Tue, 10.09.2019  |  13:00 PM
Seminar room A, 2.01

Probing the evolution of electronic coherence induced by CEP-controlled ultrashort IR pulse using VMI and ATAS

Stephan van den Wildenberg | University of Liège, Belgium

talks about

Control of molecular reactivity can be achieved using intense few-cycle pulses. Few-cycle ultrashort IR pulses allow excitation of coherently coupled electronic states, opening new ways for steering nuclear motions in molecules. We compute the coupled dynamics of electrons and nuclei of the LiH molecule by numerically integrating the Time-Dependent Schrödinger Equation (TDSE). The molecular Hamiltonian includes both the non-adiabatic coupling and the dipole coupling with the external electric field associated to a moderately strong IR pulse with a definite phase between its envelope and carrier oscillation. We also include the coupling between the electronic states of the neutral molecule and the ionization continuum in the dipole approximation, enabling us to fully account for the multi-photon excitation and ionization. During this seminar, we will discuss how these two processes interact and shape the ensuing coupled electronic-nuclear dynamics in both the neutral excited electronic states and the cationic states of the diatomic molecule.

The TDSE is integrated at nuclear grid points using the partitioning technique with a subspace of ten coupled bound states and a subspace of discretized continuous states for the photoionization continua. We show that the coherent dynamics in the neutral subspace is strongly affected by the amplitude exchanges with the ionization continua during the pulse, as well as by the onset of nuclear motion. The coupling to the cation and the resulting ionization does not preclude the control of the motion in the neutral through control of the carrier-envelope phase. Our methodology provides not only a visualization in space and in time of the entangled vibronic wave packet in the neutral states but also of the wave packet of the outgoing photoelectron. Thereby we can follow spatially and temporally the dynamics of the outgoing and bound electrons during the pulse and the nuclear motion in the bound subspace while moving through non adiabatic coupling regions after the pulse. We also show that the dynamics ensuing the excitation can be probed over time using a XUV attosecond probe pulse with varying pump-probe delays. The probing by such a pulse allows both following the evolution of the out of equilibrium electron density built by the pump pulse through analysis of the Velocity Map Imaging (VMI), and the evolution of the nuclear wave packet by recording the transient absorption spectra.