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 ${}_{}{}^1{}_{}{}^{}B_{2\mathrm{u}}^{}{}_{}{}^{}\leftarrow {}_{}{}^1{}_{}{}^{}A_g^{}{}_{}{}^{}$ electronic transition at 3739.9 Å 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.