Comparación del VEM entre experimento y simulación

Three different gas cells were used in the DLMSAA, the first of which is shown in figure 2.3.4. This cell was the most simple, consisting o f a six-way Conflat-flanged cross. LiF windows mounted on gate valves were used to transmit the light through the cell, the absorption path length of which was 160 mm. On/off valves were used to trap the gas sample within the cell, and a baratron manometer was mounted on the back of the cross to monitor the pressure. This cell allowed photoabsorption data to be measured for static gas samples at fixed temperatures.

’ In practice, up to 500 data points could be recorded in any one scan. Therefore, when small wavelength increments (0.05 nm) were used to record high resolution spectra, the maximum scan range was 25 nm. Such scan ranges were recorded at different regions o f the spectrum (e.g. 200-225 nm, 220-245 nm, etc.) and the obtained data were subsequently joined together.

Gas Inlet Baratron mounted at rear of cross On/Off valve LIF window mounted on gate valve rotary pump

Figure 2.3.4. Simple gas cell used in the DLMSAA

The second cell was constructed with the intention to investigate the effect of temperature on absorption spectra, however, this facility was not used m this work. The cell was constructed from stainless steel and mcorporated a double wall, the required cooling/heatmg medium was flowed through the cavity between the walls to govern the temperature of the gas sample (figure 2,3 .5). LiF windows were sealed onto the ends of the cell to allow the synchrotron light to pass through the gas sample. This double walled cell was mounted on a Confiât flange which was bolted onto a small vacuum chamber evacuated by a turbomolecular pump, this chamber was inserted into the DLMSAA at the position marked ‘gas cell’ in figure 2.3.3. The absorption path length of the cell was 160 mm, and a baratron gauge was used to monitor the pressure.

Baratron To rotary pump Double walled cell On/Off valve Gas inlet / ^ C o o l a n t out Coolant in Confiât flange LiF window Small vacuum

chamber To turbo pump

Figure 2.3.5. Variable temperature gas cell.

The third gas cell was constructed from borosilicate glass to enable experiments to be performed with highly reactive radical species, such radicals (e.g. CIO) react readily on metal surfaces but

less so on glass surfaces The glass cell had LiF windows fused onto the ends to allow VUV light to pass through. The cell was mounted on a Confiât flange, and was bolted into the vacuum chamber to replace the variable temperature gas cell. The glass cell had a gas inlet and a gas outlet, allowing the gas under analysis to flow through the cell. The pressure was monitored with a baratron mounted on the outlet line, and the absorption path length was 140 mm (figure 2.3.6).

Baratron

Gas Inlet To rotary

pump Confiât flange Glass cell LIF window Small vacuum

chamber To turbo pump

Figure 2.3.6. Glass gas cell.

The different gas cells were used to record photoabsorption data for the molecules studied in this work. The results of such studies are presented m this thesis, and m some cases the data are compared to those obtained using tlie techniques of electron impact spectroscopy.

2.4 Electron Optics

As described in section 1.5, the methods of electron impact spectroscopy can provide detailed mformation on the spectroscopy of molecules and their interactions with charges. This section reviews the equipment used in such experiments, and describes the principles underlying the technology used to construct the specific spectrometers described in sections 2 5-2.8.

2.4.1 Electron beam sources

In any electron impact experiment, a source of electrons is required in order to make an incident electron beam. There are several different types of electron beam sources, and the particular type used depends on the requirements of the experiment.

If low energy electrons are required, a gas-photoionisation source may be used. In this type of source, a gas (e.g. argon) is irradiated and ionised with UV light (e.g. from a synchrotron), the

ejected electrons are then formed into a beam using electron optics. Such electrons are emitted with low kinetic energies, allowing the formation of very low energy beams (e.g. -1 0 meV [R2]). The use o f monochromated synchrotron light also allows beams o f very high energy resolution to be obtained (~5 meV [R3]). However, although low energy beams at high resolution may be obtained using this technique, the beam currents obtained are typically low (-10'^^ A).

Electron beams may also be obtained by using a surface photoemission source. These systems exploit the photoelectric effect: light from a laser is made incident upon a metal/crystal surface which subsequently emits electrons. Such systems are often used as sources o f spin-polarised

electron beams, using specially prepared crystals (e.g. GaAs [G4], Pt [S3]) as the emitting surface. These sources may be used to form electron beams of high energy resolution (-40 meV) with currents of -1 0 ^ A. However, such sources have the disadvantages that the beam current and resolution do not remain constant over time [G4], and the emitting surface must be kept in an ultra-high vacuum environment of at least 10'"^ torr to prevent contamination.

The most commonly used electron beam sources are heated cathode filaments ', this type o f source was used in the electron impact work presented in this thesis. Such sources consist o f a metal wire (often bent into a hairpin shape) heated by an electric current; the metal thus emits free electrons by thermionic-emission. Sources of this type provide high beam currents (-10"^ A), are cheap and easy to use, provide stable beam currents for long lifetimes (-1000 hours), and operate in the presence of gaseous targets at pressures up to -1 0 ^ torr. However, the energy resolution o f the emitted electrons is poor (-500 meV), hence the beam must be monochromated using energy analysers if higher resolutions are required (section 2.4.3). The emission characteristics of the filament depend on the material used for the wire [M2]. The material chosen for the filaments used in this work was thoriated tungsten (ThW); further details may be found in the descriptions of the individual spectrometers (sections 2.5-2.7).