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Femtosecond Dynamics of Electron Localization at Interfaces

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Science  09 Jan 1998:
Vol. 279, Issue 5348, pp. 202-205
DOI: 10.1126/science.279.5348.202

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Abstract

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

  • * Present address: Lucent Technologies, Bell Laboratories, 2000 Northeast Expressway, Norcross, GA 30071, USA.

  • To whom correspondence should be addressed. E-mail: harris{at}socrates.berkeley.edu

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