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Femtosecond fluorescence spectroscopy by upconversion with tilted gate pulses.

Time-resolved emission bands can be observed by simultaneous sum-frequency mixing with femtosecond near-infrared gate pulses over a wide frequency range. Using tilted gate pulses in a noncollinear geometry we achieve 80 fs time resolution and background-free measurement of upconverted spectra. All components were optimized resulting in a spectrometer for routine applications. As example we report the initial fluorescence from Coumarin 153 in acetonitrile after excitation at 400 nm. The fluorescence is modulated by nuclear coherence in the excited state which affects band width and asymmetry more than the energy gap itself.