PHOTODISSOCIATION AND VIBRATIONAL DYNAMICS OF OCS IN THE VUV REGION

The photofragment excitation (PHOFEX) spectrum of jet-cooled OCS was r emeasured for the 154.5 nm peak in the (1) Sigma(+)-(1) Sigma(+) trans ition by scanning a tunable vacuum ultraviolet (VUV) laser wavelength in the deep VUV region and by probing the S(S-1) product with a second laser. Due to the efficient cooling of the sample in the free-jet exp ansion and the high-resolving power of the VUV laser, the homogeneous broadening of the dissociative state was derived directly from the obs erved PHOFEX spectrum. Based on the measured peak width, the lifetime (tau) of the dissociative state was determined to be tau congruent to 0.14 ps. The profile of the PHOFEX peak was found to be asymmetric and was fit to a Fano line profile to derive the value of the asymmetry p arameter, q=-3.5(9), which indicates a considerable mixing of the cont inuum and discrete states in that particular energy region. A Fourier- transform analysis was also performed for the whole distinct absorptio n feature in the 160-140 nm region measured by McCarthy and Vaida [J. Phys. Chem., 92, 5875 (1988)], and the period of the vibrational motio n on the dissociative potential-energy surface was derived to be 41 fs . Using these pieces of experimental evidence, the dissociation dynami cs on the repulsive (1) Sigma(+) surface in the 154.5 nm region was in ferred.