Core charge

Abstract: A metallic electrode connected to electron reservoirs by tunnel junctions has a series of charge states corresponding to the number of excess electrons in the electrode. In contrast with the charge state of an atomic or molecular ion, the charge states of such an "island" involve a macroscopic number of conduction electrons of the island. Island charge states bear some resemblance with the photon number states of the cavity in cavity QED, the phase conjugate to the number of electrons being analogous to the phase of the field in the cavity. For a normal island, charge states decay irreversibly into charge states of lower energies. However, the ground state of a superconducting island connected to superconducting reservoirs can be a coherent superposition of charge states differing by two electrons (i.e. a Cooper pair). We describe an experiment in which this Josephson effect involving only one Cooper pair is measured.

Abstract: This report describes the effect of solvent refractive index and core charge on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters (MPCs). Solution spectra of dodecanethiolate MPCs (5.2 nm average diameter) reveal an 8-nm shift in plasmon band position as the solvent refractive index is varied from nd20 = 1.33 to 1.55. The spectral shift agrees with predictions of Mie theory when the alkanethiolate monolayer is accounted for in the calculations. Electronic charging of the MPC gold core by electrolysis, from its rest potential (−0.16 V) to +0.82 V vs a Ag quasi-reference electrode, causes a 9-nm (516 to 525 nm) shift in the surface plasmon band position. The shift in surface plasmon band position with core charge is compared to the predictions of the concentric sphere model for MPC capacitance and Mie theory, as a first step toward probing effects of quantized core charging on this important optical property.