= Microwave spectroscopy
<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.
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