Absorption Cross Sections for Iodine Species of Relevance to the Photolysis of Mixtures of I2 and O3 and for the Atmosphere

Abstract

In this study the absorption spectra of iodine species andtheir absorption cross sections had been studied. To this end two independent set-ups were optimised and synchronised to allow independent static measurements as well as simultaneous time resolved UV-vis molecular absorption and VUV-UV resonance absorption measurements.Molecular absorption measurements were performed with a CCD camera providing time resolved optical multichannel data. The resonance absorption set-up used a fast single channel photo multiplier tube. With the resonance absorption set-up the absorption cross section of the I(2P3/2) 183.038nm transition was determined to be (5.42à ?à ±0.8)*10^-14 cm^2/atom at the centre of the line in agreement with previous data.With the molecular absorption set-up the absolute absorption cross section of I2 was checked in an independent measurement and the uncertainty of the result could be reduced yieldingsigma_I2(500nm)=(2.186à ?à ±0.021)*10-^18 cm^2/molec in very good agreement with previously published data. Based on all available data a weighted average of sigma_I2(500nm)=(2.191à ?à ±0.02)*10-18 cm^2/molec is recommended. With the synchronised combined set-up absorption spectra and cross sections of iodine oxides were studied by flash photolysis of mixtures of I2 and O3 in bath gases N2 and O2. A method based on Independent Component Analysis and Principal Components Analysis and least squares techniques was developed to separate overlapping absorption from different absorbers recorded in time resolved opticalmultichannel measurements. The method enables the extraction of pure curves of temporal behaviour of optical density and of pure spectra with an accuracy of à ?à ±3%. Individual spectra for ground state IO(n'<-0), vibrationally excited IO(n'<-n"), n">0, as well as OIO and three further yet unidentified absorbers were obtained. Analysis of the absorption continuum of IO provided evidence for two optically active repulsive states intersecting with the upper IO(A2Pi) state. Anomalous behaviour of the IO(2<-0) band was observed which could be explained by chemiluminescence from the IO(A2Pi), n'=2 state, but the source for IO in this state remains unclear. During the first stages of reaction the population of IO was found to be strongly inverted with vibrationally excited IO of up to n"=7 and 25% in n"=1, while in thermal equilibrium only 4% should be present. This required separate determination of cross sections for ground state and vibrationally excited state IO. The effect of low resolution and coarse binning on apparent optical density and absolute cross sections of different absorption bands was studied and a method developed to correct corresponding effects in low resolution measurements. Significant non-linear behaviour of apparent optical density with concentration was found for IO(4<-1), of relevance to studies of chemical kinetics and cross sections. A method to determine absolute absorption cross sections from curves of temporal behaviour of optical density was developed, which is independent of chemical modelling and chemical kinetics reference data using the principle ofconservation of iodine throughout the course of reaction. Absorption cross sections for iodine oxides were determined to be sigma_IO(4<-0) = (3.5à ?à ±0.3)*10-17 cm^2/molec at 0.12nm FWHM for ground state IO only, sigma_IO_eff(3<-1) = (2.0à ?à ±0.8)*10-17 cm^2/molec at 0.35nm FWHM for anoverall cross section for vibrationally excited IO of n">0, further a cross section for the IO n"=1 progression of sigma_IO(3<-1)=(4.5à ?à ±0.5)*10-17 cm^2/molec at 0.12nm FWHM, and for OIO of sigma_OIO(0,5,1<-0,0,0) = (1.3à ?à ±0.3)*10-17 cm^2/molec at 0.35nm FWHM. For two yet unidentified absorbers cross sections per iodine atom were determined of sigma_"Z"(340nm) = (1.0à ?à ±0.2)*10-18 cm^2/atom at 1.3nmFWHM and sigma_"Y"(322nm) = (1 to 3)*10-18cm^2/atom at 1.3nm FWHM.Results for ground state IO and OIO are in good agreement with literature. The other cross sections have been determined for the first time.