Title: Multielectron effects in high harmonic generation in N2 and benzene: Simulation using a non-adiabatic quantum molecular dynamics approach for laser-molecule interactions
Author(s): Dundas D.
Journal of Chemical Physics, 136, No. 19, pp. 194303-1-194303-17 (MAY 2012)
A mixed quantum-classical approach is introduced which allows the dynamical response of molecules driven far from equilibrium to be modeled. This method is applied to the interaction of molecules with intense, short-duration laser pulses. The electronic response of the molecule is described using time-dependent density functional theory (TDDFT) and the resulting Kohn-Sham equations are solved numerically using finite difference techniques in conjunction with local and global adaptations of an underlying grid in curvilinear coordinates. Using this approach, simulations can be carried out for a wide range of molecules and both all-electron and pseudopotential calculations are possible. The approach is applied to the study of high harmonic generation in N2 and benzene using linearly polarized laser pulses and, to the best of our knowledge, the results for benzene represent the first TDDFT calculations of high harmonic generation in benzene using linearly polarized laser pulses. For N2 an enhancement of the cut-off harmonics is observed whenever the laser polarization is aligned perpendicular to the molecular axis. This enhancement is attributed to the symmetry properties of the Kohn-Sham orbital that responds predominantly to the pulse. In benzene we predict that a suppression in the cut-off harmonics occurs whenever the laser polarization is aligned parallel to the molecular plane. We attribute this suppression to the symmetry-induced response of the highest-occupied molecular orbital.
Title: Current-induced atomic dynamics, instabilities, and Raman signals: Quasiclassical Langevin equation approach
Author(s): Lü J.T., Brandbyge M., Hedegård P., Todorov T.N., Dundas D.,
Physical Review B, 85, pp. 245444- (25 June 2012)
We derive and employ a semiclassical Langevin equation obtained from path integrals to describe the ionic dynamics of a molecular junction in the presence of electrical current. The electronic environment serves as an effective nonequilibrium bath. The bath results in random forces describing Joule heating, current-induced forces including the nonconservative wind force, dissipative frictional forces, and an effective Lorentz-type force due to the Berry phase of the nonequilibrium electrons. Using a generic two-level molecular model, we highlight the importance of both current-induced forces and Joule heating for the stability of the system. We compare the impact of the different forces, and the wide-band approximation for the electronic structure on our result. We examine the current-induced instabilities (excitation of runaway “waterwheel” modes) and investigate the signature of these in the Raman signals.
Title: An ignition key for atomic-scale engines
Author(s): Dundas D., Cunningham B., Buchanan C., Terasawa A., Anthony T Paxton A.T., Todorov T.N.
Journal of Physics: Condensed Matter, 24, pp. 402203-1-402203-6 (2012)
A current-carrying resonant nanoscale device, simulated by non-adiabatic
molecular dynamics, exhibits sharp activation of non-conservative
current-induced forces with bias. The result, above the critical bias,
is generalized rotational atomic motion with a large gain in kinetic energy.
The activation exploits
sharp features in the electronic structure, and constitutes, in effect, an
ignition key for atomic-scale motors. A controlling factor for the effect
is the non-equilibrium dynamical response matrix for small-amplitude atomic motion
under current. This matrix can be found from the steady-state electronic structure
by a simpler static calculation, providing a way to detect the likely appearance, or
otherwise, of non-conservative dynamics, in advance of real-time modelling.