Ultrafast structural dynamics by crystallography and coherent control

The van Thor group specialises in the investigating ultrafast structural dynamics of light-sensitive proteins,
elucidating the nuclear and electronic dynamics in key biological reactions. The advent of fourth-generation
light sources, X-ray free electron lasers (XFELS), has allowed unprecedented temporal resolution of
angstrom wavelength for pump-probe spectroscopy. The group is at the forefront of utilising these
instruments in developing novel experimental ultrafast techniques, specifically time-resolved serial
femtosecond crystallography (TR-SFX) and the corresponding theory.1,2 The objective of this PhD project
is to build upon this work and conduct experiments supported by theory to elucidate structural dynamics by
coherent control.
It is known that large vibrational coherence is generated in both the ground and excited states, following
optical excitation of biological proteins.1 Suppressing or enhancing coherence, can lead to manipulation of
the populations in states and reaction pathways. Optical control is a method whereby changing the
characteristics of the laser pulses can manipulate the coherent amplitude in both ground and excited states.
The peak power, pulse duration, carrier frequency and second-order dispersion of a pump pulse have all
been shown to impact coherent amplitude.
2,3 Additionally, it has been recently observed that secondary
(dump) pulses can enhance coherence in the ground state.
The initial work of this project will be to develop theoretical simulations in collaboration with the Buckup
group Heidelberg, to compliment historic XFEL crystallographic experimentation. The simulations will then
be used to provide theoretical support for future experimental results and influence experimental design.
The simulations employ a non-perturbative time-dependant density matrix model and monitors the transient
effect after perturbation from an external electric field (laser pulse).4 The simulations will allow full control
of the characterisation of multiple laser pulses, such that all facets of optical control described above can
be investigated. Analysis of the coherence can be discerned by a transformation of the density matrix to
the semi-classical Wigner phase space representation and coherent amplitudes resolved.5
Upon completion of the simulations, work will begin on developing the apparatus to allow pulse shaping to
manipulate the pulse characteristics in the home laser laboratory. Employing ultrafast transient absorption
spectroscopy technique, combined with active pulse shaping will allow coherent control of molecular
dynamics by impulsive Raman spectroscopy. Furthermore, the optimisation of pulse characteristics for
coherent control can then be implemented in TR-SFX experiments with beam times at XFELs.
In addition to optical control, coherence can be controlled via the structural, symmetry and directional
properties of protein crystals. Two dimensional electronic and infrared spectroscopy has been identified as
techniques which could discern the effects these properties have on vibrational coherence.6
This project hopes to elucidate structural dynamics by coherent control, incorporating multiple experimental
techniques with theory. Investigating multiple facets of coherent control both optically and structurally.