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A time-domain measurement of the relaxation time of photoexcited electrons as a function of excitation energy at a single-crystal Cu(100) surface has been made with time-resolved two-photon photoemission. The relaxation lifetime of the excited electrons is found to decrease rapidly with the amount of excitation energy above the Fermi level, and we compare the measured lifetimes with a calculation based on Fermi-liquid theory.

Photoelectrons with excess kinetic energy corresponding to several absorbed photons above the work function have been measured from atomically clean Cu͑110͒ and Cu͑100͒ surfaces under ultrahigh vacuum conditions. The power dependence of the photoemission yield does not follow a simple power law dependence corresponding to the number of photons absorbed. This behavior is reminiscent of other above threshold ionization ͑ATI͒ or tunnel ionization ͑TI͒ processes observed for atoms in the gas phase. The photoelectrons are generated with laser pulsewidths less than 100 fs in duration and peak powers as low as 100 MW/cm 2. These intensities are on the order of 10 5 times lower than that required to observe similar phenomena in the gas phase. The relatively low intensities and correlation with surface roughness suggests a contribution from a surface enhancement mechanism. Thermal heating and space charge effects have been ruled out, and the possibility of electric field enhancement at the surface due to the coupling of photons into surface plasmons is discussed. The nonlinear yield and enhancement of the photoemission produced by short pulse excitation needs to be considered when discussing photoinduced hot electron reaction channels at metal surfaces.

The temporal diffraction–difference approach of the ultrafast electron diffraction (UED) technique was used to determine the molecular structure of the transient [Fe(CO)4] (see picture) formed during the elimination of CO ligands from [Fe(CO)5]. The results clearly show that the major product, up to 200 ps, is the transient [Fe(CO)4] which is formed in the 1A1 state, rather than the ground 3B2 state.