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.