Spectroscopic techniques developed to investigate the kinetics of rapid photoprocesses occurring in the time domain 5×10−9 to 6×10−3 sec after excitation are described. The excitation system consists of a cw argon ion laser beam interrupted periodically at frequencies between 4 and 75 Hz by means of an electro-optic shutter. The shutter can be switched off within 3×10−8 sec and remains closed for approximately 6×10−3 sec, during which time the decay kinetics of the light emitted by the sample can be recorded by the detection system. The latter employs a fast photomultiplier and a sampling oscilloscope, the samples being stored digitally in a multichannel analyzer. Since an explicit decay curve is obtained from the measurements, no assumptions need be made about the form of the decay kinetics. Careful analysis of the data permits measurement of lifetimes that are much shorter than the time required to switch off the excitation light, the minimum being set by the signal-to-noise ratio alone. The use of signal averaging allows very weak signals to be measured and the emission decay curve to be followed accurately over several decades in time. Because steady state conditions are established during each cycle and the power density of the laser beam can conveniently be adjusted over the range from a few microwatts per square centimeter to several watts per square centimeter, this system is well suited to an investigation of fast photoprocesses in photosynthetic systems. The performance of the apparatus is illustrated by measuring the fluorescence decay from a solution of rubrene (5, 6, 11, 12-tetraphenylnaphthacene) in benzene. The measured lifetime is 16.3±2.1 nsec compared to a previously reported value of 16.42±1.15 nsec [J. Chem. Phys. 37, 814 (1962)]. Shortening of the lifetime due to the presence of impurities and oxygen in the solution is also discussed.
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Feb 1, 1977
J D Mollon + 2
Open Access
