Microwave Spectroscopy

Abstract

Microwave techniques have opened the way to an entirely new branch of spectroscopy. In microwave absorption spectroscopy, radiation of known frequency is passed through the material to be studied, and absorption of the radiation is detected by a reduction in the intensity of the transmitted radiation. The methods of microwave spectroscopy allow determinations not only of low-lying molecular energy levels, but also of low-lying atomic energy levels, usually due to noncoulombic interaction between the atomic nucleus and the surrounding electrons. Transitions between these low-lying atomic energy levels are associated with magnetic dipole moments, rather than electric dipole moments, hence the interaction with radiation of microwave frequency is weak. The essential components of a microwave spectrograph includes a microwave generator, with means for determining its frequency of oscillation, an absorption cell consisting of a section of waveguide that can be evacuated and filled with the gas to be studied, and a microwave detector. In addition to the immediate application of microwave absorption data to the determination of molecular structure, there are a number of other important applications in the fields of chemistry and of nuclear physics. These applications include determination of interatomic bond distances and molecular configuration; determination of electronic structure of molecules; study of internal rotation in molecules; calculation of thermodynamic quantities; determination of nuclear moments, spin, and statistics; and determination of mass differences of isotopes.