Kinetic and internal energy distributions via velocity-aligned Doppler spectroscopy: The 193 nm photodissociation of H2S and HBr

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Abstract
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.