Chapter 2 - Unstable States in Laser Assisted and Controlled MIlecular Processes

Abstract

Abstract Many aspects of intense-field molecular dynamics rely on the concept of resonance. The chapter gives a thorough review of these aspects, bringing out the specificity of laser-induced resonances, in particular those defined in the Floquet or dressed molecule picture. The role of these resonances in the time-resolved dynamics of molecules subjected to an intense, ultrafast laser pulse is discussed and basic mechanisms of molecular fragmentation and its control are reviewed. We discuss how a thorough interpretation of two-color extreme ultraviolet (XUV) + Infra-red (IR) pump–probe experiments on the dissociative ionization of H can be made in terms of adiabatic vs. non-adiabatic resonance transports (i.e., laser-induced time evolutions) and in terms of field-induced processes such as Bond-Softening (BS) and Vibrational Trapping (VT), associated with the Floquet representation or the Dynamical Dissociation Quenching (DDQ) effects associated with a time-dependent quasi-static representation. Another application of the concepts of laser-induced resonances, and of their adiabatic evolution, is devoted to laser control strategies based on Zero-Width Resonances (ZWR) and Exceptional Points (EP), the approach of which in laser parameter space corresponds to the coalescence of two laser-induced resonances. We illustrate how the concept of ZWR can be useful for the molecular cooling problem. We then show how advantage can be taken of resonance coalescence at an EP to devise new laser control strategies pertaining to vibrational energy transfer processes.