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

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.

Real time study of bimolecular interactions: Direct detection of internal conversion involving Br(2P1/2)+I2 initiated from a van der Waals dimer

“…͑1b͒ ͓Br͑ 2 P 3/2 ͒ will henceforth be referred to as Br and Br͑ 2 P 1/2 ͒ will be referred to as Br*.͔ For either branch of photodissociation, the H-atom is Ͼ15 Å removed within Ϸ100 fs ͑as determined by classical trajectories͒. The branching ratio for photodissociation of HBr, Br*/Br, has been measured at 193 nm 34,35 and was found to be 15%, but there is no report of a similar measurement at 220 nm, the pump wavelength of the current experiment. In general, the first ultraviolet continuum transition of hydrogen halides, HX, are featureless but can be decomposed into contributions from 3 ⌸ 1 , 3 ⌸ 0 and 1 ⌸.…”

Kinematic distribution function to calculate rotational populations of photofragments from photodissociation of triatomic molecules

“…Weiner et al employed LIF in a pulsed supersonic jet to measure nascent SH rotational state populations, spin-orbit state population ratios, and h-doublet state populations following the excimer laser photodissociation of H2S at 193,222, and 248 nm [ 1 I]. Wittig and co-workers used a Doppler profile technique to observe vibrational excitation of SH up to v'' = 8 at 193 nm [20]. Kleinermanns et al, using emission spectroscopy of the dissociating H2S at 193 nm, presented evidence that suggested a correlation between the bending and stretching vibrational excitation as the dissociating triatomic molecule progressed along the reaction coordinate [ 2 11.…”

Product state resolved stereodynamics: Rotational polarization of OH(2Π;υ′,N′,Ω,f ) scattered from the reaction, H+CO2→OH+CO

“…͑The dissociation dynamics have been studied extensively and the photofragment velocity distribution and translational anisotropy have been fully determined. [32][33][34][35] Although the H atom distribution generated in the photodissociation of HBr is bimodal, due to the 20% yield of Br*, the slow velocity component makes negligible contribution to reaction due to the rapidly increasing excitation function for the HϩCO 2 reaction.͒ The most probable collision energy in the center of mass was 241 kJ mol Ϫ1 ͑2.5 eV͒ with a full width at half maximum ͑FWHM͒ of 15 kJ mol Ϫ1 ͑0.15 eV͒ for a sample of CO 2 gas at 298 K. The absorption cross section of CO 2 at 193.3 nm is negligible. 36 After a delay of 75-110 ns, short enough to allow reaction ͑1͒ to proceed under single collision conditions, the scattered OH(Ј,NЈ,⍀, f ) fragments were detected via LIF, using the ⌬Јϭ0 sequence of the A 2 ⌺ ϩ ←X 2 ⌸ band system.…”