Diffractive optics based three-pulse photon echo peak shift studies of spectral diffusion in polar liquids: Evidence...

We report a three-pulse photon echo peak shift (3PEPS) apparatus adopting a diffractive beam splitter to generate triplets of femtosecond pulses with tilted wave front, which enhances the overlap over the entire aperture in a noncollinear geometry while preserving femtosecond pulse duration. The apparatus provides highly reproducible 3PEPS data including the long time peak shift, a critical parameter to recognize the spectral inhomogeneity in condensed phases. To demonstrate the high performance of our setup and to examine the inhomogeneous line broadening in liquids, we measured 3PEPS of a carbocyanine dye IR125 in several solvents. The solvation dynamics becomes slower for IR125 compared to the typical values reported previously probably due to the multipolar nature of the excitation in this large and flexible probe molecule. More importantly, inhomogeneity was observed unambiguously in all solvents, and it is excitation energy dependent. It was speculated that slow dielectric relaxation and conformational heterogeneity may be responsible for the inhomogeneity.