Velocity-Map Imaging for Emittance Characterization of Multiphoton Electron Emission from a Gold Surface

In this article we demonstrate the applicability of velocity-map imaging to characterize the emittance of photoemitted electron bunches from a gold surface. The spectrometer allows to determine both, the two-dimensional spatial- and the projected velocity distribution of photoemitted electrons. Both distributions were used to determine the normalized emittance of a planar gold surface irradiated with 45 fs long, 800 nm laser pulse operated in the multi photon regime of the sample. Furthermore we present a mathematical method to reconstruct the three dimensional velocity and energy-distribution given a known angular distribution of photoelectron velocities. The obtained distributions agree well with previous measurements. With our publication we demonstrate the ability to characterize the normalized emittance of an electron gun in a few minutes with an energy resolution up to 0.2 meV. Our new spectrometer will therefore foster fast characterization and further developments of x-ray free-electron-laser injectors and guns for ultrafast electron diffraction. In addition it opens up opportunities for studying correlated electron emission from surfaces and vacuum nanoelectronic devices.

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