THERMAL BROADENING OF AN OPTICAL-TRANSITION IN A CHROMOPROTEIN BETWEEN 50-MK AND 15-K

We measured the temperature dependence of the zero-fluence extrapolate d hole width associated with the S-1 <-- S-0 transition in protoporphy rin IX substituted myoglobin. The temperature was varied between 50 mK and 15 K. The experiments were done at two frequencies in the inhomog eneous band. We found that the hole width follows a single power law u p to 10 K. Although the characteristic features of thermal line broade ning are similar for the two frequencies, there are clearly discernibl e differences as well: The respective exponents are slightly different (1.38 vs 1.48 +/- 0.05), and the zero-temperature extrapolated hole w idths differ drastically (12 vs 18 +/- 0.5 MHz). From the experiments we conclude that the broadening mechanism is most probably spectral di ffusion and that the major contribution must stem from the protein its elf.