Polymeric alkali fullerides are stable in air

Infrared transmission, electron spin resonance, and X-ray diffraction measurements show unambiguously that RbC$_{60}$ and KC$_{60}$ are stable in air, in contrast to Rb$_{6}$C$_{60}$ which decomposes rapidly upon exposure. The specimens studied transform into pure C$_{60}$ and other byproducts when heated above 100\dd C, approximately the temperature of the orthorhombic-fcc phase transition. The stability of these compounds raises the possibility of applying them as protective layers for the superconducting fullerides.Comment: Scheduled for publication in Appl. Phys. Lett. 66, 20 Feb. 1995, typeset in REVTEX v3.0 in LaTeX. Postscript file including all figures is available on WWW http://insti.physics.sunysb.edu/~mmartin/ under my list of publications, or will be e-mailed by request

Exciton annihilation studies in poly(p-phenylene vinylene)

To verify the excitonic nature of the light-emitting state in PPV, fluorescence intensities and decay lifetimes were investigated as a function of excitation intensity. The results agree with the behavior predicted by the molecular exciton model. In particular, exciton-exciton annihilation causes the fluorescence intensity to saturate and the fluorescence lifetime to shorten at high exciton densities. In addition, the exciton annihilation, and thus diffusion, coefficients are found to be relatively large, even at low temperatures, indicating that exciton migration is important in PPV. These results indicate that the fluorescent (photoluminescent) state in PPV is excitonic in nature. The results argue against the band model where high mobility at reduced temperatures is not expected because the light-emitting species, neutral bipolarons, are associated with large lattice distortions