Rate Coefficients for Vibrational Relaxation of OH(X²Π, v = 1–4) by He

The vibrational relaxation of OH­(X²Π) 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 cm³ molecule^–1 s^–1. In this article, we have studied vibrational relaxation of the levels v = 1–4 of OH­(X²Π) 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(X²Π, v ≤ 4) was generated in the reaction O­(¹D) + H₂. A single vibrational level of OH was detected with laser-induced fluorescence (LIF) via the A²Σ⁺–X²Π 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(X²Π, 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 cm³ 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.