Formation, fragmentation, and ionization dynamics of transition metal-carbon, nitrogen, and oxygen clusters.

摘要

Several different laser and mass spectroscopic techniques are employed to investigate the formation, photo-fragmentation, and ionization dynamics of transition metal carbide, nitride, and oxide clusters. First of all, the ultraslow ionization dynamics, or delayed ionization, of transition metal-carbide clusters is studied. The delayed ionization behavior is investigated at a variety of wavelengths and the experimental data are compared with a statistical model adapted from the model for thermionic emission from a bulk metal surface, appropriately adjusted to account for the finite size of the clusters studied. The delayed ionization, which is observed at all wavelengths studied, is well described by the statistical model. The temporal dependence of the ionization is found to correlate with the ionization potentials of two systems whose values are available and the ionization potential trends of the mixed metal Met-Cars can by predicted using the thermionic emission model.; Secondly, the photofragmentation of vanadium oxide cluster cations is investigated. At low laser fluence, the major photofragmentation channel observed is the loss of O₂ or consecutive O₂ units from clusters that are oxygen rich. When the laser fluence is increased, additional fragmentation channels are observed. The results suggest that VO₂ and VO have lower ionization potentials than larger vanadium oxide clusters.; Thirdly, the formation of transition metal-nitride clusters is examined. Both neutral and cationic clusters are studied. The neutral metal nitride clusters are formed when full dehydrogenation of the ammonia is achieved, while the production of cationic “precursor” species are formed under conditions where a large fraction of the ammonia remains intact. The present study provides insight into the dehydrogenation processes involved in the production of transition metal nitrides from metal ions and ammonia.; Finally, the ultrafast ionization dynamics of titanium and vanadium carbide clusters are probed using femtosecond laser techniques. The first time-resolved measurements of the electronic relaxation dynamics in the vanadium Met-Car, V₈C₁₂ are presented. The findings reveal a similar relaxation behavior, for clusters containing four or more metal atoms, that is longer than the cross correlation width of the laser pulses. In addition, the delayed ionization behavior is absent under femtosecond ionization conditions.