PRESSURE-DEPENDENCE AND METASTABLE STATE FORMATION IN THE PHOTOLYSIS OF DICHLORINE MONOXIDE (CL2O)

The photodissociation of dichlorine monoxide (Cl2O) was studied using broadband flash photolysis to investigate the influence of variations in the photolysis wavelength domain, bath gas pressure and bath gas id entity on the yield and temporal dependence of the ClO product. ClO yi elds were independent of bath gas pressure when the photolysis spectra l band extended to 200 nm (quartz cutoff) but for photolysis restricte d to wavelengths longer than about 250 nm, ClO yields decreased with i ncreasing bath gas pressure and there was a pressure-dependent delay i n the formation of ClO. Under these conditions, a weak, highly structu red absorption spectrum was observed in the range 16 600-26 000 cm(-1) with a lifetime on the order of 500 ms. A portion of the spectrum cou ld be analyzed (22 000-26 000 cm(-1)) which showed progressions having differences of 283, 443, and 505 cm(-1). Ab initio calculations were performed to evaluate vertical excitation energies and oscillator stre ngths from the lowest-energy singlet (X(1)A(1)) or triplet (1(3)B(1)) states to various excited states. The calculations indicated that the 2(3)A(2)<--1(3)B(1) transition has an unusually large oscillator stren gth. The transition energy, 3.05 eV, is consistent with the observed m etastable spectrum. The observed pressure dependence of ClO formation could be modeled using a mechanism which assumed that Cl2O excitation at wavelengths longer than about 300 nm leads to rapid intersystem cro ssing to two metastable states in the triplet manifold. These states u ndergo competitive dissociation to ClO+Cl and collisional relaxation t o the ground state; The dynamics of Cl2O may serve as a model for othe r molecules of importance in the earth's lower stratosphere such as CI ONO2 where filtering of the solar spectrum by ozone restricts photolys is to the weak tail of the absorption continuum. (C) 1996 American Ins titute of Physics.