Effective Hamiltonians, potential energy surfaces and global calculations of vibration-rotation spectra of triatomics

Vladimir Tyuterev

GSMA, Universite de Reims, BP 1039, 51687 Reims Cedex 2, France

Among various methods and theoretical models for calculation of high-resolution molecular vibration-rotation spectra three most widespread types are to be considered: a)'local' (effective) methods, based on the perturbation theory and on effective Hamiltonians for sets of nearby vibrational states (polyads) accounting for strong rovibrational resonance interactions within these polyads; b) 'intermediate' version of calculations, which use vibration extrapolation scheme for successive polyads; c) 'global' methods (many of them use a variational technique) which allow to calculate an entire set of rovibrational states and transitions up to the dissociation from molecular properties; a potential energy surface (PES) is used to calculate energies and line positions and dipole moment surfaces (DMS) are used to calculate line and band intensities. Specific advantages and problems related with applications of these complementary approaches to molecular spectroscopy and dynamics will be discussed. Recently accurate PES of several triatomic molecules such as water[1], ozone[2,3] and H2S[4] have been obtained using extended ab initio electronic structure calculations or advanced empirical optimisation accounting for spectroscopic and chemical information (barriers, dissociation limits etc.). These surfaces were used to derive very accurate spectroscopic effective Hamiltonians for polyads of rotation-vibrational states coupled by strong resonances through a systematic application of quantum mechanical Contact Transformations [5] which have been programmed [6] to high orders of the perturbation theory. This allows building systematic links among the methods (a),(b),(c) and using information from complementary sources. Prospective of this technique for study of intensity anomalies and isotopic effects [7], for interpretation of spectroscopic parameters, for semi-classical PO analyses, and for assignments of excited states near the dissociation limit observed in recent high-resolution experiments currently in progress at the University of Reims with collaborations will be discussed.


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