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

Abstract

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 $\mathrm{np\pi \; \leftarrow \; X}$ transitions and converging to the $\text{v=0,\hspace{0.17em} 1,\hspace{0.17em} 2,\hspace{0.17em}}\mathrm{and}\text{\hspace{0.17em} 3}$ levels of the HD⁺ ground state. The other three are identified as the R(0) series of the $\mathrm{np\sigma \; \leftarrow \; X}$ transitions, converging to the $\text{v=0\hspace{0.17em}}\mathrm{and}\text{\hspace{0.17em} 1}$ levels, and the R(0) series of the $\mathrm{np\pi \; \leftarrow \; X}$ transitions converging to the $\text{v=1}$ level of the HD⁺ ground state. The lowest ionization energy $\text{(v=0,\hspace{0.17em} N=0)}$ of this molecule, obtained from the Q(1) series, is $\text{124\hspace{0.17em} 569.5± 0.6\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . Eight additional Rydberg series, observed near their series limits and identified as either R(0) or R(1) series, are also reported.