Photon-Echo Spectroscopy of Organic Mixed Crystals

We report the first observation of photon echoes in an organic mixed crystal. The echo was observed after excitation with a nitrogen-laser-pumped dye laser in the $^{1}B_{2\mathrm{u}}\ensuremath{\leftarrow}^{1}A_{g}$ electronic transition at 3739.9 \AA{} of pyrene in biphenyl at 2 K. The photon-echo excitation spectrum of the origin (zero-phonon line) region shows that drastic variations occur in the coherence lifetime of sites which, qua incoherent (fluorescent) decay, are indistinguishable.

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