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 CO/sub 2/ laser radiation. This was done by investigating experimentally the rotational spectrum of the ..nu../sub 6/ and ..nu../sub 8/ excited states in the 300--400 GHz range. The identified pump transitions and the known CO/sub 2/ laser frequencies made it possible to determine the central frequencies of the ..nu../sub 6/ (33 122 609 MHz) and ..nu../sub 8/ (31 982 778 MHz) bands to within +- 50 MHz. It was found that the ..nu../sub 6/ and ..nu../sub 8/ 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.