Two-dimensional analysis of integrated three-pulse photon echo signals of Nile blue doped in PMMA

The integrated three-pulse photon echo signal of Nile blue doped in poly(methyl methacrylate) (PMMA) was measured two-dimensionally by varying the first coherence period and the subsequent population period. The electronic dephasing time was in the femtosecond time range even at low temperatures, whereas rephasing and echo formation were still possible even when the population period was extended to > 100 ps. Oscillation was present in the echo signal even when population period was extended to longer values than the dephasing time of the vibrational coherence. Echo peak shift measurement indicated a presence of small spectral diffusion in the picosecond time range. A computer simulation was carried out from a model spectral density. The phonon mode of PMMA was represented by a Brownian oscillator centered at --> 116 cm(-1), and it reproduced the temperature dependence of the echo signal surprisingly well.