Rajakumar Balla

We report measurements of the CH3CO quantum yield, ΠCH3CO, following the 248 nm pulsed laser photolysis of acetone (CH3C(O)CH3), methyl ethyl ketone (CH3C(O)CH2CH3), and biacetyl (CH3C(O)C(O)CH3). CH3CO quantum yields at 248 nm were measured at 296 K, relative to CH3CO reference systems. CH3CO was detected using cavity ring-down spectroscopy at wavelengths between 490 and 660 nm. Measurements were performed between 60 and 670 Torr (He, N2 bath gases) and the obtained CH3CO quantum yields in the low-pressure limit, Π0CH3CO* were 0.535±0.09, 0.41±0.08, and 0.76±0.11, for acetone, methyl ethyl ketone, and biacetyl, respectively. The quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. An increase in ΠCH3CO with increasing bath gas pressure, which depended on the identity of the collision partner (He, N2), was observed. The present results are compared with previous quantum yield determinations.

We report rate coefficients for the relaxation of OH(v = 1) and OD(v = 1) by H2O and D2O as a function of temperature between 251 and 390 K. All four rate coefficients exhibit a negative dependence on temperature. In Arrhenius form, the rate coefficients for relaxation (in units of 10-12 cm3 molecule-1 s-1) can be expressed as: for OH(v = 1) + H2O between 263 and 390 K: k = (2.4 ± 0.9) exp((460 ± 115)/T); for OH(v = 1) + D2O between 256 and 371 K: k = (0.49 ± 0.16) exp((610 ± 90)/T); for OD(v = 1) + H2O between 251 and 371 K: k = (0.92 ± 0.16) exp((485 ± 48)/T); for OD(v = 1) + D2O between 253 and 366 K: k = (2.57 ± 0.09) exp((342 ± 10)/T). Rate coefficients at (297 ± 1 K) are also reported for the relaxation of OH(v = 2) by D2O and the relaxation of OD(v = 2) by H2O and D2O. The results are discussed in terms of a mechanism involving the formation of hydrogen-bonded complexes in which intramolecular vibrational energy redistribution can occur at rates competitive with re-dissociation to the initial collision partners in their original vibrational states. New ab initio calculations on the H 2O-HO system have been performed which, inter alia, yield vibrational frequencies for all four complexes: H2O-HO, D2O-HO, H2O-DO and D2O-DO. These data are then employed, adapting a formalism due to Troe (J. Troe, J. Chem. Phys., 1977, 66, 4758), in order to estimate the rates of intramolecular energy transfer from the OH (OD) vibration to other modes in the complexes in order to explain the measured relaxation rates - assuming that relaxation proceeds via the hydrogen-bonded complexes. © the Owner Societies 2006.

Rate coefficients, k, for the gas-phase reaction CH3CO + Cl 2 → products (2) were measured between 253 and 384 K at 55-200 Torr (He). Rate coefficients were measured under pseudo-first-order conditions in CH3CO with CH3CO produced by the 248-nm pulsedlaser photolysis of acetone, CH3C(O)CH3, or 2,3-butadione, CH3C(O)C(O)CH3. The loss of CH3CO was monitored by cavity ring-down spectroscopy (CRDS) at 532 nm. Rate coefficients were determined by first-order kinetic analysis of the CH3CO temporal profiles for [Cl2] < 1×1014 molecule cm-3 and the analysis of the CRDS profiles by the simultaneous kinetics and ring-down method for experiments performed with [Cl2] > 1×1014 molecule cm-3. k2 (T ) was found to be independent of pressure, with k2 (296 K) = (3.0±0.5)×10-11 cm3 molecule-1 s-1. k2 (T ) showed aweak negative temperature dependence that iswell reproduced by the Arrhenius expression k2 (T ) = (2.2±0.8)×10-11 exp[(85±120)/T ] cm3 molecule-1 s-1. The quoted uncertainties in k2 (T ) are at the 2σ level (95% confidence interval) and include estimated systematic errors. A comparison of the present work with previously reported rate coefficients for the CH 3CO + Cl2 reaction is presented. * Int J Chem Kinet 41: 543-553, 2009 © 2009 Wiley Periodicals, Inc.