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 transitions to the previously unobserved 5v(1), 6v(1), and 7v(1) levels of gas-phase trans-nitrous acid (HONO) in its electronic ground state. Observing the onset of dissociation from different rovibrational states of 5v(1) near threshold determines the HO-NO bond energy to be D-0=16 772 +/- 14 cm(-1). Observed spectral splittings and broadening of individual rovibrational transitions provide quantitative data on the rate and extent of collision free vibrational energy redistribution that would result after coherent ultrashort 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 combination of time- and frequency-resolved data allows us to estimate the linewidth 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 specific couplings and, in turn, on the vibrational character of the individual states excited. (C) 2000 American Institute of Physics. [S0021-9606(00)00320-2].