Rapid-scanning frequency-domain fluorometer with picosecond time resolution

A new frequency-domain fluorometer is described which is capable of operating over a frequency range between 10 MHz and 1 GHz. The instrument employs a mode-locked Ar-ion laser as an 82-MHz repetition-rate excitation source and an inexpensive extracavity acoustooptic light modulator for secondary-frequency generation. The instrument is very rapid and can acquire, in ˜10 s, the entire frequency spectrum necessary to determine excited-state lifetimes. Importantly, these frequency spectra are characterized by signal-to-noise ratios of at least 20 for moderate fluorescence signals (10-20 μA detector current). With this new instrument, multiple exponential decays can be easily resolved, and single exponential decays can be determined at a precision (standard deviation) of better than 20 ps.