Investigation of optical spectra of small numbers of molecules in condensed matter

最近，我们用位置选择光谱的方法研究了聚合物、玻璃和蛋白质中分子的光谱，取得了一些有意义的结果。然而，通过这些技术获得的信息仅限于具有相同光学跃迁能量的许多分子的系综平均值。为了获得更详细的信息，我们在2-5K温度范围内测量了少量丙叉分子在十六烷中的荧光激发光谱。用Ar~(++)激光泵浦的单模染料激光器作为激发源。单分子光谱的峰值频率与外加压力呈线性关系。由此确定十六烷的压缩系数为0.052 GPa^&lt；-1&gt；光谱宽度的温度依赖于每个分子，观察到的非线性依赖关系可以用二能级系统与客体分子的耦合来解释。另一方面，从理论上预测了少量分子基团的贡献表现为染料掺杂凝聚态吸收光谱中持续烧孔的光谱形状。我们在H_2取代的肌红蛋白中进行了温度循环烧孔实验，发现在4-70K温度范围内，蛋白质分子中只有一个分子基团对应于发生翻转的二能级体系。我们进一步测定了肌红蛋白中血红素电子与二能级体系之间的耦合强度。

Recently, we have obtained a number of significant results on optical spectra of molecules in polymers, glasses and proteins by means of site-selective spectroscopies. However, information obtained by these techniques is limited to ensemble averages over many molecules having the same optical transition energy. In order to obtain more detailed information, we measured fluoreascence excitation spectra of small numbers of terrylene molecules in hexadecane in the 2-5K temperature range. An Ar^+-laser-prmped single-mode dye laser was employed for the excitation. The peak frequency of the single-molecule spectrum was found to depend on the applied pressure linearly. From this result, the compressibility of hexadecane was determined to be 0.052GPa^<-1>. The temperature dependence of the spectral width was dependent on each molecule, and the observed nonlinear dependence was explained by the coupling of two-level systems to the guest molecule. On the other hand, it is theoretically predicted that the contribution fo small numbers of molecular groups manifests itself in the spectral shape of a persistent hole burned in the absorption spectrum of dyedoped condensed matter. We performed a temperature cycling hole-burning experiment in H_2-substituted myoglobin, and found that there exists only one molecular group in a protein molecule that corresponds to the two-level systems making flips in the 4-70K temperature range. We further determined the coupling strength between the heme electron and the two-level systems in myoglobin.