Novel Non-linear Optical-Fibre Sources for Time-resolved Molecular Dynamics: Towards the Next Generation of Ultrafast Spectroscopy

Developing detailed understanding of molecular interactions with light is of great importance. This is highly relevant, for example, to the fundamental biological processes of vision and photosynthesis, and also in photoresistive pathways (as seen in systems such as DNA and the melanin pigments) that protect living organisms from damage by ultraviolet (UV) light. Understanding light-molecule interactions is also of critical relevance for many other species, including photostabilizers, photochromic polymers, light harvesting complexes, sunscreens, photodynamic therapy drugs and molecules relevant to atmospheric/interstellar photochemistry. Advancing experimental techniques to improve the study of such systems is therefore imperative. In particular, learning more about the fundamental mechanisms that redistribute excess absorbed energy in molecules - and ultimately how to better utilize them - is of profound interest.

The use of "ultrafast" femtosecond laser pulses with temporal durations comparable to the timescales of molecular motion is a powerful method for studying light-matter interactions. Excess energy redistribution is followed in real time using "pump-probe" techniques: pump absorption effectively starts a dynamical "clock" on the overall process and the system is then interrogated at a series of precisely controlled delay times by the probe, mapping out the relaxation pathways. Time-resolved photoelectron imaging (TRPEI) is an extremely powerful variant of this general approach, yielding highly differential energy- and angle-resolved information offering deep insight into the underlying photophysics. A key requirement for TRPEI is the use of tuneable UV femtosecond pulses for both pump (excitation) and probe (ionization). Operating in this spectral region is, however, extremely inefficient and this places restrictions on the feasibility and scope of many studies. A rapidly emerging new technology for providing greatly improved (100-1000x) gains in UV generation efficiency makes use of hollow-core photonic crystal fibres (HC-PCFs). These also offer access to short-wavelength spectral regions (<200 nm) that are not easily realized via more conventional means. The key aim of this project is to harness the advantages afforded by HC-PCFs and undertake detailed, systematic studies of excess energy redistribution in model chromophore motifs (the light-absorbing centres in larger biomolecules). The selected motifs have all been implicated in providing UV photo-protective function and the highly-differential nature of TRPEI, supported by state-of-the-art quantum chemistry calculations, will yield much new insight into the fundamental mechanisms mediating such processes. Our study will also reveal principles relating more generally to the interplay between molecular structure, dynamics, and photochemical function that are broadly applicable to a far wider range of species - including those that may be exploited commercially.

The project brings together four researchers with complementary skills in ultrafast lasers, non-linear optics, molecular dynamics and cutting-edge computation. HC-PCF sources will be integrated into a TRPEI set-up, creating a unique state-of-the-art instrument. Detailed evaluation the device will include development of a novel single-wavelength pump-probe (SWPP) scheme that provides an expanded "view" along relaxation pathways and yields enhanced dynamical information. This opens up exciting new avenues of investigation and we will take advantage of this in using SWPP-TRPEI to perform studies of excess energy redistribution in three distinct molecular motifs providing starting models for chromophores found in nature (as detailed in the Objectives section). Our work represents a major step forward in realizing a next generation of low-cost table-top light sources for ultrafast spectroscopy and we anticipate that the dissemination of our findings will have lasting impact on this major research field.

The proposed work is fundamental in nature and it is therefore expected that the economic and societal benefits stemming from the research output will primarily be realized in the longer term. This is in addition to the academic benefits, many of which will be much more immediate. Our research will significantly advance existing methodologies for the study of energy redistribution within the excited states of molecular systems - including those with biological, environmental and/or medical relevance, yielding important new mechanistic insight. This has potential implications for a wide range of practical applications where the interplay between structure and dynamics influences light-matter interactions and associated photochemical or biological function. In addition, the tuneable HC-PCF-based spectrometer we will develop will also be of possible interest to the applied photonics community. In order to facilitate timely uptake of our findings we will exploit a wide variety of routes to maximize exposure to relevant parties. These include presenting our work at large, interdisciplinary conferences that attract several thousand delegates as well as making use of well-established industry-academia networking events at the local, national and international levels. These networking events span a number of different themes: from exploratory collaboration between universities and companies, to forums discussing policy roadmaps for major funding initiatives. It is critical for the continuity and timeliness of any follow-up research that such opportunities are fully exploited throughout the duration of the project, rather than simply at its conclusion.

There will, in addition, be very immediate impact stemming from our work in the form of highly trained personnel with technical skills in the use of cutting edge techniques in the areas of laser physics, non-linear optics, ultra-high vacuum science, molecular spectroscopic, rigorous data analysis and theoretical computational techniques. They will also have well-developed generic and widely transferrable communication, presentation and problem solving skills. They will, therefore, be ideally positioned to contribute to the growth or creation of new research projects (both pure and applied) as well as high-tech companies, enhancing innovative capacity and possible revenue generation for the UK economy. Enhanced public awareness and engagement with our research efforts will be achieved through consciously non-specialist postings on university web pages, university open days and via "case study" reports produced at regular intervals throughout the project and made available in the public domain. It is anticipated that these studies will also be beneficial for attracting future research students to the key areas of ultrafast lasers and optics, molecular spectroscopy and dynamics, and theoretical photochemistry. It will also have the potential to advertise the existence of the project to any interested parties in the commercial sector.