The ability of time- and angle-resolved two-photon photoemission to estimate the size distribution of electron localization in the plane of a metal-adsorbate interface is discussed. It is shown that the width of angular distribution of the photoelectric current is inversely proportional to the electron localization size within the most common approximations in the description of image potential states. The localization of the n=1 image potential state for two monolayers of butyronitrile on Ag(111) is used as an example. For the delocalized n=1 state, the shape of the signal amplitude as a function of momentum parallel to the surface changes rapidly with time, indicating efficient intraband relaxation on a 100 fs time scale. For the localized state, little change was observed. The latter is related to the constant size distribution of electron localization, which is estimated to be a Gaussian with a 15+/-4 A full width at half maximum in the plane of the interface. A simple model was used to study the effect of a weak localization potential on the overall width of the angular distribution of the photoemitted electrons, which exhibited little sensitivity to the details of the potential. This substantiates the validity of the localization size estimate.

At present, commercially available thin-film photovoltaic cells are evaluated for terrestrial applications. To apply thin-film photovoltaic cells for space applications, the assessment of the solar cell performance must be conducted in simulated space conditions. We investigated the temperature coefficients of the I-V characteristics of thinfilm amorphous silicon (a-Si) solar cells manufactured by Uni-Solar and Iowa Thin Film Technologies, and CuInGaSe 2 (CIGS) solar cells manufactured by MicroSat Systems with simulated space solar radiation. The temperature coefficient of the thin-film solar cells between temperatures of 15 and 100 C was measured with a temperature-controlled vacuum plate. The vacuum plate ensures maximum thermal contact between the plate and the solar cell as well as reducing the thermal gradient in the solar cell. The vacuum plate also serves as a thermal reservoir that provides temperature stability during the performance evaluation when the solar cell is exposed to simulated sunlight radiation. An X-25 sunlight simulator calibrated for AM0 conditions provides the necessary radiation in performance characterization of the thin-film solar cell. The I-V characteristics of the solar cell were obtained at various temperatures to gain a thorough knowledge of the cell's performance at different temperatures.

The two photon photoemission technique has been utilized to investigate the adsorbate electron affinity dependence of interfacial electron transfer dynamics. The comparison of calculated and experimental results highlight a brief discussion of a dielectric continuum model. For the n-heptane/Ag(lll) and the benzene/ Ag(lll) interfaces, the model effectively reproduces experimental image potential state properties. The model fails to adequately describe the influence of anthracene adsorbates on image potential state lifetimes, as demonstrated by the experimental and theoretical results. The experimental n = 1 lifetime of 1200 femtoseconds and binding energy of -0.53 eV for the anthracene bilayer differ greatly from the calculated values of 40 femtoseconds and -1.4 eV. The application offemtosecond (fs) laser technology to two photon photoemission (TPPE) has transformed the experimental investigation of electron dynamics in metals and at metal surfaces. Indirect information has long been accessible via linewidth analysis of static spectra obtained from inverse photo emission, 1 and high resolution electron energy loss spectroscopy studies;' but the difficulty in distinguishing between phase and energy relaxation, as well as the complicating influence of inhomogeneous broadening, stresses the importance of directly measuring electron dynamics.The two photon photoemission technique populates unoccupied electronic states with a pump pulse and photoemits the excited state electrons with a probe pulse, as depicted in Fig. 1(a). The binding energy of the unoccupied electronic states can be determined by measuring the kinetic energy of the photoemitted electrons and then subtracting the energy of the probe pulse photon. The excited state lifetimes can be determined by measuring the number of electrons detected as a function of time delay between the arrival of the pump and the probe pulses at the sample. The direct observation of excited state lifetimes constitutes the most significant advantage of TPPE (see Fig. 1).Time resolved TPPE has been utilized to study many aspects of electron dynamics in metals and at metal surfaces. Numerous experimental investigations of hot electron dynamics have been conducted to date in an attempt to determine the influence of the metal band structure on electron dynamics.I" The phase resolved interferometric two photon photoemission studies of Petek and coworkers have directly determined the energy dependence of both population and phase decay for noble metal surfaces 3 As expected from Fermi liquid theory, electrons with energies near the Fermi level have the longest

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