Causas exo-sistema

The major advantage of beam experiments over the flowing afterglow studies is the ability to study reactions individually and in more detail. It is possible to measure the ionisation cross section o in Equation 3.1 at a single velocity, rather than the quenching rate constant k which is integrated over all velocities. Both the ejected electrons and the positive ions are available for study. It is therefore possible to measure the ionisation cross sections for associative and dissociative reactions as well as examining the Penning ionisation electron spectrum. In addition, it is possible to measure the angular distribution of scattered metastable atoms and product ions.

Beam experiments are of two types, gas cell and crossed beam arrangements. Gas cell experiments are most useful for making absolute measurements of ionisation cross sections. Crossed beam apparatuses are more flexible but are less capable of making absolute measurements.

3.4.1 Gas cell experiments

The gas cell technique was developed by Sholette and Muschlitz (1962) and was the first technique developed to measure the absolute total ionisation cross section of rare gases by metastable helium. The technique has been further developed by many experimenters including several in the present author’s laboratory such as Dunning and Smith (1971) and Jerram and Smith (1985), whose experimental set-up is shown in Figure 3.2.

Jerram and Smith’s apparatus is as follows. The metastable beam source is a constricted- arc hot-cathode type designed by Trujillo (1975) with a wide velocity output (a more detailed analysis of the variety of sources used by experimenters is presented in chapter 4). The velocity selector designed by Trujillo (1975) is used to produce a metastable atom beam of variable velocity. A helical quench lamp of the Hotop et al (1969) type is used to remove singlet metastables from the beam (see chapter 4.3).

The gas cell, shown in some detail in Figure 3.2, is a sophisticated detector consisting of an outer earthed box and an inner box which can be biased positively or negatively and connected to either the slats or grid. Gas is admitted to the cell and the pressure carefully monitored by a calibrated Pirani gauge. Currents due to the metastable flux J are measured

at the monitor grid and currents due to positive ions Ig are measured at the fine grid. The electron current Is, due to ionisation of the reagent gas and the emission of secondary electrons from the back face of the gas cell is measured at the slats. A computer is used to record and average the outputs from two electrodes connected to the monitor grid and slats.

M etastab le Velocity Electric

atom source s e le c to r field P irani

p la te s g au g e Quench lam p G asinlet G as cell D iffusion pump Diffusion pump Diffusion pump 0.25 m To Pirani

g a u g e I n n e r_box Fine_{g rid}

To e lectro m ete r B ia s o r in n e r box voltage G as inlet M onitor g rid electrom eter S l a t s In su latin g su p p o rt O u te r box

Figure 3.2 Gas cell apparatus of Jerram and Smith (1985)

To determine the absolute ionisation cross section the following relation is used

l n (

= -floi+lntyiO

(3'3)

J

relative to the slats. With the box and slats positive relative to the grid Ig+ is measured. The secondary emission coefficient for metastable helium atoms on gold (all surfaces are coated with gold) is y. The length of the gas cell is 1, the number density of reagent gas atoms in the gas cell is n and K is a constant.

The derivation of Equation 3.3, is given by Jerram and Smith (1985) and will not be repeated here. It is clear that a plot of ln((Is"+Is+-2Ig+)/J) against n will be a straight line with gradient -ol, so that o can be determined if currents and n and 1 are known. The number density n is deduced from an absolute measurement of the pressure of the gas in the cell.

The advantage of this method over other gas cell experiments such as Woodard et al (1978) is that it is not necessary to determine y. Also the use of slats in place of the grid wires used by Dunning and Smith (1971) eliminates a problem identified by Jerram (1985) of extra ionisation caused by electrons orbiting the grid wires. The biggest problem with the experiment are the small current magnitudes and the necessity to determine the absolute number density of the reagent gas. Also the path length 1 may be larger than assumed due to elastic scattering, but this effect is likely to be small. Despite the difficulties of the measurements, fewer assumptions and uncertain calculations are necessary than in the afterglow experiments. Nevertheless the earlier flowing afterglow results are generally consistent with the results of Jerram and Smith (1985).

3.4.2 Simple crossed beam experiments

The most adaptable experimental arrangement is one in which the reagent gas is introduced to the metastable beam not in a cell but in the form of a beam. However the results are usually not absolute and have to be normalised, for example to the flowing afterglow rate constants or absolute cross sections as measured by Jerram and Smith (1985). The crossed beam technique was used for the work described in this thesis. The two beams cross at right angles and the ionisation products are separated in a mass spectrometer or detected in total. The remaining metastables may be detected as may those which have been elastically scattered. The electrons produced are also available for study.

The simplest system is that used by West et al (1975) (Figure 3.3) to measure the branching ratios of Penning and dissociative ionisation. No velocity selector is used and so

measurements can only be made over a distribution of collision velocity. This means that