Spectroscopic studies of the I 2/O 3 photochemistry : Part 1: Determination of the absolute absorption cross sections of iodine oxides of atmospheric relevance

Abstract

Multichannel time resolved absorption spectroscopy has been coupled with flash photolysis of mixtures of molecular iodine and ozone to study the spectra and determine absorption cross sections of iodine oxides. Simultaneously, the behaviour of the iodine atoms has been measured by atomic resonance spectroscopy. To separate overlapping molecular absorptions, multivariate analysis techniques have been used to yield the optical density versus time curve at an optimal wavelength for each individual molecular absorber. After the initial photolysis of I 2 and some O 3, it is assumed that the number of iodine atoms contained in the chemical system is invariant in time and that the individual optical densities of all the relevant species are observed. The solution of the resultant over-determined system of linear equations yields the absolute absorption cross sections of the iodine containing molecular species: ground state iodine monoxide, IO(X 2Π 3/2, ν″ = 0), vibrationally excited iodine monoxide, IO(X 2Π 3/2, ν″ > 0), ground state iodine dioxide, OIO( 2B 1), and the spectra of three other iodine oxides, some of which have been observed for the first time in this study. One of these oxides has been tentatively assigned to I 2O 2, and possible assignments of the other two have been discussed. The values of absolute absorption cross sections for selected vacuum wavelengths at 298 K were determined to be: σ IO(4←0)(427.2 nm) = (3.5 ± 0.3) × 10 −17 cm 2, σ IO(3←1)(459.3 nm) = (4.5 ± 0.5) × 10 −17 cm 2, σ IO(1←2)(484.9 nm) = (6.0 ± 0.5) × 10 −17 cm 2, σ OIO(0,5,1←0,0,0)(549.3 nm) = (1.3 ± 0.3) × 10 −17 cm 2 and for an up to now unidentified higher iodine oxide σ(356 nm) ≥ (7.8 ± 1.2) × 10 −19 cm 2 × I atom −1. The spectral resolution of the resultant absorption cross sections is 0.12 nm FWHM in case of IO and 0.35 nm FWHM in case of OIO. Previous determinations of these absorption cross sections have been reviewed.