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Laboratory angular distributions (LAB ADs) have been measured for the Li ] HF (v \ 1, j \ 1) reaction in a crossed molecular beam experiment at the collision energies 0.231 eV and 0.416 eV and compared with the results of extensive quasi-classical trajectory (QCT) calculations performed on the most recent ab initio potential energy surface (PES) for this system. The calculations also include the collision energy dependence of the integral and di †erential cross sections in the range 0.025È0.5 eV (2.4È48.2 kJ mol~1). In particular, the total QCT integral reactive cross sections have been found to be in very good agreement with recent quantum mechanical (QM) calculations carried out on the same PES by Lara et al. (J. Chem. Phys., 1998, 109, 9391). In addition, the triple scattering angleÈrecoil velocity di †erential cross section has been calculated in order to simulate the experimental LAB AD. An excellent concordance between both sets of data has been found, indicating that the reaction of Li with HF in v \ 1 can be very well described by QCT calculations on the mentioned PES.

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In a crossed molecular-beam study we have measured angular and time-of-flight distributions of the product LiF from the reaction Li + HF(upsilon = 0)-->LiF + H at various collision energies ranging from 97 to 363 meV for three markedly different rotational state distributions of HF obtained at nozzle temperatures close to 315, 510, and 850 K. Particularly, for the low and intermediate collision energies we observe significant effects of the varying j-state populations on the shape of the product angular distributions. At 315 K an additional feature appears in the angular distributions which is interpreted as being due to scattering from HF dimers. The experimental data are compared with simulations of the monomer reaction based on extensive quasiclassical trajectory calculations on a new state-of-the-art ab initio potential energy surface. We find an overall good agreement between the theoretical simulations and the experimental data for the title reaction, especially at the highest HF nozzle temperature.

The dynamics of the gas-phase reaction of H atoms with HCl has been studied experimentally employing the laser photolysis/vacuum-UV laser-induced fluorescence (LP/VUV-LIF) "pump-and-probe" technique and theoretically by means of quasiclassical trajectory (QCT) calculations performed on two versions of the new potential energy surface of Bian and Werner [Bian, W.; Werner, H.-J. J. Chem. Phys. 2000, 112, 220]. In the experimental studies translationally energetic H atoms with average collision energies of E col ) 1.4 and 1.7 eV were generated by pulsed laser photolysis of H 2 S and HBr at 222 nm, respectively. Ground-state Cl( 2 P 3/2 ) and spin-orbit excited Cl*( 2 P 1/2 ) atoms produced in the reactive collision of the H atoms with room-temperature HCl were detected under single collision conditions by VUV-LIF. The measurements of the Cl* formation spin-orbit branching ratio φ Cl* (1.4 eV) ) [Cl*]/[Cl + Cl*] ) 0.07 ( 0.01 and φ Cl* (1.7 eV) ) 0.19 ( 0.02 revealed the increasing importance of the nonadiabatic reaction channel H + HCl f H 2 + Cl* with increasing collision energy. To allow for comparison with the QCT calculations, total absolute reaction cross sections for chlorine atom formation, σ R (1.4 eV) ) (0.35 ( 0.16) Å 2 and σ R (1.7 eV) ) (0.13 ( 0.06) Å 2 , have been measured using a photolytic calibration method. In addition, further QCT calculations have been carried out for the H + DCl isotope reaction which can be compared with the results of previous reaction dynamics experiments of Barclay et al. [

The rotational relaxation of molecular nitrogen has been investigated down to temperatures of about 5 K with a combination of resonance-enhanced multiphoton ionization and supersonic beam time-of-flight techniques. The average rotational relaxation cross section obtained shows a maximum value of 50−60 Å2 at 20−30 K. For lower temperatures this cross section decreases and reaches a value smaller than 30 Å2 at T ≈ 5 K. For temperatures above 30 K, the cross section decreases slowly as the temperature grows and converges approximately to the determinations from other non-jet techniques and theoretical estimates available for T > 80 K. The results are compared to previous measurements from other groups using different methods, and general good agreement is found. However, we observe a significant discrepancy with some of the data from electron-beam-induced fluorescence that yield much larger cross sections for temperatures lower than 30 K.