Absorption Spectrum of HD in the Vacuum-uv Region. Rydberg States and Ionization Energy

The absorption spectrum of the HD molecule has been investigated in the 950–600 Å region with a 6.65-m normal-incidence vacuum spectrograph in the second order using the helium continuum as a light source. In the spectrum of cooled HD, seven Rydberg series are identified. Four consist of the Q(1) lines in the npπ ← X transitions and converging to the v=0, 1, 2, and 3 levels of the HD+ ground state. The other three are identified as the R(0) series of the npσ ← X transitions, converging to the v=0 and 1 levels, and the R(0) series of the npπ ← X transitions converging to the v=1 level of the HD+ ground state. The lowest ionization energy (v=0, N=0) of this molecule, obtained from the Q(1) series, is 124 569.5± 0.6 cm−1. Eight additional Rydberg series, observed near their series limits and identified as either R(0) or R(1) series, are also reported.