Rotational Spectroscopy - Historical Achievements

Historical Achievements

The first study of the microwave spectrum of a molecule (NH₃) was performed by Cleeton & Williams in 1934. Subsequent experiments exploited powerful sources of microwaves such as the klystron, many of which were developed for radio detection and ranging (RADAR) during the Second World War. Early RADAR tests illustrated starkly the significance of molecular absorption at microwave frequencies. Charles Hard Townes recalls how atmospheric water vapour absorbed the microwave radiation from one source limiting the range to eight miles in RADAR applications. The number of experiments in microwave spectroscopy surged immediately after the war. By 1948, Gordy was able to prepare a review of the results contained in approximately 100 research papers. Microwave spectroscopy became established as a uniquely powerful tool for the precise determination of structural details such as bond lengths and angles in gas phase molecules.

The invention of an instrument that achieved microwave amplification by stimulated emission of radiation (MASER) was reported by Townes et al. in 1955. The invention of the MASER ultimately stimulated the development of modern lasers which are used in applications as diverse as compact disc players, surgery and barcode readers. The identification of molecules in the interstellar medium was a highlight of the 1960s. Microwave transitions were measured in the laboratory and matched to emissions from the interstellar medium using a radio telescope. NH₃ was the first stable polyatomic molecule to be identified in the interstellar medium. Many molecules of varying size and complexity have since been identified through radio astronomy. The combination of rotational spectroscopy and radio astronomy remains important in modern astrophysical chemistry.