Raman line positions in molecular hydrogen: H 2, HD, HT, D 2, DT, and T 2

Abstract

Spontaneous Raman spectroscopy is used to determine line positions of the six isotopomers of molecular hydrogen: H 2, HD, HT, D 2, DT, and T 2. State populations as low as 1.3 × 10 8 are detected with the present experimental apparatus. This sensitivity makes possible measurement of the first overtone Q-branch line positions for H 2 and D 2 and of higher rotational transitions than previous investigations. The line positions for D 2, DT, and T 2 indicate that literature values for molecular parameters do not predict accurately line positions of transitions at J values above the observed transitions from which they were determined. The results for the six molecular isotopomers show that ab initio energy levels restricted to the adiabatic approximation do not yield line positions within the experimental uncertainty whereas recent nonadiabatic calculations reproduce the present observations. Reexamination of literature results at high energies indicates discrepancies between the theoretical calculations and experimental vibrational band origins for all vibrational levels in HT, DT, and T 2. No experimental measurements are currently available that test the accuracy of nonadiabatic ab initio rotational levels at high energies.