88 µm. All diffraction patterns were simulated assuming isotropic samples because of the random orientation of the molecules in the gen- erated molecular beam. From the simulated total scattering intensity I_t(s) the molecular scattering intensity I_m(s) could be derived using Equation 2.5. The molecular scattering intensities as expected on the detector for chosen candidate molecules are shown in Fig- ure 2.3

0 100 200 300 400

pixel x-position (m) 0

100 200 300 400

pixel y-position (m)

molecular scattering intensity 2D (hits/shot/pixel)

5 4 3 2 1 0 1 1e 172

a gauche-DIE

pixel x-position

pixely-position

0 100 200 300 400

pixel x-position (m) 0

100 200 300 400

pixel y-position (m)

molecular scattering intensity 2D (hits/shot/pixel)

6 4 2 0 2 4 1e 176

b DIBN

pixel x-position

pixely-position

0 100 200 300 400

pixel x-position (m) 0

100 200 300 400

pixel y-position (m)

molecular scattering intensity 2D (hits/shot/pixel)

0.50 0.25 0.00 0.25 0.50 0.75 1.00 1.25 1e 16

c cis-DIAB

pixel x-position

pixely-position

Figure 2.3: Simulated molecular scattering intensities for a) gauche-DIE, b) DIBN and c)cis-DIAB. The white circle in the centre of each diffraction pattern depicts the area of the detector that is covered up by the beam block.

All three simulations show visible diffraction patterns due to interference of the scattered electrons. Several rings of high intensity can be seen which are created because of the isotropic orientation of all molecules. The white circle in the centre of all diffraction patterns visualizes the area of the detector that is covered by the beam block and de- tects no signal. The radial distributions of the molecular scattering intensities are given in Figure 2.4. With these graphs estimations which candidate molecule would produce the best evaluable results can be made. The intensity in Figure 2.4 drops rapidly with increasing distance to the detector centre, corresponding to a large scattering angle θ. A suitable molecule should hence show several oscillations in its molecular scattering inten- sity close to the detector centre, where the signal is stronger, but outside of the beam block range. For gauche-DIE, only weak oscillations with two small maxima in the range

up to 140 pixels are visible. DIBN and cis-DIA, on the other hand, show much more distinct oscillations. The molecular scattering intensity of cis-DIAB has two significant maxima in the range of 40 pixels to 125 pixels. DIBN also exhibits two strong maxima up to 110 pixels and two further weaker ones in the range up to 160 pixels. Thus both molecules are preferable to gauche-DIE regarding their scattering signals.

In conclusion with the preferred molecular properties for the diffraction experiment DIBN looks the most promising. Even though it needs a higher operating temperature it should still yield the best analysable diffraction pattern due to its highest p/m ratio and the fact that 100 % of it is being deflected. Following would be cis-DIAB because of its much more distinct oscillations in the molecular scattering intensity thangauche-DIE. However, neither the proportion ofcis-DIAB in the sample nor its melting point is known and it has a much lower p/mratio of 2.33·10⁻³ D·mol/g than gauche-DIE with 8.12·10⁻³ D·mol/g.

Therefore, gauche-DIE, despite its low deflectable amount, might be considered before using cis-DIAB.

Figure 2.4:Radial distribution of simulated molecular scattering intensities for candidate molecules. The lowerx-axis depicts thes-value and the upperx-axis the pixel number of the detector. The red dashed line indicates the range of the beam block which covers the detector centre.

3 Experimental validation of electron diffraction suitability

To verify their suitability for the electron diffraction experiment the candidate molecules were introduced into the experimental set-up. A molecular beam of each sample was cre- ated and optimized with regard to unhindered flow through the chambers. The molecular composition of the beams were determined and first attempts on deflection of the target molecules were made. Successful deflection served as a criterion for further diffraction experiments with lower background signal.

