Van der Waals Complexes in Supersonic Beams. Laser Spectroscopy of Neutral-Neutral Interactions

During the past several decades there has been growing interest in the properties of van der Waals complexes. The early studies of excimers and exciplexes were carried out in discharges and vapour cells, using conventional light sources and flash photolysis techniques, and the resulting emission and fluorescence spectra yielded information on the respective potential energy diagrams, and also led to various conclusions concerning the nature of the molecular forces and potentials involved in the formation and persistence of the complexes. Some of the investigations of mercury excimers were motivated by the (unsuccessful) search for a suitable process that would lead to a Hg2 molecular dissociation laser. Developments in laser instrumentation led to corresponding refinements of the spectroscopic techniques and increased the reliability of the potentials derived from the analyses of the spectra. At the same time, theoretical calculations of the potentials became increasingly ambitious and sophisticated, permitting meaningful comparison with the experimental data.
The advent of supersonic expansion beam techniques and their application to the spectroscopy of van der Waals complexes made possible studies of many additional molecular states, including the ground states which are very weakly bound and are largely unstable at discharge and vapour-cell temperatures. However, at expansion beam temperatures of a few K, it is possible to produce adequate densities of ground-state complexes, which can be probed using techniques of laser-induced fluorescence.
The combination of these various experimental methods can now produce reasonably precise and reliable information on molecular potentials whose character is by no means straightforward, since it changes from small internuclear distances through the equilibrium bond lengths to long-range separations of the constituent atoms. Thus it is not surprising that a number of theoretical approaches has been proposed for the calculation of potentials in the various regions of internuclear separation, that would realistically reproduce those obtained from the experiments, and lead to a better understanding of the forces binding the complexes. In addition to the various “ab initio” calculations, one should note the interesting and sophisticated methods developed by LeRoy and his group for calculations of long-range interatomic potentials from experimental data. Knowledge of molecular potentials that bind diatomic (and triatomic) metal and metal-rare gas complexes becomes useful in investigations of the properties of the corresponding metal clusters, many of which are also held together by van der Waals forces and some of whose properties are intermediate between those of molecules and surfaces. There is also a burgeoning field of research on Bose-Einstein condensation and low-temperature trapping of atoms and photoassociated molecules, which can profit from the type of information yielded by investigations of van der Waals complexes, and one can foresee an expanding range of uses for results obtained from spectroscopic (and theoretical) studies of these molecules.
The author of this monograph has provided us with a fully up-to-date account of the spectroscopy of van der Waals complexes consisting of group 12 and group 2 metals and rare gases, having himself carried out a significant portion of the experimental work. The reader will find detailed descriptions of the experiments, extensive summaries of the resulting data and references up to the year 2002. There are also very useful sections describing the various theoretical potentials, methods of analysis of the spectra, and comparisons between experimental data and theoretical predictions. The extensive introductory chapters will familiarize the reader with the underlying physical principles of the subject, and this tutorial section of the book may be found to be very valuable by those readers who wish to become acquainted with this field of research or by students interested in learning about the spectroscopy of van der Waals complexes.