Photoabsorption and photoionization of HD

Relative photoabsorption and photoionization cross sections have been measured for HD at a temperature of 78 K in the wavelength region from 735 to 805 Å. The present wavelength resolution of 0.016 Å represents an improvement of more than two orders of magnitude over that of previous photoionization studies of this molecule. Bands of the 3pπ D 1Πu ← X 1Σ+g system are observed to v′=17, and ionization efficiencies are reported for a number of Rydberg states of low principal quantum number. As in the case of H2, the ionization efficiency is close to unity for Rydberg states that can autoionize with Δv=−1, but drops to zero for states that can autoionize only with a large change in vibrational quantum number and that are significantly predissociated (such as the 3pπ D 1Πu state). The breakdown of (g,u) symmetry in HD and the resulting effects on the absorption spectrum and on the decay paths of the Rydberg states are discussed.