Direct observation of a quasilocalized low-frequency vibrational mode in the fluorescence excitation spectrum of a single impurity molecule in a polymer matrix

The possibility of direct observation of a quasi-localized elementary excitation of the phonon type in polyisobutylene has been demonstrated by detecting the fluorescence excitation spectrum of a Mg-tetrazaporphyrin single probe molecule in a wide spectral range at a temperature of 6 K. The parameters of the quasilocalized low-frequency vibrational mode have been measured–the frequency (energy) in the case, when impurity molecule is in excited electronic state ($\omega_{e}$ = 13.94 $\pm$ 0.21 cm$^{-1}$) and the halfwidth of the mode spectrum ($\gamma$ = 3.82 $\pm$ 0.66 cm$^{-1}$). The measured energy of the low-frequency vibrational mode coincides with the position of a boson peak maximum in the spectrum of vibrational states of the polymer, which indicates that the weak coupling approach can be applied to the considered case of the electron–phonon interaction of an organic dye molecule with the local environment in a polymer matrix.