 Fourier transform microwave spectroscopy

Fourier transform microwave spectroscopy excites gaseous molecules with a pulse of electromagnetic radiation, then records the emitted time-dependent electric field. A Fourier transform extracts transition frequencies from the decaying signal. In a cavity implementation, two facing mirrors form a resonator; tuned excitation, receiver electronics and digitization replace the molecule-counting readout of molecular beam electric resonance. The pulsed radiation prepares quantum coherence between rotational quantum states; it does not set the whole gas spinning at one classical angular velocity.

For documented laboratory context, Evolution of Microwave Spectroscopy at the National Bureau of Standards (NBS) and the National Institute of Standards and Technology (NIST) by F. J. Lovas and others (§5, printed pp. 280–282) describes the NIST cavity instrument, phase-locked sources, mirror tuning and detector-mixer modifications. That account explains one instrument family; it does not establish the electronics of every later commercial spectrometer.