Experimental and theoretical investigation of the HH̄ 1 S g 1 state in H 2 , D 2 , and HD , and the B 9 B̄ 1 S u 1 state in HD

We present an extensive experimental and theoretical study of the energy levels of the H̄ outer-well part of the HH̄ Sg 1 state, which is the fourth adiabatic state of Sg 1 symmetry in the hydrogen molecule. Experimentally, rovibrational H̄ levels were excited in a two-step laser excitation, using wavelength tunable extreme ultraviolet radiation near 91 nm to prepare selected levels in the B Su 1 state (v518 and 19 for H2 and v 525–27 for D2 , v521–23 for HD!. A second tunable laser in the range 550–735 nm was used to excite the H̄ levels in the isotopomers H2 , D2, and HD. A third laser at 355 nm probed the excitation of the H̄ levels by dissociative ionization, producing ions for signal detection. For H2 82 quantum levels were calibrated (v52 –15, J50 –5! and 107 levels for D2 (v56 –22, J50 –5!. These level energies are compared with ab initio calculations including adiabatic and relativistic effects. Agreement between observation and calculation is of the order of 1 cm for H2 and 0.5 cm 21 for D2 throughout the rovibrational manifold. In the HD isotopomer the H̄ Sg 1 state nonadiabatically interacts with the B̄ Su 1 state. This mixing represents an example of strong g-u symmetry breaking. Apart from H̄ levels (v54 –19, J50 –3! also B̄ levels (v59 –20, J50 –3! were observed. Theoretical results for HD account for the nonadiabatic interaction with the B̄ state, based on a new ab initio calculation of the B9B̄ Su 1 potential and the coupling with the HH̄ Sg 1 state. Except for some levels near the potential barrier also for HD the experimentally calibrated levels agree with the ab initio calculated energies within 1.5 cm. @S1050-2947~99!06007-2#