New Developments in Picosecond Time-Resolved Fluorescence Spectroscopy: Vibrational Relaxation Phenomena

Abstract

Problems associated with the determination of the pathways and rates of the relaxation of radiatively prepared vibronic states have occupied much attention in recent years. The subject has been approached from two main points of interest. First, the more significant advances have been made in the study of coherence loss phenomena associated not only with vibrational energy but also with “pure” dephasing contributions. This type of approach has some advantages in that it promises much higher spectral resolution and sensitivity since it is possible to work in the frequency domain as well as to make use of coherent amplification techniques including CARS spectroscopy and photon echo studies. On the other hand, the incoherent aspects of the relaxation phenomena are much harder to get at experimentally. Incoherent anti Stokes Raman [1] and two-step excitation of molecular fluorescence [2] have been used to probe vibrational relaxation in the ground state, while studies of the corresponding process in an electronically excited state have mostly used time-resolved fluorescence approaches. One exception has been the transient absorption spectroscopic work of HOCHSTRASSER and co-workers.[3]