J. Phys. Chem. 1994,98, 6263-6271
6263
Photodissociation of A(2A2)-ExcitedOClO and Its Aggregates R. Flesch, B. Wassermann, B. Rothmund, and E. Riihl' Institut fur Physikalische und Theoretische Chemie, Freie Universitht Berlin, Takustr. 3, 0 - 1 4 1 95 Berlin, Germany Received: January 4, 1994; In Final Form: April 22, 1994'
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Photofragmentation of chlorine dioxide (OC10) and its aggregates is investigated in the UV regime [349-373 nm, A(2A2) %(2B,) transition]. The products of UV photolysis are subsequently ionized with vacuum UV laser radiation followed by time-of-flight mass analysis. The isolated molecule decays predominantly by formation of vibrationally excited C10 (X(211)) as a result of predissociation. Highly vibrationally excited C10 is formed if even quanta of the asymmetric stretching vibration are excited. The results are discussed in relation to the UV absorption cross section and competitive fragmentation routes, such as formation of molecular oxygen. The UV photolysis of homogeneous chlorine dioxide aggregates [(OClO),] yields fragments such as excited molecular oxygen, C~ZOS, and C1305. Evidence for evaporation of neutral molecules from excited aggregates is found as well. The results on aggregate photolysis are discussed in relation to experiments on chlorine dioxide in the gas and condensed phase. A b initio calculations are performed in order to rationalize the experimental results. Properties, such as structures, stabilities, and vibrational frequencies, of different isomers of the OClO dimer and the photolysis product Cl2O3 are derived. Possible implications to stratospheric photochemistry are briefly discussed.
I. Introduction Spectroscopic and photochemical properties of chlorine dioxide
OClO have been the subject of many studies in the past.l-ls The
near-UV-absorption cross section between 260 and 480 nm [A(2A2) +X(2Bl)I exhibits a long progression of vibronic transitions, which has been frequently used to identify OClO in the stratosphere.12 Predissociation into C10 0 is assumed to be the major fragmentation mechanism after UV excitation. This is evidenced from considerable line broadening of the vibronic bands,*3J4aswell as flash photolysisfollowed by product analysis,16 resonance-enhanced multiphoton-ionization (REMPI) spectroscopy: and molecular beam scattering experiments.9 An altemative fragmentation route leads to the formation of CI 0 2 . Photoisomerization of the electronically excited molecule into ClOO and a subsequent decay of the labile asymmetric isomer5,6J7J8as well as a concerted unimolecular decay9 have been discussed in this context. Various experimentalapproaches have been utilized in order to quantify the branching ratio between C1 and C10 formation. The range of isomerization yields at 360nm excitation wavelength is between
