4D attosecond imaging with free electrons: Diffraction methods and potential applications
We consider here the extension of four-dimensional (4D) electron imaging methodology to the attosecond time domain. Specifically, we discuss the generation of attosecond electron pulses and the in situ probing with electron diffraction. The free electron pulses have a de Broglie wavelength on the order of picometers and a high degree of monochromaticity (ΔE/E_0 ≈ 10−4); attosecond optical pulses have typically a wavelength of 20 nm and ΔE/E_0 ≈ 0.5, where E_0 is the central energy and ΔE is the energy bandwidth. Diffraction, and tilting of the electron pulses/specimen, permit the direct investigation of electron density changes in molecules and condensed matter. We predict the relevant changes in diffraction caused by electron density motion and give two examples as prototype applications, one that involves matter-field interaction, and the other is that of change in bonding order. This 4D imaging on the attosecond time scale is a pump–probe approach in free space and with free electrons.