Real-Time Picosecond Measurements of Electronic Energy Transfer from DODCI to Malachite Green and DQOCI

Abstract

The addition of malachite green to the saturable absorber DODCI has been reported (.1) to increase the stability of operation of a mode-locked CW Rhodamine 6G dye laser. It has been suggested (2) that this result could be due to the resonance transfer of excited electronic state energy(3) between DODCI and malachite green. DQOCI, which gives subpicosecond pulses when used by itself to mode-lock the CW Rhodamine 6G dye laser (4), also produces an improvement in performance when added to the DODCI solution. The extension (5) of the ‘Synchroscan’ streak-camera (6) for use with pulses from CW mode-locked lasers, greatly facilitates the carrying out of electronic excitation energy transfer measurements. Using low peak power (∿300W) picosecond laser pulses for excitation, nonlinear effects in the experimental solutions are avoided while precisely superimposed successive fluoresçence streak-records on the image-tube phosphor permit integration of >10⁸ pulses (in ∿0.5s) with a vidicon optical multichannel analyser (OMA) (7). Thus direct linear detection in real time of complete fluorescence decay curves, obtainable with single-shot streak cameras (4) and, effectively, photon-counting with digitization and storage can be combined in one convenient system. With the low values of instantaneous photo-electron currents involved no further image intensification is needed to avoid photocathode saturation or image-tube space charge effects (8). We report detailed measurements, of the effects on the lifetime of DODCI dissolved in the low viscosity solvent ethanol, when DQOCI or malachite green is added to the solution.