Collisional ionization studies of rubidium atoms and small halogenated molecules near reaction thresholds
Brooks, Philip R.
Doctor of Philosophy
An atomic supersonic beam of Rb (seeded in H 2 ) and a gas beam (organic molecules seeded in He or Ar) are crossed to investigate electron transfer reactions close to their energy thresholds, and to measure the electron affinities (EA) of various product negative ions. The ions are collected and identified by coincidence TOF mass spectrometry. The signal from the Rb seeded beam is orders of magnitude higher than that from a Rb charge exchange sources. Most of the energy comes from the Rb beam. Therefore the speed distribution of this beam is measured by time of flight and the speed of the gas beam estimated from theory. Then the energy distribution for the reaction is calculated. The coincidence signals of negative ions are analyzed from fundamental kinetic theory with the reaction threshold an unknown parameter in the equation. A computer program fits the experimental data and finds the optimal value of the threshold. The EA's of all negative ions are then calculated by conservation of energy. C 2 F 5 Cl, C 2 F 5 I, C 6 F 6 , C 2 Cl 4 , C 2 HCl 3 , Cl 2 C=CH 2 , trans-C 2 H 2 Cl 2 and many other molecules have been explored. The experiments produce various parent negative ions and fragments. The initial analysis suggests that C 2 F 5 has a surprisingly higher EA than what has been reported in the literature, but we believe this high value is due to the small fraction of Rb dimer. The parent ion of C 2 F 5 I, which has not been reported previously, is made in this experiment and its EA is determined. For C 6 F 6 the parent ion signal and a long tail electron signal are observed, indicating that the electrons are from autodetachment of the parent ions. The smaller EA compared to other work also supports this argument. The chlorine substituted ethene molecules produce Cl - and parent ions. Analysis shows negative EA's for all the parent ions which suggests they are probably formed in some excited states or the speed of the molecules is over estimated due to the slip between the seed and carrier gas (He).