Photoelectron spectroscopy of higher bromine and iodine oxide anions: Electron affinities and electronic structures of BrO2,3 and IO2–4 radicals Hui Wen, Gao-Lei Hou, Wei Huang, Niranjan Govind, and Xue-Bin Wang Citation: The Journal of Chemical Physics 135, 184309 (2011); doi: 10.1063/1.3658858 View online: http://dx.doi.org/10.1063/1.3658858 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/135/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An investigation into low-lying electronic states of HCS2 via threshold photoelectron imaging J. Chem. Phys. 140, 214318 (2014); 10.1063/1.4879808 Slow photoelectron velocity-map imaging spectroscopy of the C9H7 (indenyl) and C13H9 (fluorenyl) anions J. Chem. Phys. 139, 104301 (2013); 10.1063/1.4820138 Photoelectron spectroscopy of small IBr − ( CO 2 ) n ( n = 0 – 3 ) cluster anions J. Chem. 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Box 999, MS K8-88, Richland, Washington 99352, USA 2Laboratory of Environmental Spectroscopy, Anhui Institute of Optics & Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China and Environmental Protection Key Laboratory of Optical Monitoring Technology, Ministry of Environmental Protection of China, China 3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA 4Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99354, USA (Received 28 June 2011; accepted 19 October 2011; published online 14 November 2011) This report details a photoelectron spectroscopy (PES) and theoretical investigation of electron affini- ties (EAs) and electronic structures of several atmospherically relevant higher bromine and iodine − − oxide molecules in the gas phase. PES spectra of BrO2 and IO2 were recorded at 12 K and four photon energies—355 nm/3.496 eV, 266 nm/4.661 eV, 193 nm/6.424 eV, and 157 nm/7.867 eV— − − − while BrO3 ,IO3 , and IO4 were only studied at 193 and 157 nm due to their expected high electron binding energies. Spectral features corresponding to transitions from the anionic ground state to the ground and excited states of the neutral are unraveled and resolved for each species. The EAs of these bromine and iodine oxides are experimentally determined for the first time (except for IO2) to be 2.515 ± 0.010 (BrO2), 2.575 ± 0.010 (IO2), 4.60 ± 0.05 (BrO3), 4.70 ± 0.05 (IO3), and 6.05 ± 0.05 eV (IO4). Three low-lying excited states along with their respective excitation en- 2 2 2 2 ergies are obtained for BrO2 [1.69 (A B2), 1.79 (B A1), 1.99 eV (C A2)], BrO3 [0.7 (A A2), 1.6 2 2 2 2 2 (B E), 3.1 eV (C E)], and IO3 [0.60 (A A2), 1.20 (B E), ∼3.0 eV (C E)], whereas six excited 2 states of IO2 are determined along with their respective excitation energies of 1.63 (A B2), 1.73 (B 2 2 2 2 2 − A1), 1.83 (C A2), 4.23 (D A1), 4.63 (E B2), and 5.23 eV (F B1). Periodate (IO4 ) possesses a very high electron binding energy. Only one excited state feature with 0.95 eV excitation energy is shown in the 157 nm spectrum. Accompanying theoretical calculations reveal structural changes from the anions to the neutrals, and the calculated EAs are in good agreement with experimentally determined values. Franck-Condon factors simulations nicely reproduce the observed vibrational progressions for BrO2 and IO2. The low-lying excited state information is compared with theoret- ical calculations and discussed with their atmospheric implications. © 2011 American Institute of Physics. [doi:10.1063/1.3658858] I. INTRODUCTION implying that higher halogen oxides and mixed oxides can actively participate in the overall ozone depletion cycles.12, 13 Since the landmark work by Rowland and Molina1 For example, chlorine dioxide (OClO) is involved in ozone demonstrating that chlorine monoxide and chlorine atoms are depletion reactions via photodissociation to generate O related to the catalytic reactions of ozone destruction in the + ClO and Cl + O upon photoexcitation to the 2A state.6–11 stratosphere, there has been intense interest in characterizing 2 2 Therefore, the detailed mechanism and dynamics of ClO halogen oxides and understanding their implications in atmo- 2 dissociation and its atmospheric implications have drawn spheric chemistry. In the last three decades, the majority of much attention.6–11 Beyond chlorine species, the importance such research has been focused on chlorine oxides and related of bromine oxides and related species in ozone depletion also species, whose properties are now well studied.2–11 Besides has been established.14–23 It has been suggested that