The last decade has seen tremendous growth in the study of gas phase clusters. Some areas of cluster research which have received considerable attention in this regard include solvation (Lee et al. 1980), (Armirav et al. 1982), and reactivity (Dantus et al. 1991; Khudkar and Zewail 1990; Rosker et al. 1988; Scherer et al. 1987). In particular, studies of the dynamics of formation and dissociation, and the changing properties of clusters at successively higher degrees of aggregation, enable an investigation of the basic mechanisms of nucleation and the continuous transformation of matter from the gas phase to the condensed phase to be probed at the molecular level (Castleman and Keesee 1986a, 1988). In this context, the progressive clustering of a molecule involves energy transfer and redistribution within the molecular system, with attendant processes of unimolecular dissociation taking place between growth steps (Kay and Castleman 1983). Related processes of energy transfer, proton transfer, and dissociation are also operative during the reorientation of molecules about ions produced during the primary ionization event required in detecting clusters via mass spectrometry (Castleman and Keesee 1986b), providing further motivation for studies of the reaction dynamics of clusters (Begemann et al. 1986; Boesl et al 1992; Castleman and Keesee 1987; Echt et al. 1985; Levine and Bernstein 1987; Lifshitz et al. 1990; Lifshitz and Louage 1989, 1990; Märk 1987; Märk and Castleman 1984, 1986; Morgan and Castleman 1989; Stace and Moore 1983; Wei et al. 1990a,b). The real-time probing of cluster reaction dynamics is a facilitating research field through femtosecond pump-probe techniques pioneered by Zewail and coworkers (Dantus et al. 1991; Khundkar and Zewail 1990; Rosker et al. 1988; Scherer et al. 1987). Some real-time investigations have been performed on metal, van der Waals, and hydrogen-bonded clusters by employing these pump-probe spectroscopic techniques. For example, the photoionization and fragmentation of sodium clusters have been investigated by ion mass spectrometry and zero kinetic energy photoelectron spectroscopy in both picosecond (Schreiber et al. 1992) and femtosecond (Baumert et al. 1992, 1993; Bühler et al. 1992) time domains. Studies have also been made to elucidate the effect of solvation on intracluster reactions.
Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers
