U. Tappe

A high resolution crossed molecular beam investigation of the absolute cross sections and product rotational states for the reaction F+D2 (vi=0; ji=0,1)→DF(vf;jf)+D

High resolution time of flight spectra of DF products have been measured for 12 different center‐of‐mass angles in the range θc.m.=114° to 180° for the reaction F+D2→DF+D at a center‐of‐mass collision energy of Ec.m.=82.5±2.6 meV. The resolution is sufficient to clearly resolve the different final product vibrational states and to extract rotational product distributions for each of the vibrational states. Absolute reactive cross sections for the final vibrational states vf=1, 2, 3, and 4 were determined from a careful calibration of the beam source intensities and detector sensitivity. For all final vibrational states, nearly the same large rotational surprisal values of ΘR=5.3 were found. From the rotational distributions, it has also been possible to estimate opacity functions for these final vf states via the method of Elsum and Gordon [J. Chem. Phys. 76, 3009 (1982)]. The angular distributions for different vf states are compared to recent infinite order sudden approximation (IOSA) and classical tra...

State-to-state differential cross sections for the reaction F+D2 at 90 meV: A crossed molecular beam experiment and a quantum mechanical study

The F+D2→DF+D reaction has been investigated in a high resolution crossed molecular beam scattering experiment at a collision energy of 90 meV (2.07 kcal/mol). Time-of-flight spectra of the DF products have been measured covering the backward hemisphere of center-of-mass scattering angles (θcm=90°–180°). The energy resolution achieved in the spectra, as good as 20 meV, together with a careful calibration of the beam source intensities and detector sensitivity makes it possible to determine absolute differential and integral cross sections resolved in vibrational and rotational states of the DF products. Interestingly, the backward scattered DF(vf=2) and DF(vf=3) vibrational products are found to present double-peaked (i.e., bimodal) rotational distributions. A three dimensional quantum mechanical calculation of the title reaction performed on the recent ab initio potential energy surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)] is presented, which was carried out in the reagents arrangement c...

A scattering study of the dependence of the F + D2(ji = 0, 1, 2) → DF(νf,jf) + D reaction on the initial rotational state

Abstract The F + D2(ji=0, 1, 2) → DF(νf, jf) + D reaction has been studied in a high-resolution crossed molecular beam experiment at a collision energy of 3.2 kcal/mol (140.0 meV) with an energy resolution of the time-of-flight spectra of typically 25 meV. The new results for the vibrationally resolved differential cross sections for the DF(νf = 1, 2, 3, 4) products show the same general trends as an earlier experiment of Neumark et al., but some significant discrepancies are found. New evidence is presented that the vibrational product state distributions depend strongly on the initial rotational state of the D2 reagents. This effect reconciles some of the apparent discrepancies between our experiment and the earlier one.

Integral and differential state‐to‐state cross‐sections for the reactions F+D2(vi=0,ji)→DF(vf,jf)+D: A comparison between three‐dimensional quantum mechanical and experimental results

In this letter we report quantum mechanical integral and differential cross sections for the title reactions as calculated on a new ab initio potential energy surface. The calculations, all carried out in the reagents arrangement channel employing negative imaginary potentials, were done within the coupled‐states approximation. The final vibrational state‐to‐state differential and integral cross sections were compared with experiment. Altogether, a very encouraging agreement was obtained.

The H2–Ne interaction

New measurements of the elastic and rotationally inelastic differential cross sections for the Ne–D2, Ne–H2 system are compared with exact and approximate quantum calculations. The three most recent high quality, semiempirical interaction potentials available in the literature for the Ne–H2 system yield consistent theoretical scattering cross sections for Ne–H2 and for Ne–D2. They also agree with previous and with present inelastic cross section measurements for D2. However, the theory underestimates by 30% the newly measured rotational excitation in Ne–H2 collisions discussed here. We therefore propose a new potential with a modified repulsive barrier that succeeds in describing both Ne–D2 and Ne–H2 rotationally inelastic scattering experiments for j=0→j’=2 within an accuracy of a few percent.

Rotationally resolved differential scattering cross sections for the reaction F+para-H2 (v=0, j=0)→HF(v′=2, 3, j′)+H

Time-of-flight spectra of HF products in the v′=2 vibrational state from reactive scattering of F atoms from para-H2 exhibit at least four smaller peaks which are assigned to the rotational states j′=7, 8, 9, and 10. The center-of-mass rotational distributions are in good agreement with accurate quantum mechanical and approximate coupled states calculations.

F-D2 STATE RESOLVED REACTIVE SCATTERING AT 180 AND 240 MEV COLLISION ENERGIES. II. QUASI-CLASSICAL CROSS SECTIONS. A COMPARISON WITH THE EXPERIMENTAL RESULTS

Abstract Quasi-classical trajectory calculations (QCT) have been carried out for the F+D 2 reaction at the collision energies and initial rotational states necessary to simulate the molecular beam results presented in the preceding paper of this issue by Faubel et al. Although the general trends are well accounted for by the QCT calculations, there are significant differences between experiment and theoretical results. The vibrational resolved differential cross section are in an overall good agreement; however, the QCT calculations clearly underestimate both backward and forward scattering. The comparison between the product state distributions indicates that the QCT ones are somewhat broader than the experimental ones for most of the vibrational states. The limitations of the theoretical results become more clear when the laboratory frame (LAB) angular distributions (AD) and time-of-flight (TOF) spectra are simulated using the calculated DCS resolved into the final rovibrational states, v f , j f . The theoretical findings and, especially, the roles of translational energy and initial rotational momentum on the dynamics of this reaction are discussed in some detail.

An intense fluorine atom beam source

The important properties of thermal ovens for producing intense beams of F atoms by thermal dissociation of are analysed. After reviewing previous constructions a new source made of a single crystal of is described and characterized. This source has been tested for more than 3000 h at temperatures up to with a mixture of 10% in Ar at pressures up to 12 bar. A degree of dissociation of 50% and a F atom beam velocity spread of about 7% were achieved.

A further test of the shape and anisotropy of the FH2 interaction potential

Abstract A recently proposed anisotropic potential model for the interaction of a fluorine atom with a hydrogen molecule treated as a rigid rotor analysed by carrying out exact quantum calculations of elastic and rotationally inelastic differential cross sections for comparison with previoully reported FH 2 and newly measured FD 2 state selected measurements. The sensitivity of the cross sections to changes of the potential anisotropy and to isotopic substitution is examined. The results provide specific indications on the features of the best potential energy surface in terms of its average ‘size’ and its most likely anisotropy responsible for inelastic rotational excitations occuring at collision energies of about 85 meV.