State resolved rotational excitation in D2+H2 collisions

Abstract

In a crossed molecular beam experiment time-of-flight distributions of ortho D2 molecules scattered from normal H2 (nH2) and para H2 (pH2) have been measured in a center-of-mass angular range of 75° to 180°. The collision energies were 84.1 and 87.2 meV, respectively. In all spectra the rotational excitation of D2 from j=0 to j=2 has been resolved. With pH2 as secondary beam the same transition could also be observed for H2. The measurements show that the probability for rotational excitation of D2 depends on whether the scattering partner H2 is rotating (nH2) or not (pH2). In the first case the cross sections are larger by a factor of approximately 2. The reason for this behavior is the presence of an additional interaction term which is at long range distances, identical to the quadrupole–quadrupole interaction and which is absent if H2 is in the j=0 state. The experimentally derived differential cross sections for the rotational excitation of D2 and H2 are compared with theoretical results obtained by close coupling calculations based on the ab initio potential surface of Meyer and Schaefer. The comparison shows a remarkable agreement. However, small deviations in the positions of the diffraction oscillations of the elastic differential cross section curve suggest that the isotropic potential term has to be shifted to smaller distances. In order to maintain the relative position of the inelastic differential cross section curves which are well predicted by the ab initio potential the same shift has to be applied to the anisotropic potential terms.

Diffusion Monte Carlo study of the equation of state of solid para-H2

“…The popular Silvera-Goldman (SG) [1] potential, which includes a 1/r 9 term which mimicks a triple-dipole interaction, and the Buck [2,3] potential, which is instead a pure two-body interaction, have been employed indifferently in theoretical works about p-H 2 , while in the case of o-D 2 only the first one has been commonly used. However, in the case of p-H 2 , we recently discovered by means of accurate Quantum Monte Carlo simulations that an explicit three-body term is necessary in order to correctly reproduce experimental data on pressure [4], .…”

Molecular hydrogen confined within nanoporous framework materials: Comparison of density functional and classical force-field descriptions

“…For comparison, the system energies calculated using the Buck et al FF are given again. 4,5 Figure 2 shows that the agreement between the H 2 loading curves for all methods becomes increasingly worse for loadings from two to six H 2 molecules. For loadings above eight H 2 molecules, the FF calculations indicate that H 2 -H 2 repulsion becomes so large that energetically fa- vorable storage is not possible any more ͑the uptake of more hydrogen molecules would be an endothermic process͒.…”

“…In the absence of more generally appropriate functionals for confined H 2 systems we thus advocate the use of accurately parameterized interatomic potentials as employed herein and further justified in other studies. [2][3][4][5]14 Considering the large number of reported DFT studies on the storage of H 2 within various confining nanostructures and materials using one of the functionals tested herein, [15][16][17][18][19][20][21] it is important that subsequent predictions of H 2 storage capacity and energetics based upon such calculations are viewed critically. To show how the different methods can lead to disparate estimates of H 2 storage capacity we show in Table II As all calculations are effectively performed at zero Kelvin and no zero point energy correction is applied these results should not be thought to give a realistic estimate for the maximum practically achievable H 2 storage capacity in SOD.…”

“…[6][7][8][9] Such potentials have been extensively used and range in complexity from simple two parameter LennardJones forms to more accurate multiparameter potential forms ͑e.g., Buck et al, Silvera-Goldman͒. 1,4,10 Many such potentials describe the weak intermolecular hydrogen interaction in a spherically symmetric manner, which is found to be an excellent approximation when the results of their usage are compared with experimental data. The accuracy of such centrosymmetric H 2 -H 2 potentials is also evidenced by their use in the setup of high level quantum mechanical calculations of H 2 cluster systems, 11,12 the calculation of the properties of bulk dense H 2 phases, 2,3 and also for larger scale classical molecular dynamics calculations.…”

“…It is also increasingly important recently, due to the application potential of efficient storage of hydrogen as a clean portable energy resource. For the purposes of studying both bulk and clusters of H 2 molecules numerous H 2 -H 2 interaction potentials have been developed through the consideration of experimental data, [1][2][3][4][5] and via high level ab initio calculations of H 2 dimers. [6][7][8][9] Such potentials have been extensively used and range in complexity from simple two parameter LennardJones forms to more accurate multiparameter potential forms ͑e.g., Buck et al, Silvera-Goldman͒.…”