Pulsed laser-induced electron and positive-ion emission from Cu(100) under ultrahigh-vacuum conditions near the threshold for surface damage

Abstract

Results are presented from an experimental study of pulsed laser-induced electron and positive-ion emission at 590 nm from single-crystal Cu(100) under ultrahigh-vacuum conditions near the threshold for surface damage. Below 60 MW/cm2 the electron emission is consistent with a three-photon photoemission mechanism. Above 60 MW/cm2 thermionic electron emission becomes dominant. Positive ions are produced with a threshold of approximately 60 MW/cm2 and a slope of 9.5 in a log(yield) vs log(power) plot. The kinetic energy distribution of the ions is nonthermal with a maximum at approximately 0.7 eV. Ion production is most likely due to collisions of thermally desorbed atoms with electrons above the surface. Evidence is presented that suggests a critical role of substrate-emitted electrons in the formation of a laser-induced surface plasma near its low power threshold. It is also found that electron and ion yields from a given spot on the surface decrease with the number of laser pulses and spot receives. The yield from a previously unirradiated spot decreases by an order of magnitude over the course of approximately the first 25 laser pulses approaching a constant level for subsequent pulses. Enhanced yields from new spots could be eliminated by additional annealing of the surface prior to laser irradiation. These observations are consistent with enhanced charged particle yield from surface defects which are reduced in number by subsequent annealing by the laser.