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.
Scattering of NH3 by ortho and paraH2: Expansion of the potential and collisional propensity rules J. Chem. Phys. 98, 4662 (1993); 10.1063/1.464970Theoretical investigation of rotational rainbow structures in X-Na2 collisions using CI potential surfaces. I. Rigid rotor X = He scattering and comparison with statetostate experiments J. Chem. Phys. 74, 3916 (1981); 10.1063/1.441568 Penning ionization of H2 by He(23 S): Quantum mechanical scattering calculations within the rigidrotor approximation Close coupling calculations of integral and differential elastic cross sections of hydrogen ground state molecule collisions have been performed. for C.m. energies below 0.5 eV .. 1t is shown that the isotropic part of the potential. determined by the consistent ab initio potentials of five geometries, provides a very accurate (I %-2%) agreement with measured P-H2/P-H2 integral cross sections in the range of 900-2300 m/sec relative velocity. Detailed analysis of P-H2/P-H2 scattering results and the determination of a series of orbiting resonances provide a set of (virtual) quasi-bound-state energy levels. The fit formula of those levels gives two bound states of the (H 2 )2 system, for J = 0 and I, at ~0.340x 10-3 and ~0.179 X 10-3 eV, respectively, which is close to the two binding energies found for the isotropic potential. However, the more attractive plane T configuration of the (H 2 )2 system gives larger binding energies when the zero point vibrations are neglected. Lifetimes and the periods of orbiting have been evaluated for the resonances. Since the resonaces are due to pure orbiting, we have compared the spacing of the P-H2/P-H2 partial integral cross section peaks with peaks of the opacities for P-H2/0-H2 and o-H 2 /0-H 2 and found quantitative agreement almost everywhere, while the exceptions found for o-H 2 /0-H 2 at very small energies can be explained by slightly different effective moments of inertia. The undulatory structure found for the ground state P-H2/P-H2 integral cross section is clearly due to symmetry restrictions which only allow even partial waves, For o-H 2 /0-H 2 , the cross sections of solely the symmetric or the asymmetric problem show undulatory structure as well because the even or odd partial wave contributions, respectively, dominate, while in the averaged curve these structures are completely damped. A few examples of differential cross sections and differential helicity transition cross sections at resonance energies are presented. The j, conservation in the body-fixed frame used in the approximation of McGuire has been found to be not quantitatively valid in the energy range of the strong resonances while for increasing energies j, conserving cross sections become dominant. Generally for o-H 2 /p-H 2 collisions j, is better conserved than for o-H 2 /0-H 2 , Our most recently improved ab initio calculations 344
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