Journal of Physical Chemistry A, Vol.104, No.38, 8723-8729, 2000
Depletion kinetics of chromium atoms by sulfur dioxide
The gas-phase depletion kinetics of Cr(a(7)S(3), a(5)S(2), a(5)D(J)) in the presence of SO2 are reported. Chromium atoms were produced by the 248 nm photodissociation of chromium carbonyl and were detected by laser-induced fluorescence. The ground state of Cr, a(7)S(3), was found to react rapidly via a termolecular mechanism with SO2. At 297 K, the limiting low-pressure third-order rate constant is k(o) = (2.68 +/- 0.41) x 10(-28) cm(6) molecule(-2) s(-1), and the limiting high-pressure second-order rate constant is k(infinity) = (2.73 +/-0.38) x 10(-10) cm(3) molecule(-1) s(-1); the uncertainties represent +/-1 sigma in precision. k(0) was found to decrease with increasing temperature. The binding energy of CrSO2 is estimated at 53 kcal mol(-1) from combining the kinetic results with unimolecular rate theory and density functional theory. Cr(a(5)S(2)) depleted with a rate constant of k = 6.32 x 10(-10) cm(3) molecule(-1) s(-1), and the a(5)D(J) spin/orbit states depleted at the collision rate of 2.97 x 10(-10) cm(3) molecule(-1) s(-1); overall uncertainties are estimated at less than or equal to 35%. Both chemical and physical quenching are likely for the excited states. The present work completes a study on the depletion kinetics of group 6 transition metal atoms by SO2. Results are interpreted in terms of long-range attractive forces between Cr and SO2 and the orbital occupancies of the Cr atomic states.