Influence of hydrostatic pressure and temperature on two-photon absorptionof a C-60-2-thioxo-1,3-dithiole cycloadduct

Two-photon absorption (TPA) at lambda=532 nm versus applied hydrostatic pre ssure (p) (up to 20 GPa) and temperature (T) (in the 77-300 K range) was us ed to study intermolecular dynamics in powderlike fullerene-1,3-dithiolic a dduct and C-60 systems. For C-60-1,3-dithiole adduct, we observed a sharpli ke maxim of the TPA versus p-T below 230 K. Comparing the obtained results with those of C60, we have unambiguously shown that the observed behaviors are connected with the adding of the 1,3-dithiole core. To explain the expe rimental results, we performed ab initio molecular-dynamics geometry optimi zation, taking into account the superposition of all possible molecular con formations with appropriate weighting factors and higher-order intermolecul ar multipole interactions. We have shown an increase of the absolute value of the TPA coefficient in the C-60 cycloadduct by more than 30%. This fact is mainly caused by the asymmetry of intramolecular electronic charge-densi ty asymmetry. The modulatedlike dependence of the TPA versus p and T have b een observed. The latter one reflects the vibration and rotational contribu tions to the TPA values. [S0163-1829(99)07713-9].