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Photodissociation of HBr molecules and clusters : anisotropy parameters, branching ratios, and kinetic energy distributions.

Baumfalk, R. and Buck, U. and Frischkorn, C. and Nahler, N. H. and Huwel, L. (1999) 'Photodissociation of HBr molecules and clusters : anisotropy parameters, branching ratios, and kinetic energy distributions.', Journal of chemical physics., 111 (6). pp. 2595-2605.


The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around = 9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (= –0.96 ± 0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (* = 1.96 ± 0.05) with a branching ratio of R = 0.20 ± 0.03. At the wavelength of 193 nm, about the same value of R = 0.18 ± 0.03 is found, but both channels show a mainly perpendicular character with = –0.90 ± 0.10 for Br and * = 0.00 ± 0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster.

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Record Created:18 Jan 2008
Last Modified:20 Dec 2017 12:12

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