Ultrafast photo-induced coupled coherent electornic-nuclear dynamics in small molecules

Recent experimental pump-probe schemes, for example transient absorption experiments (1) or time- resolved molecular frame photoelectron angular distribution (2,3), allow following the electronic motion in molecules (4,5). This is possible when a molecule is excited by an ultrafast optical pulse with an energy bandwidth of a one or few eV. Then a band of several electronic states is coherently accessed, leading to a non-stationary superposition of electronic states. The electron density oscillates spatially and in time on the as to few fs timescale, before the onset of significant nuclear motion(6). In a short while the coherent superposition of electronic states is then coupled to nuclear motion. Investigating the coupling between nuclear motion and electronic coherence requires a computational method that suitably describes the coherent coupled dynamics of electrons and nuclei. We here discuss dynamics on several electronic states for several nuclear degrees of freedom, possibly including the photoionization process.
We will first report on the coupled electronic-nuclear dynamics in neutral HCN by numerically integrating the nuclear Time-Dependent Schrödinger Equation for 2 nuclear degrees of freedom and 6 electronic states that can be accessed in the Franck Condon region. The nuclear wave function is represented on a spatial grid using the Discrete Variable Representation (DVR). It is then propagated on a set of adiabatic potential energy surfaces that are coupled by the interaction with an external electric field through the dipole moment surfaces. Using the method described above, we computed the photodissociation dynamics of HCN excited by a few fs UV pulse tuned to selectively access the lowest excited electronic state 1A” after the end of the pulse(7). As the 1A” potential energy surface is purely repulsive, the nuclear wave packet promptly leaves the Franck Condon region to dissociate into H (S (doublet)) + CN (∏ (doublet)). The dissociation is probed over time using transient absorption spectroscopy induced by a second pulse. We show that the coherence between the ground state and 1A” is not dephased by the 2D nuclear motion on the time scale of the dissociation process taking place in 40fs on the lowest excited state.
When an intense IR pulse is used, multiphoton transitions can lead to photoionization. We will next discuss the influence of the photoionization process on the coherence between the neutral bound states in the LiH diatomic molecule using one cycle IR and few cycle UV pulses. The photoionization process is included in the nuclear Hamiltonian using the partitioning technique which allows taking into account the dynamics of the photoionization process. (8) The ionized states wave function is represented as an antisymmetrized product of the electronic state of the cation and the state of the photoelectron. The wave function of the photoelectron is represented by a set of orthogonalized plane waves. The coupling between the wave function of the neutral and the wave function of the cation is computed at each grid point in nuclear coordinates as the transition dipole between the photoelectron wave function and the Dyson orbital that is defined between the electronic state of the neutral and the electronic state of the cation.
(1) Ramasesha K. , Leone S. R. , Neumark D. M. , Ann. Rev. Phys. Chem. 67, 41–63 (2016)
(2) Hockett P. , Bisgaard C. Z. , Clarkin O. J. Stolow A., Nature Physics 7, 612–615 (2011)
(3) Li H. , Mignolet B. , et al. Phys. Rev. Lett 114, 123004 (2015)
(4) Corkum P. B. , Krausz F., Nature Physics 3, 381–387 (2007)
(5) Nisoli M. , Decleva P. , Calegari F. , Palacios A. , Martin F. , Chem. Rev. 117, 10760–10825 (2017)
(6) Remacle, F. and Levine, R. D. Proc. Natl. Acad. Sci. US, 103, 6793 (2006).
(7) van den Wildenberg, S. , Mignolet B. , Levine R. D. , Remacle F. , PCCP 19, 19837–19846
(2017)
(8) Mignolet B. , Levine R. D. , Remacle F. , Phys. Rev. A. 89, 021403 (2014)