Pressure dependence and metastable state formation in the photolysis of dichlorine monoxide (Cl2O)

Scott L. Nickolaisen, Charles E. Miller, Stanley P. Sander, Michael R. Hand, Ian H. Williams, Joseph S. Francisco

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Abstract

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 identity on the yield and temporal dependence of the ClO product. ClO yields were independent of bath gas pressure when the photolysis spectral band extended to 200 nm (quartz cutoff) but for photolysis restricted to wavelengths longer than about 250 nm, ClO yields decreased with increasing bath gas pressure and there was a pressure-dependent delay in the formation of ClO. Under these conditions, a weak, highly structured 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 could be analyzed (22000-26000 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 strengths from the lowest-energy singlet (X 1A1) or triplet (13B1) states to various excited states. The calculations indicated that the 23A2 ← 13B1 transition has an unusually large oscillator strength. The transition energy, 3.05 eV, is consistent with the observed metastable 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 crossing to two metastable states in the triplet manifold. These states undergo competitive dissociation to ClO+Cl and collisional relaxation to the ground state. The dynamics of Cl2O may serve as a model for other molecules of importance in the earth's lower stratosphere such as ClONO2 where filtering of the solar spectrum by ozone restricts photolysis to the weak tail of the absorption continuum.

Original languageEnglish
Pages (from-to)2857-2868
Number of pages12
JournalJournal of Chemical Physics
Volume104
Issue number8
DOIs
Publication statusPublished - 1 Jan 1996

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

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