Anisotropy shape control via light quenching and ground state depletion

The influence of light quenching (LQ) and ground state depletion on the sha pe of the angular distribution of the transition dipole moments of molecule s in the first excited singlet state is calculated theoretically. The theor etical results are tested experimentally using fluorescein and DCM as fluor ophores. Both effects were shown to occur under 'normal' experimental condi tions in ps-laser experiments. The initial fluorescence anisotropy can be r educed down to zero under ground state depletion or LQ. It is also shown th at, using appropriate conditions the initial anisotropy can be increased up to values larger than could be achieved even with two photon absorption. ( C) 2000 Elsevier Science B.V. All rights reserved.