Chapter 27 - Ultrafast non-franck-condon transitions: Is it possible?

Abstract

Ultrashort (femtosecond) laser pulses can be used to steer molecular dynamics including chemical reactions. Given some initial state, the aim of laser control is to guide a system into a desired final state. This chapter provides insight into the dynamics induced by ultrashort laser pulses and discusses the objectives that can be obtained. Thus, the chapter is an exploratory study that determines the type of dynamics that might be induced by laser light. It concerns not only with creation of a predetermined product, but also with the possibility of creating this product within a given time. In a Born-Oppenheimer (BO) description, the different electronic states of a molecule provide the potential energy surfaces (PESs), which govern the nuclear dynamics. An infinitely short laser pulse induces a perfect vertical or Franck-Condon transition— meaning that the nuclear part of the wave function remains unchanged by the interaction with the electric field (when transition dipole moment is considered constant over the extension of the nuclear part). The chapter reports the possibility of inducing an ultrafast non-Franck-Condon transition, which is defined to be the creation of a wave packet at “the other” turning point of the oscillation faster than the time it takes the Franck-Condon packet to reach that turning point due to the natural dynamics. This chapter explores two possible routes for inducing non-Franck-Condon transitions, namely, phase tailoring of a weak-field ultraviolet (UV) pulse and a two-pulse scheme combining a transform limited weak-field UV pulse with a strong infrared (IR) field.