Identification of active transitions in a formic acid vapor laser

Fourteen lines were identified in the submillimeter emission spectrum of an HCOOH molecular laser pumped by CO2 laser radiation. This was done by investigating experimentally the rotational spectrum of the ν6 and ν8 excited states in the 300–400 GHz range. The identified pump transitions and the known CO2 laser frequencies made it possible to determine the central frequencies of the ν6 (33 122 609 MHz) and ν6 (31 982 778 MHz) bands to within ±50 MHz. It was found that the ν6 and ν8 vibrational states interacted. Identification was also performed by a double resonance method which did not require a study of the rotational spectra of excited vibrational states.