Structure and Dynamics of van der Waals Complexes by High Resolution Spectroscopy

Abstract

Molecular beams represent an extraordinary tool for the study of molecular properties [1]. The expansion of a gas through a small nozzle into a vacuum chamber has a number of consequences which have been extensively exploited to setup different experiments. The adiabatic expansion process results in a very efficient cooling of the sample. An experiment carried out on a sample cooled in a supersonic expansion allows to observe molecular aggregates bound by weak interactions, or metastable species. It is well known that, increasing the distance from the nozzle, the frequency of molecular collisions enormously decreases: the mean free path, especially in a molecular beam experiment with differentially pumped vacuum chambers, can be as large as a few meters. Therefore, if during the first moments of the expansion some molecular aggregate bound by weak forces is formed, it can survive long enough to be observed. In a conventional gas cell a similar aggregate would be immediately broken by collisions. The sample preparation in a supersonic molecular beam makes easier its spectroscopic study. The spectrum can be highly simplified because most of the molecules are in their ground vibrational state and then the hot bands are very weak. Also only a few rotational states are significantly populated and the rotational contour becomes much simpler. The experimental spectral resolution can