The decay of valence satellite states in neon above the first double-ionization threshold has been studied experimentally and theoretically. Special emphasis was given to differentiate between two decay modes: valence Auger and valence-multiplet Auger decay. It is shown that the latter process is predominant in the low kinetic energy part of the spectrum. The main structures of this low-energy Auger spectrum could be designated by help of calculated transition energies and decay rates.PACS number(s): 32.80.Fb, 32.80.Hd, 32.80.0z, 33.60.Cv Double ionization of valence shells in atoms and molecules has recently attracted considerable interest [l -3].The reason for this recovered, still increasing interest is due to the existence of a delicate balance of competing processes contributing to the valence-shell double ionization. These processes are known as simultaneous and sequential or direct and indirect processes. Whereas the first experiments on double ionization assumed almost all intensity of doubly charged ions resulting from singlephoton ionization as due to a simultaneous or direct double-ionization process, the so-called shakeoff process, the more recent studies showed that this is not true: sequential or indirect processes contribute significantly to the valence double-ionization rate [4 -6]. Since that time, experimental and theoretical studies have tried to explore this phenomenon in more and more detail. Armen and Larkins [7] were the first to our knowledge who showed on the basis of neon data from Becker et al. [4] that the sequential processes split into two distinct decay modes: inner-valence and valence multiplet-changing Auger transitions. In both cases the excited electron does participate in the transition process, corresponding to decays where the excited core changes either its electronic configurations [for instance, Ne+ 2s2p ( P)np( S) Ne + 2s 2p ('S+'D)+e ] or its multiplet coupling [such as Ne 2s 2p ('S)np( P)~Ne + 2s 2p ( P) +e ], for example, in a spin-fiip process, using hypothetical one-electron levels for the sake of simplicity. Figure 1 shows the different processes schematically for the case of neon. The theoretical and experimental interest is focused on the latter process for two reasons: (i) its high transition probability, and (ii) the occurrence of electrons with very low kinetic energy.The valence multiplet-changing transition, also referred to as nonresonant autoionization, may, in the extreme, give rise to the ejection of 0-V kinetic energy Auger electrons. Figure 2 shows a complete electron spectrum of neon taken at a photon energy of hv=120 eV exhibiting clear evidence for all three processes: simultaneous double ionization, inner-valence Auger transitions, and valence multiplet-changing Auger transi-Valence Auger Valencemultiplet Auger 3s I zrrnzizz nI', '/( 8/ /// / AX/8 / 'il it' 2p n iL / / / / / / / /, // 2s C FIG. 1. Simplified schematic representation of neon inner-valence 2s photoionization with accompanying excitation together with possible subsequent recombin...

Isolated photosystem I (PSI) reaction center/core antenna complexes (PSI-40) were platinized by reduction of [PtCl6]2- at 20 degrees C and neutral pH. PSI particles were visualized directly on a gold surface by scanning tunneling microscopy (STM) before and after platinization. STM results showed that PSI particles were monomeric and roughly ellipsoidal with major and minor axes of 6 and 5 nm, respectively. Platinization deposited approximately 1000 platinum atoms on each PSI particle and made the average size significantly larger (9 x 7 nm). In addition to direct STM visualization, the presence of metallic platinum on the PSI complexes was detected by its effect of actinic shading and electrostatic shielding on P700 photooxidation and P700+ reduction. The reaction centers (P700) in both platinized and nonplatinized PSI-40 were photooxidized by light and reduced by ascorbate repeatedly, although at somewhat slower rates in platinized PSI because of the presence of platinum. The effect of platinization on excitation transfer and trapping dynamics was examined by measuring picosecond fluorescence decay kinetics in PSI-40. The fluorescence decay kinetics in both platinized and control samples can be described as a sum of three exponential components. The dominant (amplitude 0.98) and photochemically limited excitation lifetime remained the same (16 ps) before and after platinization. The excitation transfer and trapping in platinized PSI-40 was essentially as efficient as that in the control (without platinization) PSI. The platinization also did not affect the intermediate-lifetime (400-600 ps) and long-lifetime (> 2500 ps) components, which likely are related to intrinsic electron transport and to functionally uncoupled chlorophylls, respectively. The amplitudes of these two components were exceptionally small in both of the samples. These results provide direct evidence that although platinization dramatically alters the photocatalytic properties of PSI, it does not alter the intrinsic excitation dynamics and initial electron transfer reactions in PSI.

