Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

with the most intense electron emission. Similar studies of solid metal nanoparticles using a range
of sizes (20 to 120 nm in diameter) are underway.
PEEM is a charged particle (photoelectrons) imaging technique and has therefore been applied
almost exclusively to conducting materials and in particular metals. We have found, however,
that wide-gap semiconductors and even insulating materials, in some cases, yield high quality
PEEM images. We have imaged 3 µm diameter polystyrene spheres supported on a thin metal
substrate illuminated by 400 nm (~ 3.1 eV) and 800 nm (~ 1.5 eV) femtosecond (fs) laser pulses.
Intense photoemission is generated by microspheres even though polystyrene is an insulator and
its ionization threshold is well above the photon energies employed. We observe the most intense
photoemission from the far side (the side opposite the incident light) of the illuminated
microsphere that is attributed to light focusing within the microsphere. The light focused though
the microsphere then propagates to the thin film surface where photoemission from the metal
substrate can be imaged. For the case of p-polarized, 800 nm fs laser pulses, we observe
photoemission exclusively from the far side of the microsphere and additionally resolve sub-50
nm hot spots in the supporting Pt/Pd thin film that are located only within the focal region of the
microsphere. We find that the fs PEEM images at both 400 and 800 nm can be modeled using
finite difference time domain (FDTD) electrodynamic simulations. The FDTD simulations
predict light focusing in the optically transparent microsphere and subsequent focusing of the
transmitted field on the supporting metal surface. The ability to obtain high resolution and high
contrast PEEM image from an insulator is attributed to photoinduced conductivity of the
polystyrene microspheres.
Future Directions
If exciton-based desorption can be generalized from alkali halides to metal oxides then selective
excitation of specific surface sites could lead to controllable surface modification, on an atomic
scale, for a general class of technologically important materials. While exciton-based desorption
is plausible for MgO and CaO, we note that the higher valence requires a more complex
mechanism. With the aid of DFT calculations we have developed such a hyperthermal desorption
mechanism that relies on the combination of a surface exciton with a three-coordinated surfacetrapped
hole, a so-called “hole plus exciton” mechanism. In every instance we have studied, a
hyperthermal O-atom KE distribution can be linked to an electronic surface excited state
desorption mechanism. In contrast, a thermal O-atom KE distribution clearly indicates a bulk
derived origin for desorption. In analogy to alkali halide thermal desorption, we have considered
a bulk-based thermal desorption mechanism involving trapping of two holes at a threecoordinated
site (a “two-hole localization” mechanism). Our calculations, however, do not
indicate that two-hole localization is likely without invoking a dynamical trapping process. Since
extensive calculations have not yielded an enduring theoretical model for thermal desorption we
are exploring possible nonthermal mechanisms that yield low kinetic energy particle desorption in
the “thermal energy” range. Initial experiments on CsBr thin films suggest intriguing possibilities
of near-thermal desorption from surface molecular anion centers (H centers). We will therefore
study low energy desorption from thin films of CsBr, CsI, and perhaps RbI grown on insulators
and metals. We also plan to grow and study other oxide surfaces in the near term such as ZrO2,
BaO, and ZnO.
PEEM combined with interferometric time-resolved two-photon photoemission is an
experimental method which offers both nanometer spatial and fs time resolution. Using these
techniques it should be possible to time-resolve PEEM images of localized surface plasmons for
various plasmonics nanostructures such as wires, spheres or particle aggregates. Using PEEM we
have directly recorded plasmonic enhancement factors, of core-shell particles, without the
presence of a molecular emitter as required using surface enhanced Raman scattering (SERS).
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