Rate coefficients for vibrational relaxation of OH(X2Π, v = 1-4) by He.

The vibrational relaxation of OH(X(2)Π) by collisions with rare gases is very slow due to small molecular interactions. No measurement of the rate coefficients has been made for relaxation of relatively low vibrational levels v ≤ 4 of OH by He, and there is only one report of the upper limit for v = 2, <1 × 10(-14) cm(3) molecule(-1) s(-1). In this article, we have studied vibrational relaxation of the levels v = 1-4 of OH(X(2)Π) by collisions with He. A gaseous mixture of O3 and H2 in a carrier gas at 70-130 Torr of He was irradiated at 266 nm, and OH(X(2)Π, v ≤ 4) was generated in the reaction O((1)D) + H2. A single vibrational level of OH was detected with laser-induced fluorescence (LIF) via the A(2)Σ(+)-X(2)Π transition. Time-resolved LIF intensities of OH(v) were recorded, and kinetic analysis was made by an originally developed integrated profiles method (IPM). On the basis of the evaluation of the pressure-dependent rate coefficients of diffusion loss and the effect of impurities on the kinetics, the rate coefficients of vibrational relaxation for OH(X(2)Π, v = 1-4) by He have been determined to be (2.9 ± 1.5) × 10(-17), (1.4 ± 0.4) × 10(-16), (5.2 ± 0.5) × 10(-16), and (1.6 ± 0.2) × 10(-15) cm(3) molecule(-1) s(-1) for v = 1, 2, 3, and 4, respectively (the confidence limits are 2σ). The rate coefficients are larger at higher vibrational levels and smoothly correlate to those reported previously for v = 10-12.