Ultrafast electron flux calibration for nanoscale dynamic imaging

The far shorter wavelength of electrons as compared to photons of the same energy has long been recognized as revolutionary to imaging applications. Here we consider a low-energy form of electron microscopy whereby pulses of electrons are generated by an ultrafast (femtosecond) laser pulse [1,2]. Unlike photons, pulses of electrons are heavily dispersed in vacuum when there is on average more than one particle per pulse through the action of space-charge (SC). As a consequence, when trying to minimize the temporal resolution at the target under investigation, it is vitally important that the number of electrons per pulse is well known. The effect of SC is negated when there is on average less than one electron per pulse hence any electron flux calibration needs to be calibrated and sensitive down to the single particle level [3]. Here we demonstrate just such an observation requiring no assumptions about the experimental apparatus allowing the number of particles per electron pulse to be determined.