FIG. 4.

Determination of the brightness for ultrafast electron pulses. (a) Image of the converged electron beam (convergent beam electron diffraction mode) with a spot size of 1.15 nm, as determined from a Gaussian fit (red line) of an intensity profile (blue dots) of the area marked with a black rectangle. Scale bar, 2 nm. (b) Diffraction pattern of the converged electron beam. Scale bar, 2 mrad. [(c) and (d)] Time-energy distributions of electron pulses with 0.48 and 63 electrons per pulse, respectively. The projections of the pulses into the time and energy domains are presented on the top and on the right, respectively. (e) Pulse duration as a function of the number of electrons per pulse [spot size 1, alpha 3, no condenser lens aperture (CLA) inserted]. For pulses with two maxima in the time domain, we report the full width at half the height of the smaller maximum. A linear fit serves as a guide for the eye. (f) Brightness as a function of the number of electrons per pulse (spot size 1, alpha 3, CLA 2). The brightness is measured with the second condenser lens aperture inserted, which reduces the number of electrons in the sample plane by a factor of about 15. For easy comparison with (e), we report the number of electrons without condenser lens aperture inserted on the bottom axis and the actual number of electrons on the top axis.