Dephasing times and linewidths of optical transitions in molecular crystals: Temperature dependence of line shapes, linewidths, and frequencies of Raman active phonons in naphthalene

The temperature dependence of linewidths, line shapes, and frequencies is investigated for Raman active fibrational phonons of naphthalene in order to derive the mechanism of optical dephasing for phonon excitations and to measure their dephasing times. Theoretical background is presented on various manifestations of anharmonic interactions particularly in relation to dephasing and relaxation. For the case of the naphthalene crystal, it is shown that the temperature dependences of the phonon frequencies and of their respective linewidths do not manifest by the same mechanism. The dominant contribution to the temperature dependence of phonon frequencies is from the thermal expansion of the lattice. On the other hand, the line shapes are asymmetric Lorentzian and the temperature dependence of the linewidth is consistent with the dephasing process due to T₁ relaxations in which a phonon exchanges energy with two other phonons through cubic anharmonic interaction.