Vibrational relaxation of OH by oxygen atoms Ani Khachatrian, Paul J. Dagdigian * Department of Chemistry, Zanvyl Krieger School of Arts and Sciences, The Johns Hopkins University, 3400 North Charles Street (Remsen B42), Baltimore, MD 21218-2685, USA Received 27 May 2005; in final form 22 August 2005 Available online 19 September 2005 Abstract The collisional removal of OH(v = 1) by O( 3 P) atoms is investigated. OH is generated by 193 nm photolysis of H 2 O 2 , and O( 3 P) atoms are generated by a microwave discharge in O 2 diluted in Ar. OH(v = 0 and 1) concentrations are monitored by laser-induced fluorescence vs. the time after the photolysis laser pulse. From comparison of these concentrations with kinetic simulations, the room-temperature total removal rate constant for OH(v = 1) in collisions with O( 3 P) is determined to be (3.9 ± 0.6) · 10 11 cm 3 molecule 1 s 1 . This value is slightly larger than the OH(v = 0)–O( 3 P) reaction rate constant, but the difference is within the experimental uncertainty. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction The OH radical is an important species in the earthÕs atmosphere and other environments. An understanding of its collisional vibrational relaxation rates is important in modeling OH chemistry in the upper atmosphere [1,2]. Collisional vibrational relaxation of OH in a wide range of vibrational levels by stable collision partners has been investigated [3–10]. Investigation of collisional relaxation of vibrationally excited OH by oxygen atoms, present at significant concentration in the upper atmosphere, is also relevant. This energy transfer process is interesting from a fundamental point of view [11] since it involves two open-shell species and occurs through formation and decay of a transient HO 2 complex (see Fig. 1). OH(v = 0) can be removed in collisions with oxygen atoms through the reaction OHðX 2 PÞþ Oð 3 PÞ! H þ O 2 DH  ð0KÞ¼16:29  0:09 kcal=mol. ð1Þ Because of its importance in atmospheric and combustion chemistry, the rate of reaction 1 has been extensively studied; the IUPAC subcommittee for gas phase data evaluation [12] and the NASA panel [13] recommend k v =0 -(OH–O) = (3.5 ± 0.4) and (3.3 ± 0.7) · 10 11 cm 3 molecule 1 s 1 at 298 K, respectively. Collisional removal of OH(v P 1) by O( 3 P) can occur by both chemical reaction and vibrational relaxation. In EPR studies, Spenser and Glass [14] report values of (1.45 ± 0.25) and (1.05 ± 0.53) · 10 10 cm 3 molecule 1 s 1 for OH(v = 1)–O( 3 P) vibrational relaxation and chemical reaction rate constants, respectively. Marschall et al. [15] have investigated collisional removal of OH(v) by O( 3 P) through laser-induced fluorescence measurements of time- dependent OH(v) concentrations after 248 nm photolysis of ozone and subsequent reaction of O( 1 D) with water to form vibrationally excited OH. They reported a prelimin- ary value k v =2 (OH–O) = 4.6 · 10 11 cm 3 molecule 1 s 1 (quoted in [16]) for the room-temperature OH(v = 2)– O( 3 P) total removal rate constant. Varandas [17] has computed rate constants for reactive and non-reactive OH(v)–O( 3 P) collisions at several temperatures through trajectory calculations on the lowest HO 2 potential energy surface (PES). We present here an investigation of the collisional re- moval of OH(v = 1) by O( 3 P). Oxygen atoms are prepared in a flow system by a microwave discharge in O 2 diluted in argon. Hydroxyl is generated by 193 nm photolysis of 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.131 * Corresponding author. Fax: +1 410 516 8420. E-mail address: pjdagdigian@jhu.edu (P.J. Dagdigian). www.elsevier.com/locate/cplett Chemical Physics Letters 415 (2005) 1–5