Chemical Analysis by Microwave Spectroscopy

I. INTRODUCTION Microwave spectroscopy is concerned with the observation and analysis of transitions between molecular rotational energy levels. These transitions fall within the segment of the electromagnetic spectrum called the microwave region. This region lies between the conventional radio wave region and the infrared region and extends from frequencies around 1,000 MHz (λ=30 cm) to around 1,000,000 MHz (λ=0.3 mm). In this spectral region, it is common practice to express frequencies in megahertz (MHz) or gigahertz (GHz), where the Hertz (Hz) unit denotes cycles per second, 1 MHz=106 Hz, and 1 GHz=109 Hz. The region may be further divided into the centimeter-wave (λ= 1 to 30 cm), millimeter-wave (λ=1 to 10 mm), and submillimeter-wave regions (λ< 1 mm). Whereas the infrared region is associated with the phenomena of molecular vibrations, the microwave region is associated with molecular rotations. Microwave spectroscopy usually implies gas-phase molecular rotational spectroscopy, though other areas of spectroscopy, such as EPR, operate in the microwave region. The term molecular rotational resonance (MRR) spectroscopy has also been applied.