Resonance energy transfer, motional narrowing, and vibrational dephasing in molecular crystals: 390 cm−1 internal vibration of naphthalene

For the naphthalene-h8 crystal, the vibrational dephasing of the 390 cm−1 Raman active vibration is investigated by the temperature dependence of the linewidth and the line shape in the Raman spectra. This mode shows a relatively large resonance transfer interaction with a consequent factor group splitting of ∼5.5 cm−1. The corresponding transition in the isotopic mixed crystal, composed of 10% naphthalene-h8 and 90% naphthalene-d8, is, predominantly, due to the monomer. This monomer transition exhibits a Gaussian line shape at 4 K and has a width of ∼1.5 cm−1. In the neat naphthalene-h8 crystal at 4 K, the line shape of the higher energy Davydov component is found to be a motionally narrowed Lorentzian with a width much smaller than that of the monomer transition in the isotopic mixed crystal. In contrast to this, the line shape of the lower energy Davydov component is asymmetric. The low energy side of the lower Davydov component is found to obey the Urbach rule, while its high energy side is a Lorentzian. Furthermore, the width of the lower Davydov component is larger than that of the upper Davydov component but still smaller than what is observed for the corresponding monomer transition in the isotopic mixed crystal. This result suggests that the vibrational exciton band k-scattering mechanism, which involves the exciton–phonon scattering, does not contribute significantly to the vibrational dephasing. It is because such a mechanism predicts that at 4 K the upper Davydov component would be broader than the lower Davydov component as observed for the electronic excitons in molecular crystals. A detailed study of temperature dependence of the linewidth of the upper Davydov component also supports the assumption that the k scattering within the vibrational exciton band is not the main contributor to the dephasing of the 390 cm−1 vibration. This observed temperature dependence of the linewidth is found to be consistent with the mechanism of pure dephasing which involves an off-diagonal quartic anharmonic interaction with the high-frequency in-plane librational phonon motions.