FABRICACIÓN DEL CONL

7.1 Conclusions

Total and partial cross sections for collisions of Ar, C 0 2, N20 , CH4, C2H2, NH3 with helium atoms both in the He(23S) and He(21S) metastable states of helium have been made and also for SF6 with the triplet state only. The total ionisation cross section for He(21S) + Ar is in good agreement with other available data at low collision velocity (below 5kms_1), and there is no reason to believe that the results at high velocity (5-20 kms'1) are not equally reliable, although the error bars are larger. In every other case the variation of total ionisation cross section for collisions with He(21S) are the first to be measured.

The procedure of normalising the singlet cross section with the same normalisation constant as for the corresponding triplet cross section has been tested in the case of argon and is found to be very satisfactory. This technique has allowed comparisons to be made between absolute measurements of total ionisation cross sections and both present triplet and singlet results, where several absolute measurements exist. The other available absolute results are at single energies, and normally fall within the error bars of the present cross sections.

The triplet measurements agree well with the previous results of Jerram (1985) and in most cases extend his measurements into higher and lower energy regions. The only significant disagreement is in the case of methane, which is likely to be the result of much better performance of the present quench lamp than that used by Jerram.

The branching ratios are the first energy-dependent ratios measured in the case of collisions with the singlet metastable state, as are the cases of N20 and SF6 for the triplet metastable state. Where comparisons are possible in the singlet case, agreement with other single energy measurements is within the error bars, which are large at energies above 0.2 eV. For the triplet case, the agreement with the energy-dependent measurements of Jerram is good, and the present results increase the accuracy by an order of magnitude. In most cases the fraction of Penning ionisation decreases with energy and associative ionisation is only apparent for argon.

The present total ionisation cross sections for both metastable species with argon have been compared with theoretically generated cross sections. In the triplet case it is possible to obtain a good agreement between experiment and cross sections calculated using any of the model potentials, provided a saturating form of imaginary potential is used. The potential which gives the best agreement is that of Siska (1979).

In the singlet case, the potential devised by Olsen (1972) is not satisfactory, as it does not reproduce the steeply rising cross section at low energy. The interaction potential derived by the Pittsburg group (Martin et al 1978) is superior to that of the Freiburg group (Haberland and Schmidt 1977), again using the saturating form of imaginary potential.

In both singlet and triplet cases, the necessary saturation in the imaginary part of the interaction potential reduces the ability of these potentials to produce theoretical high energy differential elastic cross sections which fit experimental data. To reproduce the present experimental results, the saturation in the imaginary part of the interaction potential was introduced at lower internuclear separation than in the work of Burdenski et al (1981). The present work allows the theoretical interaction potentials to describe both elastic and inelastic processes satisfactorily to higher interaction energies than any previous experiments.

7.2 Possible improvements and suggestions for further work

It is almost certainly possible to improve the detection efficiency of the present ion detector. This would be done by replacing the electron multiplier with the available multi-

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channel plates of larger detecting area (1000 mm compared with 314 mm ). A mounting identical to that described in chapter 4 could be used.

The quench lamp used in the present experiment has proved to be more than three times as effective as the previous lamp introduced by Harper (1977). However it is not ideal, tending to overheat. This leads to unwanted outgassing in the lamp and makes refilling and baking the lamp frequent necessities. This might be overcome if the present air cooling system were replaced by a water cooling system. Alternatively the 2058 nm resonant radiation could be produced by means of a 1064 nm YAG laser pumping a tunable dye laser. This would be expensive but is the most likely way to achieve full quenching at velocities up to and above 2 0 kms' 1

As stated in chapter 5, in order to reduce by a factor of three the magnitude of the error bars in the present experiment, it would be necessary to increase the amount of data recorded by a factor of ten. This would best be achieved by increasing the count rate of the experiment but to do this it would be necessary to change from a single-shot to a multi-shot TOF system. This would require a change to a faster computer, fast enough to accept counts a few microseconds apart, and to operate in the multi-channel-analyser mode. The TOF mass spectrometer would need to be replaced in favour of a quadropole mass spectrometer because the time delay between the detection of electron and ion in the TOF type would lead to multiple start pulses in a very high count rate experiment.

With a redesigned interaction region it would be possible to observe the ejected electron energy distributions for different molecules. This could be done by using the multi-channel plate detector to its full capacity as a position sensitive device. Electrons would be collimated as in the Hotop et al (1977) system and then spread out according to energy by passing them through an electric field perpendicular to their trajectory. The multi-channel plates could be inclined with respect to this trajectory to increase sensitivity. A one­ dimensional position sensitive anode would be used (or a combination of a phosphor strip and charge-coupled device). The present TOF system would be useful for recording ejected electron energy distributions (EEED) as a function of collision energy, which has been rarely done.

By further collimation of the metastable atom beam and the introduction of a second channeltron detector displaced from the present one by several degrees, it would be possible to perform differential elastic scattering experiments, in a similar manner to that of Kroon (1985), with the cross section measured as a function of energy at a fixed angle. This would help, as it already has in the case of argon, in the formulation of theoretical potential surfaces for the interaction of the systems described in this thesis.

Appendix 1