Abstract and subjects
Reducing the mean transverse energy (MTE) of electrons emitted from photocathodes will improve the performance of linear accelerator applications like x-ray free electrons lasers (XFELs) and ultrafast electron diffraction (UED) and microscopy (UEM) experiments. Metallic photocathodes are popular for these applications due to their convenience of use. However, they typically have a very low quantum efficiency and require a large laser fluence in order to extract the desired charge density for applications like XFELs and single-shot UED/UEM experiments. Recent theoretical investigations have shown that nonlinear photoemission effects of multiphoton emission and electron heating increase the MTE dramatically at these large laser fluences. In this paper, we report on measurements of nonlinear near-threshold photoemission from a graphene-coated Cu(110) photocathode at laser pulse lengths of 130 fs, 1 ps, and 10 ps. We extrapolate our measured data to find the ideal irradiating photon energy that results in a minimum MTE from such metallic photocathodes for charge densities relevant to photoinjectors and specify quantum efficiency requirements to obtain the thermally limited MTE at these charge densities.