Published: December 12, 2011 r2011 American Chemical Society 166 dx.doi.org/10.1021/jp2061943 | J. Phys. Chem. A 2012, 116, 166–173 ARTICLE pubs.acs.org/JPCA Photolysis (193 nm) of SO 2 : Nascent Product Energy Distribution Examined through IR Emission Jianqiang Ma, Michael J. Wilhelm, Jonathan M. Smith, and Hai-Lung Dai* Department of Chemistry, Temple University, Philadelphia, Pennsylvania, United States I. INTRODUCTION The dissociation of SO 2 has been the subject of many studies in the past. Not only is SO 2 an important molecule in the chemistry of the atmosphere and the combustion of petroleum products, but the overall complexity of the dissociation process alone is worthy of note. 113 A particular type of dissociation processes of interest is the predissociation of electronically excited molecules following photoexcitation. Couplings of the electronically excited state through vibronic interactions 14 with other states cause radiationless transitions, 15 including those leading to dissociation. In the case of SO 2 , Okabe reported a sudden drop in the laser-induced fluorescence (LIF) quantum yield near 219 nm and predicted the predissociation threshold to be 5.66 eV. 13 Later studies, detailed below, suggested that the predissociation involves coupling between the ~ C 1 B 2 state and three other states: the ground electronic ~ X 1 A 1 state, the repulsive 3 1 A 1 state, and the repulsive 2 3 A 0 state. Katagiri et al. 10 combined fluorescence quantum yields, mea- sured from the LIF spectrum in the 220210 nm region, and predissociation rates, from rotational line broadening in high- resolution absorption spectra, to conclude that near 200 nm the predissociation mechanism is not much different from that at excitation wavelengths just above the dissociation threshold (∼219 nm). The main mechanism of predissociation is through vibronic coupling of the ~ C and ~ X states. Cosofret et al. 5 examined the photodissociation of SO 2 in the 202207 nm region through detection of the photofragments with resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) mass spectrometry. They observed a change in the population of the nascent SO radical at 203.0 nm, which they attributed to the presence of two predissociation mechanisms. It was suggested that the dissociation mechanism for wavelengths shorter than 203 nm involves an avoided crossing with the repulsive state 1 A 1 . This conjecture is supported by the dispersed emission spectroscopy study 2 between 197 and 212 nm as well as the theoretical calculation of the vibrational states up to the dissociation limit. 8 Kanamori et al. used tunable infrared diode laser spectroscopy to characterize the nascent SO product from 193 nm photolysis of SO 2 . 4 They observed correlations between the SO levels with different electronic spin and rotational angular momentum quantum numbers and suggested that this result was indicative of spinorbit mixing between the ~ C state and a 3 A 0 repulsive state of SO 2 that facilitates its dissociation. Several other studies have used a variety of methods to examine the population distribution of the SO radical from photolysis at 193 nm. 1,3,7,9,11 A summary on the population distribution of the nascent SO was presented by Yamasaki et al. 3 The nascent SO was determined to be in an inverted vibrational population with the maximum at v = 2. Populations of v > 2, on the other hand, remain unclear as one report indicated no substantial population for v > 2, 11 while others have reported that there is small but non-negligible (<10%) population in v = 5. 3,4 The nascent SO vibrational population distribution is an important subject for study, not only as a novel example of a predissociation reaction, but also due to the general importance of SO energy transfer in the characterization of combustion and planetary environments. 18 As a diatomic radical, SO is a model system for studying radical energy transfer, a topic of fundamental interest. Weiner and co-workers performed a study of the 193 nm photolysis of SO 2 using time-resolved FTIR for detecting IR emission following photoexcitation. 1 Their study highlighted the effectiveness of time-resolved IR emission in analyzing the nascent SO product distribution. The results showed evidence Received: June 30, 2011 Revised: November 8, 2011 ABSTRACT: Infrared emission following the photolysis of SO 2 by a 193 nm laser pulse (20 ns duration) was recorded with 500 ns time and 10 cm 1 spectral resolution. Spectral analyses of the time-resolved spectra revealed the vibrationally excited nascent SO population distribution as (v = 1)/ (v = 2)/(v = 3)/(v = 4)/(v = 5) = 0.54 ( 0.04, 1.00 ( 0.03, 0.00 ( 0.03, 0.01 ( 0.03, and 0.10 ( 0.03. The nascent SO was found to be rotationally excited with an average rotational temperature around 1000 K for v = 1 and v = 2 levels and 300 K for the v = 5 level. The vibrationally excited SO likely originates from two distinct dissociation mechanisms; the v = 1 and 2 populations are generated through intersystem crossing between the ~ C state and a repulsive state (2 3 A 0 ), and the v = 5 population is generated through internal conversion from the ~ C to the ~ X state. Efficient VV energy transfer from nascent vibrationally excited SO to SO 2 (ν 1 ) is also observed. The appearance of the SO 2 (ν 1 ) ν 1 = 2 emission, before that from the ν 1 = 1 population is consistent with the previous report that the Δν = 2 channel is more efficient than the Δν = 1 channel.