Mode‐specific relaxation dynamics of photoexcited fe(II) protoporphyrin IX in hemoglobin

The behavior of the resonance Raman spectra of deoxyhemoglobin in the presence of strong resonant fields was studied by using ca. 30 ps laser pulses. The line width of the strongest Raman‐active mode in the Soret absorption band region, v₄, broadens as a function of laser fluence, and its anti‐Stokes/Stokes intensity ratio is power dependent. Previous studies have suggested that equipartition of excess vibrational energy occurs on very fast time scales in the heme macrocycle subsequent to absorption of a photon. The mode‐specific vibrational dynamics of v₄, however, indicate that the vibrational relaxation of the chromophore cannot be treated within the context of an equipartition of energy, but rather should be evaluated in terms of the relaxation phenomena of the specific vibrational mode. These data are evaluated in terms of relaxation mechanisms of the vibrational mode with reference to power‐broadening processes and transient vibrational population redistribution.