FEMTOSECOND STOKES SHIFT IN STYRYL DYES - SOLVATION OR INTRAMOLECULARRELAXATION

Transient absorption and gain spectra of the styryl dye LDS-750 in sol ution have been studied by the pump/supercontinuum probe (PSCP) techni que with excitation at 530 nm. The pump/probe intensity correlation wi dth was 70 fs, providing a time resolution of 40 fs. Spectra were dete cted in the range 400-800 nm with 1.5 nm resolution. Before 70 fs, pro minent spectral structure is observed due to resonant Raman scattering from a 1500 cm(-1) active mode of the chromophore. At later time, the gain spectrum undergoes an ultrafast redshift and change of shape, wi th time constants of similar to 200 and similar to 600 fs for acetonit rile and chloroform solutions, respectively. At high pumping energy (1 .2 mu J), the final emitting state is reached by internal conversion f rom higher electronic states without a further essential Stokes shift. The emitting state is assigned to an excited isomeric form of the mol ecule. At low pumping energy (0.3 mu J), the first excited electronic state isomerizes in an ultrafast process followed by a slower process, the dynamics of which is controlled by the solvent. The geometrical a nd electronic nature of these processes and their coupling to the solv ent needs further clarification. (C) 1997 American Institute of Physic s.