Deviations from Integer Power Laws in fs-Laser Excited PEEM
Due to Coexisting Electron Emission Mechanisms
A. Gloskovskii, D. Valdaitsev, S. A. Nepijko, G. Schönhense
Institut für Physik, Johannes Gutenberg-Universität, D-55099 Mainz, Germany
The observation of non-integer (i. e. fractional) power laws observed in fs-laser excited
PEEM has been a puzzle [1,2] because it is obviously in conflict with the concept ofmultiphoton photoemission (nPPE) predicting Ielectron∼ Iphotonn (n being integer). In thiscontribution we discuss results for well-defined Ag nanocluster films deposited on Si(characterized by SEM, AFM and HiRes-TEM). Excitation by the fundamental (hv=1.451.65eV) and first harmonic (hv=2.9-3.3eV) of a Ti:sapphire laser excludes 1PPE due to theworkfunction of the clusters being >3.8eV. The cluster films deposited as stepped wedges
appear bright in the PEEM images, see Fig.1a. Corresponding cluster sizes as determinedby HR-TEM (see inset) vary from 3-4nm in stripe 1 up to >50nm in stripe 4. Stripe 5 is apercolated film followed by bulk silver.
Electron kinetic energy spectra have been measured as function of photon energy andintensity (i.e. power of the fs-laser pulse) using an imaging high-pass energy filter oralternatively a time-of-flight technique as described in [3]. Upon variation of the photon
energy of the first harmonic, the Fermi level onset in the spectra shifts by 2∆hv inaccordance with 2PPE, see Fig. 1b. However, the power dependence is non-uniform overthe spectra, see Fig. 1c. We discuss several alternative emission mechanisms that may
become significant for certain conditions. The transient “hot” (non-Fermi-Dirac [4])electron gas confined in small clusters may cause possible changes in the optical constants,
absorption coefficients, matrix elements of the electron transition probability andparameters in the Richardson-Dushman equation. Plasmon-mediated nPPE is special
because the localized surface plasmon enhances the brightness in PEEM due to the nearfield [5] and “saves” photons for several fs leading to a high concentration ofelectromagnetic energy in nanometer-sized clusters. Finally, optical field emission may
occur at very high laser power (more precisely, local electrical field). For excitation at
hv=1.45-1.65eV the local intensity vs laser power curves show different regimes withdifferent non-integer slopes. Fermi level onset shifts by 2-2.5∆hv when increasing the laser
frequency and appears significantly smeared out in electron energy spectra.

Fig.1: (a) PEEM image of Ag cluster films with varying mass thickness (stripes 1 to 5 correspond to 1, 2, 5,
10 and 20nm, respectively) for fs-laser excitation at hv=3.1eV; the inset shows a HR-TEM image of two
individual clusters. (b) Kinetic energy spectra of stripe 2 taken at different photon energies. (c) Ratio ofenergy spectra taken at hv=2.9eV with two different laser powers (normalized at the maximum at 4 eV).
[1] O. Schmidt et al., Surf. Sci. 482-485 (2001)687; G.H. Fecher et al., J. El. Spectr. Rel. Phen. 126 (2002) 77[2] J. Ernst, private communication[3] M. Cinchetti, Appl. Phys. Lett. 83 (2003) 1503[4] B. Rethfeld et al., Phys. Rev. B 65 (2002) 214303[5] M. Cinchetti et al., Phys Rev. Lett. 95 (2005) 047601

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