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The two-photon absorption cross sections were determined by two-photon-excited fluorescence measurements, with 5-ns, 10-Hz pulses from a Nd:yttrium-aluminum-garnet-pumped optical parametric oscillator-amplifier system. The setup consisted of two arms to allow relative measurements with respect to bis-1,4-(2-methylstyryl)benzene (in cyclohexane), fluorescein (in pH 11 water), and rhodamine B (in methanol). The sample chromophores were dissolved in toluene (10-4 M concentration); an approximately collimated beam was used to excite the samples over the 1-cm length of the cuvettes. The fluorescence emitted at right angles to the excitation was collected and focused onto photomultiplier tube detectors. The intensity of the incident beam was adjusted to assure excitation in the intensity-squared regime.
S. M. Kennedy and F. E. Lytle, Anal. Chem. 58, 2643 (1986). We scaled the peak δ value reported in this paper by a factor chosen to match the long-wave-length data reported therein with that of (7).
M. Lipson, M. Levin, S. R. Marder, J. W. Perry, unpublished results.
B. H. Cumpston et al., in preparation.
Support from the NSF (Chemistry Division), Office of Naval Research, and the Air Force Office of Scientific Research (AFOSR) and its Defense University Research Instrumentation Program at the California Institute of Technology is gratefully acknowledged. The research described in this paper was performed in part by the Jet Propulsion Laboratory, California Institute of Technology, as part of its Center for Space Microelectronics Technology, and was supported by the Ballistic Missile Defense Organization, Innovative Science and Technology Office and AFOSR through an agreement with NASA. The work in Mons was carried out within the framework of the Belgium Prime Minister Office of Science Policy "Pôle d'Attraction Interuniversitaire en Chimie Supramoléculaire et Catalyse" PAI 4/11 and is partly supported by the Belgium National Fund for Scientific Research (FNRS-FRFC) and an IBM-Belgium Academic Joint Study; D.B. is a Chercheur Qualifié FNRS, and T.K. is a Ph.D. grant holder of Fund for Research in Industry and Agriculture. The femtosecond two-photon fluorescence excitation cross-section measurements were carried out in the Developmental Resource for Biophysical Imaging Opto-electronics at Cornell University with support of NIH-National Center of Research Resources and NSF. G.S. acknowledges support from the NASA-JOVE program.
