We have measured the full two-photon absorption (TPA) spectrum in the
transparency range of single-crystal polydiacetylene paratoluene-sulfo
nate (PTS) using z scan. The spectrum consists of three well-defined t
wo-photon excited states superimposed on a continuum of even-parity st
ates. Comparison of the TPA and the one-photon absorption spectra iden
tifies a two-photon accessible state as the lowest excited state of PT
S. The accompanying dispersion in the intensity-dependent refractive i
ndex coefficient n2 is also reported.