Bound energy levels at the n=2 dissociation threshold in HD

Level energies of g symmetry states lying just below the n=2 dissociation threshold have been determined in a XUV + IR multi-step laser excitation experiment in HD, with an absolute accuracy of the excitation energy of 0.015 cm−1. Intermediate B1Σu+ energy levels have been calibrated by measuring transition frequencies in the B1Σu+–X1Σg+(16,0) Lyman band with an accuracy of 0.007 cm−1. It is shown that v=3 is the highest bound vibrational level in the shallow I′1Πg outer well potential in HD, and that only two rotational levels exist.