Attosecond X-ray Spectroscopy of Ultrafast Dynamics in the Condensed Phase

We are all familiar with the idea that X-rays can "see" inside matter, after all this is the basis of much imaging technology from medical X-rays to Superman's astounding vision. We plan to use new sources of X-rays with the unique property that they are in pulses of duration less than 1 femtosecond (1 fs= 10^-15 s that is a millionth of a billionth of a second) to allow us, for the first time, to take snapshots and even make movies of some of the fastest events that occur within matter.
Our ability to produce and use such short pulses of X-rays has come out of our research over the past six years (funded by an ERC Advanced Grant and an EPSRC Programme Grant). We showed that high harmonic generation (HHG) based sources of ~0.3 fs pulse duration could be generated from 150 - 600 eV, an unprecedented achievement for ultrafast X-ray sources. Combined with the ultra-thin liquid sheet jets that we have recently invented and few-femtosecond optical pulses available in our laboratory that can start the clock by electronic excitation of the material, we have now in place the tools that enable a new kind of ultrafast X-ray spectroscopy that can be applied to any system whether it is in gas, liquid or solid state. We now want to advance this research in new directions by using this method to investigate some of the fastest processes in physics and chemistry.
In parallel with our laser research we have been working with international teams to develop the methods of using X-ray free electron laser facilities to make measurements on the few-femtosecond timescale. This has included work with LCLS (SLAC, California) in a two-pulse/two-colour mode with ~ 3 fs temporal resolution. Later this year it is anticipated that LCLS will produce the first FEL based sub-femtosecond X-ray pulses of extreme brightness (> 10^6 times greater than our HHG source). We plan to use this new capability to develop a new concept in X-ray non-linear spectroscopy that will allow us to precisely follow electron motions in matter at atomic spatial resolution and with time-scales faster than a femtosecond.
What will we investigate with these remarkable new tools? The answer is the fundamental dynamical events in physics, such as exciton formation and charge migration, and key processes in chemistry, such as electron transfer and bond-breaking/making. These can occur within 10 fs of initial electronic excitation and have hitherto not been accessible to direct measurement. Moreover with our tools we can track the dynamics of microscopic systems across the boundary in the temporal domain between quantum and classical behaviour. In condensed phase systems, where the quantum coherence of the initial state is lost in a few 10's of femtoseconds, this will allow us to see into a new quantum regime of dynamics. In particular we will focus on structures containing delocalised electrons (pi-conjugated systems) as in this case the electrons are highly mobile and so can display the very fastest dynamics. Additionally pi-conjugated molecules are the building blocks of polymers and molecular complexes of great interest in photochemistry and solar-energy conversion. Not only will our measurements capture the ultrafast electronic motion in these systems they will also allow us to measure the structural dynamics associated with isomerization, ring opening and other chemical changes.
Our research will extend the frontier of science's measurement capability in the time domain with the likelihood of measuring physical processes at timescales 100 times faster than before. This will lead to new breakthroughs in our understanding of quantum dynamical processes in nature and technology.

As well as academic beneficiaries the research outcomes in photocatalysis and optoelectronics have a high potential to impact the Catalysis and Renewable Energy industrial sectors. In parallel there is a resulting societal benefit in terms of efficient use of resources for chemical synthesis and renewable energy production. To capture this we will continue to work with industrial contacts (e.g. in Johnson-Mathey, GSK and Rolls-Royce) and will organise a follow up meeting to the very successful one on "Applications of Ultrafast X-rays" that was hosted at Imperial College in 2016.
An important impact to the national science and technology workforce will be in providing a number of highly skilled researchers trained in the methods of ultrafast X-ray measurement. This will address a developing skills gap in the UK as we don't yet host, or plan to host, an X-ray FEL facility of our own, but we do plan to make use of the internationally available facilities for both scientific and technological research in the future (see report issued by STFC on X-ray FELs in 2016).
The research will result in the development of technology that may be widely used by the technology community in the private sector and public research arena as well as having potential for commercial development. The technology that may be most relevant are compact soft X-ray ultrafast coherent light sources, few-cycle high power laser pulses tuneable in the IR and new liquid jet technology. First the sub-fs soft X-ray pulses we can generate from table top (and hence portable) apparatus represents a new technological capability. They can be applied to X-ray spectroscopic analysis of a wide range of materials and can capture transients at all timescales. This may prove critical for future methods of in-situ measurements on chemical reactors, combustion and other industrial processes. Second the few cycle waveform controlled mid-IR pulses, again generated from a relatively compact apparatus, may find application in remote sensing (e.g. for environmental monitoring) and defence/security (stand-off detection of chemicals). Important wavelength bands we plan to open up by this research include the 1100-1600 nm, and in the UV, and along with the mid-IR these may find application in spectroscopic analysis and environmental monitoring. Aspects of the work already completed on few cycle sources are now being developed towards commercialisation via funding from an ERC Proof-of-Concept grant. Third the new liquid sheet jet technology is already exciting the interest of an industrial partner ("Rho nano" UAB, Lithuania) and through the new project we hope to greatly extend the capability of this technology to handle a large range of liquids and to handle smaller volumes of high value sample. This may have impact in the Chemical Analysis, Biomedical and Pharmaceuticals sectors.