Spin‐lattice relaxation and the anisotropic part of the H2–He and H2–Ne intermolecular potential

Survival of rotational alignment in H2 scattering from Si(100)

We report a state-prepared, state-resolved study of rotational scattering of a diatomic molecule from a solid surface. Specifically, H₂ molecules with 80 meV kinetic energy are rotationally aligned in the j = 3 rotational state via stimulated Raman pumping and then scattered from a Si(100) surface at normal incidence. The rotational alignment of the scattered molecules is determined by measuring, for both the incident and scattered molecules, the ionization yield of a probe laser, tuned to selectively ionize molecules in the j = 3 rotation level, as the probe laser polarization is rotated. The measurement is performed for two initial rotational alignments: a "helicoptering" alignment with the bonds constrained to lie primarily parallel to the surface and a "cartwheeling" alignment with the bonds lying primarily normal to the surface. For both initial alignments, the modulation of the probe ionization yield with laser polarization for the scattered molecules is pronounced, although significantly weaker than the modulation measured for the incident molecules. This indicates a significant modification but not a complete elimination of the initial alignment. The modulation is found to be stronger for the scattered molecules originating in the cartwheeling alignment than for the helicoptering alignment. These results contribute toward an improved understanding of the role of rotational motion in molecule-surface dynamics.

Collisional cross-section of water molecules in vapour studied by means of 1H relaxation in NMR

In gas phase, collisions that affect the rotational angular momentum lead to the return of the magnetization to its equilibrium (relaxation) in Nuclear Magnetic Resonance (NMR). To the best of our knowledge, the longitudinal relaxation rates R₁ = 1/T₁ of protons in H₂O and HDO have never been measured in gas phase. We report R₁ in gas phase in a field of 18.8 T, i.e., at a proton Larmor frequency ν₀ = 800 MHz, at temperatures between 353 and 373 K and pressures between 9 and 101 kPa. By assuming that spin rotation is the dominant relaxation mechanism, we estimated the effective cross-section σ_J for the transfer of angular momentum due to H₂O-H₂O and HDO-D₂O collisions. Our results allow one to test theoretical predictions of the intermolecular potential of water in gas phase.