Discrete Kinetic Energy Research Articles

Kinematically complete final state investigations of molecular photodissociation: two- and three-body decay of laser-prepared H33 s 2 A1'

We have performed kinematically complete investigations of molecular photodissociation of triatomic hydrogen in a fast beam translational spectrometer recently built in Freiburg. The apparatus allows us to investigate laser-induced dissociation of neutral molecules into two, three, or more neutral products. The fragments are detected in coincidence and their vectorial momenta in the center-of-mass frame are determined. We demonstrate the potential of the method at the fragmentation of the 3 s 2 A 1 ′ ( N = 1, K = 0) state of triatomic hydrogen. In this state, three-body decay into ground state hydrogen atoms H+H+H, two-body predissociation into H+H 2 (v , J), and photoemission to the H 3 ground state surface with subsequent two-body decay are competing channels. In the case of two-body predissociation, we determine the rovibrational population in the H 2 (v , J) fragment. The vibrational distribution of H 2 is compared with approximate theoretical predictions. For three-body decay, we measure the six-fold differential photodissociation cross-section. To determine accurate final state distributions, the geometric collection efficiency of the apparatus is calculated by a Monte Carlo simulation, and the raw data are corrected for apparatus efficiency. The final state momentum distribution shows pronounced correlation patterns which are characteristic for the dissociation mechanism. For a three-body decay process with a discrete kinetic energy release we have developed a novel data reduction procedure based on the detection of two fragments. The final state distribution determined by this independent method agrees extremely well with that observed in the triple-coincidence data. In addition, this method allows us to fully explore the phase space of the final state and to determine the branching ratios between the two- and three-body decay processes.

A method for calculating the shapes of peaks resulting from fragmentations of metastable ions in a mass spectrometer. II. Peak shapes arising from a distribution of kinetic energy releases: determination of distribution function

Following the calculations described in the preceding paper (part I), which determine the MIKE peak shape arising from a discrete kinetic energy release, a method is presented for extending the calculations for the determination of the kinetic energy release distribution, n (T), from any experimental peak shape. This new approach has the advantage, compared to previous work, that the distribution can be obtained directly and does not involve any trial and error methods. It applies equally well where discrimination occurs against some of the product ions having components of velocity parallel to the length of the instrument slits. A variety of peak shapes have been investigated and several examples are given of the energy release distribution for various ionic reactions. Charge separation reactions of doubly-charged ions have been examined and in one case, the reaction 91 2+ -> 52 + in toluene, the energy release function exhibits fine structure, which has not previously been observed.