  The dependence of the quantum efficiency on the angle and polarization of the
incident photon needs to be formulated for a precise description of the
response of photomultiplier tubes. A simplified one-step model of photoelectron
emission was derived from Spicer's three-step model, and it enabled the
formulation of the dependence of the quantum efficiency in the visible range
for thin multi-alkali (NaKSbCs) photocathodes. The expression of the quantum
efficiency was proved by a measurement of the photocurrent for linearly
polarized light at various incident angles. Meanwhile, the measurement revealed
the complex refractive indices and thicknesses both of the stratified
photocathode and antireflection coating. It is indicated that the angular
dependence of the quantum efficiency is dictated by the optical properties of
the photocathode, which are discussed in detail on the basis of the obtained
parameters.
