Power-induced broadening of transient resonance Raman spectra of deoxyhemoglobin

The power-dependent behavior of the resonance Raman spectra of deoxyhemoglobin has been studied by using ∼30-ps laser pulses. The line width of a Raman-active mode sensitive to porphyrin π-electron density, v₄, broadens asymmetrically as a function of laser fluence. Several possible origins of this phenomenon are examined including the presence of a transient photoproduct, unrelaxed internal vibrational population, and a nonlinear change in the resonance Raman scattering process. Our data are qualitatively consistent with the last of these possibilities. The implications of these results for future transient Raman studies are discussed.