Abstract

We discuss the technique of resonance Raman saturation spectroscopy and present experimental results that probe relaxation processes in heme proteins following electronic excitation in the Soret band. The observable relaxation time scales are limited by the laser excitation rate, k_L, rather than by the laser pulse width (~10 ns). Analysis of the data using a model that allows for parallel relaxation channels leads to effective electronic relaxation times for deoxymyoglobin (2ps), ferrocytochrome c (3ps), and deoxyhemoglobin (1ps). The more rapid relaxation times found for hemoglobin and its isolated chains are not currently understood. The Raman depolarization ratio is predicted to increase at high laser flux, due to the preparation of a partially oriented sample by photoselective excitation. Such effects are observed in heme systems and the relaxation times extracted from the depolarization analysis are in agreement with the measurements of Raman intensity saturation. Studies of the asymmetric broadening of the ν₄ mode of cytochrome c at high laser flux, reveal that the lineshapes in the Stokes and anti-Stokes region are inequivalent. Time-reversal symmetry dictates that this broadening is due to an underlying Raman band, associated with an excited electronic state that is populated at high laser flux. Similar linebroadening effects, observed in hemoglobin and myoglobin samples, are also shown to arise from Raman scattering of excited electronic states rather than Rabi broadening¹ or anharmonic coupling to vibrationally hot low frequency modes².

© 1992 The Author(s)