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

点击翻译按钮获取中文摘要

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.