An RF cavity-based ultrafast electron energy loss spectrometer: A new tool for unraveling dynamic processes in materials

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Electron energy loss spectroscopy (EELS) is an extremely powerful and well developed technique that has become extremely valuable for materials physics studies, providing spectroscopic information on a number of low energy (0 – 10eV) elementary excitations like valence shell electron excitations, phonons, plasmons and charge density oscillations of all kinds at all wavevectors. High-energy loss excitations (10 – 1000 eV) like core-electron excitations can also be measured, providing information similar to xray absorption spectroscopy on valency, local geometry and bonding of the absorbing species that has been so important for fundamental studies. Conventional EELS, however, is only capable of studying materials at equilibrium. Here we propose to construct a novel momentum-resolved ultrafast electron energy loss spectroscopy (q-uEELS) instrument that seamlessly integrates with the existing ultrafast electron diffractometer in the Siwick lab. This q-uEELS spectrometer makes use of RF cavities in a time-of-flight geometry to enable high resolution in both energy and time, providing energy loss spectra with a time resolution of femtoseconds to picoseconds. The new measurement capabilities provided by q-uEELS will help uncover the effects of photoinduced structural changes on material properties and the dynamics of optical excitation in materials on their natural scales of length (subA - nm), time (fs–ps) and energy (meV - eV) simultaneously by revealing the previously hidden information about characteristic excitations contained in the inelastic electron scattering. This proposal leverages two very recent advances in the Siwick group and represents an exciting new scientific opportunity to develop EELS for studies of material properties far from equilibrium.