Direct Measurement of Correlation Driven Electron Dynamics in an Amino Acid

All material systems are made up of positively charged nuclei, and negatively charged electrons. The way in which the arrangement of electrons and nuclei respond to external stimuli, such as photons from light, determines their physical and chemical properties. The electrons in materials do not behave as independent particles, and their position and momenta are highly correlated due in part to their mutual Coulombic repulsion. An approach to probing and understanding the correlated behaviour of electrons is to remove one of the electrons and then observe how the remaining electrons adjust in response to the sudden removal of an electron. That is precisely the approach we will follow here; we will employ a short X-ray pulse from a free-electron laser to remove an electron from the Glycine molecule leaving a "hole" in the molecule, and then, with a second time-delayed short X-ray pulse we will probe the evolution of that hole as the electrons adjust. The second "probe" pulse will excite an inner core electron to the energy where the hole was created. As the other electrons readjust, the accessibility of that hole to the core electron will vary. As such, the probability of re-populating the hole with the core electron will evolve in time, providing us with a way to view the electron motion in the molecule. This experiment requires two synchronised laser pulses with duration of 5 fs and with a photon energy of 280 eV. The only place in the world that light pulses with these characteristics are available is at the Linear Coherent Light Source (LCLS) facility at Stanfrod, US.

The successful implementation of the exposed experiment at LCLS will provide a new and general approach to measuring the effects of electron correlation in molecular systems. As such, this method will provide new insights on the structure of matter at the atomic level and provide stringent tests of our current methods for calculating molecular structure. As the LCLS Proposal Panel point out "This is a very challenging experiment but has the potential of opening a new frontier in ultrafast molecular dynamics... the panel feels this is an important experiment that has to be done to pave the way towards this frontier area of AMO science". These experiments will ultimately benefit our understanding of how material properties relate to underlying structure and dynamics, and so will have impact beyond the molecular physics community, being relevant to material and biological sciences, for example.