We report center-of-mass kinetic energy distributions for the 193 nm photodissociation of H2S and HBr using the method of velocity–aligned Doppler spectroscopy. Nascent H atoms are detected by sequential two-photon photoionization via Lyman-α (121.6 nm + 364.7 nm), and internal SH(X 2Π) and Br excitations are observed directly in the H-atom kinetic energy distributions. The kinetic energy resolution is much better than in ‘‘conventional’’ sub-Doppler resolution spectroscopy and results from detecting spatially selected species whose velocities are aligned with the wave vector of the probe radiation, kprobe, thereby providing a kinetic energy distribution for a specific laboratory direction. This improved resolution is achieved in the present experiments by using pulsed, collimated, and overlapped photolysis and probe beams, but the vital aspect of the technique involves increasing the delay between the two lasers in order to discriminate against species having velocity components perpendicular to kprobe. In the case of HBr, we identify the Br(2P3/2) and Br(2P1/2) contributions and find that the Br(2P1/2) channel accounts for approximately 14% of the fragmentation associated with perpendicular electronic transitions. Concerning H2S, SH(X 2Π) vibrational structure is clearly evident in the H-atom kinetic energy distribution, and the SH vibrational distribution shows oscillations, with [v″=0]>[v″=1], [v″=1]<[v″=2], [v″=2]>[v″=3], [v″=3]<[v″=4], and [v″=4]>[v″=5]. Such oscillatory behavior was predicted theoretically by Kulander. A simulation of our data places 32% of the SH in v″>0 (〈Evib(SH)〉∼2700 cm−1, which is approximately 14% of the available energy, hν-D0), while the general features of our H2S data are in accord with the TOF study of van Veen et al. Presently, our measurements appear to be limited by the dye laser resolution (∼0.06 cm−1 at 364.7 nm), but a significant improvement of the laser bandwidth is possible using commercially available sources. The velocity-aligned Doppler spectroscopy technique is not limited to detecting atoms, and species can be monitored using ionization, LIF, and absorption spectroscopy. As a result, this method should find applications in many areas of molecular physics.

The technique of velocity-aligned Doppler spectroscopy (VADS) is used to investigate the photodissociation of HBr (193 nm) and HI (193 and 248 nm). Doppler profiles at Lyman-for the H-atom photofragment are reported, and the corresponding populations of the halogen atom 2P3/2 and 2P1/2 spin-orbit states are determined. The VADS technique facilitates measurement of spatial anisotropy (0's) for each spin-orbit state. For HI photolyzed at 248 nm, I(2P3/2) and I(2Pi/2) are produced with relative populations 0.54 ± 0.05 (0 = -1.0 ± 0.2) and 0.46 ± 0.05 (0 = 1.7 ± 0.2), respectively, while 193-nm HI photodissociation yields an I(2P3/2) population of >0.9 (0 = -1.0). For 193-nm HBr photodissociation, the Br(2P3/2) and Br(2P1/2) populations are 0.86 and 0.14, respectively. Surprisingly, both channels appear to originate primarily from a perpendicular transition, since 0 is -1.0 for each state. This is in contrast to the case of HI, and plausible excitation and dissociation mechanisms are discussed in terms of possible coupling schemes.* 0+) and one parallel (3 + ' 0+).8,9 In these systems, the

The results of laser-assisted reactivity in constrained gas expansions involving either triethylgallium (TEGa) or trimethylgallium (TMGa) with ammonia are reported. NH3 (or ND3) along with TEGa or TMGa are introduced into a high vacuum chamber through a customized dual-source pulsed nozzle assembly. Excimer laser output (193 or 248 nm) is focused into the mixing and reaction region of the nozzle source, and a quadrupole mass spectrometer interrogates the photolyzed expansion. During gas mixing and expansion into the vacuum, the adduct signals from TMGa:NH3 or TEGa:NH3 are observed when the nozzle assembly is cooled. Laser irradiation of the gas mixtures results in the formation of new adducts, TMGa:NH2 or TEGa:NH2, and diverse, higher mass GaN-containing species. Power and wavelength dependence studies are used to reveal the photolytic origins of individual mass spectral features, and the influence of sample pressure on cluster formation processes is also examined.

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Photoexcitation is used to prepare species whose subsequent fragmentation can be exploited for the purposes of studying, controlling and manipulating different kinds of molecular processes. First, we show how a form of sub-Doppler resolution spectroscopy can be used to determine centre-ofmass kinetic energy distributions, thereby enabling internal energy distributions to be obtained for elementary processes which occur at a fixed total energy. We present data for H atoms monitored at the Lyman-a wavelength. Secondly, we show how such processes can be used to study binary interactions by preparing van der Waals complexes (e.g. C0,HBr