Infrared absorption spectroscopy using a free-electron laser

The spectral bandwidth of an infrared free-electron laser is rather broad (≥10 cm−1 FWHM) due to the short duration of the micropulse (1 ps). It is demonstrated that by resolving the transmitted laser beam using a spectrometer, it is possible to record IR spectra of gases that are limited in resolution only by that of the spectrometer. Over 20 air absorption lines (due to water vapor) are recorded over one free spectral range of the free-electron laser. As a further demonstration of the spectroscopic technique, an IR absorption spectrum for ammonia is recorded for two vibration–rotational transitions.