Thin Film Photoemission Experiments

The "promptness " of the photoemission process is an important is-sue in the production of ultra-short electron bunches from photoinjector guns. One would ideally hope to obtain electron emission whose dura-tion faithfully mimics the photon pulse for photon pulses on the order 100 fs for many advanced accelerator applications. If all the photoelec-trons were emitted at the surface, the electron current would indeed mimic the photon pulse. However, it is important o experimentally determine the r lative contributions of the surface versus the bulk of the material to the net photoelectron current. It is the purpose of these experiments o address this issue. PHOTOELECTRON DYNAMICS The s tandard model of volume photoemiss ion consists of a three step pro-cess: (1) the electron absorbs a photon of energy hu which elevates it to an excited energy state, (2) the excited electron travels to the surface, and (3) the electron crosses the surface barr ier to escape the material. The height of th