Study of Linear and Nonlinear Microscopic Polarizabilities of C 60
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STUDY OF LINEAR AND NONLINEAR MICROSCOPIC POLARIZABILITIES OF C60 ERIK WESTIN AND ARNE ROS9N Department of Physics, Chalmers University of University of Goteborg, S-412 96 GOteborg, Sweden
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ABSTRACT The low energy part of the linear optical response spectrum 3 and lowest order hyperpolarizability y( ) of C6 0 are calculated by a sum over state approach using single particle wavefunctions and. energy levels determined from local density calculations. Lorentz local field factors, as well as a RPA correction are introduced to facilitate comparison with the dielectric function £(W) determined from films of C60. y( 2 ) for a centrosymmetric molecule such as C6 0 is zero and the lowest non-zero contribution to the polarizability is y( 3 ). Reasonable agreement is found with linear optical response experiments if a RPA screening is used. However, SHG and THG experiments on C 6 0 in solid or solution form, yields values closer to the unscreened results.
INTRODUCTION As soon as macroscopic quantities of C 6 0 and C 7 0 fullerene molecules in solution or crystalline form became available after the development of the electric arc evaporation method of Kratschmer, Huffman and co-workers [1], very extensive research have been going on elucidating optical and transport phenomena of these new materials. The first resolved optical spectrum of C60 was taken by Kratschmer et al in 1990 [1] and later a number of refined spectra have been recorded on C 6 0 in different solvents, Ajie et al [2], Hare et al [3] and Leach et al [4]. Further fundamental optical properties as the dielectric function c((O) and refraction index (n,k) have been determined from EELS [5] and ellipsometric measurements on films [6]. A large third-order nonlinear optical response of C 6 0 , in solution or deposited as thin films, have also been measured by a number of groups [7-12]. The second-order response has also been found to be large [12), which may be due to the presence of oxygen in the film. The access to all these new experimental data and others [13] have given good possibilities to characterize the electronic and geometrical structure of the earlier discovered C6 0 or Buckminsterfullerene [14). Shortly after the proposal by Kroto et al. [14] of the existence of C 6 0 , we evaluated the excitation energies and oscillator strengths of the optical transitions using the CNDO/S-CI method [15]. These types of CI calculations have recently been extended, now including also catand anions [16]. Rather good agreement was obtained with some
Mat. Res. Soc. Symp. Proc. Vol. 270. @1992 Materials Research Society
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experiments by Heath et al. [17]. However, more interesting was our prediction of very strong transitions in the UV region and around 215 nm. Theoretical calculations are rather cumbersome since in in the principle one has to solve for a 360 electron system i.e. case of C 6 0 although very detailed information have been achieved from extended H~ckel calculations [18]. We used earlier the local density approximation, LDA, and evalua
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