▎ 摘　　要

The brightness of electron beams emitted from photocathode sources plays a critical role in determining the performance of x-ray free-electron lasers and ultrafast electron-diffraction applications. In order to achieve the maximum brightness, the electrons need to be emitted from a photocathode with the lowest-possible mean transverse energy (MTE). Recent investigations have shown that capping a Cu(110) photocathode with a monolayer of graphene can protect the quantum efficiency (QE) from long-term exposure to varying vacuum conditions. However, there have been no studies that investigate the effects that a monolayer of graphene has on the MTE. Here, we report on measurements of a graphene-coated Cu(110) single crystal near the photoemission threshold for room and liquid-nitrogen temperatures. At room temperature, a minimum MTE of 25 meV is measured at 295 nm. At liquid-nitrogen temperatures, a minimum MTE of 9 meV is measured at the photoemission threshold of 290 nm.