Femtosecond relaxation of an iron porphyrin observed with polarization spectroscopy in a three-level system.

An experimental and theoretical study is presented of the nonlinear response of a dilute solution being pumped by two laser fields having different frequencies omega(1) and omega(2) that are both in resonance with electronic transitions of the solute. Experimental results were obtained for the dispersion of the third-order susceptibility chi((3))(omega(2))(2) for a dilute solution of iron(III) tetraphenylporphyrin chloride in chloroform with a fixed intense field (omega(1)) resonant with the Q(o) transition and a tunable weaker field (omega(2)) resonant with the Soret transition. The observed Lorentzian dispersion is slightly narrower than the linear absorption spectrum. The theoretical model for the third-order polarization incorporates three levels coupled to two electromagnetic fields and a population reservoir. The equations of motion for the density operator were solved in third order for the appropriate Fourier component. The resulting dispersion curve depends on both total dephasing and population decay rates, thereby demonstrating a general method for the determination of subpicosecond relaxation processes in systems to which the model is applicable. Theoretical fits to the results for iron(III) tetraphenylporphyrin chloride imply that the Soret transition is essentially homogeneously broadened with a total dephasing width of 900 +/- 200 cm(-1), and it is argued that the population decay time in this case is approximately 3.5 fsec. The broad linear absorption spectrum of the Soret band is interpreted as resulting from intramolecular perturbations between the Soret and lower energy excited states of iron(III) tetraphenylporphyrin chloride.