Robert C. Forrey

Vibrational relaxation of CO by collisions with 4He at ultracold temperatures

Quantum mechanical coupled channel scattering calculations are performed for the ro-vibrational relaxation of CO in collisions with ultracold He atoms. The van der Waals well in the interaction potential supports a number of shape resonances which significantly influence the relaxation cross sections at energies less than the well depth. Feshbach resonances are also found to occur near channel thresholds corresponding to the j=1 rotational level in the v=0 and v=1 vibrational levels. Their existence influences dramatically the limiting values of the elastic scattering cross sections and the rotational quenching rate coefficients from the j=1 level. We present complex scattering lengths for several low lying rotational levels of CO which characterize both elastic and inelastic collisions in the limit of zero temperature. Our results for the vibrational relaxation of CO (v=1) are in good agreement with available experimental and theoretical results.

Complex scattering lengths in multi-channel atom–molecule collisions

Abstract It is shown that in the presence of inelastic scattering, zero energy elastic and inelastic scattering can be characterized by a complex scattering length, the imaginary part of which is related directly to the total inelastic scattering cross section. Collisions between H atoms and vibrationally excited H 2 molecules are investigated. Zero energy cross sections for all vibrational states of H 2 and the corresponding complex scattering lengths are reported using accurate quantum mechanical calculations. We obtain large values of the elastic cross sections which we attribute to s -wave bound and quasi-bound states of H⋯H 2 ( v ). The energies and lifetimes of the quasi-bound states are extracted from the complex scattering lengths.

THRESHOLD PHENOMENA IN ULTRACOLD ATOM-MOLECULE COLLISIONS

Abstract Vibrational relaxation cross sections for an atom–molecule collision are calculated by explicit numerical solution of the coupled equations of scattering theory and shown to follow an inverse velocity dependence at very low initial kinetic energies of the incident atom, in accordance with Wigners threshold law. Rate constants for H+H 2 ( v , j =0)→H+H 2 ( v ′ v , j =0) are calculated for all the vibrational levels of H 2 and found to vary by seven orders of magnitude between the deepest level v =0 and the vibrational level v =12.

Hydrogen dissociative chemisorption and desorption on saturated subnano palladium clusters (Pdn, n = 2–9)

H2 sequential dissociative chemisorption on small palladium clusters was studied using density functional theory. The chosen clusters Pdn (n = 2-9) are of the lowest energy structures for each n. H2 dissociative chemisorption and subsequent H atom migration on the bare Pd clusters were found to be nearly barrierless. The dissociative chemisorption energy of H2 and the desorption energy of H atom in general decrease with the coverage of H atoms and thus the catalytic efficiency decreases as the H loading increases. These energies at full cluster saturation were identified and found to vary in small energy ranges regardless of cluster size. As H loading increases, the clusters gradually change their bonding from metallic character to covalent character. For the selected Pd clusters, the capacity to adsorb H atoms increases almost proportionally with cluster size; however, it was found that the capacity of Pd clusters to adsorb H atoms is, on average, substantially smaller than that of small Pt clusters, suggesting that the catalytic efficiency of Pt nanoparticles is superior to Pd nanoparticles in catalyzing dissociative chemisorption of H2 molecules.

Quantum dynamics of CO-H2 in full dimensionality

Accurate rate coefficients for molecular vibrational transitions due to collisions with H₂, critical for interpreting infrared astronomical observations, are lacking for most molecules. Quantum calculations are the primary source of such data, but reliable values that consider all internal degrees of freedom of the collision complex have only been reported for H₂-H₂ due to the difficulty of the computations. Here we present essentially exact, full-dimensional dynamics computations for rovibrational quenching of CO due to H₂ impact. Using a high-level six-dimensional potential surface, time-independent scattering calculations, within a full angular momentum coupling formulation, were performed for the de-excitation of vibrationally excited CO. Agreement with experimentally determined results confirms the accuracy of the potential and scattering computations, representing the largest of such calculations performed to date. This investigation advances computational quantum dynamical studies representing initial steps towards obtaining CO-H₂ rovibrational quenching data needed for astrophysical modelling.

