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

We report on a study of the I' (1)Pi(g) outer well state of HD. Via a resonance-enhanced XUV + IR (extreme ultraviolet + infrared) excitation scheme, rovibronic levels (v = 0-2, J = 1-4) are populated and probed by pulsed lasers. Level energies are measured with an accuracy of approximate to 0.03 cm(-1). Due to gerade-ungerade symmetry breaking, the long-range behavior of the I' potential in HD deviates from that of H-2 and D-2. When this deviation is taken into account a semi-empirical potential for the I' (1)Pi(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 I' well can sustain only 4 vibrational levels, with the v = 3 level having a binding energy of approximate to 1.38(3) cm(-1).