Ultrafast Ultraviolet Spectroscopy

Funded by: EPSRC

Time-resolved photoelectron imaging of molecular dynamics with our deep and vacuum ultraviolet pulse sources

This project is led by Dr. Dave Townsend of the Ultrafast Dynamics Group. LUPO is providing the techniques and technology for ultrafast deep and vacuum ultraviolet generation in gas-filled hollow-core photonic crystal fibres.

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. 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.

Publications

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable …

Talks

We demonstrate time-resolved photoelectron velocity-map imaging using tunable DUV pulses generated through resonant dispersive-wave …