| |
The laser-assisted photoelectric effect (LAPE) is a powerful tool for characterizing femtosecond-to-attosecond EUV pulses, and for time-resolved spectroscopy of electron dynamics in atoms. In these experiments, EUV and IR pulses are focused into a gas with various relative time delays. The EUV radiation photoionizes an atomic gas and, in the presence of the IR field, the photoemitted electrons can simultaneously absorb or emit an IR photon simultaneously with the EUV photon, leading to sidebands in the photoelectron spectrum. These sidebands change as a function of time delay between the EUV and IR fields. This change has been used in past experiments to measure the pulse duration of attosecond EUV pulses and to measure an 8 fs core hole Auger decay lifetime. To date, the laser-assisted photoelectric effect has only been observed in gas-phase atoms, where the discrete nature of the photoelectron spectra from atomic levels, as well as the sidebands, make them easy to distinguish.
In our experiment, we observed the laser-assisted photoelectric effect from solids for the first time. This result represents the laser-assisted version of the original manifestation of photoelectric effect. We illuminate a platinum crystal simultaneously with EUV and IR pulses and then record the kinetic energy spectra of the photoemitted electrons for different relative delays. For long time delays (> 30fs), the photoemission spectra are unaffected by the presence of the IR field. However, when the intense laser field overlaps with the EUV field, one or more IR photons can be absorbed or emitted simultaneously with the EUV photon.
This result is significant for three reasons. First, surface LAPE has the potential to study ultrafast, femtosecond-to-attosecond time-scale electron dynamics in solids and in surface-adsorbate systems - where complex, correlated, electron relaxation processes are expected. This is in contrast with measurements in atomic systems, where dynamics are generally homogeneously broadened and where time-domain studies have duplicated information that can be obtained from spectroscopic studies. Second, surface LAPE will make it possible to characterize lower-flux and higher-energy EUV pulses, because of the orders-of-magnitude higher density of target atoms on a surface compared with a gas, which is typically limited to <<1 torr pressure to allow for sufficient electron mean-free-path in the experiment. Finally, this result represents new physics - the extension of laser-assisted photoemission from atoms to the case of a surface. |
|
 |
