Three-photon excitation ofp-quaterphenyl with a mode-locked femtosecond Ti:sapphire laser.
We observed emission fromp-quaterphenyl (p-QT) at 360 nm when exposed to the focused light from a femtosecond (fs) Ti:sapphire laser at 850 nm. This wavelength is too long to allow two-photon excitation of p-QT. The emission intensity of p-QT was found to depend on the cube of the laser power at 850 nm, suggesting that excitation occurs due to a three-photon process. The same emission spectrum and single exponential decay times were observed for three-photon excitation at 850 nm as for two-photon excitation at 586 nm and for one-photon excitation at 283 nm. The same rotational correlation times were observed for one-, two-, and three-photon excitation, but higher time-zero anisotropies were observed for two- and three-photon excitation. The steady-state anisotropies for one-, two-, and three-photon excitation are precisely consistent with cos(2)θ, cos(4)θ, and cos(6)θ excitation photoselection, where θ is the angle between the electric field of the incident light and the absorption dipole. These experiments were performed with 3×10(-5) M solutions of p-QT. Use of such low concentrations was possible because p-QT displays one of the highest apparent cross sections we have observed to date for three-photon excitation. The spatial distribution of the excited fluorescence was less for three-photon excitation than for two-photon excitation of Coumarin 102 at the same 850-nm excitation wavelength. The high cross section, photostability, and clear cos(6)θ photoselection of p-QT make it an ideal three-photon standard for spectroscopy and microscopy.
Center for Fluorescence Spectroscopy, Department of Biological Chemistry, University of Maryland School of Medicine, 108 North Greene Street, 21201, Baltimore, Maryland.
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Gryczynski
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Malak
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Lakowicz