Optical dephasing in the light-harvesting complex II: A two-pulse photon echo study

Time-resolved and integrated two-pulse photon echo signals were measured at 5 K in the wavelength range from 640 to 690 nm (with an increment of 5 nm) in samples of solubilized light-harvesting complex Il (LHC II) from spinac h. Furthermore, the dephasing time was determined at 680 nm in the temperat ure range 5 T less than or equal to 300 K. The following results and conclu sions were gathered from these experiments: (a) the photon echo intensity a s a function of the temporal distance between both excitation pulses exhibi ts a nonexponential decay; (b) data analysis on the basis of a superpositio n of several dephasing processes leads to three characteristic dephasing ti me domains (A, B, and C) with markedly different wavelength dependencies of dephasing time and relative amplitude, T-2(A) less than or equal to 1.7 ps from 640 to 675 nm, T-2(B) = 4-13 ps over the whole wavelength region of 6 40 to 690 mn, and T-2(c) greater than or equal to 40 ps from 675 to 685 nm; (c) the dephasing times T-2(A) and T-2(B) are attributed to the kinetics o f excitation energy transfer and to multiexciton processes above 670 nm, an d (d) the longest dephasing time T-2(C) reaches values up to 350 ps at 684 nm. The temperature dependence of the dephasing rate (T2(c))(-1) at 680 nm is characterized by two different domains: below 20 K the rate steeply decr eases due to pure dephasing, similar to the well-known T-1.3 dependence of the homogeneous line widths at low temperatures in organic glasses. At temp eratures above 20 K a linear increase of (T-2(C))(-1) with temperature and a comparatively weak slope are observed, ascribed to uphill energy transfer . The results obtained in this study are discussed in comparison with data in the literature that were gathered from hole burning, transient absorptio n measurements, and spectra simulations.