Purely Electronic and Vibronic Fluorescence Excitation of Single Terrylene Molecules in a Naphthalene Crystal

Fluorescence excitation spectra of spatially resolved single terrylene molecules in a naphthalene crystal were studied at 5 K as a function of exciting laser light intensity. The fully saturated fluorescence detection rate for purely electronic transition (0,0) was found to be about twice smaller than that for the two main vibronic components located 239 and 251 $cm^{-1}$ above. This relation is fulfilled when the triplet population rate $k_{23}$ is much weaker than the depopulation rate $k_{31}$. We propose that the weak fluorescence excitation lines observed at the frequencies of 38, 67, 97, and 145 $cm^{-1}$ correspond to nontotally symmetric low-frequency vibration modes of terrylene.