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In this paper, quantitative analysis of the evolution of GaAs(Cs,O) surface was made during negative electron affinity (NEA) activation using angle-dependent X-ray photoelectron spectroscopy (XPS). The thickness, chemical compositions and percentage of each element of GaAs(Cs,O) surface layers after "high-temperature" single-step activation and "high-low temperature" two-step activation were obtained. It was found that compared to single-step activation, the thickness of GaAs-O interface barrier had a remarkable decrease, the degree of As-O bond became much smaller and the Ga-O bond became dominating, and at the same time the thickness of (Cs,O) layer also had a small deduction but the ratio of Cs to O had no change after two-step activation. To explore the influences of the evolution of GaAs(Cs,O) surface layers on photoemission, surface barrier models on basis of the XPS results were built for single-step activation and two-step activation, and the respective theoretic surface escape probability of electrons was calculated. The results of theoretic calculation can be consistent with the ones of experiment. According to calculation the decrease of thickness of GaAs-O interface barrier has a main effect on the increase of surface escape probability, which explains why higher sensitivity is achieved after two-step activation than single-step activation. It was also found that to explain the variations of long threshold wavelength of photoemission during activation, the evolution of the height of GaAs-O interface barrier and scattering characteristics of (Cs,O) layer with GaAs(Cs,O) surface structure is necessary to further consider.