We have studied electroabsorption (EA) in both films and dispersed molecules of C6o and C7o. In contrast to a recent model for the third harmonic generation (THG) spectrum, we find that the two-photon states important to the nonresonant XΒ°)(0) in molecular C6o are above the 3.6 eV T~, state, not below it. We present a model consistent with spectra of EA, THG and degenerate four-wave mixing (DFWM). The lowest-energy resonances in XO)(to) are dominated by 'forbidden' transitions made allowed by vibronic coupling in the molecule and intermolecular coupling in the solid.
Nonlinear optical spectroscopies provide a valuable study of the electronic structure of C6o and C70, and are a foundation for possible applications of fullerene materials to optical devices. Large thirdorder nonlinear susceptibilities X O) have been measured (see recent references in Ref. [1]) in films and solutions at scattered photon energies. Recent thirdharmonic generation (THG) spectra of C6o films have been reported: Meth et al. [2] observed a peak at 0.95 eV, and proposed a two-state model with a three-photon resonance to the absorption peak at 2.8 eV. Kajzar et al. [1] reported peaks at both 1.2 and 0.95 eV, and proposed a three-level model with a three-photon resonance at 3 to = 3.6 eV (first allowed molecular transition), and a two-photon resonance at 2 to = 1.9 eV (lowest singlet state [3]). THG spectra in C70 were likewise modeled with a three-level system [4]. A degenerate four-wave mixing (DFWM) study [5] in solid C6o between 1.4 and 1.7 eV has found the signal decreases toward higher energies, signifying a resonance below 1.4 eV.
Electroabsorption (EA) probes the same interactions between excited states as does THG and DFWM, but displays only one-photon resonances. EA has the advantage of a broad, continuous energy range, good signal to noise, and phase information difficult to obtain from pulsed laser spectroscopies. EA in solid C6o was measured [6,7] soon after thin films of C60 were available. EA of C6o in polysilane was reported [8], but the polysilane EA masked most of the C6o EA.
Here we report EA in both dispersed and solidstate C60 and C7o. We present a model that is consistent with the available EA, THG and DFWM spectra. The results show that states above 3.6 eV determine the nonresonant XΒ°)(0) in the molecule. Dipole-forbidden states above 2.3 eV are prominent in the resonant XΒ°)(to) spectra in the molecule and