Non-Markovian relaxation observed in photon echoes of iron-free myoglobin

Non-Markovian relaxation has been observed from photon echo signals of iron-free myoglobin for temperatures below 8 K. The transition to the fast modulation limit above 10 K is considered to be initiated by the phonon-assisted conformational relaxation of the protein. From the analysis using stochastic theory, we point out that although the observed echo signals exhibit nonexponential behavior, the corresponding hole-burning spectra happen to be quasi-Lorentzian.