As described earlier, measurements of the sticking coefficient give valuable information on the dynamics of the adsorption process and the number of adsorbed/reacting molecules can be ob-tained in case the molecular flux is known. Detection of the background pressure is possible by mass spectrometry, which will be explained in more detail after discussing the general principle of sticking measurements.
3.3.1 Measurement principle
An early method which is still applied today for determining sticking coefficients is to measure the increase in the surface coverage with flash desorption methods [185] as a function of the deposition time. The experimental error of this method, however, has been reported as ∼50
% [186]. A more accurate procedure for the determination of sticking coefficients has been introduced by Bell and Gomer, in which the reflected flux of molecules is sampled with a Field emission tip [187]. In 1972, King and Wells introduced a method which is comparably simple for determining the sticking coefficient and coverage accurately [188].
The principle of a King-Wells sticking type measurement is shown in Figure 3.3. Initially, a gold flag is situated in front of the sample, on which the molecular beam is directed to. As the sticking coefficient of many molecules at room temperature is zero on gold, the impinging molecules are usually reflected from the gold flag. This causes a transient pressure rise in the chamber, which is detected with a QMS. The QMS signal during this molecular beam experiment is indicated in the inset of Figure 3.3.
Subsequently, the gold flag is removed and the molecular beam is directed onto the sample surface. This results in the adsorption of molecules from the molecular beam on the surface, which gives rise to a smaller pressure increase compared to the measurement on the gold flag.
As the surface is being covered by adsorbates with increasing exposure, less molecules are being adsorbed and the pressure rise in the chamber increases with time.
As the gold flag is in a slightly different position than the sample, the scattering geometry is
Abbildung 3.3: Representation of the measurement principle, used for measuring the King-Wells sticking coefficient.
changed when removing the gold flag. This influences the QMS signal. To take this into account, an additional experiment with a gold sample is performed. By comparing the QMS signal during impingement of the beam on the sample with the signal during impingement of the beam on the gold flag, the sticking coefficient can be obtained with additionally taking into account this change in the scattering geometry. For details to the evaluation procedure, I refer to section 5.2.
In the present setup, pulses of molecules impinge on the sample which is necessary for the energy measurements. The measurement and detection principle in this special case of a King-Wells
sticking type measurement is described above. For the case that desorption of a surface species occurs in the time scale of the pulse, the situation arises in which molecules adsorb during the pulse and desorb in between the pulses. This has to be taken into account when calculating the sticking coefficient and the coverage.
3.3.2 Quadrupole mass spectrometry
In this work, a quadrupole mass spectrometer has been used for the sticking probability measu-rements and to detect gas phase products that are evolved during the molecular beam measure-ments.
In the first stage of the mass spectrometer, gas is ionized. This is usually achieved by electron impact. Electrons, which are emitted from a cathode, are accelerated onto the initially neutral molecules with energies in the range 10 eV - 100 eV. A fraction of this energy is thereby trans-fered to some of the impinging molecules, which may thereby form ions. These ions are often unstable and form fragments, which is very common for large organic molecules [189].
In the second stage, the ionized fragments are selected according to the mass: charge ratios
Abbildung 3.4: Schematic representation of the quadropole mass filter
(m/z). This is achieved by four electrodes which are arranged parallel to each other, as repre-sented in Fig. 3.4. The voltage on the electrodes consists of a direct current component and of an alternating current component. The direct current is the same on opposing electrodes, whi-le neighboring ewhi-lectrodes are on opposite potentials. The ions, which travel on a trajectory as shown in Fig. 3.4 are deflected by the electric field, produced by the electrodes, unless they have a specific m/z ratio. This ratio is determined by the direct-current potential and the amplitude and frequency of the alternating current component.
After passing the mass filter, the selected molecules are directed onto a detector. The use of chan-neltron electron multipliers are very common for detecting ions. Such a detector is used in our setup. However, they can be only used in low pressure regions (p<10-6mbar). CEM detectors consist of a small tube on which a potential of∼1-3 kV is applied. The impact of the positively charged ions leads to the emission of electrons, which are accelerated towards the anode. On their way, the electrons create an avalanche of secondary electrons, which leads to amplification factors of up to 108.