Coherent Excitation of Multilevel Systems by Laser Light

Abstract

The excitation and dissociation of polyatomic molecules have attracted much attention since the demonstration of isotopic selectivity in the multiple-photon dissociation of BCl3 [5.1] and SF6 [5.2], which has been shown to occur in the absence of collisions [5.3]. Whether the infrared-laser-induced dissociation of SF6 and other polyatomic molecules must be described as a coherent process obeying the Schrödinger equation, or as a unimolecular chemical reaction obeying usual chemical kinetic equations, is currently the subject of much discussion in the scientific community. Certain effects such as collisions, which may be important in practical applications of laser isotope separation, may tend to reduce the importance of coherent effects. However, regardless of practical considerations, there remains the fundamental question of the process of excitation of polyatomic molecules at low laser intensities or fluences, under collisionless conditions. It is clear from experimental high-resolution spectra (for example, the Doppler-limited 3v3 spectrum of SF6 [5.4]) that the vibration-rotation states of polyatomic molecules at excitations well below the dissociation threshold are discrete and free from perceptible lifetime broadening. One would therefore expect the dynamics of laser excitation of these discrete states to possess distinctively quantum-mechanical features, such as multiphoton resonances. In this chapter we review the theoretical tools and concepts needed for a first-principles-quantum-mechanical study of the coherent laser excitation of polyatomic molecules under collision-free conditions, and illustrate these methods with selected numerical results.