Electronic decoherence following photoionization: full quantum-dynamical treatment of the influence of nuclear motion

Photoionization using attosecond pulses can lead to the formation of coherent superpositions of the electronic states of the parent ion. However, ultrafast electron ejection triggers not only electronic but also nuclear dynamics---leading to electronic decoherence, which is typically neglected on time scales up to tens of femtoseconds. We propose a full quantum-dynamical treatment of nuclear motion in an adiabatic framework, where nuclear wavepackets move on adiabatic potential energy surfaces expanded up to second order at the Franck-Condon point. We show that electronic decoherence is caused by the interplay of a large number of nuclear degrees of freedom and by the relative topology of the potential energy surfaces. Application to $\mathrm{H_2O}$, paraxylene, and phenylalanine shows that an initially coherent state evolves to an electronically mixed state within just a few femtoseconds. In these examples the fast vibrations involving hydrogen atoms do not affect electronic coherence at short times. Conversely, vibrational modes involving the whole molecular skeleton, which are slow in the ground electronic state, quickly destroy it upon photoionization.Comment: 18 pages, 8 figure

Sub-picosecond energy transfer from a highly intense THz pulse to water: a computational study based on the TIP4P/2005 model

The dynamics of ultrafast energy transfer to water clusters and to bulk water by a highly intense, sub-cycle THz pulse of duration $\approx$~150~fs is investigated in the context of force-field molecular dynamics simulations. We focus our attention on the mechanisms by which rotational and translational degrees of freedom of the water monomers gain energy from these sub-cycle pulses with an electric field amplitude of up to about 0.6~V/{\AA}. It has been recently shown that pulses with these characteristics can be generated in the laboratory [PRL 112, 213901 (2014)]. Through their permanent dipole moment, water molecules are acted upon by the electric field and forced off their preferred hydrogen-bond network conformation. This immediately sets them in motion with respect to one another as energy quickly transfers to their relative center of mass displacements. We find that, in the bulk, the operation of these mechanisms is strongly dependent on the initial temperature and density of the system. In low density systems, the equilibration between rotational and translational modes is slow due to the lack of collisions between monomers. As the initial density of the system approaches 1~g/cm$^3$, equilibration between rotational and translational modes after the pulse becomes more efficient. In turn, low temperatures hinder the direct energy transfer from the pulse to rotational motion owing to the resulting stiffness of the hydrogen bond network. For small clusters of just a few water molecules we find that fragmentation due to the interaction with the pulse is faster than equilibration between rotations and translations, meaning that the latter remain colder than the former after the pulse

Full dimensional (15D) quantum-dynamical simulation of the protonated water-dimer II: infrared spectrum and vibrational dynamics

The infrared absorption spectrum of the protonated water dimer (H5O2+) is simulated in full dimensionality (15D) in the spectral range 0-4000 cm-1. The calculations are performed using the Multiconfiguration Time-Dependent Hartree (MCTDH) method for propagation of wavepackets. All the fundamentals and several overtones of the vibrational motion are computed. The spectrum of H5O2+ is shaped to a large extent by couplings of the proton-transfer motion to large amplitude fluxional motions of the water molecules, water bending and water-water stretch motions. These couplings are identified and discussed, and the corresponding spectral lines assigned. The large couplings featured by H5O2+ do not hinder, however, to describe the coupled vibrational motion by well defined simple types of vibration (stretching, bending, etc.) based on well defined modes of vibration, in terms of which the spectral lines are assigned. Comparison of our results to recent experiments and calculations on the system is given. The reported MCTDH IR-spectrum is in very good agreement to the recently measured spectrum by Hammer et al. [JCP, 122, 244301, (2005)].Comment: 30 pages, 6 figures, submitted to J. Chem. Phy

Multiconfiguration time-dependent Hartree impurity solver for nonequilibrium dynamical mean-field theory

Nonequilibrium dynamical mean-field theory (DMFT) solves correlated lattice models by obtaining their local correlation functions from an effective model consisting of a single impurity in a self-consistently determined bath. The recently developed mapping of this impurity problem from the Keldysh time contour onto a time-dependent single-impurity Anderson model (SIAM) [C. Gramsch et al., Phys. Rev. B 88, 235106 (2013)] allows one to use wave function-based methods in the context of nonequilibrium DMFT. Within this mapping, long times in the DMFT simulation become accessible by an increasing number of bath orbitals, which requires efficient representations of the time-dependent SIAM wave function. These can be achieved by the multiconfiguration time-dependent Hartree (MCTDH) method and its multi-layer extensions. We find that MCTDH outperforms exact diagonalization for large baths in which the latter approach is still within reach and allows for the calculation of SIAMs beyond the system size accessible by exact diagonalization. Moreover, we illustrate the computation of the self-consistent two-time impurity Green's function within the MCTDH second quantization representation.Comment: 12 pages, 8 figure

Towards attochemistry: Control of nuclear motion through conical intersections and electronic coherences

The effect of nuclear dynamics and conical intersections on electronic coherences is investigated employing a two-state, two-mode linear vibronic coupling model. Exact quantum dynamical calculations are performed using the multi-configuration time-dependent Hartree method (MCTDH). It is found that the presence of a non-adiabatic coupling close to the Franck-Condon point can preserve electronic coherence to some extent. Additionally, the possibility of steering the nuclear wavepackets by imprinting a relative phase between the electronic states during the photoionization process is discussed. It is found that the steering of nuclear wavepackets is possible given that a coherent electronic wavepacket embodying the phase difference passes through a conical intersection. A conical intersection close to the Franck-Condon point is thus a necessary prerequisite for control, providing a clear path towards attochemistry.Comment: 12 pages, 3 figure

Mixed Quantum-Classical Dynamics for Near Term Quantum Computers

Mixed quantum-classical dynamics is a set of methods often used to understand systems too complex to treat fully quantum mechanically. Many techniques exist for full quantum mechanical evolution on quantum computers, but mixed quantum-classical dynamics are less explored. We present a modular algorithm for general mixed quantum-classical dynamics where the quantum subsystem is coupled with the classical subsystem. We test it on a modified Shin-Metiu model in the first quantization through Ehrenfest propagation. We find that the Time-Dependent Variational Time Propagation algorithm performs well for short-time evolutions and retains qualitative results for longer-time evolutions.Comment: 13 pages, 19 figures, revision after first referee roun

Collective rovibronic dynamics of a diatomic gas coupled by cavity

We consider an ensemble of homonuclear diatomic molecules coupled to the two polarization directions of a Fabry-P\'erot cavity via fully quantum simulations. Accompanied by analytical results, we identify a coupling mechanism mediated simultaneously by the two perpendicular polarizations, and inducing polaritonic relaxation towards molecular rotations. This mechanism is related to the concept of light-induced conical intersections (LICI). However, unlike LICIs, these non-adiabatic pathways are of collective nature, since they depend on the \emph{relative} intermolecular orientation of all electronic transition dipoles in the polarization plane. Notably, this rotational mechanism directly couples the bright upper and lower polaritonic states, and it stays in direct competition with the collective relaxation towards dark-states. Our simulations indicate that the molecular rotational dynamics in gas-phase cavity-coupled systems can serve as a novel probe for non-radiative polaritonic decay towards the dark-states manifold.Comment: 6 pages, 5 figures and supporting materia