Photoelectric field emission from needle cathodes

Experiments have been carried out at laser intensities of the order of 108 W/m2 using a cw argon laser and tungsten needles with roughly 1-μm tip radius operated at voltages up to 50 kV. The results show a nonlinear dependence of the photocurrent on the laser intensity and a strong dependence of the photocurrent on the electric field. Comparison with theoretical calculations based on the Fowler–Nordheim tunneling theory of field emission indicates that the photoexcited electrons rapidly relax to energy levels just above the Fermi level, from which they tunnel out.