Mechanisms of Multiphoton Photoemission from Metal Surfaces Xuefeng Cui, Cong Wang, Adam Argondizzo, and Hrvoje Petek Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA 15260 USA [email protected] Abstract Multiphoton photoemission (MPP) from metal surfaces with energy, momentum, time, and resolution enables probing the electronic structure, optical response, and electron dynamics at metal surfaces. We discuss recent progress in understanding of the MPP excitation process for Ag(111) and Cu(111) surfaces using tunable femtosecond laser excitation and photoelectron E(k) imaging. Introduction Interferometric time-resolved two-photon photoemission (ITR-2PP) has proven capable of measuring quasiparticle correlations in noble metals.1 For instance, the dephasing of Shockley surface state and top of the d-band holes in copper has been studied in a nonresonant 2PP process with 3.1 eV, 10 fs excitation.2,3 Modeling of the coherent 2PP process by optical Bloch equations proved inadequate to reproduce experimental data, suggesting that the coherent polarization dynamics in a metal are more complex than could be reproduced by a simple excitation scheme treating the optically coupled levels at the momentum of observation. We reexamine MPP processes from Shockley surface (SS) states of Ag(111) and Cu(111) surfaces with a broadly tunable <15 fsexcitation NOPA source for various resonance conditions using 3D (energy, momentum, and time resolved) MPP. Results and Discussion Three and four photon photoemission processes are excited with a 1-2 MHz repetition rate fiber-pumped NOPA system (Clark MXR). Photoemitted electrons are detected with an energy and momentum imaging photoelectron spectrometer (SPECS). The degenerate interferometric pump-probe measurements are taken by splitting the excitation pulses into two replicas in a Mach-Zehnder interferometer; acquiring photoelectron images in ~100 as delay intervals obtains movies of the coherent polarization dynamics excited in the single crystal metal samples. Ó Springer International Publishing Switzerland 2015 J.-Y. Bigot et al. (eds), Ultrafast Magnetism I, Springer Proceedings in Physics 159 DOI 10.1007/978-3-319-07743-7_86 278 Mechanisms of Multiphoton Photoemission from Metal Surfaces 279 Figure 1 shows the MPP spectra of Ag(111) surface excited with and 1.81 and 2.20 photon excitation showing dispersive bands due to the initial SS and intermediate image potential (IP) states at -0.063 and 3.79 eV relative to the Fermi level (EF). The range of SS is limited to |k|||=0.07 Å-1by its intersection with EF. The band dispersions are consistent with the known band structure of Ag(111). Tuning the laser into two-photon resonance between the SS and IP state causes dramatic changes in the MPP spectra, which can be interpreted through strong surface electron-hole correlation. 3D photoelectron measurements show evidence for this correlation in the time domain. In short, the MPP process is found to be driven by the local field of the correlated state. Fig. 1.E(k) images of MPP from Ag(111) surface excited with 1.81 and 2.20 eV light. Similar experiments have suggested that the intermediate “virtual” state in nonresonant 2PP from SS of Cu(111) surface has a finite “lifetime”, which is inconsistent with its detuning from real states.2 Measurements over a broad excitation photon energy range suggest that the apparent slow dephasing of the linear polarization is associated with the coherent polarization induced in the sample at E(k) regions that are distant from that of the observation. Apparently, the coherent polarization dynamics detected at the SS feature reflect the optical response of bulk Cu, and coupling between the surface and bulk polarizations. The detailed 3D measurements on Ag and Cu provide unprecedented view into optically induced correlation in noble metals, which is hidden in the linear optical response. The lessons learned from noble metals are applicable to more complicated materials where novel properties emerge from quasiparticle correlations. Acknowledgments This research was supported by Division of Chemical Sciences, Geosciences, and biosciences, office of Basic Energy Science of the U.S. Department of Energy through grant DE-FG02-09ER16056. 280 X. Cui et al. References [1] H. Petek, S. Ogawa,“Femtosecond Time-Resolved Two-Photon Photoemission Studies of Electron Dynamics in Metals”Prog. Surf. Sci. 56, 239-310 (1997). [2] S. Ogawa, H. Nagano, H. Petek, A. P. Heberle, “Optical Dephasing in Cu(111) Measured by Interferometric Two-photon Time-resolved Photoemission”Phys. Rev. Lett.78, 1339-1342 (1997). [3] H. Petek, H. Nagano, S. Ogawa, “Hole Decoherence of d-Bands in Copper”Phys. Rev. Lett.83, 832-835 (1999).