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This article describes the study of rotational energy transfer in the strongly polar LiH–HCN system. A supersonic beam of LiH, rotationally state selected in ja=1 by an electric quadrupole field, is scattered by HCN gas at room temperature. Laser fluorescence detection is used to determine integral cross sections for ja=1→j′a LiH transitions. The measured cross sections (in Å2) are 245±30 (2σ), 519±88, 222±47, 125±26, 64±17, and 41±12 for j′=0,2,3,4,5, and 6, respectively. The large magnitudes of the cross sections reflect the strong, long-range dipolar coupling. A comparison is made with the predictions of various theoretical models, based on the approximate solution of the time-dependent classical-path equations of motion for rectilinear trajectories. The usual Born approximation significantly overestimates the cross sections for the dipole allowed (ja→ja±1) transitions, even when statistical microreversibility is imposed. Worse, extension of the Born approximation to second order appears to introduce even larger errors. Better agreement with experiment is obtained within the sudden approximation, although the predicted ja=1→j′a=0,2 cross sections are still too large, due to the breakdown of this approximation at large impact parameter for this long-range system. This can be remedied in the adiabatically corrected sudden (ACS) approximation, which we have recently developed, by the introduction of an effective dephasing frequency into the sudden action integral. In a confirmation of our earlier study of the LiH–HCl, DCl systems, we find that the ACS cross sections are in good agreement with the experimental values, especially for the strongest transitions.
This article describes the study of rotational energy transfer in the strongly polar LiH–HCN system. A supersonic beam of LiH, rotationally state selected in ja=1 by an electric quadrupole field, is scattered by HCN gas at room temperature. Laser fluorescence detection is used to determine integral cross sections for ja=1→j′a LiH transitions. The measured cross sections (in Å2) are 245±30 (2σ), 519±88, 222±47, 125±26, 64±17, and 41±12 for j′=0,2,3,4,5, and 6, respectively. The large magnitudes of the cross sections reflect the strong, long-range dipolar coupling. A comparison is made with the predictions of various theoretical models, based on the approximate solution of the time-dependent classical-path equations of motion for rectilinear trajectories. The usual Born approximation significantly overestimates the cross sections for the dipole allowed (ja→ja±1) transitions, even when statistical microreversibility is imposed. Worse, extension of the Born approximation to second order appears to introduce even larger errors. Better agreement with experiment is obtained within the sudden approximation, although the predicted ja=1→j′a=0,2 cross sections are still too large, due to the breakdown of this approximation at large impact parameter for this long-range system. This can be remedied in the adiabatically corrected sudden (ACS) approximation, which we have recently developed, by the introduction of an effective dephasing frequency into the sudden action integral. In a confirmation of our earlier study of the LiH–HCl, DCl systems, we find that the ACS cross sections are in good agreement with the experimental values, especially for the strongest transitions.
Earlier close-coupling studies on the HF–HF system [A. E. DePristo and M. H. Alexander, J. Chem. Phys. 66, 1334 (1977)] have been extended to larger channel bases, allowing the determination of converged integral cross sections for excitation out of the lower rotational levels of the bimolecular system. The calculations were confined to collision energies appropriate to supersonic beam experiments (E =0.5–1.5 eV). Two potential surfaces were used, both taken from our earlier fit to ab initio points [M. H. Alexander and A. E. DePristo, J. Chem. Phys. 65, 5009 (1976)]. In the first surface the symmetry of the only anisotropic term included corresponds to the standard dipole–dipole interaction; to which were added, in the second surface, a primarily repulsive anisotropy as well as the long-range dipole–quadrupole interaction. The largest cross sections (40–60 Å2) are associated with R–R processes of the type j1 j2→j1±1, j2∓1 which are dipole-allowed in first order. The magnitudes of these cross sections are little affected by the presence of the shorter-range anisotropic terms, since much of the inelasticity occurs at large impact parameter. Cross sections for processes which are dipole-allowed only in second or higher order are considerably smaller (1–10 Å2), have classical dynamical thresholds at high energy, and are substantially lowered when the additional anisotropic terms are added to the potential, which has the effect of redirecting inelastic flux into the dipole forbidden channels. The cross sections for first order dipole–quadrupole transitions are also small, even in cases of near resonance. By contrast we find sizeable cross sections (7–15 Å2) for transitions which are coupled only by the short range anisotropy, which implies that rotational energy transfer between polar molecules cannot be fully described by models which rely solely on the standard long-range multipole expansion of the potential.
Isotopes of O, C, and S have been separated by two-step, laser photodissociation of OCS. The technique utilizes isotopically selective vibrational excitation of OCS in the ν2 (bending) vibrational mode with a line-tuned CO2 laser (λ∼9.4 μm), followed by photodissociation with a KrF excimer laser (λ=249 nm) and chemical scavenging of the sulfur atoms. Enrichment factors ranging from 1.5 to 3.5 were obtained for different isotopes. A sensitive IR absorption apparatus was used to measure absorption of the high energy CO2 laser pulse by OCS as a function of pressure and laser fluence. Average absorptions ranging up to 1.5 photons/molecule were attributed to rapid rotational relaxation and in part to sequential absorption up the ladder of ν2 vibrational levels. The 249 nm photodissociation cross section of OCS was measured as a function of absorbed IR energy. An excitation of one CO2 laser photon per molecule, or two quanta of ν2 vibration per molecule, increased the photodissociation cross section by a factor of 9 over the thermal (295 K) cross section. A rate constant of (4.8±1.0)×105 s−1 Torr−1 was inferred for the exchange of ν2 vibrational energy between isotopic varieties of OCS.
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