On the preservation of coherence in the electronic wavepacket of a neutral and rigid polyatomic molecule

Abstract

We present various types of reduced models including five vibrational modes and three electronic states for the pyrazine molecule in order to investigate the lifetime of electronic coherence in a rigid and neutral system. Using ultrafast optical pumping in the ground state (11Ag), we prepare a coherent superposition of two bright excited states, 11B2u and 11B1u, and reveal the effect of the nuclear motion on the preservation of the electronic coherence induced by the laser pulse. More specifically, two aspects are considered: the anharmonicity of the potential energy surfaces and the dependence of the transition dipole moments (TDMs) with respect to the nuclear coordinates. To this end, we define an ‘ideal model’ by making three approximations: (i) only the five totally symmetric modes move, (ii) which correspond to uncoupled harmonic oscillators, and (iii) the TDMs from the ground electronic state to the two bright states are constant (Franck–Condon approximation). We then lift the second and third approximations by considering, first, the effect of anharmonicity, second, the effect of coordinate-dependence of the TDMs (first-order Herzberg–Teller contribution), third, both. Our detailed numerical study with quantum dynamics is meant to be realistic for pyrazine over about 20 femtoseconds, and was further extended so as to probe the effect of such approximations on a model system. We show that long-term revivals of the electronic coherence persist up to the picosecond time range even for the most realistic model.

Attochemistry: Is Controlling Electrons the Future of Photochemistry?

“…64,83 The fast electronic decoherence occurring on the femtosecond timescale was then confirmed on other molecules by one of us using the DD-vMCG (Direct-Dynamics variational Multi-Configuration Gaussian) method, 62 and by other groups using the MCTDH (Multi-Configuration Time-Dependent Hartree) method. 50,[84][85][86] Both of these approaches treat quantum mechanically the nuclear motion. While the latter typically requires the de-velopment of a model Hamiltonian, a reduced number of nuclear coordinates being often taken into account, 87 the former calculates the potential energy surfaces on-the-fly aiming at describing all nuclear coordinates.…”

“…The coherence lifetime was shown to depend on molecular properties such as the non-adiabatic coupling strength, the relative shape of the coupled state potential energy surfaces, and the nuclear wavepacket widths. 50,62,63,86 The idea of attosecond charge-directed reactivity -using an attosecond pulse to directly control the formation and breaking of chemical bonds by controlling electron rearrangement in molecules -unsurprisingly greatly inspires chemists. One can imagine that by controlling the initial electronic wavepacket created, one could control the electron dynamics that occurs following excitation.…”

“…These results were obtained using single Ehrenfest trajectories, i.e., the nuclei are moving classically on mean-field potentials taking into account all nuclear coordinates. Later simulations on xylene and polycyclic norbornadiene cations including ensembles of Ehrenfest trajectories predicted a fast electronic decoherence because of the nuclear wavepacket width. , The fast electronic decoherence occurring on the femtosecond time scale was then confirmed on other molecules by one of us using the direct-dynamics variational multi-configuration Gaussian (DD-vMCG) method and by other groups using the multi-configuration time-dependent Hartree (MCTDH) method. ,− Both of these approaches treat quantum mechanically the nuclear motion. While the latter typically requires the development of a model Hamiltonian, a reduced number of nuclear coordinates being often taken into account, the former calculates the potential energy surfaces on-the-fly aiming at describing all nuclear coordinates .…”

“…This ensures a faster convergence to the in principle exact result. The coherence lifetime was shown to depend on molecular properties such as the nonadiabatic coupling strength, the relative shape of the coupled state potential energy surfaces, and the nuclear wavepacket widths. ,,, …”

Ultrafast chiroptical switching in UV-excited molecules

“…The time it takes for decoherence to occur in photoionized [40][41][42][43][44][45][46][47] and photoexcited 18,48 molecules is currently the subject of extensive investigations.…”