State selected unimolecular dissociation of HOCl near threshold:: The 6νOH vibrational state

The spectroscopy and unimolecular dissociation dynamics of HOCl are examined by accessing rotational resonances of the 6 nu(OH) vibrational level over the K-a=0-5 manifolds using overtone-overtone double resonance. The spectroscopic analysis indicates that state mixing between the zeroth-order "bright" O-H stretching overtone state, 6 0 0, and "dark" background vibrational levels is incomplete as the bright state couples to only a fraction of the available states. The coupling of 6 0 0 to a set of nearby dark states is mediated primarily by anharmonic coupling with the fourth-order vibrational resonance k(1,223) playing a particularly important role through its ability to couple the 6 0 0 state directly to the 5 2 1 vibration and indirectly to the 4 4 2 vibration. The measured state-specific unimolecular dissociation rates for 6 0 0 show large fluctuations with J and K-a and are substantially slower than that expected on the basis of statistical theory. The rate fluctuations are interpreted on the basis of spectroscopic data which suggest that the fluctuations arise as a result of variation in state mixing as different dark vibrational states come in and out of resonance with the bright state for different values of J and K-a. (C) 1999 American Institute of Physics. [S0021-9606(99)01333-1].