Cluster Spectroscopy: Variations in Ionization Potentials and Spectral Shifts as a Function of Degree of Aggregation and Studies of Cluster Fragmentation
Studies of trends in the variations of spectral features, ionization potentials, and dissociation processes of clusters as a function of degree of aggregation are presented. The results bear on such questions as the changing properties of systems undergoing transitions between the gas and the condensed phase, as well as the origin of magic numbers. Investigations of the spectral shifts of an electronic transition in a chromophore such as paraxylene or phenylacetylene show that clusters containing from 3 to 15 argon atoms all undergo a red shift of about 50 cm-1 as a limiting value in the S1 state. Evidence for spectroscopic changes between the gaseous and the condensed state is also apparent from the broadening of linewidths.
A major advance in the study of unimolecular dissociation has become available through the use of multiphoton ionization coupled with a reflectron introduced into the drift region of a time-of-flight mass spectrometer. Using single and two-color tunable pulsed lasers, the excess energy introduced into a cluster can be well controlled. The power of this method is demonstrated by the results of recent investigations of hydrogen bonded clusters such as ammonia and methyl alcohol and also clusters of rare gas atoms which, following ionization, lead to an internal ion-molecule reaction and subsequent cluster fragmentation. The role of dissociation and the influence of the thermochemical stability of cluster ions in effecting the appearance of magic numbers in certain cluster distributions is discussed. The application of this method in determining ionization potentials of probe molecules following successive clustering with a solvent species is also presented. The results of studies of dielectrics are contrasted with trends found for alkali metal systems.
A final related topic is that of internal ion-molecule reactions following multiphoton excitation of clusters. A finding of some importance is an internal Penning ionization process taking place in certain clusters leading to electron transfer between the chromophore and the solvent molecules. Findings of a delayed electron transfer reaction, having implications to the bulk condensed state, are also presented.
KeywordsIonization Potential Spectral Shift Magic Number Argon Atom Multiphoton Ionization
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