3.1 The experimental set-up

All preparatory studies on candidate molecules were done using the machine shown in Figure 3.1. This set-up can be used for both the characterization of the sample and the actual diffraction experiment. It was operated under high-vacuum with two turbo

Figure 3.1: Photograph of the used experimental set-up. The machine consist of three combined vacuum chambers referred to as source chamber (2), deflector chamber (3) and velocity-map imaging (VMI) chamber (5). The source chamber is connected to a gas feed through (1). Between deflector and VMI chamber a controller (4) for positioning of a skimmer that connects the chambers is located. The detector sits on top of the VMI chamber with a CMOS camera (6) mounted right above it.

pumps attached onto the first chamber, in the following referred to as source chamber, and one turbo pump each for the second and third chamber, referred to as deflector and VMI chamber. The chambers are separated by walls with molecular beam skimmers connecting them with each other. The pumping speed of about 2000 l/s for each pump achieves pressures on the order of 10⁻⁶mbar for the source and 10⁻⁹mbar for deflector and VMI chamber when the ELV is running. The high vacuum was essential for the creation of a cold molecular beam [36] and for the reduction of background signals when running experiments. The inside of the machine is depicted schematically in Figure 3.2.

Figure 3.2: Schema of the experimental set-up used for characterization of the molecular beam and actual electron diffraction experiment. The molecular beam (yellow) is created by a pulsed valve and passes three skimmers and a deflector before entering the interaction region. When the deflector is used the third skimmer can be moved to select specific deflected parts of the beam. A probing laser (red) with a central wave length of 800 nm is used for ionizing molecules in the beam which are accelerated towards a detector by a VMI spectrometer. For electron diffraction, electrons (green) are generated from a copper cathode by a 400 nm laser (blue) and focussed with the electron gun electrodes.

The Faraday cup and a beam block of 6 mm diameter can be inserted into the chamber.

Molecular beam, ionization laser and electrons were all crossing perpendicular to each other.

A pulsed Even-Lavie valve (ELV) [37] with an integrated cartridge containing the sample molecule was located in the source chamber (Figure 3.3a) and connected to the gas feed through. With a usual backing pressure of 100 bar Helium, the ELV is opened by a short current pulse (20 µs) which produces a pulsed molecular beam depicted as the yellow line in Figure 3.2. To transition the solid sample in the cartridge into the gas phase the ELV could be heated. For an even warming of the sample, aluminium foil was wrapped around the cartridge which is depicted in Figure 3.3b. A cold beam of molecules was created due

to supersonic expansion and had an internal temperature of about 1 K [37, 38] which was crucial for later deflection of the molecules. The position of the valve could be aligned in all dimensions to ensure perfect alignment with the set-up and unrestrained arrival of the beam in the interaction region. The molecular beam passes two Skimmers, with diameters of 3 mm and 1 mm, before entering the deflector. Skimming the gas-jet leads to a cold collimated beam with high molecular density of about 10⁸molecules/cm³ and allows for strong differential pumping into the next vacuum chamber which is operated at much lower pressure [39].

a b

Figure 3.3: Photograph a) shows the inside of the source chamber with the ELV mounted onto a holder and connected to the seeding gas feed through. In photograph b) the same ELV but with aluminium foil fixed around the sample cartridge can be seen. The foil was necessary for good conduction of heat throughout the whole sample, because the heating element was only heating the tip of the cartridge.

An electric deflector, positioned in the second chamber, was used to deflect molecules with an apparent dipole moment [40]. The deflector consists of two electrodes, a rod and a trough, mounted at a vertical distance. For deflection, a strong inhomogeneous electric field is created within the deflector by applying 10 kV to 12 kV on the deflector rod and keeping the lower electrode on ground potential. This leads to an electric field gradient in respective y-direction in the deflector. When passing the inhomogeneous electric field Stark forces act upon polar molecules causing them in this set-up to deflect upwards [30]. The rate of deflection hereby depends on the electric field gradient and the dipole- moment-to-mass ratio. Molecules with a high µ/m ratio are deflected the most which allows for a separation from non polar seeding gas like Helium. The deflected molecules are further dispersed according to their rotational quantum states, whereby molecules in ground or low rotational states show the strongest deflection [30]. To achieve strong deflection a cold molecular beam is preferable, since only a few low rotational states are populated [41]. A perfect alignment of beam and set-up is important to prevent collisions with mechanical parts which would lead to an increased temperature in the beam and hence diminish the deflection rate. Strong deflection is necessary to separate the molecule of interest from seeding gas and other impurities in the beam. Moving the third Skimmer, diameter = 1.5 mm, upwards allows for selecting only the deflected molecule of interest

