Fluorescence-Encoded Infrared Spectroscopy: Ultrafast Vibrational Spectroscopy on Small Ensembles of Molecules in Solution

Fluorescence-encoded infrared (FEIR) spectroscopy is an ultrafast technique that uses a visible pulse to up-convert information about IR-driven vibrations into a fluorescent electronic population. Here we present an updated experimental approach to FEIR that achieves high sensitivity through confocal microscopy, high repetition rate excitation, and single-photon counting. We demonstrate the sensitivity of our experiment by measuring ultrafast vibrational transients and Fourier transform spectra of increasingly dilute solutions of a coumarin dye. We collect high-quality data at 40 mu M (similar to 2 orders of magnitude below the limit for conventional IR) and make measurements down to the 10-100 nM range (similar to 5 orders of magnitude) before background signals become overwhelming. At 10 nM we measure the average number of molecules in the focal volume to be similar to 20 using fluorescence correlation spectroscopy. This level of sensitivity opens up the possibility of performing fluctuation correlation vibrational spectroscopy or-with further improvement-single-molecule measurements.