Theory of ultrafast electron diffraction: The role of the electron bunch properties.

We present a general formalism for scattering of electron bunches used in ultrafast electron diffraction experiments. To perform the scattering calculation, we associate the classical distribution function that describes the electron bunch just before scattering with the asymptotic-in Wigner distribution. Using single scattering and far-field approximations, we derive an expression for the diffracted signal and discuss the effects of the different bunch parameters on the measured diffracted flux. We identify the transverse and longitudinal coherence lengths and discuss the importance of these length scales in diffraction pattern formation. We present sample numerical calculations for scattering by nanosize particles based on our model and discuss the results in terms of bunch and scattering target parameters. [ABSTRACT FROM AUTHOR]