Chapter 15 - Microwave Spectroscopy: Molecular Systems

Abstract

Microwave (MW) spectroscopy has mostly been—and sometimes even by spectroscopists still is—considered to be a tool to obtain molecular structures. The development of Fourier transform microwave (FTMW) spectroscopy and, most notably, its combination with supersonic-jet expansion techniques allow for challenging investigations of chemically and physically interesting molecular systems. The combination of Fourier transform MW (FTMW) spectroscopy with supersonic-jet expansions allowed the study of the rotational spectra of weakly bound molecular complexes. These are, among others, in situ prepared molecular species such as radicals, ions, and other transient species, generated by combining an electrical discharge, laser ablation, or laser photolysis, and highly dynamical systems including weakly bound molecular complexes. From these latter systems, information on nonbonding intermolecular interactions and on the internal dynamics is easily obtained. Many chemical problems, difficult to unravel with other techniques, were solved only by rotational spectroscopy experiments on supersonic jets. The widely unexplored interactions in the intermediate regime between bonding and nonbonding, molecular recognition, molecular aggregation, and many more are investigated with these techniques without having to rely on ab initio calculations—their support is of great help both in guiding spectroscopic searches and in interpreting the spectra.