Understanding how charge and energy transfer processes occur in molecules is of extreme importance in the design of electronic devices (e.g. solar cells) and in understanding chemical and biological processes (e.g. photosynthesis). In order to capture, and ultimately control, the dynamics of atoms and molecules, we need a method of probing the system of interest with sufficiently short resolution in time. For observing electronic motion, this requires resolution on the attosecond timescale (1 attosecond = 10-18s = 1 quintillionth of a second). One way to acheive this resolution is through the use of intense, ultrashort laser pulses.
An accurate theoretical description of a molecule interacting with a short, intense laser pulse in principle requires the solution of the time-dependent Schroedinger equation (TDSE). Currently however, this is only possible for the simplest molecules, which have one or two electrons. Beyond systems with more than two electrons, solving the TDSE directly in full dimensionality becomes impossible due to massive increases in computational cost. To consider larger molecules, we therefore turn to time-dependent density functional theory (TDDFT).
The aim of this project is to study dynamical processes in these two classes of molecules (1- and 2-electron systems described through direct solution of the TDSE; larger molecules treated using TDDFT) by employing large-scale, highly parallelized computer codes, which have been developed within the Atomistic Simulation Centre. Current work is focused on simulating the response of the hydrogen molecular ion (H2+), the nitrogen molecule (N2) and the acetylene molecule (C2H2) to laser pulses of varying wavelengths, intensities, durations and polarizations.
Ionization and dissociation of H2+: Probability density of H2+ during and after the interaction of a 10 cycle, 780nm laser pulse with peak intensity 2.0 x 1014W/cm2, as a function of z and internuclear separation R. The molecule is aligned along the z axis and is initially in an excited vibrational state. The laser pulse is linearly polarized, also along the z axis. As the electric field of the laser pulse increases (shown in left plot), we begin to see tunnel ionization in the positive and negative z directions. After the laser pulse has ended, we see parts of the wavefunction continuing to move towards larger values of R, indicating molecular dissociation.