<title>Evaluation of forbidden-transition dipole moments of quasi-rigid molecules in limiting ordering schemes of vibrational-rotational interactions: numerical example for the CH<formula><inf><roman>4</roman></inf></formula> molecule</title>

Abstract

ABSTRACT The theory of linked groupings of vibrational-rotational interactions in molecules has been applied for the construction ofthe infinite series for effective dipole moments of onequantum vibrational transitions. The necessary and sufficient symmet-nc selection rules for forbidden transitions dipole moments have been developed. All aspects of the removal of prohibitionfrom fully symmetric fundamentals of nonpolar molecules have been investigated. Mechanisms of activation of other mac-tive fundamentals of nonpolar molecules are considered also. Ab initio calculation of dipole moment function and theevaluation ofthe effective transition dipole moments for CH4 molecule are presented.Keywords: Contact transformations, Effective dipole moment, Irreducible representation, Fundamentals, Selection rules 1. INTRODUCTION Two models, which take into account the weak forbidden vibrational-rotational transitions, have been proposed someyears ago to explain a number of singularities in spectra of multiphoton absorption of strong IR laser radiation and collision-free dissociation of molecules. On the first it was suggested that multiphoton excitation of molecules such as SF6 or 0504occurs participation of weak transitions with R>O, in the second was favour the transitions with M>l, which are allowedbecause ofthe external field. At the treatment ofthis experiments and the planing ofthe JR-JR double resonance experimentstypes it is necessary to decide the problems: (i) how will be change the classification of vibrations, first of all fundamentalsof nonpolar molecules from the point of view their dependence on the degeneracy of initial state oftransition; (ii) to developnot only necessary but sufficient symmetric selection rules for appropriate effective dipole matrix elements. In the doubleharmonic approximation the fully symmetric fundamentals of the nonponlar molecules have been forbidden in JR absorptionand emission spectra [1]. In the contribution have been investigated all aspects of the removal of the prohibition from fullysymmetric fundamentals of nonpolar molecules. Mechanisms of activation of other inactive fundamentals of nonpolar mole-cules are considered also. The necessary condition for activation in JR spectra of fully symmetric fundamentals of nonpolar