Absorption spectrum of nitrous acid for the ν1+2ν3 band studied with continuous-wave cavity ring-down spectroscopy and theoretical calculations

We present the high resolution absorption measurements of gaseous HONO at room temperature using continuous-wave cavity ring-down spectroscopy in the near-infrared region between 6017 and 6067 cm−1 at a resolution of 1 pm (0.037 cm−1). For the trans-HONO isomer an extensive analysis of the ν1+2ν3 combination band 6045.8089 cm-1 was performed starting from the results of a previous study for the 11 and 31 vibrational states [Guilmot J-M, Godefroid M, Herman M. Rovibrational parameters for trans-nitrous acid. J Mol Spectrosc 1993;160:387-400]. The present combination band is perturbed because of the existence of several dark states of HONO which could not be identified unambiguously. The rotational constants achieved for the 1132 state deviate slightly from the values which are predicted from the rotational constants achieved in the previous studies for the 11 and 31 vibrational states of trans-HONO.