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Applications
Unlike NMR, Infrared and UV-Visible spectroscopies, microwave spectroscopy has not yet found widespread application in analytical chemistry. It has primarily been used to investigate fundamental aspects of molecular physics. It is a uniquely precise tool for the determination of molecular structure in gas phase molecules. It can be used to establish barriers to internal rotation such as that associated with the rotation of the CH3 group relative to the C6H4Cl group in chlorotoluene (C7H7Cl). When fine or hyperfine structure can be observed, the technique also provides information on the electronic structures of molecules. Much of current understanding of the nature of weak molecular interactions such as van der Waals, hydrogen and halogen bonds has been established through rotational spectroscopy. In connection with radio astronomy, the technique has a key role in exploration of the chemical composition of the interstellar medium. Microwave transitions are 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. . Current projects in astrochemistry involve both laboratory microwave spectroscopy and observations made using modern radiotelescopes such as the Atacama Large Millimetre Array (ALMA). . Fundamental research applying microwave spectroscopy can sometimes yield commercially-significant outcomes. For example, 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.

Absorption cells and Stark modulation
A microwave spectrometer can be most simply constructed using a source of microwave radiation, an absorption cell into which sample gas can be introduced and a detector such as a superheterodyne receiver. A spectrum can be obtained by sweeping the frequency of the source while detecting the intensity of transmitted radiation. A simple section of waveguide can serve as an absorption cell. An important variation of the technique in which an alternating current is applied across electrodes within the absorption cell results in a modulation of the frequencies of rotational transitions. This is referred to as Stark modulation and allows the use of phase-sensitive detection methods offering improved sensitivity. Absorption spectroscopy allows the study of samples that are thermodynamically-stable at room temperature. 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. The number of experiments in microwave spectroscopy surged immediately after the war. By 1948, Walter Gordy was able to prepare a review of the results contained in approximately 100 research papers. Commercial versions of microwave absorption spectrometer were developed by Hewlett Packard in the 1970's and were once widely used for fundamental research. Most research laboratories now exploit either Balle-Flygare or chirped-pulse Fourier transform microwave (FTMW) spectrometers.