Subnanosecond time-correlated photon counting with tunable lasers.

We present several laser based methods to improve the technique of time-correlated photon counting. Our Ar(+) laser pumped tunable dye laser can be operated in three timing configurations: acousto-optically mode locked, cavity dumped, and cavity dumped-mode locked. Performance characteristics of the laser system in various operational modes are described along with measurement techniques for both gas and liquid phase. The subnanosecond pulses generated by mode locking are extremely stable and they maintain identical pulse shapes over a 6-h period, as shown via photon counting measurements at a 15-psec channel resolution. Our RCA C31034 photomultiplier with a red sensitive GaAs photocathode provides wavelength-independent response to detected fluorescence in both the visible and ultraviolet. The present limit of our apparatus is controlled by the accuracy of deconvoluting fluorescence decay from the finite response width caused by photomultiplier transit time dispersion (0.8 nsec FWHM). Our system stability is sufficient to accurately determine exponential decays as short as 50 psec. Furthermore, we can successfully analyze dual exponential decays such as those arising from solution reorientation times of 390 psec competing with a fluorescence lifetime of 725 psec. Examples of the laser performance are selected from a variety of measurements in the gas phase and from the fluorescent dye rose bengal in the liquid phase.