of presentation made at Meeting of the American Physical Society, Los Angeles, CA, March 1998) The two-photon action cross-sections for twelve molecules, including stilbene derivatives and several paraphenylene vinylene oligomers, were measured over the wavelength range 700-1005 nm using femtosecond pulsed excitation. One molecule shows cross-sections in excess of 3000 GM (1 GM=10^-50 cm^4.s/photon), while several others are near 500 GM. Another of the molecules shows a nearly constant cross-section of 300 GM when excited between 720-920 nm, with rhodamine-like emission. The cross-sections of related molecules, differing chemically by the addition of hydroxyl or methyl groups, iodonation, or length of carbon chains, are compared. Comparison between one-photon and two-photon excitation is made. A theory describing the absolute magnitude and peak excitation wavelength of two-photon cross-sections if being developed in conjunction with these measurements.
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