Control of attosecond electronic dynamics in molecules

In the last decade, the development of ultrashort, attosecond and few-femtosecond laser pulses opened new avenues towards probing and controlling molecular electron dynamics, and thereby molecular reactivity. The aim of the thesis is to show by dynamical simulations that the purely electronic dynamics in molecules can be controlled and probed by ultrashort pulses. When an ultrashort pulse is used to excite a molecule, an electronic reorganization occurs before the onset of nuclear motion. In the first dozen of femtoseconds following the excitation, there is a timescale where the dynamics is purely electronic and where the nuclei can be considered as fixed. It is in this time windows that we showed that we can control the spatial localization of the electronic density by tuning the parameters of the pulse. It is important to control this density because at the end of the pulse, the electronic density is out equilibrium and it creates a force on the nuclei that is different from that which we would get from the Born Oppenheimer separation. So by controlling the electronic density, we could trigger a specific outcome of a chemical reaction or rearrangement, which would offer a new way to control chemical reactivity. This control is different from conventional photochemistry where the electrons are equilibrated with the instantaneous position of the nuclei.

The research has been focused on the development of theories and methodologies for the description of the non-equilibrium electronic dynamics and its probing and on the applications to small and large molecules for the comparison with experimental results.

The description of the dynamics induced by short and intense electric field requires the use a non perturbative method that takes into account the time profile of the strong electric field. We used a time-dependent multiconfigurational method where the time-dependent electronic wavefunction is expressed on a basis of the time-independent field free electronic states of the molecule. This method is particularly well fitted for the description of the electronic dynamics of large systems since it only requires the electronic structure of the field free excited states, which can be computed using quantum chemistry methods adapted to the size of the molecule. We integrate the time-dependent Schrödinger equation at a frozen nuclear geometry with an electronic Hamiltonian that is time-dependent and includes the effect of the electric field of the pulse. The field free electronic states of the molecule are coupled due to the dipole interaction induced by the strong field, which creates a non stationary coherent superposition of states with the electronic density localized in different regions of the molecule as a function of time. The control of the spatial localization of the electronic density can be obtained for aligned molecules by tailoring the parameters of the pulse, mainly the carrier frequency and the polarization. We demonstrated control in the LiH molecule and in a larger molecule, ABCU (C10H19N), a cage molecule composed of 86 electrons.

The ultrafast electronic dynamics can be probed by a second pulse that photoionizes it. We showed that the molecular frame photoelectron angular distributions (MFPAD) can be used to probe the interferences between the states and that a given coherent superposition of states yields a unique MFPAD, which makes the ionization an ideal probe of the electronic dynamics. The computation of the MFPAD requires the evaluation of the dipole-coupling matrix element between a neutral state and an ionized state. We chose to express the ionized state as the antisymmetrized product of a cationic state and an orthogonal plane wave that describes the wavefunction of the ionized electron. The dipole-coupling matrix element is a n electron integral that can be reduced to a one electron integral composed of the dipole coupling between an orthogonalized plane wave and a Dyson orbital. The Dyson orbital is the overlap between the neutral and cationic state and represents the orbital from which the electron has been ionized.

We first modeled a sequential pump-probe experiment on ABCU (C10H19N) and LiH at a fixed nuclear geometry. The pump pulse induces a motion of the electronic density in the neutral electronic states of the molecule that is subsequently probed by a sudden ionization. Our computations show that the MFPAD vary significantly as a function of the pump probe delay, and reflect the electronic dynamics. We also used the sudden ionization approximation to probe the dynamics in the cationic states of tetrapeptides. In that case, the pump-probe scheme is slightly different. The neutral molecule is first suddenly photoionized to the cation states, which creates a coherent superposition of states with amplitudes depending on the photoionization coupling elements. The motion of the electronic density along the molecular backbone of the tetrapeptide cation is then probed by a second sudden ionization.

We also developed a coupled equations scheme based on the partitioning technique with the aim to describe the ionization dynamics and the electronic dynamics on the same level. The complete space is partitioned into a subspace composed of the neutral bound states and a subspace composed of the ionized states. We then integrate the time-dependent Schrödinger equation where the bound and ionized subspaces are coupled by the electric field. Using such formalism, we can describe pump probe experiments involving multiphoton excitation (and ionization) by an IR pulse and ionization by a train of attosecond pulses, as it is often encountered in attosecond experiment. We showed in a realistic IR pump – attosecond pulse train (APT) probe experiment on LiH that the non stationary electron dynamics can be triggered by an ultrashort IR pulse and probed by angularly resolved ultrafast ionization induced by a train of attopulses. We proposed a new probing scheme where the APT acts as a frequency filter that only probes the superposition of states with a beating frequency matching the time interval between two XUV attopulses of the train. The coupled equation scheme can be applied to larger systems since it only requires the electronic structure of the excited states of the molecule. We used coupled equations to investigate the charge migration in the cationic states of PENNA (C10H15N), a relatively large molecule composed on a phenyl chromophore on one side of the molecule and an amine chromophore on the other side. The IR pulse launches the dynamics in the cationic states that is then probed by a femtosecond XUV pulse.

In the thesis, we also investigated the ionization of Super Atom Molecular Orbitals (SAMO), which are diffuse hydrogenoid like orbitals that can be found in fullerenes like C60 or in nanostructure. This work was carried out in collaboration E. Campbell group’s (University of Edinburgh) where angularly resolved photoelectron spectra were measured for gas phase C60 with c.a. 100fs pulse. The angularly resolved photoelectron spectrum only exhibits peaks corresponding the SAMO states. In order to support this attribution we computed the electronic structure and the photoionization lifetime of the 500 lowest excited states of C60 for a sudden photoionization. The SAMO states have a lifetime of the order of a femtosecond while the other states have photoionization lifetimes 3 to 4 orders of magnitudes larger. This difference in photoionization time scales explains why the SAMO states are the only states that photoionize during the c.a. 100fs pulse. The SAMO states act as doorway states for the photoionization.