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Communications presence of these levels is likely to be responsible for the filling of the "valley" near 9 eV in the spectrum of PPV.In conclusion, it has been shown that exposure of PPV films to air and in the dark lead to inclusion of water molecules in three different types of site. In particular, the formation of complex, C-like structures leads to increases in torsion angles in such a way as to reduce the conjugation and increase the HOMO-LUMO band gap. The effects of the geometrical changes, and the associated electronic structural changes, are clearly seen in the UPS measurements. Effects seen in the optical absorption spectra are consistent with the occurrence of water-induced changes in the frontier electronic structure, but appear to be more complicated than the UPS data and the modelling might suggest. The effects reported are completely reversible, disappearing upon heat treatment of PPV in UHV. A recipe for preparing clean PPV films is proposed. ExperimentalThe PPV samples were prepared from a soluble THT-leaving group precursor polymer [14] obtained from Cambridge Display Technology, Cambridge. UK. Thin films of thickness about 10,20, or 30 nm were spin-coated onto optically flat Si(ll0) wafers, and converted at 240°C for 10 h under a mild vacuum of 5 x mbar. Samples were then sealed in dry nitrogen gas for shipment to Linkoping. For subsequent studies, "as-prepared", "clean" films were made by heating each sample at 150 "C overnight in a vacuum of better than 10. ' mbar, prior to measurements, as discussed previously [9]. A complete set of XPS and UPS valence band spectra were recorded, establishing the quality of the films. In the XPS data, the residual oxygen was found to be

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We investigate experimentally the transition to phase synchronization in coupled Nd:YAG lasers. As the coupling strength increases, the phase difference of two chaotic laser outputs develops from nonsynchronous to a phase-synchronous state via +/-2pi phase jumps. We analyze this transition.

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