Title: Femtosecond Mega-electron-volt Electron Energy-Loss Spectroscopy,
Abstract: 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.