First Determination of the Branching Ratios between Reactive Removal and Vibrational Relaxation in Collisions of OH(X2Πi, v = 1 and 2) with CH4

Abstract

Absolute rate constants for reactive removal and vibrational relaxation have been determined for the collisions of OH({Chi}{sup 2}{Pi}{sub i}, {upsilon} = 1 and 2) with CH{sub 4}. The hydroxyl radical was produced in the reaction O({sup 1}D) + CH{sub 4} {r_arrow} OH({Chi}{sup 2}{Pi}{sub i}, {upsilon} {le} 4) + CH{sub 3} initiated by the 248 nm photolysis of O{sub 3} in the presence of CH{sub 4}. All the vibrational levels of OH({upsilon}) were detected by laser-induced fluorescence (LIF) via the sequences {Delta}{upsilon} = 0 ({upsilon}{double_prime} = 0, 1, and 2) and {Delta}{upsilon} = {minus}3 ({upsilon}{double_prime} = 3 and 4) of the {Alpha}{sup 2}{Sigma}{sup +}{minus}{Chi}{sup 2}{Pi}{sub i} transition. Temporal profiles of the LIF intensities were analyzed using a new linear regression method, and total removal rate constants for the vibrational levels ({upsilon} = 1--4) were determined. The linear analysis coupled with the previously reported nascent vibrational distributions of OH gave absolute rate constants for the reactive and nonreactive removal of OH({upsilon}). Rate constants for reactive removal, OH({upsilon}) + CH{sub 4} {r_arrow} products, were determined to be {kappa}({upsilon} = 1) = (2.1 {+-} 0.6) {times} 10{sup {minus}13} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} and {kappa}({upsilon}=2) = (0.9 {+-} 0.4) {times} 10{sup {minus}12} cm{supmore » 3} molecule{sup {minus}1} s{sup {minus}1}, and for vibrational relaxation, OH({upsilon}) + CH{sub 4} {r_arrow} OH({upsilon} {minus} 1) + CH{sub 4}, are {kappa}(1{r_arrow}0) = (3.5 {+-} 0.6) {times} 10{sup {minus}13} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} and {kappa}(2{r_arrow}1) = (1.1 {+-} 0.4) {times} 10{sup {minus}12} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. This article is the first report on the branching ratios between reactive removal and vibrational relaxation.« less