Abstract

Electronic dephasing time T₂ can be studied in “real time” by using fs-pulse lasers or by using broadband noisy light with ps or ns pulse duration. In this case, the short coherence time (tc) of the noisy light, not the duration of the pulses, is the quantity that determines the time resolution in self diffraction (SD) experiments [1]. In this kind of experiments, when the transition under study is homogeneously broadened and T₂ > t_C, T₂ is determined from the asymmetric experimental curves, which relates the intensity of the signal and the time delay between the incident pulses. On the other hand, if T₂ < t_C, this technique allows one to measure T₂ for the inhomogeneously broadened transition by recording simultaneously the two first-order SD signals and measuring the time separation τ_s between the peaks of the two signals. In this work we study electronic dephasing in a solution of polyaniline emeraldine base in dimethylsulfoxide (PANI) at room temperature using a SD scheme, in which a linearly polarized broadband noisy light beam is divided in two beams, k₁ and k₂. One of the beams is delayed relatively to the other and both are combined again at the sample, forming a small angle (~3°). The SD signals along the quasi phase-matched directions k₃=2k₁-k₂ and k₄=2k₂-k₁ are readily seen. The broadband light sources were two dye lasers pumped by the second harmonic of a Q-Switched Nd:YAG laser (10 ns, 5 Hz). They had emission peaks at 583 nm and 710 nm. The first one was blue-shifted relative to the 636 nm absorption maximum of the S₀-S₁ transition in PANI and the second was red-shifted, as can be seen in Fig 1. The coherence time of both dye lasers was =120 fs. The SD signals as a function of the relative time delay t were measured simultaneously for both directions and fitted by Gaussian curves, which allowed to determine the peak’s locations, as well as their widths. Then we recorded τ_s as a function of the intensities, of both dye lasers. When exciting PANI at 710 nm, the separation between the SD peaks is (11±3)fs over the entire intensity range used. On the other hand, when exciting the sample at 583 nm, we observed that τ_s increased from (31±3)fs to (106±3)fs as the incident intensity increased and reached a kind of saturation regime. In [1] it is shown that when inhomogeneous broadening dominates, one gets values of τ_s that grows with the degree of inhomogeneous broadening. Considering that the transition S₀-S₁ is homogeneously broadened, we attribute the present results to changes in the population of vibrational states, which leads the system to behave as inhomogeneously broadened. Similar effect was observed in dye solutions [2], but for PANI it is being reported here for the first time.

© 2000 IEEE