Femtosecond Mega-electron-volt Electron Energy-Loss Spectroscopy

Pump-probe electron energy-loss spectroscopy (EELS) with femtosecond temporal resolution will be a transformative research tool for studying non-equilibrium chemistry and electronic dynamics of matter. In this paper, we propose a new concept of femtosecond EELS utilizing mega-electron-volt electron beams from a radio-frequency (rf) photocathode source. The high acceleration gradient and high beam energy of the rf gun are critical to the generation of 10-femtosecond electron beams, which enables improvement of the temporal resolution by more than one order of magnitude beyond the state of the art. The major innovation in our proposal - the `reference-beam technique', relaxes the energy stability requirement on the rf power source by roughly two orders of magnitude. Requirements on the electron beam quality, photocathode, spectrometer and detector are also discussed. Supported by particle-tracking simulations, we demonstrate the feasibility of achieving sub-electron-volt energy resolution and ~10-femtosecond temporal resolution with existing or near-future hardware performances.