and probing it individually. This decreases the background signal enormously since mainly chosen molecules arrive in the interaction region.

The Mentioned interaction region is reached when the molecular beam crosses with the probe laser or the electron beam in the third vacuum chamber. Ionization laser, electron beam and molecular beam all cross perpendicular to each other. The electron beam (green) is created by photo-ionization with short 400 nm laser pulses (red) irradiating a copper cathode. The laser pulses are generated via second-harmonic generation of near- infrared pulses from a Ti:sapphire laser system (Coherent Astrella) with a repetition rate of 1 kHz. Electrons are collimated and accelerated by electric fields of the electron gun consisting of three electrodes in velocity-map imaging (VMI) spectrometer configuration [42]. The number of electrons per pulse can be measured by inserting a Faraday cup into the beam path. When running the actual electron diffraction experiment, a beam block can be placed into the beam path to protect the detector from high-energetic unscattered electrons. The photograph in Figure 3.4a shows the inside of the VMI chamber with the ion-VMI electrodes taken out. The top electrode of the electron gun, Faraday cup and the beam block can be seen. In Figure 3.4b the VMI spectrometer was put into the chamber and Faraday cup and beam block were inserted through holes in the µ-metal that shields the chamber from electric or magnetic stray fields.

a b

Figure 3.4: Photograph a) depicts the open VMI chamber with the VMI spectrometer taken out. Faraday cup (left), beam block (right) and the upper electrode of the electron gun (mid) are shown. In photograph b) the inside of the VMI chamber with the VMI spectrometer inserted is shown. The spectrometer consist of three electrodes, whereas the upper electrode can be seen in the photograph, and a µ-metal shield with holes for inserting Faraday cup or beam block.

For characterization of the molecular beam, a probing laser (red beam in Figure 3.2) with a central wavelength of 800 nm was shot onto the molecular beam. The probing laser pulses were generated by the previous mentioned laser system and allowed for strong field ionization of molecules. A VMI spectrometer was mounted inside the chamber to guide generated ions onto the detector. The VMI spectrometer, consisting of three electrodes, can be operated in either spatial [43] or velocity-map imaging mode [44] depending on the applied voltages of the electrodes. In spatial imaging mode, ions that are created

in a specific spot in the interaction region reach the detector at a specific point. The resulting image on the detector hence corresponds to the spatial distribution of all ions in the interaction region. In VMI mode, on the other hand, ions with the same transversal momentum hit the same point on the detector and the resulting image shows a momentum distribution independent on where the ions were generated. Both modes can be used to characterize different properties of the incoming molecular beam, which is further explained in subsection 3.2.1.

Generated ions are lead onto a detector (Photonis) consisting of a 42 mm diameter Z- stack of multi-channel-plates (MCPs), a fast phosphor screen (P-46) and a high frame- rate CMOS camera (Optronis CL600x2). The camera is operated at a repetition rate of 1 kHz with a camera-link-readout-resolution of 480 x 480 pixel. This corresponds to a edge length of 80 µm for each pixel on the detector. Recorded data is read out at the same rate by a computer software that creates background corrected images. Background correction was achieved by installing a green LED onto the side of the phosphor screen which is depicted in Figure 3.5. The repetition rate of the LED can be synced with the pulse rate of the ELV so that every signal recorded on the detector when the LED is on, originates from the actual molecular beam. When the LED is turned off, all recorded data is therefore considered background and subtracted when creating the actual image.

