Stability of rare gas cluster ionsPhotoionization efficiency curves of rare gas clusters generated by supersonic expansion of pure argon, krypton, and xenon gas have been measured as a function of photon energy using light from the Berlin Electron Synchrotron Facility (BESSY). Assuming that the relative shapes of the efficiency curves are independent of cluster size, ionization potentials for each cluster can be extracted from the data by using the known absolute ionization thresholds of the dimer or trimer ions. The data agree with the model that ionic clusters are composed of an ionic substructure surrounded by shells of atoms. In particular, it has been confirmed that Ar 3+ is the central substructure in Ar n+ for n < 15 ± 1. In addition we have estimated the ratio of the covalent binding energy to the charge-induced dipole binding energy for the ions. The larger dipole-induced binding energy in the case of xenon may explain the differences between the mass spectra of Xe and Ar clusters.

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With the use of near-edge x-ray-absorption fine structure spectroscopy (NEXAFS), the unoccupied n* and o* bands of poly-(3-methylthiophene) electrochemically grafted on to a Pt surface have been probed. The C E-edge NEXAFS characteristics confirm that (i) the polymeric chain unit is composed of the same structural architecture as the monomer and (ii) the doping proceeds via a narrowing of the band gap with the appearance of metalliclike behavior. The same tendencies are observed by varying the structure of the monomer and the nature of the dopant which evidences a similar conduction mechanism for these poly-five-membered-heterocycles.Polythiophene and its derivatives have been extensively studied during recent years because of their high stability against oxygen and moisture compared to other organic conducting polymers. ' Such a property has been put forward for practical applications in the field of catalysis, energy storage, or photovoltaic conversion.Moreover, by varying the structure of the monomer and the nature of the dopant, long-range order and partial crystallization have been obtained. Several theoretical models have been proposed to explain the large change in conductivity between the doped and undoped states. Based essentially on the appearance of localized levels in the gap, they involve spinless bipolarons, related to local lattice deformation s.X-ray (XPS) and ultraviolet (UPS) photoemission spectroscopy data have shown that (i) a tr-bonding band located at 1.1 eV below the Fermi level is formed during the coupling of the monomeric units, in good agreement with theoretical calculations and (ii) doping proceeds through uniform extraction of z-bonding electrons resulting in a band-gap narrowing. In order to confirm these electronic characteristics, we have probed the unoccupied levels of poly-(3-methylthiophene) (PMeT). We report, in this Rapid Communication, the evolution of the z* and o* bands of thin PMeT films grafted on Pt by using near-edge x-ray-absorption fine-structure (NEXAFS) spectroscopy around the C K edge as a function of the doping level.Films of PMeT were electrochemically deposited on a Oat Pt electrode immersed in a CH3CN-5 x 10 'M LiC104-5X10 M 3-methylthiophene electrolytic medium. The electrode was polarized at +1.35 V relative to a saturated calomel electrode (SCE) for obtaining the 30%-C104 fully doped film. Intermediate-doped (15% and 3%) and fully undoped films were synthesized by changing the potential from +1.35 to +0.8, +0.1 and -0.2 V with respect to SCE, respectively. Film thicknesses were adjusted between 500 and 1000 A according to the chosen deposition time. Samples were rinsed with acetone, dried in a N2 stream, and then transferred through a series of interlocks into an ultrahigh-vacuum sample chamber.NEXAFS experiments were carried out at the electron storage ring BESSY-Berlin using the high-energy TGM2 beam line' with a resolution of 0.5 eV at the C K edge.The incident photon beam Io is monitored by collecting the total electron yield from a high-transmission met...

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