Abstract:
On account of high electron density, the Coulomb interaction in metals is efficiently screened on the length scale of interatomic distance and the time scale of inverse plasma frequency, i.e., typically in the sub-femtosecond regime. Nevertheless, transient excitonic interactions have been invoked in the interpretation of optical spectra and electron dynamics in noble metals. We explore the surface electronic structure of silver and copper surfaces by energy, momentum, and time-resolved multiphoton photoemission. Features observed in the near-resonant multi-photon photoemission spectra cannot be understood from the joint density of occupied and unoccupied states that are coupled by optical transitions and a Fermi Golden Rule transition rate formalism, suggesting that quasiparticle correlations, for example, transient excitonic interaction between photoexcited electron and hole propagating in the occupied and unoccupied surface states may affect the transition amplitudes and quasiparticle lifetimes as observed in momentum-resolved multi-photon photoemission spectra. Photoelectron energy-momentum distributions provide clear evidence for electron-hole pair correlation, and related excitonic interactions, on the time scale for screening in noble metals.

Biosketch:
Hrvoje Petek obtained BS degree in 1980 at MIT, and PhD in 1985 at U. C. Berkeley, both in Chemistry. His PhD research concerned the spectroscopy and dynamics of singlet methylene in the gas phase. As a Postdoctoral Fellow and later Research Associate he worked from 1985 to 1993 at the Institute for Molecular Science, in Japan, where he established a research program on ultrafast isomerization of molecules in supersonic molecular beams. Subsequently, he became a Group Leader at Hitachi Advanced Research Laboratory, in Saitama, Japan, where he initiated his current research on ultrafast surface dynamics. In 2000, he became a Professor of Physics, and later Professor of Chemistry, as well as the co-Director of the Petersen Institute for NanoScience and Engineering at the University of Pittsburgh. His research interests focus on the electronic structure and ultrafast electron and nuclear dynamics of clean and molecule covered metal and semiconductor surfaces. He is a pioneer in the coherent ultrafast spectroscopy and microscopy of solid-state materials. Petek has been an Alexander von Humboldt Senior Scholar at the Fritz-Haber Institute and Max Planck-Institute for Microstructure Physics, and a visiting professor in numerous universities throughout Europe and Asia. He is a Fellow of the American Physical Society, and since 2006, has been the Editor-in-Chief of Progress in Surface Science.