The Use of Monochromatically Excited Resonant Light Scattering as a Probe of Ultra Short T₁ and T₂ Type Relaxation Processes in Excited Electronic States

Abstract

Raman scattering has traditionally been a tool of the vibrational spectros-copist. Consequently the phenomenon of Raman scattering has been primarily described in the language of vibrational spectroscopy with little emphasis on the explicit contributions of excited electronic states. these approaches typically prove inadequate in the description of light scattering phenomena when the excitation frequency is resonant with a radiative transition of the probed system. In particular it is conceptually difficult to understand the connection between ordinary Raman scattering and the re-emission associated with resonance. With ordinary Raman scattering the low intensity Raman scattered radiation is spectrally sharp and tracks in frequency with the changes in incident frequency. However for resonant excitations of condensed phase molecules, the re-emission can display a wide variety of properties both with respect to spectral resolution, efficiency of re-emission and spectral sensitivity to shifts in incident frequency. In some systems effectively all the re-emission is in the form of thermally relaxed fluorescence whereas other systems display re-emission that is Raman like for all excitation frequencies.