Photoelectric quantum efficiency of niobium for λ = 193 nm and λ = 248 nm

We report a measurement of the quantum efficiency of a niobium photocathode for the wavelengths λ = 193 nm and λ = 248 nm. Charge yields for a 20 ns excimer laser pulse were measured as a function of DC accelerating anode voltage. Our experimental results for the quantum efficiency are ε(248) = 3.2±0.6 × 10−6 and ε(193) = 4.5±0.5 × 10−4. We conclude that niobium is a promising photocathode material for a superconducting rf electron gun if the laser wavelength is RE200 nm. Laser pulse energy requirements are estimated for such a gun.