Surface plasmons excited by a Nd: YAG (YAG, yttrium aluminum garnet) laser in an attenuated-total-refiection (ATR) geometry were used to desorb Al atoms from smooth Al films.The ejected Al atoms were ionized by a pulse-excimer laser and measured by time-of-flight mass spectroscopy. The desorption rate exhibits a sharp maximum at the surface-plasmon resonance angle. The experiment demonstrates that surface-plasmon-induced desorption can be accomplished on smooth surfaces. Since ATR excitation allows the use of a wide range of wavelengths, this method will be useful in the study of electronic desorption processes.Photodesorption by laser light has been accomplished by rapid surface heating, '2 by coupling to phonon modes, and by coupling to electronic excitation processes." Electronic excitation has attracted much interest recently, since it is a nonthermal mechanism that may avoid the fragmentation of desorbed molecules. Hoheisel et al. have observed desorption of metal atoms when laser light is coupled to the surface plasmon modes of small metal particles. "We report here the desorption of Al atoms from smooth Al films by surface plasmons excited by a Nd:YAG (YAG, yttrium aluminum garnet) laser in an attenuated total reflection (ATR) geometry. In contrast to the earlier work where plasmon coupling occurs only at selected wavelengths determined by the particle size and the metal's electron density, ATR excitation allows the study of plasmon-induced desorption on smooth surfaces over a broad range of frequencies below the plasmon frequency of the metal film. The existence of a tunable plasmon excitation mechanism that generates strong fields and field gradients at the surface will provide a novel and powerful tool for the study of electronic desorption mechanisms on surfaces.In the ATR method, a thin metal film is deposited on the base of a prism (or semicylinder). Surface plasmons are generated at the metal-vacuum interface when ppolarized light is incident through the prism onto the film at angles just above the angle for total internal reAection. ' The plasmon coupling occurs over a very narrow range of angles that varies with the frequency of the light. The plasmon excitation is most easily observed as a strong minimum in reflectance measured as a function of incident angle, ' but has also been observed in photoemission' and in other processes such as surface-enhanced Raman spectroscopy (SERS) that are sensitive to the strong surface fields associated with the plasmon excitation. In the experiments reported here, the existence of the plasmon-induced desorption mechanism is confirmed by comparing the angular dependence of the plasmongenerated peak in optical absorption with that of the desorption rate. The samples in our experiment were prepared by evaporating 99.999% aluminum onto the base of a glass prism. The data reported here were taken on a 27-nm film, as measured by a quartz-crystal thickness monitor. The desorption experiments were performed in a cryopumIied vacuum system with a base pressure of less th...

Photosynthetic reaction centers are integral plant membrane protein complexes and molecular photovoltaic structures. We report here that addition of Photosystem I (PSI)-proteoliposomes to retinoblastoma cells imparts photosensitivity to these mammalian cells, as demonstrated by light-induced movement of calcium ions. Control experiments with liposomes lacking PSI demonstrated no photosensitivity. The data demonstrate that PSI, a nanoscale molecular photovoltaic structure extracted from plants, can impart a photoresponse to mammalian cells in vitro.

In oxygenic plants, photons are captured with high quantum efficiency by two specialized reaction centers (RC) called Photosystem I (PS I) and Photosystem II (PS II). The captured photon triggers rapid charge separation and the photon energy is converted into an electrostatic potential across the nanometer-scale (~6 nm) reaction centers. The exogenous photovoltages from a single PS I RC have been previously measured using the technique of Kelvin force probe microscopy (KFM). However, biomolecular photovoltaic applications require two-terminal devices. This paper presents for the first time, a micro-device for detection and characterization of isolated PS I RCs. The device is based on an AlGaN/GaN high electron mobility transistor (HEMT) structure. AlGaN/GaN HEMTs show high current throughputs and greater sensitivity to surface charges compared to other field-effect devices. PS I complexes immobilized on the floating gate of AlGaN/GaN HEMTs resulted in significant changes in the device characteristics under illumination. An analytical model has been developed to estimate the RCs of a major orientation on the functionalized gate surface of the HEMTs.