The evolution of the escape probability of hot and thermalized photoelectrons to vacuum from GaAs(001) with adsorbed layers of cesium and oxygen under the transition from the negative to positive effective electron affinity is studied by photoemission quantum yield spectroscopy. A minimum in the dependence of the escape probability of thermalized electrons at zero affinity is observed. The minimum is caused by a photoelectron capture into the two-dimensional subband states in the near-surface band bending region. Lower escape probability values for the cesium deposition, as compared to the oxygen deposition on the GaAs(Cs,O) surface with negative electron affinity, are explained by the photoelectron reflection or scattering at two-dimensional "metallic" Cs clusters.