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Two-photon-induced conductivity has been observed in Si and GaAsP photodiodes and in a CdS photo-conductive cell at room temperature and applied to the ultrashort optical pulse measurement with a simplified Michelson-type arrangement. The efficiency of the two-photon conductivity is found to be 3 x 10(-14)I A/W, where I is the intensity of the incident pulse in watts per squared centimeter. A single-shot pulse-width measurement has also been performed by using a two-dimensional silicon CCD array. Use of these inexpensive photodetectors with nonlinear characteristics has proved to be convenient for intensity correlators because of their instantaneous response to temporal coincidence of optical pulses and ease of optical arrangement.

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Dynamic properties of the picosecond fluorescence of highly enriched reaction-center particles of photosystem I (8 -10 chlorophylls/P700) prepared from spinach have been investigated. The number ( N ) of photons used to excite chlorophyll molecules per reaction center was controlled between 0.06 and 80. The lie lifetime was ca. 25 ps for N 5 1. which is much shorter than previously measured lifetimes of photosystem I particles. The initial fluorescence intensity saturated at higher excitation intensities ( N 2 1). This was interpreted in terms of interaction and annihilation among excited chlorophyll molecules which occur almost entirely within the duration of a laser flash. The spectrum-resolved fluorescence decay was faster at 690 than at 680 nm. This implies that two kinds of antenna chlorophylls. apart from and in close proximity to P700, have different lifetimes. Upon heat treatment a component with a much longer fluorescence decay time was observed. The growth of this component upon heat treatment at increasing temperatures showed a correlation with a decrease in the amount of P700 that could be photooxidized.