Fiber-Optic Probe for Collection of Two-Photon Excited Fluorescence Spectra in Low-Temperature Glassy Solvents

Low-repetition-rate pulsed lasers are excellent sources for two-photon excitation of samples frozen in low-temperature glassy solvents. Unfortunately, similar efforts to combine synchronously-pumped dye laser excitation with normal cryostats and sample cells have consistently failed. The continuous nature of the laser creates a weak refractive index gradient which causes the beam to wander randomly in space. We have found that a fiber-optic probe directly frozen in the glassy sample permits two-photon excited fluorescence spectra to be obtained. This arrangement is stable because the fiber restricts beam-induced refractive index changes to harmless expansion. Studies of fluxional fluorophores indicate that the viscosity at the fiber tip is not significantly changed under conditions of average power known to cause beam wander in ordinary cryostat cells.