The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCl in the v(OH)=6 region, probed by infrared-visible double resonance overtone excitation

We use infrared-visible double resonance overtone excitation to promote HOC l molecules to single, well-characterized rotational levels of high OH stre tching states just above the HOCl-->HO+Cl dissociation threshold on the gro und potential energy surface. Double resonance spectra are monitored by las er induced fluorescence detection of the OH dissociation products. We prese nt here the results obtained in the 6 nu(1) region of (HOCl)-Cl-35 where we have studied states with J ranging from 4 to 25, K-a from 0 to 5 and energ y up to 300 cm(-1) above the dissociation threshold. In the spectra for K-a =0-3 states, the zeroth-order (n(OH),n(theta),n(OCl))=(6,0,0) level is spli t by mixing with a nearby dark state. Because the two states have very diff erent A rotational constants, their separation increases with K-a, but the effects of the mixing remain observable in the spectrum up to K-a=3. Compar ison with preliminary results from (HOCl)-Cl-37, together with analysis of the rotational constants, allows us to identify the perturbing state as (4, 4,2). The lack of further strong perturbations compared to the average dens ity of states allows us to infer that most of the matrix elements for coupl ings between the (6,0,0) bright state and other dark states are less than s imilar to 0.1 cm(-1). The average intramolecular vibrational energy redistr ibution (IVR) rate implied by these matrix elements (2.5x10(9) s) is two or ders of magnitude longer than the predictions of statistical rate theory, i ndicating that IVR is likely to be the rate limiting step in the unimolecul ar dissociation process from (6,0,0). The present work provides the spectro scopic foundation for direct time-resolved studies of the unimolecular diss ociation dynamics presented in a forthcoming paper. (C) 1999 American Insti tute of Physics. [S0021-9606(99)00725-4].