Four-photon absorption in the single-crystal polymer bis(paratoluene) sulfonate.

Strong four-photon absorption corresponding to Im[x((7)) (-?; ?, -?, ?, -?, ?, -?, ?)] has been measured for the first time to the authors' knowledge with 100-fs pulses at 1600 nm in the single-crystal polymer poly[bis (p-toluene sulfonate)] of 2, 4-hexadiyne-1, 6-diol. The transition involved is from the even-symmetry ground state into the vibronic subband of the dominant, even-symmetry, excited two-photon state at an energy of 2.7 eV.