THEORETICAL-STUDY OF THE ABSORPTION-SPECTRUM OF THE PSEUDOROTATING NA-3(B)

Interpretation of the vibrational absorption spectrum for Na-3(X) --> Na-3(B) based on full ab-initio quantum mechanical description is pres ented. Potential energy surfaces (PES) for the ground and excited stat e and dipole transitions have been calculated employing ab-initio accu rate correlated wavefunctions. All three vibrational modes of the mole cule are included in the theoretical treatment. The absorption spectru m is calculated in two ways, namely a time-independent approach employ ing the sine-function discrete variable representation (DVR) method in which eigenvalues as well as eigenfunctions are evaluated directly, a nd a time-dependent approach performing a three-dimensional wave packe t propagation in the B state, after instantaneous absorption from the X state. On the basis of our calculations we discuss two possible assi gnments of the absorption lines up to 330 cm(-1) above the origin. The calculated spectrum agrees satisfactorily well with the measured spec trum and most of its features can be explained, including not only the dominant progressions of angular and radial pseudorotations but also the rather weak signatures of the symmetric stretch. Nevertheless some open questions remain concerning comparison between theoretical and e xperimental findings. (C) 1997 Elsevier Science B.V.