Electronic photon echo spectroscopy and vibrations

Coupling between electronic and nuclear degrees of freedom in Rhodamine 800 has been studied using fluorescence line narrowing (FLN) and three-pulse photon echo peak shift (3PEPS). From the FLN measurement we extract the Huang-Rhys S-factors and frequencies of 16 Franck-Condon active vibrations clustered in two groups centred around 350 cm−1 and 1400 cm−1. The five modes below 500 cm−1 covered by the excitation pulse spectral band create a complex beating pattern in the 3PEPS measurements. After determining the reorganisation energy of the modes involved in the peak shift measurement we are able to obtain the S-factors of the modes. The S-factors from two different experiments are in good agreement. The dephasing rates of these vibrations deviate significantly from the band-gap law. We propose that the main channel for the dephasing of the modes above 225 cm−1 is a second order process involving one phonon and a molecular vibration.