Extraction of representative kinetic energy parameters from photofragmentation time-of-flight spectra

The shape and width of lines observed in time-of-flight mass spectroscopy applied to molecular photofragmentation contain information on the initial momentum and energy distributions of the fragment ions. The problem of extracting this information is discussed in perspective. Usual determinations of a single kinetic energy parameter may, depending on the line shape, lead to quite biased results. A simple and intuitively representative solution is suggested instead. It is shown that, under the conditions of isotropic emission and absence of kinematical discrimination, the mean kinetic energy is proportional to the variance of the line distribution. This parameter is rather insensitive to noise and easy to correct for instrumental and thermal broadening, as is demonstrated on spectra of the decay CS22+→S+ + CS+, observed at various energies of the mother ion.