Messung der Zerfallszeiten von Molekülionen

Abstract

The time interval between the ionization of a molecule by electron impact and its dissociation was determined from the kinetic energy of the fragment ions. This technique allows a direct differential sampling of the dissociation rate at various times, in contrast to previous, integral methods. The use of a disc-shaped molecular beam of 28 μ half-width and a draw-out field of 10 000 V/cm in the ionization region resulted in a minimum measured time interval of 7.4·10-9 sec. The longest times measured were between 5 and 6 μsecs. The time dependence of the decomposition rate was measured for two decompositions in butane, five in heptane, three in benzene, one in CD4 and one in CO2++, with rather similar results. It was established that the decay is not exponential. All decay curves, plotted semi-logarithmically, were found to have an increasing slope towards shorter times, indicating contributions from a variety of rate constants, in agreement with the predictions of the statistical theory of ion decompositions. 50 delayed decompositions in the μsec region were investigated, about half of which had not been reported previously. For these measurements the detection limit was about 10-5 of the base peak intensity. In particular, such "metastables" were found in methane and the deutero-methanes. They were explained by considering the external rotation of the ion. Small fragmentation energies set free during the decomposition can be determined from the kinetic ion energy. The detection limit is as low as 5 mV for the total fragmentation energy, since the method is unaffected by thermal translational energy. Some rules concerning these energies were found.