Figure 3.5: Photograph of the phosphor screen (P-46) with a green LED fixed onto the right side of the screen for background correction.

Using a high voltage switch (Photek), gating of the detector is possible which allows for detection of selected ions with a specific time of flight (TOF). This can be used for characterizing the different ions created by coulomb explosion of the molecular beam and derive which molecules are actually present in the beam. To synchronize the delay between laser, ELV and detector a delay generator (Stanford Research Systems, DG645) was used which also controlled the gating settings of the detector.

3.2 Preparing candidates for electron diffraction

3.2.1 Characterization of 2,5-Diiodobenzonitrile molecular beam

For first trials, the sample DIBN was introduced into the set-up because it had already been successfully used in the machine in the past [27]. Additionally, it exhibited the largest µ/m ratio of all three presented candidate molecules (Table 2.1) which was as- sumed to lead to the strongest deflection. Before placing the sample into the set-up it was ground to increase the surface to volume ratio and ease the transition of molecules into the gas phase. The powder shown in Figure 3.6a was filled into the sample cartridge and both ends sealed with filter paper (Figure 3.6b). This had the capacity to absorb any molten product and prevented the sample from getting into the gas line. The cartridge was not filled up completely to ensure unhindered flow of the seeding gas over the sample.

a b

Figure 3.6: Photograph a) shows the ground sample of DIBN. In Photograph b) the ELV sample cartridge with the inserted DIBN is shown. A piece of filter paper can be seen in the front opening of the sample tube.

To produce a molecular beam of DIBN the ELV was heated up to 120^◦C. This temper- ature was lower than the actual meting point of about 165^◦C [32], but chosen because higher temperatures with DIBN often led to disfunctions of the valve after some hours of operation [27]. The ELV produced a pulsed molecular beam seeded in 100 bar of He- lium at a rate of 100 Hz. To find the best temporal overlap between molecular beam and ionization laser the number of generated ions from the beam was recorded depending on different delay times between laser and molecular beam. The ions were created due to strong field ionization of the beam molecules by the 800 nm laser pulses. The detector was gated onto a specific TOF to detect only ions originating from the molecular beam which were determined beforehand. Since the camera read-out was at a 1 kHz rate but the valve was operated at 100 Hz, 9/10 of the recorded data was used for background

correction. The background corrected temporal beam profile of DIBN is depicted in Fig- ure 3.7. For this beam profile the detector was gated onto N2⁺ ions originating from the DIBN beam. The number of events per shot is equivalent to the number of N2⁺ ions hitting the detector. Figure 3.7 shows the experimental obtained data as blue dots. To determine the pulse duration the data was fitted with a Gaussian depicted as the red graph. The fit shows good agreement with the experiment, expect for very early times where the experimental data is higher in intensity. Higher intensities for shorter delay times between the laser and the valve are due to the ELV not closing fast enough and molecules leaking out [45]. That is why at the end of the pulse the intensity does not decrease as abrupt as it increases in the beginning of the pulse. According to the fit the full width half maximum (FWHM) of the DIBN pulse is about 32 µs with the maximum at a delay time of 595 µs. For all further measurements the time delay between laser and molecular beam was set to 590 µs to have the best temporal overlap.

Figure 3.7: Temporal beam profile of the N2⁺ signal in the DIBN beam. The blue dots represent the number of detected ions at different delay times between laser and molecular beam. The red line shows the Gaussian fit of the experimental data with the standard deviation σ and the expected value µ.

