Zerfallszeiten von Molekülionen. II

Abstract

The previously described apparatus for the measurement of decomposition times of short-lived organic ions has been modified. With the improved version the accessible range of times between ionization and decomposition extends now from about 5·10-9 sec to 5·10-6 sec, although in one case times as short as 3 · 10-9 sec could be distinguished. The substances studied were benzonitrile, n-butane, n-heptane and benzene. The logarithm of the differentially measured decomposition rate was plotted versus the logarithm of the time. All of these plots were found to be straight lines, indicating approximate decomposition laws di/dt ∼ t-r. From the empirical parameter r the distribution of rate constants contributing to the overall decay was evaluated. This distribution led to a determination of the function k(E) which describes the increase of the rate constant k with increasing internal energy E in the ion and is an important molecular property, hitherto only calculable under various assumptions. The calibration of the energy scale of k(E) was based on previous measurements of ionization curves of “normal” and “metastable” fragments. The k(E)-curves for butane and heptane are compared with the most recent experimental and theoretical breakdown curves, with generally good agreement with regard to the energy scale and relative rate constants. The absolute magnitude of the rate constants is surprisingly small and the rise with energy quite slow. It appears that for the decomposition of benzonitrile no rate constants larger than 5·108 sec-1 occur. The amount of fragmentation taking place in times shorter than 10-8 sec was accordingly found to be much smaller than generally assumed; in benzonitrile only 35% of all mass 76 fragments present after 10-6 sec are formed within the first 10-8 sec. For butane and heptane partial mass spectra for an observation time of 10-8 sec could be evaluated. They are significantly different from the ordinary mass spectra due to different decomposition laws for different fragments. For benzonitrile, the decomposition function was also measured for different electron energies, starting near threshold. The expected result, a decreasing contribution from high rate constants with decreasing Uel, was established and also reproduced by a calculation.