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D. R. Johnson coauthored research on microwave spectroscopy.^[ref]

Microwave spectroscopy measures a system's response in the microwave frequency range. Molecular rotation, atomic hyperfine structure and transitions shifted by external fields can occur at these frequencies; the physical transition must be identified rather than inferred from the word microwave alone.

D. R. Lide, Jr. coauthored research on microwave spectroscopy.^[ref]

Microwave spectroscopy measures a system's response in the microwave frequency range. Molecular rotation, atomic hyperfine structure and transitions shifted by external fields can occur at these frequencies; the physical transition must be identified rather than inferred from the word microwave alone.

F. J. Lovas coauthored research on microwave spectroscopy.^[ref]

Microwave spectroscopy measures a system's response in the microwave frequency range. Molecular rotation, atomic hyperfine structure and transitions shifted by external fields can occur at these frequencies; the physical transition must be identified rather than inferred from the word microwave alone.

A maser uses stimulated emission to amplify microwaves. With population inversion and feedback it can sustain a coherent wave field as an oscillator. The name abbreviates "microwave amplification by stimulated emission of radiation".^[ref]

Why make molecules supply an oscillator? Their energy levels offer a reproducible frequency reference, while stimulated emission lets successive molecules reinforce one coherent wave field. In his 1964 lecture, Charles Hard Townes traced his motivation to microwave spectroscopy: resolving molecular structure required useful coherent sources.^[ref] The interesting balance is between molecular amplification and escaping energy: even a beam with population inversion fails to oscillate if cavity losses win. This is the microwave counterpart of laser threshold.

Microwave spectroscopy measures a system's response in the microwave frequency range. Molecular rotation, atomic hyperfine structure and transitions shifted by external fields can occur at these frequencies; the physical transition must be identified rather than inferred from the word microwave alone.

A frequency-scanned source can drive a sample while a detector records absorption or a changed state population. The Lamb shift experiment provides a historical atomic example: a microwave field transfers metastable hydrogen atoms to a state that decays rapidly, allowing a downstream state-sensitive signal to reveal the resonance. The measured quantity is a change in detector response versus frequency, from which a transition energy is inferred.

R. D. Suenram coauthored research on microwave spectroscopy.^[ref]

Microwave spectroscopy measures a system's response in the microwave frequency range. Molecular rotation, atomic hyperfine structure and transitions shifted by external fields can occur at these frequencies; the physical transition must be identified rather than inferred from the word microwave alone.