Towards universal potentials for (H2)2 and isotopic variants: post-Born-Oppenheimer contributions

Vibrational Raman Shifts of Spin Isomer Combinations of Hydrogen Dimers and Isotopologues

Binding energies for para-para, ortho-para, and ortho-ortho hydrogen dimers (H₂)₂ are calculated using the six-dimensional (6D) interaction potential developed by Hinde [ J. Chem. Phys. 2008, 128, 154308]. The eigenstates of the dimers are computed by diagonalization using, as a basis, products of the rovibrational states of the monomers, a radial grid for the distance between the monomers, and spherical harmonics for the end-over-end rotation of the dimer. We describe the overall nuclear spin symmetry and use these properties to determine the relative population of various states, making use of a Boltzmann factor for each spin isomer to assess the effect of temperature. A predicted Raman spectrum in the Q(0) and Q(1) region of the hydrogen dimer is produced. To assess the accuracy of our model, we verify our produced shifts with experimental results obtained previously by Montero et al. [ Eur. Phys. J. D 2009, 52, 31-34] and find good agreement. These results are extended to other cases involving the deuterium (D₂)₂ and tritium dimer (T₂)₂ isotopologues, to predict Raman shifts.