Two-photon absorption (TPA) at $\lambda =532\mathrm{}\mathrm{nm}$ versus applied hydrostatic pressure (p) (up to 20 GPa) and temperature (T) (in the 77–300 K range) was used to study intermolecular dynamics in powderlike fullerene-1,3-dithiolic adduct and ${\mathrm{C}}_{60}$ systems. For ${\mathrm{C}}_{60}$-1,3-dithiole adduct, we observed a sharplike maxim of the TPA versus $p-T$ below 230 K. Comparing the obtained results with those of ${\mathrm{C}}_{60},$ we have unambiguously shown that the observed behaviors are connected with the adding of the 1,3-dithiole core. To explain the experimental results, we performed ab initio molecular-dynamics geometry optimization, taking into account the superposition of all possible molecular conformations with appropriate weighting factors and higher-order intermolecular multipole interactions. We have shown an increase of the absolute value of the TPA coefficient in the ${\mathrm{C}}_{60}$ cycloadduct by more than 30%. This fact is mainly caused by the asymmetry of intramolecular electronic charge-density asymmetry. The modulatedlike dependence of the TPA versus p and T have been observed. The latter one reflects the vibration and rotational contributions to the TPA values.

• Received 9 February 1998

DOI:https://doi.org/10.1103/PhysRevB.59.9229