EQUILIBRIUM PHASES IN K-DOPED AND RB-DOPED C(60) FROM IN-SITU INFRARED REFLECTIVITY MEASUREMENTS

In situ IR reflectivity was used to study the doping of thin C60 films with K and Rb by a load and equilibrate procedure. Nearly pure phases of AxC60 for x = 1, 3, 4, and 6 were obtained in this way and the IR spectra were characterized. A quantitative analysis of the reflectivity spectra using a multilayer response function revealed the line positions, linewidths and the oscillator strengths for the F1u modes. The second-lowest mode and the highest mode showed good agreement with the charged phonon model with respect to line shift and absorption intensity and were thus assigned as charged phonon modes. The coupling constants for these modes to the virtual t1u --> t1g transition were found to be of the order 0.01. This value is smaller but not negligible compared to the coupling of the gerade modes with A(g) and H(g) symmetry. Thus, the coupling from the virtual t1u --> t1g transition may contribute as well to the pairing process in the superconducting phase.