Time-of-flight analysis

(5.4) For the simulation of spectra of scattered molecules in this work the parameters Γ= 0.1 cm⁻¹, σ = 0.4 cm⁻¹, A_L = 0.3, and A_G = 0.7 are used. If no saturation ef-fects are present and the efficiency of the ionization step does not depend on the initial and intermediate state the intensity of a 1+1⁰ REMPI transition is proportional to the transition dipole moment squared. Equation 2.24 gives the relevant matrix element for the calculation of the transition dipole moment. The matrix element can be split into a vibronic and a rotational part. Since in this work only the rotational state distribution in the ground state has been probed the vibronic part is the same for all analyzed transitions and can be used as a constant factor to match the integrated ion signal over the whole band. The rotational part can be solved by evaluation of Equation 2.24. The central part of this task is the calculation of direction-cosine matrix elements, which are tabulated in reference [109]. All the calculations mentioned above are included in the MATLAB code given in the Appendix A.1.

5.2. Time-of-flight analysis

The scattering experiments on nitric oxide and formaldehyde differ fundamentally in the way the starting point of the time-of-flight experiment is defined. In the scattering of highly vibrationally excited nitric oxide only direct-scattering is observed. The narrow angular distribution simplifies the analysis. Furthermore, the starting point is defined by the nanosecond laser preparation step in a small part of the molecular beam and thus the initial beam of vibrationally excited molecules can be approximated as a point source.

This is clearly not the case in the formaldehyde ground state scattering since the initial

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5.2. Time-of-flight analysis distribution is given by the whole molecular beam pulse which is on the order of ten microseconds and depends on the seeding conditions. Moreover, for the formaldehyde scattering broad angular distributions are observed in some cases, which indicates that an analysis must include velocity components parallel to the laser beam used for detection (y-axis). Thus the analysis of the two experiments is discussed separately in the following two sections.

5.2.1. Nitric oxide

The analysis described in the following has been developed following the methods de-scribed in reference [101]. The procedure for time-of-flight experiments on surface scattering of highly vibrationally excited NO surface scattering including the position-ing of preparation and probe lasers is described in Section 4.1.3. First, the analysis of time-of-flight experiments in the incoming beam is described. Second, the analysis of time-of-flight experiments for scattered molecules is described. The flight distance l_in between pump and probe laser can be determined following the methods described in Section 4.1.3. The arrival time of the Pump laser pulse serves as the starting pointt =0 in the time-of-flight experiment. By delaying the probe laser with respect to the preparation lasers the time-of-flight of the molecules from the preparation position to the probe posi-tiont_incan be varied precisely. The probe laser is set to a fixed wavelength for the detection of a single quantum state characterized by the rotational and vibrational quantum numbers J,Ω, andvvia 1+1 REMPI spectroscopy. The ion signalI_ionis recorded as a function of the time-of-flightt_in. In general it has been shown that the flux velocity distributionF(v) in a molecular beam can be described by a floating Maxwell Boltzmann distribution given in Equation 5.5.[110]

In this equation N₁is a normalization factor,vis the velocity, andv₀is the center velocity.

The parameterαdescribes the width of the velocity distribution. Since in the time-of-flight experiment the velocity of a single molecule is constant the relation v = l_in/t holds. We can transform Equation 5.5 into a flux arrival time distribution F(t)by multiplying with the Jacobian ^dv_dt =−^l

5. Analysis

In the experiment the molecules are detected in a small volume by a nanosecond laser pulse (probe laser diameter 1 mm). Because the velocity of the molecule is on the order of 10²−10³ m/s the molecules are approximately stationary during the laser pulse. For a realistic estimate of the velocity spread (10 %) and a flight length of l_in = 20 mm the spatial dimension of the molecular beam pulse will be larger than the dimension of the detection region. Thus, under the conditions of the experiment the recorded ion signal is proportional to the density, which means that slower molecules are detected more efficiently than faster molecules. Similarly, the preparation step also selects a higher number of slower molecules. Since the density can be converted into flux by multiplying with the velocity the relationship between the ion signal recorded in the experiment and the flux is given byI_ion,in(t_in) ∝ ^F(tin)

The parameters N₃,v₀, andαcan be fitted to the experimental arrival time distributions.

The determination of the best fit parameters allows the extraction of relevant quantities like the average velocity in the incoming beamhvi_in according to Equation 5.8.

hvi_in =

The normalization factor can be obtained from _N¹₁ =∫∞

0 v³exp

− ^v−v_α⁰2 dv.

