Rabi broadening of transient resonance Raman spectra of deoxy hemoglobin

The power-dependent behavior of the resonance Raman spectra of deoxy hemoglobin was studied by using ~30-psec laser pulses. The linewidth of a Raman active mode that is sensitive to porphyrin π-electron density, ν4, broadens asymmetrically as a function of laser fluence. Several possible origins of this phenomenon are examined, including the presence of a transient photoproduct and a nonlinear change in the resonance Raman scattering process through Rabi broadening. Our data are consistent with the second possibility and can be qualitatively modeled in simulations that use realistic values of T1 and T2 for the heme Soret electronic transition.