The 3 nu(1) overtone band of trans-nitrous acid: Rotational and perturbation analysis and absolute intensity

High-resolution (0.013 cm(-1)) spectra of the second overtone of the OH str etch of trans-nitrous acid were recorded between 10 230 and 10 350 cm(-1). The spectra were analyzed to yield a complete set of rotational parameters, including the A, B, and C rotational constants and the 5 quartic distortio n constants. Two groups of anharmonic perturbations were observed and analy zed, and for one of these groups, the spectra were fit with a second model to extract an improved set of parameters that took into account these inter actions. Much less information could be obtained from our spectra about the second group of perturbations, except that the same dark vibrational state was probably not responsible for both groups of perturbations. The anharmo nic coupling matrix element between the 3 nu (1) "bright" state and one "da rk" vibrational state was found to be 0.0338(31) cm(-1), and possible assig nments for the "dark" level are discussed. In addition, we report the first measurement of the absolute intensity of the 3 nu (1) overtone band, which was found to be 2.1 +/-0.6x10(-21) cm(2) molecule(-1) cm(-1). Our findings are discussed in the context of the contribution of overtone-mediated proc esses to OH radical production in the upper atmosphere. (C) 2001 American I nstitute of Physics.