Picosecond dynamics of Eosin B at the air/water interface by time-resolved second harmonic generation: orientational randomization and rotational relaxation

Time-resolved surface second harmonic generation has been used to study the picosecond dynamics of the dianion Eosin B dye molecule at the air/water interface. It is found that the induced second harmonic signal depolarization obtained with the perturbation of the distribution of Eosin B molecules at the interface can be attributed to an out-of-plane rotational diffusion motion of the molecules. The data also suggest that the high power densities of the laser pulses randomize the orientation distribution of the molecules at the interface. A rotational diffusion time of 90±20 ps is measured, a value significantly faster than the one obtained in bulk aqueous phases.