Linear and Nonlinear Continuum Resonance Raman Scattering in Diatomic Molecules: Experiment and Theory

Abstract

Resonance Raman spectra of the diatomic halogen molecules iodine and bromine with excitation above the dissociation limit of excited electronic states have been topics of experimental and theoretical interest for some time [1,2]. Excellent agreement between experiment and time-independent numerical calculations based on the dispersion relations found by Kramers and Heisenberg and derived by Dirac using second-order perturbation theory has been obtained particularly for the bromine molecule. This system is also of special interest since it shows scattering via two interfering excited states [3]. We became reinterested in this type of continuum resonance Raman scattering for the following reasons. First, the introduction of a time-dependent approach [4] allows the numerical calculation of continuum resonance Raman spectra without the summation over continuous states and therefore offers an alternative method which in addition nicely illustrates the scattering process in an instructive wavepacket picture. Second, the time-dependent approach also gives quantitative information on the scattering time of this type of resonance Raman scattering. Third, continuum resonance Raman spectra are extremely sensitive to changes in the potential functions [1–3,5] and represent therefore valuable experimental data for the precise determination of diatomic excited states. Fourth, transition to the repulsive potentials of electronic excited states induces competitive resonance Raman scattering and unimolecular dissociative processes. The dissociation itself can be monitored via electronic Raman scattering by the produced atoms [6]. In this paper we give illustrative examples for each of the points mentioned.