Vibration-vibration and vibration-translation energy transfer in H2-H2 collisions: A critical test of experiment with full-dimensional quantum dynamics

Abstract

Quantum scattering calculations of vibration-vibration (VV) and vibration-translation (VT) energy transfer for non-reactive H2-H2 collisions on a full-dimensional potential energy surface are reported for energies ranging from the ultracold to the thermal regime. The efficiency of VV and VT transfer is known to strongly correlate with the energy gap between the initial and final states. In H2(v = 1, j = 0) + H2(v = 0, j = 1) collisions, the inelastic cross section at low energies is dominated by a VV process leading to H2(v = 0, j = 0) + H2(v = 1, j = 1) products. At energies above the opening of the v = 1, j = 2 rotational channel, pure rotational excitation of the para-H2 molecule leading to the formation of H2(v = 1, j = 2) + H2(v = 0, j = 1) dominates the inelastic cross section. For vibrationally excited H2 in the v = 2 vibrational level colliding with H2(v = 0), the efficiency of both VV and VT process is examined. It is found that the VV process leading to the formation of 2H2(v = 1) molecules dominates over the VT process leading to H2(v = 1) + H2(v = 0) products, consistent with available experimental data, but in contrast to earlier semiclassical results. Overall, VV processes are found to be more efficient than VT processes, for both distinguishable and indistinguishable H2-H2 collisions confirming room temperature measurements for v = 1 and v = 2.

Recent Advances in Development and Applications of the Mixed Quantum/Classical Theory for Inelastic Scattering

“…55,56 The corresponding effective collision energy U is defined as (13) It does not appear in any equations listed above but is used to set up the initial conditions in the actual calculations (namely, in eq 8 instead of E). Using simple algebra, one can express U, E, and E′ through each other: 55,56 (14) (15) (16) (17) Note that E′ < U < E. Also note that at the threshold for excitation, when E = ΔE and E′ = 0, we obtain U = ΔE/4. This means that the effective or actual collision energy U of MQCT trajectories is never less than one-quarter of the quantum for the state-to-state transition of interest.…”

“…We also studied one case when the rotational energy released by one collision partner is absorbed by rotation of the other partner, the so-called quasi-resonant energy transfer between two molecules. Among the molecules we studied there were such important as H2O, 2,16,17,19,38 and such large as HCOOCH3 which, to our best knowledge, is the most complicated system ever considered for the inelastic scattering calculations. 12 In all cases we compared our MQCT results against results of the fullquantum calculations carried out with MOLSCAT 64 or Hibridon.…”

“…13,14 Third, the range of scattering energies we have to cover is often broad, with different mechanisms dominating in different collision regimes (e.g., scattering resonances at low energies 2,12,15 in contrast to vibrational excitation at high energies). [16][17][18] Finally, very detailed information about the process may be needed, such as differential over scattering angle cross section for state-to-state transition. 19,20 If several of these complications have to be tackled simultaneously, the inelastic scattering process does not look simple anymore.…”

A full-dimensional quantum dynamical study of H2+H2 collisions: Coupled-states versus close-coupling formulation

“…Later studies provided a full-dimensional PES for the four atom system [13][14][15][16] which allowed the rigid rotor formulation to be extended to include vibrational motion. 17,18 More recently, a series of quantum dynamics calculations have been reported [19][20][21][22][23][24] which use a full-dimensional CC formulation to compute the collision cross sections. The results of these calculations were shown to provide good agreement with experimental data in the limited cases where experimental data are available.…”

A quasiclassical trajectory study of H2 + H2 energy transfer: a survey of applicability of detailed balance

“…Tremendous challenge has remained for the full-dimensional quantum mechanical treatment of H 2 + H 2 systems due to the need to treat all bonds as reactive ones. [7][8][9][10][11][12][13] The size of quantum calculations rises with the number of channels required for convergence which in turn rises with increases of total energy. Therefore, quantum calculations can be possible in principle, but not practically, at higher energies for the H 2 + H 2 system and its isotopic analogs.…”