NEAR-UV ABSORPTION CROSS-SECTIONS AND TRANS CIS EQUILIBRIUM OF NITROUS-ACID

The A 1A" <-- X 1A' absorption spectrum of gaseous nitrous acid has been measured in the 300-400-nm range. Absolute cross sections were determined by a combination of gas-phase and wet chemical analysis. The cross sections of prominent bands are 25% larger than the recommended values of Stockwell and Calvert. The influence of spectral resolution on absolute and differential absorption cross sections was also investigated. The integrated band area of the n-pi-* transition yields an oscillator strength f = (8.90 +/- 0.36) X 10(-4), less than the reported liquid phase value of 2 X 10(-3). The equilibrium constant K = rho-trans/rho-cis, based on the assumption that the oscillator strength of the n-pi-* transition is the same for both rotamers, was found to be 3.25 +/- 0.30 at 277 K. This yields an energy difference DELTA-E between trans- and cis-HONO of -2700 J mol-1 in the electronic ground state, and -6000 J mol-1 in the excited state.