EVALUATION OF LINEAR AND NONLINEAR MICROSCOPIC POLARIZABILITIES OF C60

Abstract

The electronic structure of C ₆₀ and some of its doped compounds have earlier been investigated at the level of the local density approximation, LDA, using a numerical LCAO approach. In this study, single particle wavefunctions determined from ground state LDA calculations are used for evaluation of dipole matrix elements which combined with a sum over state approach yield linear and nonlinear microscopic polarizabilities γ⁽¹⁾ and γ⁽³⁾, respectively. Lorentz local field factors, as well as a simple RPA type correction are introduced to facilitate comparison with the dielectric function ε(ω) determined from films of C ₆₀. γ⁽²⁾ for a centrosymmetric molecule such as C ₆₀ is zero and the lowest non-zero contribution to the polarizability is γ⁽³⁾. Reasonable agreement is found with results from linear optical response experiments i.e. ellipsometric and EELS measurements on C ₆₀ in solid or solution form. SHG, THG and DFWM experiments yield values close to the unscreened result, while invoking a RPA screening results in a non-resonant value about 2 orders of magnitude lower than most experiments.