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The photodissociation dynamics of ICl has been studied near 304 and 280 nm on a simple miniature time of flight (mini-TOF) photofragment translational spectrometer with a short pulse of a weak acceleration field. An intense hot band effect was observed. Many small peaks were resolved in each photofragment translational spectrum (PTS). Based on simulations, the principal peaks were assigned not only to the different photodissociation channels (1) I + Cl, (2) I + Cl*, (3) I* + Cl, or (4) I* + Cl*, but also to the different chlorine isotopes (Cl and Cl). Moreover, some extra peaks showed the existence of an intense hot band effect from vibrationally excited ICl molecules, though only a few percent of ICl molecules remained in the vibrationally excited states in our supersonic molecular beam. Based on the spectra near 304 nm, the quantum yield Φ of each channel, the curve crossing, and the branching fraction σ from each transition state were determined. [ABSTRACT FROM AUTHOR]

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths (λphot), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by X—H (X = 0, 5) bond fission after excitation at their respective S1(¹ππ*)-S0 origins and at all shorter wavelengths. The achieved kinetic energy resolution reveals population of selected vibrational levels of the various phenoxyl and thiophenoxyl coproducts, providing uniquely detailed insights into the fragmentation dynamics. Dissociation in all cases is deduced to involve nuclear motion on the ¹πσ* potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to λphot and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long λphot is rationalized in terms of radiationless transfer from S1 to S0 levels carrying sufficient O—H stretch vibrational energy to allow coupling via the conical intersection between the S0 and ¹πσ* PESs at longer O—H bond lengths. In contrast, H + C6H5O(X²B1) products formed after excitation at short λphot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared ¹ππ* excited state and the ¹πσ* PES. The fragmentation dynamics of thiophenol at all λphot matches the latter behavior more closely, reflecting the different relative dispositions of the ¹ππ* and ¹πσ* PESs. Additional insights are provided by the observed branching into the ground (X²B1) and first excited (²B2) states of the resulting C6H5S radicals. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, the photodissociation dynamics of aryl halides has been a subject of intensive studies, which is closely related to the atmospheric chemistry. Here we present a review on the photochemistry of aryl halides, with emphasis on the recent progress in photodissociation dynamics at 266nm by using photofragment translational spectroscopy. The ab initio calculations have also been employed to investigate those photodissociation processes. It has been found that the photodissociation of aryl halides at 266nm is attributed to the nonadiabatic process via intersystem crossings from bound singlet excited state to triplet excited state and/or via internal conversion from bound singlet excited state to ground state. Also, the substitution effects in the photodissociation dynamics of aryl halides are discussed. [Copyright &y& Elsevier]

The photodissociation of ethyl iodide at 279.71, 281.73, 304.02 and 304.67 nm has been studied on our new mini-photofragment translational spectrometer with a total flight path of only 5 cm. Some vibrational peaks are firstly resolved in the TOF spectra of I*(2 P 1/2) and I(2 P 3/2) channels. These vibrational peaks are assigned to the excitation states ( v 2 = 0, 1, 2, ...) of the umbrella mode ( v 2, 540 cm−1) of the photofragment C2H5, and the distribution of the vibrational states is obtained. The dissociation energy has been determined to be D 0(C-I)=2.314 ± 0.03 eV. The energy partitioning of the available energy ( E avl= E T+ E int= E T+ E V,R) calculated from our experimental data Ē int / E avl = 22.1% at 281.73 nm, 22.4% at 304.02 nm for the I* channel, and Ē int / E avl = 25.2% at 279.71 nm, 25.9% at 304.67 nm for the I channel, seem to be more reliable. [ABSTRACT FROM AUTHOR]