Variation of kinetic energy release and average internal energy with temperature and electron energy for unimolecular ionic transitions

Abstract

The reactions of metastable ions occur from a narrow band of internal energies arising from the broad distribution of energies which results from electron ionization. Small changes in the kinetic energy release can be measured accurately, as a function of the temperature of the ion source2-6, or as a function of the energy of the ionizing in a conventional mass spectrometer. Previous m e a s u r e r n e n t~~.~ of the temperature dependence of the kinetic energy release have shown only slight changes, although in some case^^'^,^ the fractional change with temperature was observed to be comparatively large. Beynon and coworkers3. have observed a pronounced temperature dependence for H' loss from n-propano14 and H' loss from methane3 and this has been explained by assuming tunnelling of the H' atom through the centrifugal barrier.The effect of variation of energy of the ionizing electrons on the kinetic energy release have been studied by some Tajima et ~l . ,~ and Curtis et al. ,9 had indicated that the kinetic energy released was in fact independent of the electron energy. However, Rabbih and Selim' had found previously that the kinetic energy released with metastable peaks produced from methylacetophenone isomers changed significantly with changing ionizing-electron energy, for simple bond-cleavage processes.The present communication deals with an investigation of the kinetic energy released in metastable transitions generated by electron ionization of benzyl alcohol and benzylamine. Six out of the seven transitions studied are unimolecular rearrangement transitions. The influence of ion-source temperature and of electron-beam energy on the kinetic energy release value, for these unimolecular ionic transitions will be considered. Furthermore, the effect of temperature and electron energy on the average internal energy (of the reacting ions) for the same transitions, as determined by the intensity ratios (where m* is a metastable peak and [MI+ and [F]' are molecular-ion and fragment-ion peaks, respectively) is also considered. EXPERIMENTALThe metastable peaks were studied using a singlefocusing model MM16F mass spectrometer (VG Micromass, Manchester, UK). The instrument was equipped with a combined electron-ionization/ chemical-ionization ion source and was used in the electron-ionization mode. The conditions were: ion accelerating voltage 3 kV, emission current 100 pA and chamber temperature 473 K during electron-energy measurements.All the metastable peaks observed in this study have a Gaussian shape. The kinetic energy release (T,,,) with each metastable peak was calculated" from the measured width at half height, after correction" for the energy spread of the main ion beam. Multiple measurements were carried out, and the energy release values were reproducible to better than 7%, while the intensity ratios were reproducible to better than 8%. All the peaks were checked for pressure dependence. The results indicate that none of the peaks are pressure induced i.e., they are truly metastable and due to unimolecul...