DYNAMIC STOKES SHIFT IN COUMARIN - IS IT ONLY RELAXATION

Picosecond fluorescence spectra of coumarin 153 in solution show transient line narrowing. The peak's frequency shift and its area decay are nonexponential in time. A biexponential fit reveals a correlation between the fast exponents for the shift and area decay at different temperatures. A plot of shift as a function of area is temperature independent at short times. These findings are consistent with an inhomogeneous mechanism for the early part of the dynamic Stokes shift. Several suggested experiments may help distinguish between inhomogeneous and relaxation mechanisms.