Vibration-rotation spectroscopy of the HD⁺ ion near the dissociation limit

Abstract

We have observed and measured five rotational components of the 21–17 band in the HD⁺ molecular ion. The highest bound rotational level in υ=21 is predicted to be N = 3, which lies only 1·8 cm⁻¹ below the H⁺+D dissociation limit, and we have observed transitions involving all four rotational levels of υ=21. The measured transition frequencies are close to those predicted by recent non-adiabatic calculations. Each vibration-rotation line is split into six hyperfine components, which arise primarily because of Fermi contact interactions with the proton and deuteron nuclei. Analysis of these hyperfine multiplets yields values of the Fermi contact hyperfine constants for the lower and upper states in each vibration-rotation transition. These values show that in υ=17 the electron distribution is essentially symmetric, but in υ=21 it becomes increasingly asymmetric as N increases. The proton Fermi contact hyperfine constant is close to 715 MHz in υ=17, but decreases to only 53 MHz in 21,3. Correspondingly the deuteron hyperfine constant increases from 112 MHz in υ=17 to 213 MHz in 21, 3, which is only slightly less than the free deuterium atom value of 218 MHz. These results demonstrate that in 21, 3 the electron is virtually localized at the deuteron, so that the molecule is perhaps best regarded as a long-range H⁺…D Langevin complex.