Absorption of light by a molecule can trigger a wealth of photoinduced processes. Bond breaking, bond formation, large changes in the molecular structure… these are just some of the new realms of chemical and physical processes opened by light. The challenge is capturing and understanding these processes that often occur on the timescale of femtoseconds. They key motivation for my research is developing tools and techniques that allow us to do just this in the field of ultrafast spectroscopy.
Development of ultrafast spectroscopic techniques
One of the challenges of studying light-initiated processes is that they occur in a regime where the Born-Oppenheimer approximation breaks down, making it impossible to separate the molecular electronic and nuclear degrees of freedom. This means that, to successfully map all aspects of a photoinduced process experimentally, the technique of choice must be sensitive to both the electronic and nuclear changes the molecule undergoes.
Ultrafast optical techniques have excellent time-resolution and sensitivity to the evolution of the valence electronic structure. Of this family of techniques, multidimensional electronic spectroscopy (2D-ES) makes it possible to gain unprecedented levels of detail about the energy relaxation dynamics and behaviour of coherences even in large, complex molecular systems. This is because with 2D-ES it is possible to frequency-resolve both excitation and detection signals, avoiding the spectral congestion issues found with more widely used approaches such as transient electronic absorption (TEA) spectroscopy.
While the valence electronic structure of a molecule is dependent on its nuclear configuration, it can be challenging to retrieve structural information directly from optical experiments. For this, time-resolved X-ray techniques are better suited. By studying transitions from the tightly-bound core electrons, element-specific, localised information can be retrieved on both local changes in electronic structure and structural information such as bond angles and lengths. A variety of detection methods, including absorption, emission and photoelectrons, can be used in the X-ray domain to recover that maximum amount of chemical information.
Applications to complex molecular systems
My motivation for developing the aforementioned experimental techniques is to make it possible to study a wider range of complex molecular systems, including porphyrin complexes for photodynamic therapy, and inorganic and organic compounds for photocatalysis. As well as being excellent candidates for developing our knowledge of photophysical behaviour, there are key scientific questions concerning their energy relaxation pathways, and how chemical substitution and local environment influence these. Understanding this is key for driving future molecular design for applications.
Combining Experiment and Theory
Developments in excited state electronic structure methods over the last decade have meant that it is possible to directly simulate the observed signals in ultrafast experiments. Using ab initio methods alongside experiments makes it possible to gain deeper insight into the exact mechanisms of energy relaxation and can aid in the interpretation of complex experimental spectra. We perform a variety of such calculations here at UCL and in collaboration with several international groups, particularly for benchmarking and testing new methodological approaches for the simulation of experimental spectra.