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 reson ance-enhanced XUV + IR (extreme ultraviolet + infrared) excitation scheme, rovibronic levels (v = 0-2, J = 1-4) are populated and probed by pulsed las ers. 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 deviati on is taken into account a semi-empirical potential for the I' (1)Pi(g) sta te may be constructed, resulting in better agreement with the observed leve l energies than derived from an adiabatic ab initio potential. With this ne w 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 1.3 8(3) cm(-1).