Quantum dynamics of rovibrational transitions in H2-H2 collisions: Internal energy and rotational angular momentum conservation effects

We present a full dimensional quantum mechanical treatment of collisions between two H(2) molecules over a wide range of energies. Elastic and state-to-state inelastic cross sections for ortho-H(2) + para-H(2) and ortho-H(2) + ortho-H(2) collisions have been computed for different initial rovibrational levels of the molecules. For rovibrationally excited molecules, it has been found that state-to-state transitions are highly specific. Inelastic collisions that conserve the total rotational angular momentum of the diatoms and that involve small changes in the internal energy are found to be highly efficient. The effectiveness of these quasiresonant processes increases with decreasing collision energy and they become highly state-selective at ultracold temperatures. They are found to be more dominant for rotational energy exchange than for vibrational transitions. For non-reactive collisions between ortho- and para-H(2) molecules for which rotational energy exchange is forbidden, the quasiresonant mechanism involves a purely vibrational energy transfer albeit with less efficiency. When inelastic collisions are dominated by a quasiresonant transition calculations using a reduced basis set involving only the quasiresonant channels yield nearly identical results as the full basis set calculation leading to dramatic savings in computational cost.

State-to-state rotational transitions in H2+H2 collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the superthermal region. The two most recently published potential energy surfaces for the H(2)-H(2) complex, the so-called Diep-Johnson (DJ) [J. Chem. Phys. 112, 4465 (2000); 113, 3480 (2000)] and Boothroyd-Martin-Keogh-Peterson (BMKP) [J. Chem. Phys. 116, 666 (2002)] surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H(2)+H(2) collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H(2)-H(2) interaction. We found significant differences in rotational excitation/deexcitation cross sections computed on the two surfaces in collisions between two para-H(2) molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment B. Mate et al., [J. Chem. Phys. 122, 064313 (2005)] for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.

Quantum Mechanical Calculations of Rotational Transitions in H-H2 Collisions

We report quantum mechanical cross sections and rate coefficients for rotational transitions due to collisions of hydrogen atoms with hydrogen molecules. The sensitivity of the results to the nature of the potential energy surface and to the scattering formulation is investigated. We find that the rigid rotor and harmonic oscillator approximations are inadequate to describe the dynamics. We make use of the most reliable of the potential energy surfaces in a close-coupled description of the nonreactive scattering that takes the vibrational motion into account, and we calculate rate coefficients for rotational transitions at temperatures below 1000 K, where the reactive channels may be neglected.

RATE OF FORMATION OF HYDROGEN MOLECULES BY THREE-BODY RECOMBINATION DURING PRIMORDIAL STAR FORMATION

Astrophysical models of primordial star formation require rate constants for three-body recombination as input. The current status of these rates for H2 due to collisions with H is far from satisfactory, with published rate constants showing orders of magnitude disagreement at the temperatures relevant for H2 formation in primordial gas. This letter presents an independent calculation of this recombination rate constant as a function of temperature. An analytic expression is provided for the rate constant which should be more reliable than ones currently being used in astrophysical models.

Full-dimensional quantum dynamics calculations of H2–H2 collisions

We report quantum dynamics calculations of rotational and vibrational energy transfer in collisions between two para-H(2) molecules over collision energies spanning from the ultracold limit to thermal energies. Results obtained using a recent full-dimensional H(2)-H(2) potential energy surface (PES) developed by Hinde [J. Chem. Phys. 128, 154308 (2008)] are compared with those derived from the Boothroyd, Martin, Keogh, and Peterson (BMKP) PES [J. Chem. Phys. 116, 666 (2002)]. For vibrational relaxation of H(2)(v=1,j=0) by collisions with H(2)(v=0,j=0) as well as rotational excitations in collisions between ground state H(2) molecules, the PES of Hinde is found to yield results in better agreement with available experimental data. A highly efficient near-resonant energy transfer mechanism that conserves internal rotational angular momentum and was identified in our previous study of the H(2)-H(2) system [Phys. Rev. A 77, 030704(R) (2008)] using the BMKP PES is also found to be reproduced by the Hinde PES, demonstrating that the process is largely insensitive to the details of the PES. In the absence of the near-resonance mechanism, vibrational relaxation is driven by the anisotropy of the potential energy surface. Based on a comparison of results obtained using the Hinde and BMKP PESs with available experimental data, it appears that the Hinde PES provides a more accurate description of rotational and vibrational transitions in H(2)-H(2) collisions, at least for vibrational quantum numbers v ≤ 1.