The diffraction signatures of individual vibrational modes in polyatomic molecules

Though one normally thinks of single-molecule diffraction studies as tools for eliciting molecular geometry, molecular diffraction patterns are really the Fourier transforms of complete molecular wave functions. There is thus at least the possibility of imaging the vibrational wave functions of poly atomic molecules by means of a pump-probe diffraction experiment: the pump laser could prepare a specific vibrational state and an electron or x-ray c ould then be diffracted off the molecule some short time later. The present paper develops the general theory of diffraction signatures for individual vibrational wave functions in polyatomic molecules and investigates the fe asibility of seeing such signatures experimentally using the example of a l inear triatomic molecule modeled after CS2. Although aligned molecules in s pecific vibrational quantum states turn out to exhibit very characteristic diffraction signatures, the signatures of the vibrational wave functions ar e partially washed out for the complete isotropy expected from gas phase mo lecules. Nonetheless, it is possible to design a diffraction experiment usi ng a pump-dump sequence with a polarized laser beam which will select a non isotropic sample of vibrationally excited molecules. We show that the resul ting level of anisotropy should enhance the diffraction signature, helping to distinguish different vibrational components. These model calculations t herefore suggest the possibility of observing the dynamics of vibrational w ave packets using experimentally realizable diffraction techniques. (C) 200 0 American Institute of Physics. [S0021-9606(00)02303-5].