Polyatomic molecules typically exhibit a dense manifold of electronic states in the UV-VIS range. When excited with few fs optical pulses, a coherent superposition of states which has a non equilibrium electronic density is built and the density beats in space and time between the different parts of the molecule. The electronic coherences between optically active electronic states that have been excited can be tailored and probed using strong sub and few optical fs pulses. Their dynamics is manifestation of a purely electronic time scale, before the coupling to the nuclei takes place. I will discuss the onset of the coupling of the non equilibrium electronic density with nuclear motion and how the fate of a chemical reaction can be controlled by tailoring the electronic coherences through excitation by a ultra short, sub fs to few fs, pulses. We simulate realistic pump-probe experiments by solving the time-dependent Schroedinger equation on multidimensional nuclear grids for several electronic states coupled by non adiabatic interactions. The photoexcitation and photoionization dynamics are explicitly taken into account by including the dipole coupling to neutral and cationic states in the time-dependent Hamiltonian. I will report on fully quantal simulation of the photoinduced coupled electronic-nuclear dynamics in several molecular systems: LiH, HCN, and norbornadiene (C7H8). We show that the electronic coherences built by the pump pulse are modulated by different periods of the nuclear motion, the non adiabatic couplings and the photoionization process, but can be followed in space and in time for several nuclear periods. In LiH we show that one can steer the fragmentation yield to specific channels by controlling the value of the carrier envelope phase (CEP) of an essentially one cycle IR strong pulse. The simulations include a dense manifold of Σ and Π states that converge to the ionization potential and multiphoton photoionization. We show that the CEP of the pulse is imprinted in the electronic coherences that beat with opposite phases all the way to the dissociation asymptotes for pump pulses with opposite CEP phase equal to 0 or π. We will also report on tailoring electronic coherences for getting specific products in the HCN molecule, simulating the nuclear motion on a three dimensional grid for a dozen of electronic states. For norbornadiene, we will discuss how to steer the photoisomerization process to quadricyclane using a short UV fs pulse.

Coherent ultrafast photoinduced coupled electronic-nuclear dynamics in dense manifolds of electronic states in molecules

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.