Beyond Born-Oppenheimer spectroscopic study for the C state of LiH

A theoretical spectroscopic study is performed for the C state of LiH using eight diabatic or adiabatic states of symmetry (1)Sigma beyond the Born-Op penheimer approximation. For the position of the vibrational levels, an exc ellent agreement is found with the ten recently observed ones giving confid ence to the present study. The vibrational levels present rather strong vib ronic shifts (rising to about 10 cm(-1) for upsilon = 37) particularly for the highest ones. Two approximate methods are tested for their estimation a nd a new method is presented for the evaluation of the vibronic part of the adiabatic correction. The new method involves the diabatic picture and the discrete variable representation. It leads to nonlocal corrections but avo ids the calculation of radial couplings. Nonradiative lifetimes are accurat ely determined. They range over six orders of magnitude, from nanoseconds t o milliseconds. Similarities of the LiH molecule in the C state and in high er excited states at large internuclear distances with highly excited Rydbe rg atoms are emphasized. (C) 1999 American Institute of Physics. [S0021-960 6(99)01423-3].