Two-color two-photon excitation of indole

We observed two-color two-photon (2C2P) excitation of indole upon simultaneous illumination at 380 and 760 nm with picosecond pulses from a cavity-dumped dye laser. The emission spectrum of indole with 2C2P excitation was the same as observed for one-photon excitation with an equivalent energy of 250 nm. Observation of the 2C2P signal required temporal and spatial overlap of the 380- and 760-nm pulses. Illumination at 380 nm alone resulted in a background emission due to one-color two-photon excitation at 380 nm. This background was rendered insignificant compared to the 2C2P signal by attenuation of the 380-nm beam and amplification of the 760-nm beam. The ability to control the intensity of each beam is a unique advantage of 2C2P excitation. The amplitude of the 2C2P signal depended on the angle between the polarization of each beam in a manner which suggests participation of both the ¹L_a and ¹L_b states of indole to the 2C2P transitions. 2C2P excitation can provide a new tool to investigate the photophysical properties of indole, tryptophan, and proteins.