The technique of H Rydberg atom photofragment translational spectroscopy has been applied to investigate the ultraviolet photodissociation dynamics of hydrogen bromide. Branching fractions between the channels forming ground Br( 2 P 3/2 ) and spin-orbit excited Br( 2 P 1/2 ) atoms have been determined at 15 independent wavelengths in the range 201-253 nm, and photofragment recoil anisotropies for these two channels have been characterized at six different wavelengths within the same wavelength range. The channel forming ground state products, HϩBr( 2 P 3/2 ), is observed to arise solely from a perpendicular ͑i.e., ⌬⍀ϭ1͒ transition at all excitation energies, whereas the channel to formation of excited state products, HϩBr( 2 P 1/2 ), has a marked wavelength dependence: at long wavelengths ͑ϭ243 nm͒, the photofragments are produced by a parallel ͑i.e., ⌬⍀ϭ0͒ photodissociation mechanism, which becomes more perpendicular in character as the photolysis energy is increased. Within the wavelength range studied, the branching fractions indicate that Br( 2 P 3/2 ) products are formed in preference to Br( 2 P 1/2 ) products, with propensities that are relatively invariant to excitation wavelength, although a small, yet pronounced, cusp appears at ϳ235 nm. The observations are discussed with reference to the known behavior of the other hydrogen halides and highlight the influence of spin-orbit interactions in the photofragmentation dynamics of this series of molecules.

Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl (X 1 ⌺ ϩ ) molecules. Jet-cooled H 35 Cl molecules within a time-of-flight mass spectrometer were prepared by infra-red absorption in the following quantum states: vϭ1, Jϭ0 and Jϭ5; vϭ2, Jϭ0 and Jϭ11; vϭ3, Jϭ0 and Jϭ7. The excited molecules were then photodissociated at ϳ235 nm and the Cl( 2 P j ) photofragments detected using ͑2ϩ1͒ resonance enhanced multiphoton ionization. The results are presented as the fraction of total chlorine yield formed in the spin-orbit excited state, Cl( 2 P 1/2 ). The experimental measurements are compared with the theoretical predictions from a time-dependent, quantum dynamical treatment of the photodissociation dynamics of HCl (vϭ1Ϫ3, Jϭ0͒. These calculations involved wavepacket propagation using the ab initio potential energy curves and coupling elements previously reported by Alexander, Pouilly, and Duhoo ͓J. Chem. Phys. 99, 1752 ͑1993͔͒. The experimental results and theoretical predictions share a common qualitative trend, although quantitative agreement occurs only for HCl (vϭ2).

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The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201È210 nm has been investigated. Ground state hydrogen photofragments, H(2S), were detected using the H Rydberg atom time-of-Ñight (HRTOF) technique to obtain directly the relative yields of the available product channels : and The product branching fractions are reported and compared with). recent experimental measurements and theoretical calculations. In addition, the two spinÈorbit components of ground state chlorine photofragments formed by photolysis of HCl at 205.5 nm were monitored using (2 ] 1) resonance enhanced multiphoton ionization (REMPI). The relative sensitivity of this detection method for and atoms is found by comparing the relative REMPI signal intensities with the product Cl(2P 3@2 ) Cl(2P 1@2 ) branching fraction determined by the HRTOF technique.

In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg timeof-flight (HRTOF) spectroscopy to study the primary photofragmentation channels of allyl iodide excited at 193 nm and the ensuing dissociation of the nascent allyl radicals as a function of their internal energy. Two C-I bond fission channels were found to produce the allyl radical, one channel forming I( 2 P 3/2 ) and the other forming I( 2 P 1/2 ). The nascent allyl radicals are dispersed as a function of the translational energy imparted from the photolysis and therefore by their internal energy. Although all of the I( 2 P 3/2 ) and a portion of the I( 2 P 1/2 ) channel allyl radical products have enough internal energy to overcome the 60 kcal/mol barrier to form allene + H, the data showed that a substantial fraction of the allyl radicals from the I( 2 P 1/2 ) channel that formed with internal energies as high as 15 kcal/mol above the 60 kcal/mol barrier were stable to H atom loss. The stability is due to centrifugal effects caused by significant rotational energy imparted to the allyl radical during photolysis and the small impact parameter and reduced mass characterizing the loss of an H atom from an allyl radical to form allene + H. A photoionization efficiency (PIE) curve identified the major C 3 H 4 secondary dissociation products as allene. Comparison of the mass 40 signal in the TOF spectra at two photoionization energies showed that branching to H + propyne does not occur at near-threshold internal energies, indicating that the experimentally determined allyl f 2-propenyl radical isomerization barrier, which is lower than recent ab initio calculations of the barrier by ∼15 kcal/mol, is far too low.