Linear intensity dependence of photo-induced field emission from tungsten

As a first step in an investigation of the polarization dependence of photo-induced field emission from tungsten, the dependence of the photocurrent on the intensity of illumination has been studied. The beam from a CW krypton ion laser operating at selected wavelengths in the visible region of the spectrum was focussed onto the field emitter, and the emission current in a narrow energy range (chosen to minimize tip heating effects) was measured as a function of intensity. The apparatus was designed to ensure that the position of the focal spot and the intensity of illumination could be adjusted without mutual influence. The photocurrent from each crystal plane was found to be linearly proportional to the luminous intensity over the range of intensity investigated (0–35 MW m−2). It is concluded that in tungsten there exists a low intensity regime where the external current in photo-induced field emission is caused predominantly by one-photon excitations.