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

Absolute rate constants for reactive removal and vibrational relaxation have been determined for the collisions of OH(X 2 Π i, v = 1 and 2) with CH 4 . The hydroxyl radical was produced in the reaction O( 1 D) + CH 4 → OH(X 2 Π i, v ≤ 4) + CH 3 initiated by the 248 nm photolysis of O 3 in the presence of CH 4 . All the vibrational levels of OH( v ) were detected by laser-induced fluorescence (LIF) via the sequences Δ v = 0 ( v ‘ ‘ = 0, 1, and 2) and Δ v = −3 ( v ‘ ‘ = 3 and 4) of the A 2 Σ + −X 2 Π i transition. Temporal profiles of the LIF intensities were analyzed using a new linear regression method (refs 10 and 11), and total removal rate constants for the vibrational levels ( v = 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( v ). Rate constants for reactive removal, OH( v ) + CH 4 → products, were determined to be k ( v =1) = (2.1 ± 0.6) × 10 -13 cm 3 molecule -1 s -1 and k ( v =2) = (0.9 ± 0.4) × 10 -12 cm 3 molecule -1 s -1, and for vibrational relaxation, OH( v ) + CH 4 → OH( v − 1) + CH 4, are k (1→0) = (3.5 ± 0.6) × 10 -13 cm 3 molecule -1 s -1 and k (2→1) = (1.1 ± 0.4) × 10 -12 cm 3 molecule -1 s -1 . This article is the first report on the branching ratios between reactive removal and vibrational relaxation.