Molecular Excitations Created by an Arbitrarily Shaped Laser Pulse: An Adiabatic Wavepacket Propagation Study

Abstract

In pump-probe experiments, femtosecond laser pulses permit excited states to be accessed on a time-scale shorter than typical times for molecular motions, and are being used to investigate the dynamics of transitory species¹. The theoretical descriptions of these experiments using wavepacket propagation techniques are currently limited to a first-order perturbative treatment of the time-dependent Schrödinger equation and rely on the rotating-wave approximation (RWA)². A non-perturbative, non-RWA wavepacket description of molecular excitations created by ultra-short and intense laser pulses is proposed in the present communication, where a time-dependent electronic representation is introduced by solving the local, nuclear-coordinate parameterized Schrödinger equation describing the laser-driven, multistate electronic system. As shown in previous works³, these solutions can be identified with the eigenstates of a time-dependent effective hamiltonian with respect to which the exact time evolution of the N-state electronic system is adiabatic. For a two-channel system, the time-dependent adiabatic representation depends on an effective area of the laser pulse and geometrical phases which are also functionals of the laser pulse-shape. Due to this explicit dependence on the characteristics of the laser pulse, the representation is appropriate for studying pulse-shape effects in the short-time dynamics of laser-driven molecular systems.