The triplet nf Rydberg states of HD with n=9–17, L=3, v=1, and R=1,3 have been observed and identified with the laser–molecular-beam method. A 20-eV electron beam excites a collimated HD beam to the metastable 2p ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }$ state. Two counterpropagating cw single-frequency laser beams excite the metastable molecule first to the intermediate 3d ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }$ state, then to a triplet nf Rydberg state. The transition frequencies between the 3d and nf states have been measured to an accuracy of 0.02 ${\mathrm{cm}}^{\mathrm{-}1}$. The long-range model gives the transition frequencies to an accuracy of approximately 0.1 ${\mathrm{cm}}^{\mathrm{-}1}$. Using these energy-level measurements, the ionization potential of the metastable state has been determined to be 27 460.98(30) ${\mathrm{cm}}^{\mathrm{-}1}$ and the energy difference between the triplet metastable and the singlet ground state has been determined to be 97 105.50(30) ${\mathrm{cm}}^{\mathrm{-}1}$. In addition, the linewidths of the triplet Rydberg states have been measured, giving the autoionization rates.

• Received 21 September 1994

DOI:https://doi.org/10.1103/PhysRevA.52.992