Outline

Method: IR spectroscopy

• Mix of tabletop and large scale experiments
• Measuring optical transmission, reflection as the function of frequency.
• Two fundamental aspects:
  • molecular vibrations of simple and complicated molecules and
  • Example: linear molecule (Like CO₂ or CS₂)
  • free electron excitations

Materials: Fullerenes and fullerides

• Fullerenes discovered in 1985 by Curl, Kroto and Smalley
• Late breaking news
• The most abundant fullerene, the C₆₀ molecule has icosahedral molecular symmetry,
• Solid C₆₀ forms an approximate face centered cubic fcc structure
• Solid C₆₀ can be doped with alkali metals and other elements and molecules.

Modes in pure C₆₀

• Four IR allowed modes
• Spectrum of pure C₆₀ single crystal looks a bit more complex....
  • temperature dependence
  • pressure dependence
  • comparing the natural and C¹³ enriched single crystals
• Model system: Liquid CS₂
• Model calculation: Perturbed linear molecule
• Two sources of "extra" modes:
  • Many more modes than the four "allowed" IR active modes: symmetery breaking
  • Modes extend to high frequencies: anharmonicity
  • Not sensitive to C¹³, except for slight line broadening
  • Pressure dependence and temperature dependence suggests intramolecular coupling.
  • All 46 fundamental vibrational modes can be deduced.

Superconductivity in K₃C₆₀