LEED is one of the first and most commonly used methods for investigating the structure of periodic systems such as surfaces [190]. Similarly to X-ray scattering techniques, LEED is based on the constructive interference of an incident (k) and an outgoing wavevector (k’) so that
k2=k’2 (3.3)
The incident and outgoing electrons are typically in the energy range 20-1000 ev and thus have typically a mean free path between 5 and 20 Å. Thus LEED is a surface sensitive technique (note that LEED has a high surface sensitivity as both the incident and outgoing electron have a low mean free path). The components of the wavevector parallel and perpendicular to the surface can be separated and only the component parallel to the surface is conserved.k’kin equation 3.3 can be written as
k’k+ =k2k+ghk (3.4)
whereghk is a combination of the reciprocal lattice vectors. Thus, from a pattern on the LEED screen, the periodicity of the surface or adsorbate structure can be directly obtained.
To deduce information on the actual location of the atoms within the unit mesh, the diffracted beam intensities have to be compared to results from a quantitative scattering theory. For further information on this subject, I refer to the references [190, 191].
A schematic representation of a LEED apparatus is shown in Figure 3.5. The electrons are
Abbildung 3.5: Schematics representation of the essential parts for a LEED experiment emmitted from the electron gun and the scattered electrons leave the sample in the direction of the LEED screen. To ensure that only the electrons with the energy of the source reach the screen, a negative potential is applied on the inner grids G2 and G3. The screen is biased at a
positive voltage (5-6 keV) to accelerate the transmitted electrons to a high kinetic energy so that light emission is caused on the coated fluorescent glass screen.
4 Experimental Setup
In this chapter, explicit information on the experimental setup, that was used for this work will be given. This subject can be separated into two parts, one concerning the setup for the SCAC experiment with calibration and the other the setup for the preparation of the model catalysts.
The SCAC experiment, which is used in this work is based on the pyroelectric heat detection technique of Campbell et al. [62]. To allow for the study of supported model catalysts, facilities for the preparation of such have been integrated into the UHV-apparatus.
Figure 4.1 shows an overview of the apparatus, applied for the present studies. Preparations of
Abbildung 4.1: Schematic overview of the experimental setup, consisting of two separate cham-bers (from [123]). The chamber on the right hand side is used for preparing the model catalysts and the adsorption/reaction chamber is used for performing the SCAC experiments with the according calibration
the model catalysts are performed in the preparation chamber, while the the adsorption/reaction chamber is used for the SCAC experiments. Both chambers are independently pumped and sepa-rated by a gate valve. A translational rod is used to transfer the sample between both chambers.
The transfer stage, indicated on the right part of Figure 4.1, is separated from the preparation chamber by a gate valve and pumped by a 50 L/s turbo molecular pump (Pfeiffer, TMH/U 071).
On top of the Mo sample holders, a T-piece is added, which allows for an easy transfer between the transfer rod and the sample holders in the adsorption and in the preparation chamber by using wobble sticks.
4.1 Preparation chamber
The preparation chamber is continuously pumped by a 500 L/s turbo molecular pump (TMP) (Pfeiffer, TMU 521 P), the typical base pressure in this chamber is 1−2·10−10mbar. During sample preparation, the mounted crystal remains in a rotatable xyz-manipulator (VAb GmbH), which is equipped with a filament for electron bombardment and K-Type thermocouple connec-tors for temperature measurements. Figure 4.1 shows the equipment, situated around the cen-tral part of the preparation chamber: two metal evaporators (Omicron, EFM 3), a sputter gun (Omicron, ISE-10), a quartz crystal microbalance (Sigma Instruments) for calibrating the flux of the evaporating metals, a combined LEED/Auger electron spectrometer (AES) unit (SPECS, ErLEED) and a quadrupole mass spectrometer (Hiden, Halo 201). A gas doser is fixed on a translational stage with a 10µmopening to the gas supply. Either thick (1 mm) samples or thin (1µm) samples are mounted on the Mo sample holders with a central whole, which is 8 mm in diameter.
Besides AES and LEED, which can be used on both samples, TPD studies can be performed on the 1 mm thick single crystal. Therefore, the K-type thermocouple connectors are point-welded to the crystal, which can be heated by radiation from the filament or electron bombardment.
The temperature ramp is controlled by an external Labview program, while the desorbing gas is detected with a QMS. Details on the standard setup of AES/LEED and TPD experiments are described elsewhere [190, 191]. To allow for low temperature measurements, a reservoir inside the manipulator can be filled with liquid nitrogen. In this assembly, it is possible to maintain the crystal temperatures in the range 100 K- 1200 K. The temperature of the 1µmthin single crystal is monitored by a pyrometer (Sensotherm, MP25), as mounting of a contact probe would destroy the crystal. As the pyrometer is only calibrated for a fixed emissivity, which is different from the emissivity of Pt, calibration to the output voltage of the pyrometer by a K-type thermocouple on a 1mmthick Pt(111) crystal has been performed.
4.1.1 Setup for the reflectivity measurement
A reliable knowledge of the sample reflectivity is crucial for the energy calibration with a laser during the microcalorimetric measurement. The reflectivity measurement is performed with a Helium-Neon laser (Lasos, 632.8 nm, 2 mW, cw) in a setup shown schematically in Figure 4.2.
Laser, polarizer, beam splitter and the two photodiodes are mounted on a platform and can be attached to the UHV system, so that the laser points through the borosilicate window to the center of the chamber. To monitor the intensity of the incident laser beam, a fraction of the laser light is deflected by a prism to a photodiode before passing through the window. The fraction of the laser light, which is not deflected by the beam splitter impinges on the sample at an angle of∼5◦ to the surface normal. The reflected laser light passes through the window again, its intensity is measured by a second photodiode (Silicon Sensor, PS100-2). The reflectivity is obtained by the following equation:
IRe f lection/ILaser=C·re f l (4.1)
IReflectionis the intensity of the reflected laser light and ILaseris the intensity of the incident laser light. C is a factor, which is dependent on the experimental setup andre f l is the reflectivity.
In order to obtain C, calibration with five dielectric mirrors (LayerTec) of known reflectivities
(41.4pm0.4 %, 59.7±0.9 %, 76.7±1.1 %, 86.2±0.2 % and 96.6±0.1 %) is carried out. Since the beam splitter is polarisation dependent and the polarisation of the initial laser beam changes with time, the usage of a linear polarizer, which is indicated in Figure 4.2, is necessary.
Abbildung 4.2: Setup for the reflectivity measurement, Laser, polarizer, beam splitter and the photodiodes are attached on the outside of the UHV system (from [123]).