To determine whether the molecular beam was propagating through the vacuum chambers without being cut off density profiles of the beam were recorded. For this, the focus of the ionization laser was scanned vertically (corresponding y-direction in this set-up as shown in section 3.1) across the molecular beam. For every focus height ions were created along the focus range (corresponding x-direction) and and recorded onto the detector in spatial imaging mode. The number of generated ions varied with the ionization laser height and lead to a spatial profile of the beam. Position of the ELV was adjusted till the molecular beam was arriving in the interaction region without any large cut-offs. In Figure 3.8a the spatial beam profile of the N2⁺ ions originating from the DIBN beam for different ELV heights is depicted. When moving the ELV up the graphs shift to the left which indicates

a

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Figure 3.8: a) Spatial beam profiles of the N2⁺ signal in the DIBN beam for different heights of the ELV. Higher negative values of the ELV height correspond to a higher position in real space. The best beam profile was measured at an ELV height of y = -13.0 mm. The respective 2D beam profile at this ELV height is depicted in b), where only a cut out of 160 x 160 pixel of the detector is shown.

that the molecular beam is moving down in the interaction region. The low slope of the graphs on the left side show that the density of the beam slowly decreases. This indicates that the lower edge of the beam is not significantly cut off. The slope on the right side of the graphs, on the other hand, exhibits a much steeper slope. This shows that the upper edge of the molecular beam has been cropped. Since the right sides of the graphs do not shift as strongly depending on the ELV height as the left sides, it is likely that the beam is cut off close to the interaction region. The small distance between the point where the beam is cropped and interaction region does not allow for large derivation of the beam from its actual path and hence the difference in height for the upper edge is not that significant. From the spatial profile the expansion of the molecular beam in y-direction could be determined. The determined diameter of the beam at an ELV height of y= -13.0 mm was about 2.88 mm.

To visualize the density profile, measured images for every focus height were sampled over each other and created a two dimensional beam profile depicted in Figure 3.8b. The 2D beam profile predicts the density distribution in bothxandy-direction. As expected from the beam profile graphs in Figure 3.8a the lower edge of the beam is blurred, meaning that the beam is not cut off. The upper edge, on the other hand, is very sharp and round shaped. This shape and the sudden decrease in intensity are proof that the upper part of the beam is cut off by the third skimmer. The overall round shape of the beam profile with the maximum intensity almost in the centre indicates that the majority of the beam reaches the interaction region which is to be preferred for following experiments. The beam shape and profile show that the beam does not hit any set-up components with its lower edge. The upper edge is cut off by the third skimmer. It is furthermore possible that the upper edge is hitting the deflector rod. This edge might not be apparent in the

2D beam profile when the third skimmer is positioned lower than the deflector rod. The edge created by the rod might be hidden above the edge created by the third Skimmer.

Moving the Skimmer further up would probably reveal this additional edge from the de- flector but this was not experimentally verified with the DIBN beam. Measurements with DIE discussed in subsection 3.2.2 show that this might be the case. A contact with the deflector rod would lead to a warmed beam which would impede deflection of the DIBN molecules.

A time-of-flight (TOF) spectrum of the DIBN beam was recorded to examine the appar- ent molecules in the beam. In order to record this spectrum, the time delay between laser and detector was scanned and for each time the generated ions detected. For the TOF measurements the MCP was coupled to an oscilloscope which displayed the incoming sig- nal as a function of time delay. Since the TOF of the ions is dependent on their mass and charge, a mass spectrum was calibrated from the TOF spectrum. The mass spectrum depicted in Figure 3.9 was recorded with an ELV temperature of 160^◦C.

mass/charge ratio in u/e

0 50 100 150 200 250 300 350 400

oscilloscope signal in mV

-0.04 -0.03 -0.02 -0.01 0

Mass spectrum

+DIBN

+[DIBN-I]

I+

+[DIBN-2I]

+O2H+xCH +xH2C 2+[DIBN]

+xH3C +xH4C

+H +Aniline

+xH5C +3H6C

Figure 3.9: Mass spectrum of the DIBN sample heated to 160^◦C. Molecules were frag- mented due to laser induced Coulomb explosion and fragments were recorded according to their mass-to-charge ratio. All signals were assigned to either DIBN fragments, impurities or background gases.

