The material reviewed herein gives some idea of the value and promise of nonlinear Raman spectroscopies in studies of molecular complexes and clusters. It is clear that such spectroscopies have addressed, and will continue to address, important issues that are difficult or impossible to address by other means. It is also apparent that the different nonlinear Raman methods have different strengths with regard to these kinds of studies. SRL and CRS are particularly suited to the study of large homogeneous clusters. They have allowed for (1) the unprecedented characterization of condensation dynamics in free-jet expansions,22,74,75 (2) the elucidation of cluster phases,19,20,22,75 (3) the determination of cluster temperatures,19,20,22(4) the characterization of low-frequency collective modes in cluster ground states,11,19,20 and (5) the assessment of the degree of long-range order in finite-size systems.19,20,22,75 Mass-selective IDSRS, on the other hand, is particularly valuable in size- or species-selective studies of small-to-medium size species, be they homogeneous or heterogeneous in nature. Its strengths have allowed for or facilitated (1) the characterization of the size dependence of ground-state collective modes in small clusters,58,60,65,73,80 (2) the elucidation of solvent-shell accretion in solute‒solventn clusters,84,85,96–98 (3) the precise characterization of complexation-induced splittings, shifts, and widths of ground-state intramolecular vibrational resonances, results relevant to the structure and dynamics of small complexes and clusters,12,28,29,47,61 (4) the correlation of ground-state and excited-state vibrational intervals,58,65,73 and (5) the measurement of the vibronic spectra that arise from excited vibrational levels in the ground-state manifold.28,29 Thus, one sees that the nonlinear Raman methods are complementary to one another, as well as being complementary to other classes of spectroscopic techniques. The nonlinear Raman spectroscopy of molecularbeam samples is very far from being a mature field. One expects that many more studies of the type reviewed herein will be reported in the future. Of course, one also expects that there will be developments with regard to increasing the information content and extending the range of such studies. An increase in the routine spectral resolution available is one development that is obviously desirable and quite feasible. The extension of size-selective experiments to larger and larger clusters is also both desirable and feasible. Besides these developments, one expects that new classes of Raman-based experiments will soon become prevalent. A picosecondresolved version of mass-selective IGSRS is conceivable and may well provide a widely applicable means by which to study ground-state cluster dynamics directly in the time domain. Such a method would also make possible, by means of rotational coherence spectroscopy,102 the measurement of rotational constants associated with vibrational levels in the groundstate manifold. Application of mass-selective IDSRS to the study of laser-generated, jet-cooled species such as radicals, metal clusters, and biomolecules is another promising future direction. So is the use of stimulated Raman action spectroscopies in the study of mass-selected cluster ions. Given all this, it is reasonable to expect that nonlinear Raman methods will remain a fruitful source of ground-state spectroscopic information on jet-cooled species for many years to come. © 1994, American Chemical Society. All rights reserved.
