Gerade/ungerade symmetry-breaking in HD at the n = 2 dissociation limit

We report on a study of the Iʹ 1 g outer well state of HD. Via a resonance-enhanced XUV + IR (extreme ultraviolet + infrared) excitation scheme, rovibronic levels (upsilony = 0-2, J = 1-4) are populated and probed by pulsed lasers. Level energies are measured with an accuracy of ~0.03 cm^-1. Due to gerade-ungerade symmetry breaking, the long-range behavior of the Iʹ potential in HD deviates from that of H2 and D2. When this deviation is taken into account a semi-empirical potential for the Iʹ1 g state may be constructed, resulting in better agreement with the observed level energies than derived from an adiabatic ab initio potential. With this new potential it is predicted that the Iʹ well can sustain only 4 vibrational levels, with the upsilony = 3 level having a binding energy of ~1.38(3) cm ^