Each signal was assigned to a suitable fragment originating from the DIBN beam. The parent ion DIBN⁺ had the largest mass-to-charge ratio of 355 u/e. Also fragments of DIBN could be assigned to several signals. The weak water signal and the signal of C2Hx⁺, which could also be assigned to N2, were originating from residual gas in the gas feed through. The strongest signal with a mass-to-charge ratio of 93 u/e was assigned to Aniline with the help of previous recorded DIBN mass spectra at this machine [27].

Aniline might be a byproduct or impurity in the used DIBN sample. The high intensity of the Aniline signal is due to the much higher vapor pressure of Aniline compared to DIBN at 160^◦C [46, 47]. The fact that the Aniline signal decreased over time much faster while all other signals showed the same intensity supports this assumption. After one hour of operating the valve the Aniline signal was completely gone. All signals between the Aniline and water peak were assigned to CxHx⁺ cascades. These signals were also

found in previous DIBN mass spectra at this machine [27] and could origin from Aniline fragmentation when compared to Aniline mass spectra [48, 49]. However, their intensity did not decrease over time like the Aniline parent ion signal, which could also mean that they are actually fragments of DIBN.

After four hours of continuous heating and operating the ELV all signals originating from the molecular beam vanished. There were only H⁺ and H2O⁺ signals in the TOF spectra visible coming from background ions in the chamber. The spectra showed no changes whether the ELV was turned on or not. The ELV seemed to be blocked when DIBN was heated over several hours. This behaviour was observed previously on this machine at an operating temperature of 190^◦C and an ELV repetition rate of 250 Hz [27]. Since the here discussed TOFs were obtained at even lower repetition rates (100 Hz) and lower temperatures, it has to be considered that DIBN might not be the best first sample for electron diffraction on this machine. A failure of the ELV with DIBN was unexpected because previous measurements carried out with DIBN on other machines had been suc- cessful before [26]. However, within those measurements the ELV was operated at an even lower repetition rate of 60 Hz and the valve was only opened for 14 µs [9]. In the course of this thesis all measurements with DIBN were carried out with an ELV opening time of 20 µs. The longer opening time might cause enhanced damaging of DIBN molecules on the valve and lead to a rapid failure. Unfortunately, a high repetition rate and long opening times are necessary to have a high molecular density in the interaction region and hence increased signal. For future measurements with DIBN a balance between ELV operating rate and risk of damaging the valve has to be found. Parts of the here used ELV might already be damaged quite a lot by DIBN and it should be considered to exchange them.

Further characterization of DIBN as a candidate for electron diffraction was not realizable because a stable molecular beam could not be generated.

3.2.2 Characterization of 1,2-Diiodoethane molecular beam

DIE was chosen over DIAB as the second candidate for electron diffraction because pro- vision of DIE was much faster. Furthermore, it exhibits a higher µ/m ratio (Table 2.1) allowing for strong deflection, which had been successfully shown in previous works [50].

For preparation the sample was ground and inserted into the ELV cartridge with some fil- ter paper similar to DIBN (subsection 3.2.1). The crystalline DIE, shown in Figure 3.10, had a light brownish colour before it was ground. Since the melting point of DIE lies around 82^◦C the sample had to be heated. To create a molecular beam the molecules were seeded in 90 bar of Helium. The molecular beam was roughly aligned with the set-up and a TOF spectrum was recorded with an ELV temperature of 75^◦C and VMI mode of the spectrometer. The TOF spectrum was calibrated into a mass spectrum to determine the molecular composition of the beam. In Figure 3.11 all signals were assigned to ions or fragments according to their µ/mratio. Signals originating from DIE fragments could be assigned to several peaks when compared with literature DIE mass spectra [51].The signal at 254 u/e most likely origins from I2⁺ impurities which were probably generated during the synthesis of DIE [50]. The strong signal at 28 u/e was assigned to N2⁺ impurities in the beam which could origin from atmospheric nitrogen leaking into the gas line. Even though the gas line was pumped several times the nitrogen signal did not vanish com- pletely. When applying 90 bar of Helium onto the gas line no nitrogen can leak into the gas line. The remaining signal at 28 u/e therefore could also origin from C₂H_x fragments of DIBN. The impurity at 60 u/e was assigned to singly charged carbonyl sulfide (OCS) that was used in prior experiments on the machine. This signal could easily be removed by pumping away the rest OCS and did not appear in later mass spectra. For the sig- nals at 42 u/e and 170 u/e the assumption was made that they might origin from singly charged iodopropane and its fragment propane. Iodopropane could be a left over reagent that was used for synthesizing DIE or it was generated as a side product. To verify these