The velocity distribution in the scattered beam is derived in a similar manner. However, the velocity of the molecules in the incoming beam has to be known in order to derive the time of the collision with the surface. Since the preparation step takes place very close to the surface it is justified to approximate that there is no spread of the beam pulse during the short flight length between preparation position and surface. In this case the time difference between the laser preparation step and the collision is given by∆t = hvi_ind_ps. The relevant time-of-flight for scattered moleculest_sc can be calculated from the delay between probe and preparation laser pulsest_Laser byt_sc = t_Laser −∆t. The function used to described the recorded ion signal for scattered moleculesI_ion,sc is from a mathematical point of view identical to the one used for the incoming beam and given in Equation 5.9.

The relationship between recorded ion signal and fluxI_ion,sc(t_in) ∝ ^F(tsc)

v² still holds because

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5.2. Time-of-flight analysis the directly scattered molecule’s velocity is determined by the incidence velocity of the molecule. Equation 5.9 is used to fit the arrival time distributions obtained for scattered molecules.

I_ion,sc(t_sc)dt_sc = N₃· l_sc²

t_sc³ exp −

l_sc/t_sc−v₀ α

₂!

dt_sc (5.9)

The average scattered velocity can be extracted in analogy to Equation 5.8.

5.2.2. Formaldehyde

In the following discussion of the analysis it is helpful to remember the chosen coordinate system given in Figure 4.1. The most probable velocity of formaldehyde in the molecular beam can be measured experimentally by a simple method. The arrival time distribution of the molecules in the incoming beam I_ion,in(x,z,t₀)is measured at different positions on the x-axis along the propagation direction of the incoming beam by delaying the probe laser firing with respect to the nozzle opening. Then the probe position x can be plotted against the maximum pointt_max,inof the arrival time distribution at the respective position.

The slope of this plot yields the most probable velocity in the incoming beamv_mp,in. The intersection with thet-axis yields the time of the collision with the surface. One can also analyze the arrival time distributions for scattered molecules following a similar approach.

Again one can extract the most probable scattered velocityv_mp,scfrom the slope of a plot of the flight lengthl = x²+z²against the most probable arrival timet_max,sc. This method neglects the y-coordinate and is only applicable if the distribution over the scattering angle θ is very narrow. For direct-scattering this requirement is fulfilled. However, for trapping-desorption this is not the case and another approach described below is more reliable in extracting the velocity distribution.

In the following an expression for the expected ion signal for desorbing formaldehyde molecules detected at a position defined by the coordinates x,y and z will be derived. The flight distance from the surface to the probe position is defined by d = (x²+ y²+z²)^0.5. The flux velocity distribution of molecules desorbing from a surface can be described by Equation 5.10.[110]

F(v)dv = N₄·v³exp

− mv² 2k_BT

cos(θ)dv (5.10)

As long as effects of detailed balance are unimportant the effective temperatureTis equal to the surface temperature T_surf. With the substitutions v = d/t and cos(θ) = x/d we

5. Analysis

The measured ion signal for a single starting point in timeI_ion,tdcan now be related to the flux by taking into account that the detector is sensitive to the velocity along thexz-plane v_xz =(x²+y²)^0.5/t. Thus, the ion signal is proportional to the density: I_ion,td ∝ F(v)/v_xz. Following this argumentI_ion,td can be described by Equation 5.12.

I_ion,tddt = N₅· x(x²+y²+z²)^3/2

Under the laser focusing conditions applied in the experiment the ion detector is sensitive to all formaldehyde molecules betweeny₀ =1.5 cm andy₁=−1.5 cm with a sharp cutoff for higher and lower values for y. Thus, we have to integrate over the y-coordinate in order to obtain a reasonable description of the arrival time distribution. In addition, the FWHM of the molecular beam pulse at the surface is on the order of ten microseconds.

Thus, the simulation of the ion signal for a single starting point in time is convoluted with the temporal profile of the incoming beam at the position of the surface I_ion,in(0,0,t₀). This can be accurately measured by moving the surface out of the detection region.

Equation 5.13 describes a simulation of the observable arrival time distribution taking the two considerations mentioned above into account.

I_ion,td,conv(t)=N₅

The parametersN₅andTcan be varied to optimize the fit of the simulation to the observed arrival time distribution. The difference between simulation and experimental observation is minimized in a nonlinear least-square fitting procedure: The integrals in Equation 5.13 are solved numerically and subsequently the simulation is compared to the experimental observation.

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6. Scattering of highly vibrationally