Third-order nonlinear spectra in C60 and C70: reconciling electroabsorption third-harmonic generation and degenerate four-wave mixing spectra

摘要

We present an overview of what has been learned form nonlinear optical spectral studies in C$-60$/ and C$-70$/ by our group and others. These include third harmonic generation (THG), degenerate four-wave mixing (DFWM), and electroabsorption (EA). We present empirical models of interacting states consistent with spectra of EA, THG, and DFWM in C$-60$/ and C$-70$/. The spectra in the dispersed and solid forms differ significantly. In C$-60$/ the lowest- energy resonances in $chi$+(3_$/ are dominated by 'forbidden' transitions made allowed by vibronic coupling in the molecule and intermolecular coupling in the solid. These interactions must be included in theoretical models of C$- 60$/ if comparison is to be made between calculated and experimental nonlinear optical spectra. Major features of $chi$+(3)$/ spectra in C$-70$/ can be modeled with similar states redshifted from those of C$-60$/.