Lecturer in Applied Mathematics and Theoretical Physics

Address

Room A7
Atomistic Simulation Centre
Department of Physics and Astronomy
School of Mathematics and Physics
Queen's University Belfast
University Road
Belfast BT7 1NN
Northern Ireland

Ultra-fast electron and photon driven dynamics in molecular systems

The interaction of molecular systems with ultra short laser pulses
provide fundamental examples of complex quantum many body systems
driven far from equilibrium. A highly non perturbative and non adiabatic
coupling exists between electronic and nuclear degrees of freedom which
induces both charge and energy flow within the molecule. These charge and
energy transfer processes occur on the femtosecond timescale, and are of
extreme importance in the design of electronic devices, probes and
sensors, and in the areas of condensed matter and plasma physics, medicine
and biochemistry. The development of non adiabatic quantum approaches is
therefore one of the great challenges in Physics. The challenge comes
about through the diversity of time scales that occur in the problem.
These time scales range from a few femtosecond for electron transfer
through tens of femtoseconds for excitation processes to hundreds of
femtoseconds characterizing the ionic motion. All these processes need to
be described within a consistent dynamical picture.

We have developed a number of approaches for describing the inteaction
of molecules using both full quantum descriptions of electron and ions
for small molecules and mixed, quantum classical approaches for large
molecules.

Quantum electron-ion dynamics of small molecules

For one- and two-electron diatomic molecules such as H_{2}^{+} and H_{2} we can treat both
the electronic and vibrational degrees of freedom exactly through the solution of the time-dependent Schroedinger equation (TDSE),
assuming that the laser light is linearly polarised along the intermolecular axis. Studying such systems interacting with
ultrashort intense laser pulse allows us to gain an understanding of the fundamental roles of electron-electron and
electron-ion interactions in ultrafast processes and can act as a benchmark for high-precision laboratory experiment.

We have developed computer codes based on a mixed Lagrange mesh and finite difference approach for solving the TDSE for these molecules.
These include a code called THeREMIN (vibraTing HydRogEn Molecular IoN) for describing H_{2}^{+} and a code called H2MOL
for describing H_{2}.

Pre-ionization dynamics of H_{2}^{+} by an ultrashort laser pulse Dissociation of H_{2}^{+} by a 6-cycle linearly polarized Ti:sapphire laser pulse.
The molecule lies along the z-axis with the laser polarization aligned along this axis. The TDSE is solved in cylindrical
coordinates with -150 ≤ z ≤ 150, 0 ≤ ρ ≤ 100, 0 ≤ R ≤ 20. In the plot the ρ coordiante has been integrated over and we focus on that part
of the grid neat the atoms. We see electron wavepacket responding in antiphase to the field with very little ionization occuring. After
the pulse has finished, we see wavepackets moving out in R which is indicative of dissociation.

Relevant Papers

Title: Dissociative ionization of molecules in intense laser fields

Author(s): Dundas D., Meharg K.J., McCann J.F., Taylor K.T.

European Physical Journal D, 26, No. 1, pp. 51-57 (OCT 2003)

Title: Efficient grid treatment of the ionization dynamics of laser-driven H-2(+)

The basis of this research is a time dependent density functional
theory approach implemented in a real space, massively parallel computer
code called EDAMAME (Ehrenfest DynAMics on Adaptive MEshes). This code was
developed in both the ASC and
CTAMOP.

The aim of this research is to
develop an experimental and theoretical capability that will lead to a
novel method for peptide sequencing using ultrashort laser pulses. This
work is being carried out with
Abi Wardlow, in collaboration with Dr Jason Greenwood of the
Centre
for Plasma Physics at QUB.

Ionization of Benzene by an intense, ultrashort laser pulse Ionization of benzene by a 5-cycle Ti:sapphire laser pulse. The benzene molecule lies in the plane and
the laser pulse is linearly-polarised with the polarization direction horizontal in the plane.
The laser wavelength is λ = 780 nm and its peak intensity is I = 4.0x10^{14} W/cm^{2}.
Ionizing electron wavepacket is emitted each half-cycle, in anti-phase to the field, as the laser
electric field strength passes through maxima and minima.

Relevant Papers

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)

Title: Molecular effects in the ionization of N2, O2, and F2 by intense laser fields

Non-conservative current-induced forces in nanoscale devices

We work on the real-time simulation of current flow in these systems, and of the dynamics of the atoms
driven by the huge current densities possible in atomic wires. A recent breakthrough was to prove theoretically that the forces on atoms that current flow exerts are
non-conservative, and to simulate the resultant operation of a one-atom 'waterwheel'. This work was featured
in two News and Views articles in the Nature Journals and sparked off experimental and theoretical interest
internationally.

Atomic waterwheels
An open-boundary non-adiabatic molecular dynamics simulation of the corner atom in a bent
atomic wire. The current in the wire is in the region of 70 μ A. The atom is driven in
an expanding orbit by the non-conservative current-induced force on it. Its kinetic energy
grows exponentially in time, till other factors kick in to slow it down.

Relevant Papers

Title: Current-driven atomic waterwheels

Author(s): Dundas D., McEniry E.J., Todorov T.N.

Nature Nanotechnology, 4, No. 2, pp. 99-102 (2009)

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)