bromine halogen atoms and monoxides, there is increasing evidence species is roughly 45 times more effective than chlorine for global ozone destruction.18 Despite the important roles they a)Visiting student of alternate-sponsored fellowships (Pacific Northwest play in the stratospheric chemistry, bromine oxides are rela- National Laboratory). b)Permanent address: State Key Laboratory of Molecular Reaction Dynam- tively less studied compared with their chlorine counterparts, 5, 19–21 ics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, particularly for higher bromine oxides. Experimental China. characterization of BrOx is challenging, in part, due to their c)Authors to whom correspondence should be addressed. Electronic addresses: [email protected], [email protected], and thermal meta-stability against decomposition ultimately into 21 24–39 [email protected]. Br2 and O2. Most of studies have been focused on BrO2, 0021-9606/2011/135(18)/184309/11/$30.00135, 184309-1 © 2011 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.127.202.94 On: Wed, 04 Mar 2015 07:18:31 184309-2 Wen et al. J. Chem. Phys. 135, 184309 (2011) 21, 40–47 with a only handful devoted to BrO3. Recently, there Understanding the role of iodine chemistry in ozone de- has been increasing interest in understanding the role of pletion has attracted a lot of attention recently13, 48–53 and iodine chemistry in ozone depletion as well.13, 48–53 However, should provide a more complete view of halogen atmospheric the knowledge about iodine oxides is even less robust than chemistry. Iodine species undergo similar reactions in the tro- bromine species. Therefore, it is necessary to have a system- posphere due to the formation of its compounds in the ma- atic spectroscopic approach to investigate intrinsic molecular rine boundary layer.13, 51–53 Iodine oxides have been known properties of these bromine and iodine higher oxides. The ob- about for more than 100 years, but their properties remain tained thermodynamic and spectroscopic information may as- poorly characterized. The detection of gas phase IO2 was first 54 sist in understanding their roles in the atmospheric chemistry. reported in the thermal decomposition from I2O5, and in 55 5 Bromine dioxide has been relatively well studied. BrO2 halogen monoxide inter-reactions. Gilles et al. carried out has been detected in the stratosphere as a possible night- a negative ion photoelectron spectroscopic study on OIO− to 22, 23 time bromine reservoir. The first observation of gas- probe the ground state of IO2. The EA of OIO was determined 56 phase BrO2 in the laboratory was reported by Butkovskaya to be 2.577 eV. The rotational spectrum of the OIO radical, 24 57 et al., which suggested that paramagnetic BrO2 molecules as well as visible absorption spectrum, has been reported. 57 with lifetime >10 s among the products of the O + Br2 re- Despite similar visible absorption and excitation transition, action were found with the discharge flow-mass spectrome- IO2 displays different photochemistry when compared to both 2 2 try technique. The first spectroscopic study (visible absorp- ClO2 and BrO2. Upon the C( A2) ← X( B1) excitation, the 25 tion) of gas phase BrO2 was reported by Rattigan et al. excited IO2 exclusively dissociates to I + O2 with the quan- Since then, a numbers of studies have been devoted to inves- tum yield close to one51 with no IO + O being detected58— tigate the properties of BrO2 experimentally employing a va- in contrast to both channels being observed for its lighter riety of techniques,23–29 including: rotational spectroscopy,26 congeners.6, 38 Several theoretical calculations on thermody- 27, 28 19, 29 13, 52, 59 visible absorption in solid matrices, photoionization, namic properties of IO2 have been reported. The dis- and high-resolution ultraviolet/visable (UV/Vis) absorption tinctly different dissociation mechanism of IO2 and its atmo- spectroscopy.30, 31 The highly structured visible absorption spheric implications have stimulated a detailed, high-level ab spectrum of bromine dioxide radical, OBrO, observed in the initio computation study on the low-lying electronic states of 15 500–26 000 cm−1 region by Miller et al.,31 was augmented OIO and their connection path to the dissociated products,53 2 by Franck-Condon simulations and high-level ab initio calcu- which predicted the C( A2) state of IO2 is still more stable 2 lations, and concluded due to the dipole-allowed C( A2) ← relative to IO + O product.