Spectroscopic studies of the I 2/O 3 photochemistry : Part 2. Improved spectra of iodine oxides and analysis of the IO absorption spectrum

Abstract

Based on multivariate statistical separation techniques spectra of iodine oxides formed in the photolysis of I 2 + O 3 have been obtained. Data was recorded by time resolved UV–vis absorption spectroscopy. Overlapped spectra of ground state IO( ν′ ← 0), vibrationally excited IO( ν′ ← ν″) with ν″ > 0, and OIO have been separated from each other and from underlying absorptions of further iodine oxides. The uncertainty due to uncorrected foreign absorptions is of no more than ±3%. Relative errors of spectra are of the order of a few percent for the main parts of the spectra. The up to now unknown continuous parts of spectra are thereby determined. Previous uncertainties in differential absolute absorption cross-section are removed. By the separation of the ground state IO spectrum from that of vibrationally excited IO spectroscopic measurements under non-equilibrium conditions are enabled. Three further absorber spectra have been extracted in the 200–600 nm window. Two of them are most likely caused by I 2O 2 and possibly I 2O 3 thereby providing part of the missing link between IO and OIO consumption and the formation of higher oxides and possibly aerosol. All spectra are available as supplementary data. Given the unprecedented quality of extracted spectra for the first time an analysis of band strength of the IO(A 2Π 3/2 ← X 2Π 3/2) transition could be made. The continuum absorption of the ground state IO spectrum was resolved into overlapped bands of bound–bound transitions and two bound-free transitions. From the bound-free transitions found the existence of two optically active repulsive states intersecting with the IO(A 2Π 3/2) potential has been inferred and tangents to the two repulsive potentials have been determined. Correlating the tangents to probable dissociation products gives a plausible rough picture of the shape of the repulsive states which is in good agreement with previous observations on predissociation of states. There is evidence that one of the continuous absorptions observed could have its origin in the IO( 2Π 1/2) sub-system. Relative band strength was determined for absorption bands of IO(A 2Π 3/2 ← X 2Π 3/2). They are in good agreement with literature and with simple calculations based on a Morse approximation. An anomalous behaviour of IO(2 ← 0) in time resolved measurements was observed and studied. It could be explained by a hypothetical partial population inversion of the IO(A 2Π 3/2) state, but the source of it remains unclear.