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The quantitative description of low kinetic energy photoelectron emission in semiconductors is still an open question. In this article a model is developed to simulate the photoexcitation and transport of low kinetic energy electrons in Cs2Te. The statistical extension of the model, by Monte Carlo trajectory calculations, gives photon energy dependent quantum yields in agreement with experimental data. This is regarded as evidence that for near threshold photoemission in Cs2Te the dominant scattering mechanism is electron-phonon, while the mean energy scattering energy loss is 5 meV and the electron mean free path is 3 nm. The spatial distribution and the time response (0.4 ps) of the photoemitted electrons are estimated.

Nonlinear photoelectric emission from Cu and W is investigated by subpicosecond laser pulses. Using a pump and probe technique, total-yield correlation measurements are obtained without space-charge saturation. The effects of the heating of the conduction electrons induced by the laser pulses are shown, considering the peak to background contrast ratio of the experimental correlation trace. The present analysis clarifies the consequences of a transient nonequilibrium temperature difference between electrons and the lattice on the correlation trace, confirming for W the presence of a thermally assisted photoemission mechanism. @S0163-1829~97!05628-2#

The total charge emitted from a polycrystalline Mo surface by 500 fs-264 nm laser pulses has been measured. Though a one-photon photoelectric effect is expected, a non-linear increase of the photoelectric yield was observed as a function of laser peak intensity, confirming earlier observations on Au, W and Zr. The threshold intensity for this non-linearity is 2 between 0.1 and 0.2 GW/ cm . The linear and non-linear regimes were clearly discerned in the experimental data. The non-equilibrium heating of the conduction electrons is considered as the cause of the observed non-linear behaviour. © 1999 Elsevier Science B.V. All rights reserved.

The morphological and mechanical properties of nanoparticles-based ultrathin Ag films, synthesized by supersonic cluster beam deposition over a sapphire substrate, are unveiled exploiting ultrafast optoacoustic, atomic force microscopy, X-ray photoelectron spectroscopies, and X-ray diffraction techniques. The films, with thicknesses in the 10–50 nm range, have a porous structure composed of metallic Ag nanoparticles with a crystalline structure and average diameter of 6 nm. The films acoustic modes are in the hypersonic frequency range, the thinner films frequencies exceeding 100 GHz. The acoustic spectra are well accounted for modeling the nanoparticles film as an effective continuous medium. The modes quality factors show the existence of acoustically quasi-dark and bright states. The film effective density and effective elastic stiffness constants are respectively 0.8 and 0.5 that of bulk Ag. The present results are relevant in view of applications for optoacoustic transducers in the hypersonic frequen...

The total charge emitted from a polycrystalline Mo sample by 500 fs laser pulses at normal incidence is measured as a function of the laser peak intensity. Total yield data are taken at wavelengths of 527 and 264 nm. In both cases, a nonlinearity higher than expected is measured. A thermally enhanced regime is clearly observed when using 264 nm pulses for laser peak intensity larger than 0.1--0.2 ${\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. This effect is interpreted on the basis of the nonequilibrium heating of the conduction electrons. Pump and probe photoemission data at 527 nm show a significant enhancement of the photoelectric sensitivity when the probe pulse is delayed by 1 ps from the pump. This enhancement is related to the growth of the available electron density induced by the nonequilibrium heating. Single pulse photoemission at this wavelength is not properly explained by a thermally assisted photoemission regime. This may indicate that other processes have a role in determining the photoemission yield.

The total yield photoemission of a polycrystalline molybdenum sample irradiated by 790 nm–150 fs laser pulses is measured in the absence of space charge saturation. The extracted charge versus the laser intensity, measured over three decades of the extracted charge, which represents a significant extension of the previous experiments, gives us the opportunity to test the so-called thermally assisted non-linear photoemission model. The present results show non-linear photoemission behavior that can be explained, as expected, by a thermally assisted 3-photon emission. However a pure 4-photon emission could not be discarded. To take aim at clarifying this question the related non-linear coefficients are calculated and a scaling law for the probability of different multiphoton orders is proposed.

We measure the transient reflectivity of an high-Tc superconductor with an unprecedented temporal resolution (~15fs) demonstrating that on the 20-fs timescale electron dynamics is described in terms of charge carriers interacting with short-range antiferromagnetic fluctuations.

Ultrafast spectroscopy techniques tailored to image/detect thin films and surface molecular complexes are expected to become fundamental tools in various scientific and technological contexts. Here we review the advances of specific surface-sensitive ultrafast spectroscopies aimed at developing surface biomolecular sensors.