Time-resolved Spectroscopic Studies on the basic Photo-induced Processes in C_<60> Single Crystals

研究了原始C_<60>单晶在90K以下低温相的光致发光和近带边电子结构。利用双光子激发光谱，从几种晶体中提取出单态光谱，这些晶体在普通单光子激发下表现出明显的多样性。在1.815 eV以下的1.685 eV处，存在具有离散子峰的局域吸收带。通过位置选择性激发进入局部吸收带获得的发光线窄化光谱的总体特征被分配给单个C60分子（即8个h_g， 2个a_g及其组合）的拉曼活性阶对称模式耦合的振动进展，伴随着由于协同振动和分子间平移而产生的声子侧带。本文讨论了光发光的起源及其弛豫动力学，并讨论了非均匀展宽局域态，这些局域态可能是由相邻的本征缺陷或晶格失序降低对称性稳定的扰动Frenkel激子引起的。

Photoluminescence and near-band-edge electronic structure of pristine C_<60> single crystals have been investigated in the low temperature phase below 90K. By applying two-photon excitation spectroscopy, a monomorphic spectrum is extracted from the several crystals which exhibit apparent diversity for ordinary one-photon excitation. Also, a localized absorption band with discrete vibronic sub-peaks is clearly confirmed at 1.685 eV below the fundamental absorption edge (1.815 eV). Overall features of the luminescence-line-narrowing spectrum obtained by site-selective excitation into the localized absorp tion band are assigned to the vibronic progressions coupled with the Raman-active gerade-symmetry modes of individual C60 molecules (i.e., eight h_g, two a_g, and their combinations), accompanied by phonon-side-bands due to cooperative librations and intermolecular translations. The origin of photo luminescence and its relaxtion kinetics are fully discussed in connection with the inhomogeneously broadened localized states which are presumably due to perturbed Frenkel excitons stabilized with symmetry-lowering by adjacent intrinsic defect or some lattice disorder.