Quantitative imaging of ultrashort photoelectron pulse dynamics*

Understanding and mitigating the space charge effects is a pressing issue in the development of ultrafast electron diffraction and imaging. Using a novel ultrafast projection imaging technique, quantitative imaging of transient space charge effects in the generation of high density ultrashort electron pulses is performed, which offers a means to directly compare with multi-electron calculations. We establish that the pulse width exhibits a fractional power-law scaling with the sheet density of the emitted electron pulses. By comparing to multi-electron simulations, the initial longitudinal phase space of the photoelectrons is extracted, demonstrating a strong dependence of the initial momentum spread on the sheet density. Multielectron effects are treated using a simple extension of single electron photoemission theory yielding qualitatively correct estimates of the quantum efficiency.