Photo-Emission Studies from Zn Cathodes under Plasma Phase

In this paper we report investigations of the electron emission from pure Zn cathodes irradiated by UV laser pulses of 23 ns (FWHM) at a wavelength of 248 nm (5 eV). The metal cathodes were tested in a vacuum photodiode chamber at 10-7 Pa. They were irradiated at normal incidence and the anode-cathode distance was set at 3 mm. The maximum applied accelerating voltage was 18 kV, limited by the electrical breakdown of the photodiode gap. Under the above experimental conditions a maximum applied electric field of 6 MV/m resulted. In the saturation regime, the measured quantum efficiency (QE) value increased with the accelerating voltage due to the plasma formation. The highest output current was achieved with 14 mJ laser energy, 18 kV accelerating voltage and its value was 12 A, corresponding to a global quantum efficiency (GQE) approximately of 1 x 10-4. The temporal quantum efficiency (TQE) was 1.0 x 10-4 at the laser pulse onset time, and 1.4 x 10-4 at the pulse tail. We calculated the target temperature at the maximum laser energy. Its value allowed us to obtain output pulses of the same laser temporal profile. Tests performed with a lower laser photon energy (4.02 eV) demonstrated a GQE of two orders of magnitude lower.