A Resonance Raman Scattering Study of Vibrational Dephasing in the Mixed Crystal of Pentacene in Naphthalene

Resonance Raman scattering is used to investigate vibrational dephasing in the mixed crystal of pentacene in naphthalene. It is shown that, as for the pure electronic transition, uncorrelated resonant phonon scattering processes in the ground and vibrationally excited state induce vibrational dephasing in this system

Optical Free Induction Decay by Extracavity Laser-Frequency Switching

In this letter we demonstrate the possibility of performing optical free induction decay by extra-cavity laser-frequency switching. The method is used to study optical dephasing of pentacene in p-terphenyl and the results are found to be consistent with those previously obtained by the intra-cavity switching technique. The prospects of extra-cavity frequency switching as a tool for studying optical dephasing on a subnanosecond time-scale and in the uv spectral range are emphasized.

Application of Redfield theory to optical dephasing and line shape of electronic transitions in molecular mixed crystals

Redfield relaxation theory is used to study optical dephasing in molecular mixed crystals where pseudolocalized phonons play a dominant role. As a model an effective four-level system is defined which consists of the ground and excited electronic state of the guest impurity and one pseudolocalized phonon in each of these states. It is shown that coherence transfer between the purely electronic and the phonon-promoted transition is due to interference between the scattering amplitudes in the ground and excited state and only occurs when the transitions are near resonant. When the scattering amplitudes in the ground and excited state are identical the optical Redfield equations reduce to the modified Bloch equations and coherence transfer may appreciably contribute to optical dephasing. It further proves essential for coherence exchange to occur that next to electron–phonon coupling the phonon anharmonicity is taken into account. The stochastic and correlation function theories of line shape, using the same model system, have also been examined and we conclude that in these theories exchange of coherence is not properly taken into account. Next to the effect of pseudolocalized phonons the effect of band phonons on optical dephasing is also examined and within our model both effects are additive.

Optical Free Induction and Fluorescence Decay of Pentacene in p-Terphenyl Following Narrow Band Laser Excitation

Optical free induction and fluorescence decay experiments on pentacene following narrow band laser excitation are reported. From the results we conclude that pentacene for its lowest 1B2u ← 1A1g transition is a perfect two-level system and from the power saturation term in the OFID we calculate a transition dipole moment of 1.5 ± 0.2 debye. A long-lived transient observed in the fluorescence during the onset of cw dye laser excitation is ascribed to a bottle-neck in the optical pumping cycle.