Toward a table-top synchrotron.
Dr Adrien Chauvet, recently appointed as lecturer in Physical Chemistry, has recently been published in the prestigious journal Science. The full article can be read through here: Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source. A brief summary of the techniques written by Dr Chauvet is included below.
Better understanding our environment is the key for growth and success. With this goal in mind scientists keep developing new pieces of equipment that enables to see nature in more detail. We are now capable of watching nature at a molecular and even atomic level. But at this stage, a magnifying glass is not enough. Instead, we use high power lasers to resolve the minute displacements of atoms and electrons.
Recently, a group of scientists in Geneva, Switzerland, conducted a pioneer experiment, in which Adrien Chauvet, lecturer at The University of Sheffield, took part. The experiment used the latest laser technology to produce ultrashort bursts of high energy photons. With such pulses it is possible to watch the electronic rearrangement of specific atoms while photochemical reactions are taking place. But the technique comes at a price, which is power: the more powerful the light pulses are, the deeper into the electronic structure you can see with greater precision.
The experiment was important because it not only shines light onto molecular dynamics that were never seen before, but is as much a breakthrough in terms of technological advancement. Indeed, using table-top lasers, the team reports the generation of photons with energies up to 350 eV, thus entering the so-called “water-window”.
This spectral window situated in the x-ray region is of significant importance for the study of biology as it offers sufficient contrast to distinguish atoms such as Carbon, Nitrogen and Sulfur from Oxygen, consequently offering to “see” organic molecules in solution (as is the case with visible light). While such photon energies were initially reserved to large scale facilities like the Diamond Light Source (UK), they are produced here from a table-top laser system. The advantages are considerable: going from a facility that is about the size of 3 football fields down to the size of a single laboratory room, the cut in terms of cost is huge. Furthermore, table-top lasers can provide femto-second time resolution with an inherent synchronisation that is otherwise hard to achieve. Resolution of this scale is necessary to follow the molecular dynamics.
From a technical point of view, high energy photons are generated by focussing pulses of light in a confined cell filled with neon. Ironically, the longer the wavelength of the pulses used to ionise the neon, the more efficient is the generation of high energy photons, i.e. with short wavelength, up to the x-ray region. The pulses are initially produced by a typical titanium:sapphire driver and are amplified in two successive stages to reach an outstanding 17 Watts (of 800 nm, at 1 kHz). To put this number into perspective, it is about 3000 time more powerful than a typical laser pointer.
This much energy is required to tune the colour of your pulses to a longer wavelength: from 800 to 1800 nm. When tightly focussed, these pulses are destructive as they ionize air and cut through metal. And this is effectively their purpose: they efficiently ionize neon atoms, and it is the subsequent recombination between the ejected electron and the ionized neon atom that produces the desired x-ray burst. In this particular experiment, the x-ray pulses are used to probe the different electronic levels of the Sulfur and Carbon atoms while the molecules are dissociating with one of their constituent. The electronic reorganisation was followed with a resolution of about 50 fs.