Double-resonance overtone photofragment spectroscopy of trans-HONO. I. Spectroscopy and intramolecular dynamics

Using the technique of double-resonance overtone photofragment spectroscopy (DROPS), we have measured rotationally resolved vibrational overtone trans itions to the previously unobserved 5v(1), 6v(1), and 7v(1) levels of gas-p hase trans-nitrous acid (HONO) in its electronic ground state. Observing th e onset of dissociation from different rovibrational states of 5v(1) near t hreshold determines the HO-NO bond energy to be D-0=16 772 +/- 14 cm(-1). O bserved spectral splittings and broadening of individual rovibrational tran sitions provide quantitative data on the rate and extent of collision free vibrational energy redistribution that would result after coherent ultrasho rt pulse excitation. In parallel with these frequency domain measurements, we determine the unimolecular dissociation rates directly in time for trans -HONO molecules excited to several rotational states near threshold. The co mbination of time- and frequency-resolved data allows us to estimate the li newidth contributions from the finite dissociation lifetime of the molecule . Our results reveal intramolecular dynamics that are clearly not a simple function of the vibrational energy but rather depend sensitively upon speci fic couplings and, in turn, on the vibrational character of the individual states excited. (C) 2000 American Institute of Physics. [S0021-9606(00)0032 0-2].