Multipole Excitation of Fullerene Molecules in a Semi-Classical Model
AuthorLamichhane, Krishna Prasad
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The local current approximation (LCA) is introduced and derived from a generalvariational principle. This approach serves as a semiclassical description of strongly collective excitations in finite fermion systems. Here applied for the first time to study the coupling of surface and volume dipole oscillations in the fullerene C60 molecules.The spectrum obtained for the coupling of the pure translational mode with compressional volume modes in the semiclassical LCA shows close agreement with the experimentally observed spectra. Applying the same approach to the photoionization cross section of C60, we discuss the results of higher multipole resonances of fullerene molecules. The comparison to data obtained from electron scattering experiments reveals the adequacy of the semi-classical approach as well as the collective nature of several high angular momentum resonances. Similar to the results of the dipole case, the coupling of surface and volume plasmon modes in C60 molecules is seen to shift both peaks slightly towards lower energies. Finally we study a simple mechanical model to describe the response of fullerene molecules to an oscillating electric field. In this model, we treat the pi and sigma electrons of fullerene molecules as two independent harmonic oscillators with differeing mass, electrical charge and restoring force, connected by a weak coupling. A third force couples the harmonic oscillators. We explicitly solve the model's equation of motion under the influence of a driving harmonic electric field and a variable damping term. The power consumption of the coupled system is calculated for a range of driving frequencies and damping terms. For an appropriate choice of parameters, the results of this mechanical model show similarity to the experimentally determined photo-excitation cross-section of C60 molecules.