Theoretical study of the absorption spectrum of the pseudorotating Na3(B) Original Research Article

Interpretation of the vibrational absorption spectrum for Na3(X) → Na3(B) based on full ab-initio quantum mechanical description is presented. Potential energy surfaces (PES) for the ground and excited state and dipole transitions have been calculated employing ab-initio accurate correlated wavefunctions. All three vibrational modes of the molecule are included in the theoretical treatment. The absorption spectrum is calculated in two ways, namely a time-independent approach employing the sinc-function discrete variable representation (DVR) method in which eigenvalues as well as eigenfunctions are evaluated directly, and a time-dependent approach performing a three-dimensional wave packet propagation in the B state, after instantaneous absorption from the X state. On the basis of our calculations we discuss two possible assignments of the absorption lines up to 330 cm−1 above the origin. The calculated spectrum agrees satisfactorily well with the measured spectrum 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 experimental findings.