Resonance Raman scattering (RRS) in conjugated polydiacetylene solutions is employed in order to determine the vibrational spectra of resonance-selected chromophores within a single conformationally disordered polymer chain. Although the polymer-chain length contains up to 1000 repeat units, the conjugation length (i.e, , the length over which backbone planarity is maintained without interruption and which defines the chromophore) may be as smail as three units. Variation of the solvent system can increase the conjugation length to essentially infinity. The absorption spectrum of the polymer in chloroform consists of an inhomogeneously broadened line shape with contributions from a distribution of individual chromophores. RRS can photoselect chromophores with specific transition energies and, therefore, with specific conjugation lengths. The delocalization of electron density along the conjugated unit, which decreases the absorption energy of the chromophore, also causes a decrease in frequency for those Rarnan modes which primarily correspond to multiple bond stretching, such as v~~. The change of~and~as the incident laser energy is tuned through the absorption band is reported here. Far from the absorption band an average value for v~is obtained. As the incident energy approaches the absorption band, long-conjugation-length chromophores are photoselected and v~decreases markedly. At higher energies within the absorption band, shorter-conjugation-length chromophores are photoselected and v increases, becoming larger than the average value of~~~. Similar results are obtained for v~~. A model for the corrugation-length dispersion. is presented and found to be in good agreement with absorption profiles, frequency shifts, and RRS intensities.

Three-wave-mixing spectroscopy is used to determine the dispersive and absorptive parts of a strongly allowed twophoton transition in a series of polydiacetylene solutions. The data analysis yields the energy, width, symmetry assignment, and oscillator strength for the two-photon transition, The data conclusively demonstrate that strong two-photon absorption is a fundamental property of the polydiacetylene backbone. The remarkably large twophoton absorption coeAicients are explained by large oscillator strengths for both transitions involved in the twophoton absorption combined with strong one-photon resonance effects. The experimental results are shown to be consistent with a simple theoretical model for the energies and oscillator strengths of the oneand two-photonallowed transitions.

Excited-state absorption in alexandrite has been measured from 420 to 680 nm by using pulsed flash lamp pumping and a dye laser probe. The excited-state absorption cross section is generally of the same order of magnitude as the ground-state absorption cross section (10-19 cm 2 ) and therefore may represent a large loss of energy from the pump source in a laser system. The results were confirmed at two wavelengths with a cw experiment in which 568-or 590-nm radiation was used for pumping and probing.

In materials with an excited-state absorption cross section larger than the ground-state absorption cross section, increasing the incident light intensity (thus populating the excited state) increases the absorption. We show that such a reverse saturable absorber can function as a power limiter and pulse smoother for long pulses and as an energy limiter and pulse shortener for short pulses. The necessary properties for such a material are described. Reverse saturable absorption is demonstrated at 488 nm in alexandrite. A simple model describing these effects is presented.

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