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 (1). 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 (Figure 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 a short XUV pulse (2). The Jahn-Teller effect makes the methane cation unstable in the Td geometry of the neutral. The sudden ionization of the neutral leads to a coherent superposition of the three lowest electronic states of the cation that are strongly coupled by non adiabatic interactions (Figure 1B). This non equilibrium electronic density is driving the ultrafast nuclear rearrangement occurring in the first few fs, through non adiabatic population transfers in regions of different geometrical configurations. Our quantum dynamical simulation on an ensemble of suddenly ionized randomly oriented molecules show that on the ground state of the cation, the less distorted D2d configurations are populated before the C2v minimum is reached. The isotope effect on the coherent dynamics leads to a ratio of three of the CD4+/CH4+ autocorrelation functions in the first 2 fs of the dynamics, in agreement with the ratio of the yields in high harmonics that has been experimentally measured (3). Such a strong isotopic effect on the non adiabatic population transfers between electronic states in a coherent superposition has already been reported by us for N2 (4) and LiH (5).
Figure 1 A. Norbornadiene (ND) photoisomerization to quadricyclane(QC). Paths followed by the non equilibrium density for two different linear polarizations of a fs short UV exciting pulse. B. Schematic view of the PES of the neutral ground state of methane and of the three lowest excited states of the cation involved in the Jahn-Teller effect.
(1)Valentini, A.; van den Wildenberg, S.; Remacle, F. PCCP 2020, 22, 22302-22313. (2) Gonçalves, C. G. M.; Levine, R. D.; Remacle, F. ChemRxiv. Preprint. 2021, https://doi.org/10.26434/chemrxiv.13702456.v1 (3) Baker, S., et al. Science 2006, 312, 424. (4)Ajay, J. S.; Komarova, K. G.; Remacle, F.; Levine, R. D. Proc. Natl. Acad. USA 2018, 115, 5890-5895; Komarova, K. G.; Remacle, F.; Levine, R. D. J. Chem. Phys. 2019, 151, 114308. (5) Komarova, K.; vandenWildenberg, S.; Remacle, F.; Levine, R. D. J. Phys. B 2020, 53, 134001.