1—The photoelectric effect at metal surfaces is of great importance and many attempts have been made to explain its main features, but an exact theory presents great difficulty. One of the best and most recent is that of Mitchell (1934, 1936), who considered an idealized model in which the potential barrier at the surface was a simple step; the field of the light wave was calculated by the classical optical theory, assuming that the optical constants change abruptly at the surface. The latter approximation is not a good one, even for the simple potential jump, sines the electron density does not tall sharply to zero, but decreases in a distance of about 3 × 10
-8
cm. from a high value inside the metal to a negligible value outside the jump. It is well known that a free electron gives no photoelectric effect, and so the photo-emission from a metal is almost entirely due to the surface-potential gradient; it is in this region that the light wave is least accurately given by the simple optical theory and fluctuates most.
Publisher Correction: Resonant phase-matching between a light wave and a free-electron wavefunction
