Population and rotational kinetics of the rhodamine B monomer and dimer: Picosecond transient spectrometry

We use picosecond excite-and-probe techniques at 532 nm, a subnanosecond fluorescence spectrometer, and a transient-transmission spectrometer with picosecond temporal resolution to measure the population kinetics and rotational diffusion of aqueous rhodamine B monomer and dimer. The excited-state lifetimes of the monomer and dimer are determined to be 1.6 ns and 100 ps, respectively, and the rotational randomization time of the monomer is measured to be 250 ps. The dimer rotational diffusion time is determined to be longer than that of the monomer, indicating that there is a large molecular species in equilibrium with the monomer.