INFRARED DIODE LASER SPECTROSCOPY OF TRIPLET $C_{3}$: THE $\nu_{3}$ BAND OF THE $\tilde{a}{^{3}\Pi_{u}}$ ELECTRONIC STATE

Abstract

The lowest-lying electronically excited state of $C_{3}$, denoted $\tilde{a}{^{3}\Pi_{u}}$ is metastable because its decay to the singlet ground state is spin forbidden. This work reports the observation of a gas-phase infrared absorption spectrum between 1428 and $1473 cm^{-1}$ which is assigned to the $V_{3}$ fundamental band of the $\tilde{a}{^{3}\Pi_{u}}$ state. Metastable $C_{3}$ was produced in flowing mixtures of helium and acetylene discharged in a methanol-cooled hollow cathode cell, and the resulting plasma was probed with an infrared diode laser source. The observed band consists of three R and three P branches, which show staggering due to A-type doubling effects and nuclear spin statistics. Since the lower level has been observed previously in the (000)-(000) band of the $\tilde{b}{^{3}\Pi_{g}} - \tilde{a}{^{3}\Pi_{u}}$ electronic $transition,^{1}$ unambiguous rotational numbering of the new spectrum could be made by comparing lower state combination differences. The band origin, near $1450 cm^{-1}$, is close to the value of $1455.3 cm^{-1}$ determined previously from matrix-isolation $experiments.^{2}$