Self-modulation of photoemission at the picosecond time scale

The pulse response of negative electron affinity GaAs photocathode over a wide range of quantum efficiencies has been measured. Low quantum efficiencies, where an intense excitation is needed, may lead to rapid dynamic change of the transport conditions on a time scale of a few picoseconds. This phenomenon is attributed to the surface photovoltage effect. The amount of charge that is trapped at the surface is calculated by a one-dimensional diffusion model. Then, the resulting surface photovoltage (SPV) is determined. Our findings indicate that the charge is trapped mainly during the first 40 picoseconds (ps) after excitation, which establishes an almost constant SPV afterward. This characteristic can be approximated to a continuous pump–probe in which self-modulation of the electron pulses at picosecond time scale is reached. Such a situation implies that after 40 ps the time evolution of the signal will remain the same as without SPV, but with a reduced quantum efficiency. This assumption was confirmed with a reasonable accuracy by rescaling the data for times larger than 40 ps.