Figure 3.10: Photograph of the DIE sample before it was ground to powder. After grinding the powder had turned colourless.

mass/charge ratio in u/e

0 50 100 150 200 250 300

oscilloscope signal in mV

-0.010 -0.005 0.000

Mass spectrum

[DIE-I]+

170

I+ DIE⁺

+

CHx

OCS+

+ 43 N2

+

I2

H+

+

Hx

C2

Figure 3.11: Mass spectrum of the DIE molecular beam heated to 75^◦C. All signals were assigned to DIE fragments or left over molecules from previous experiments. Signals labelled in red are impurities in the beam.

assumptions images of all impurities in velocity map configuration of the spectrometer were recorded. In this configuration all fragments with the same velocity and mass hit the same spot on the detector. Therefore, these images show the velocity distribution of the recorded fragments which served as a support in assigning them to specific molecules.

For comparison, the velocity map image of the parent ion DIE⁺ is shown in Figure 3.12a.

All parent ions have about the same velocity from the molecular beam because there is no additional momentum added when they are ionized in the 800 nm laser focus. That is why only one major signal is detected on the velocity map image. The VMI of I₂⁺ (Fig- ure 3.12b), on the other hand, shows a higher velocities distribution. This counteracts the assumption that I2⁺ would be present in the sample as a molecule. If there was molecular iodine in the sample they should all have the same velocity after ionization as typical for parent ions. The VMI predicts that either molecular iodine is gaining momentum or that iodine atoms gain some momentum before they form I2. It is not yet fully understood how this I2⁺ signal is generated and if it origins from molecular iodine in the sample or is generated during the ionization process of another iodine containing molecule. The VMI of the signal at 170 u/e, depicted in Figure 3.12c, shows the typical velocity distribution of a parent ion with mainly one apparent velocity. This supports the assumption that this signal is caused by iodopropane. The corresponding VMI of the presumably propane frag- ment, shown in Figure 3.12d, underlines this assumption. The propane fragments should show some velocity distribution because they gain momentum when iodine is split off of iodopropane in the ionization process. This distribution is clearly visible in the VMI and proves that there is iodopropane present in the molecular beam. The strong 28 u/e signal assigned to N2⁺could additionally origin from iodopropane and DIE fragments like C2Hx

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170 u/e

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43 u/e

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Figure 3.12: VMIs of DIE molecular beam impurities and DIE parent ion as reference.

Parent ions like DIE⁺ show a narrow velocity distribution indicating that the 170 u/e signal in c) also origins from a parent ion. The velocity distribution in the VMI of I2+ in b) and 43 u/e in d) is larger indicating that these signals origin from smaller fragments.

as literature mass spectra predict [51, 52] and explain the high intensity. The presumably iodopropane impurity in the DIE sample should not amount over 1 % according to the sample supplier. The much higher intensity of iodopropane signals over the DIE signals can be explained by its higher vapour pressure of 57 hPa [53] compared to 30 hPa of DIE [54]. This leads to a higher ratio between impurity and target molecule than the initial 1 % in the powder sample. The iodopropane signals could be removed by running the valve for some time, leaving a pure DIE sample in the valve. The impurities also vanished when the valve was heated to higher temperatures. A corresponding mass spectrum at an ELV temperature of 110^◦C is shown in Figure 3.13. In this spectrum only DIE related signals were recorded but the used temperature lead to burning of the sample over time and hence to a decreasing signal. For this reason, all following measurements with DIE