Temperature Dependence of the Optical Linewidth and Lineshift

As mentioned before, an increase in temperature activates low frequency modes of the matrix (phonons) or the molecule (librations) which lead to a broadening and a frequency shift of the homogeneous absorption line. In experiments with ensembles of molecules the homogeneous optical linewidth and its temperature dependence 

The first temperature dependent study of the optical linewidth of individual molecules was performed on single pentacene molecules doped into p-terphenyl [10]. As can be seen in Fig. 4, below 4 K the optical linewidth remains essentially constant at the lifetime-limited value of 7.8 MHz. Above 4 K, temperature dependent dephasing processes contribute to the linewidth. In previous photon echo experiments, the 

It is interesting to see that for both dopants (pentacene and terrylene) in p-ter-phenyl optical dephasing seems to be induced by a librational mode. This model is strengthened by the observation of dilferent activation energies for the two chromo-phores which gives further evidence that the low frequency excitations are localized at the impurities. 

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Figure 5. Temperature dependence of the optical linewidth and lineshift for a single terrylene molecule in p-terphenyl. Both data sets could be approximated by fitting an activated process (Eq. 4) to the data. The width as well as the shift yield an activation energy of 17 2cm. ...

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