Picosecond transient dichroism and birefringence spectroscopy on pheophorbide-a molecules in solution

Abstract

Picosecond transient dichroism and birefringence on pheophorbide-a in solution is investigated using a pump-and-probe experiment. In this experiment the sample is pumped resonantly to the S0-S1 transition by linear polarized light and probed by a continuum polarized 45° with respect to the pump beam polarization plane. The photoselective excitation of the sample generates a transient anisotropy resulting in a rotation of the probe beam polarization plane due to light-induced dichroism and in the generation of elliptically polarized light due to light-induced birefringence. The polarization state of the probe beam after passing the sample is analysed by means of angle-resolved measurements carried out at different time delays. From these measurements (i) the light-induced changes of the absorptive index and (ii) the light-induced changes of the refractive index are obtained separately. The results are in good agreement with calculations using Kramers-Kronig transforms. Moreover, (iii) the anisotropy spectrum of the absorptive index is measured to obtain information about the orientation of the transition dipole moments of the excited-state transitions and (iv) the orientation relaxation time of the S1-state is determined from the decay time of the transient anisotropy.