Few-cycle short strong pulses allow excitation of coherently coupled different electronic states towards steering nuclear motions in neutral molecules and cations. The progress of the non stationary vibronic density towards the products can be monitored using a second pulse, by transient absorption spectroscopy or by photoionization. The Hamiltonian used to simulate the quantum dynamics includes the pump and probe pulses which provides a quantum mechanical description of both multiphoton excitation and ionization. We will first report on selective bond formation in a four-center ring closure induced by an ultrashort UV pulse. Norbornadiene consists of two ethylene moieties connected by a rigid C3H4 bridge. Excitation with a short fs UV pulse yields a non-equilibrium electronic density in the open norbornadiene that evolves towards the closed four-atom ring quadricyclane (Fig. 1A). We show that short UV pulses of different linear polarizations allow tailoring the electronic coherences that make up the initial non-equilibrium electronic density. They in turn follow different dynamical paths that produce different yields in the open and closed form, thereby opening the way to controlling bond-making by selective pumping of electronic states with attopulses. We will next report on the coherence driven isotope effect in the ultrafast Jahn-Teller structural rearrangement that follows the sudden ionization of methane by short XUV pulses of 13.95 and 17.05 eV. The sudden ionization of the neutral leads to a coherent superposition of the three lowest electronic states of the cation that are strongly coupled (Fig. 1B). Our quantum dynamical simulation on an ensemble of suddenly ionized randomly oriented molecules show that the less distorted D2d configurations are populated before the C2v minimum. The 17.05eV XUV pulse leads a higher value (=3) of the ratio between the autocorrelation functions of the initial states than the 13.95 XUV (ratio=1.5), which is due to the crucial role of the electronic coherences involving the D2 state, that is not accessed by the 13.95 XUV pulse.

Exploiting electronic coherences for steering selective bond formation in molecules

ACS Fall 2021

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