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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Microwave spectroscopy is a technique used to study the interactions of molecules with microwave radiation. It is primarily concerned with the rotational energy levels of molecules, which correspond to transitions between different rotational states. Microwave spectroscopy involves exposing a sample to microwave radiation and measuring the absorption or emission of this radiation as the molecules transition between their rotational states. The technique takes advantage of the fact that different molecules have unique rotational spectra, allowing researchers to identify and characterize them based on their rotational transitions.