S.L. Shapiro

Exciton interactions in crystalline tetracene studied by single picosecond pulse excitation

Abstract The efficiency of fluorescence in crystalline tetracene was studied as a function of excitation intensity with single picosecond laser pulses. Theoretical fits to the experimental fluorescence-quenching curves, at room temperature and at 170 K, for both a and b -incident polarizations, gave a singlet-singlet annihilation rate of γ ss = 10 −7 cm 3 s −1 , to within a factor of two. This rate constant was confirmed by directly measuring the dependence of fast radiative lifetimes on excitation intensity. We also infer a value for the singlet diffusion rate, D s , or 4 × 10 −2 cm 2 s −1 to within a factor of two.

Energy transfer in photosynthesis: Pigment concentration effects and fluorescent lifetimes☆

The first steps in photosynthesis involve the transfer of energy from bulk pigment molecules to photosynthetic reaction centers. Because the energy transfer processes are of subnanosecond duration, it has been difficult to make accurate measurements. With the introduction of picosecond techniques such as the optical gate to the study of the fluorescent properties of photosynthetic materials [ 1,2], it has become possible to directly measure rapid energy transfer processes. In this letter we report results obtained on photosynthetic systems by a powerful streak camera technique [3]. These results indicate that the decay of the fluorescence from two photosynthetic species, Anacystis nidulans and Chlorella pyrenoidosa, is nonexponential with much shorter lifetimes than reported previously. Furthermore, studies of the dependence of the lifetime on concentration of several pigments including chlorophylls a and b, show that at the high pigment concentrations known to be present in living cells, the lifetime can be exceedingly short allowing us to conclude that the high dye concentrations are responsible for the short decay times observed experimentally in the living cells.

Single-pulse picosecond determination of 735 nm fluorescence risetime in spinach chloroplasts.

1. Introduction Within the last few years there have been extensive investigations of the fluorescence from photosynthetic systems using picosecond lasers as excitation sources [l-l 11. Such studies have established that the quenching of the prompt fluorescence, with increasing incident excitation intensity employing a single pulse, arises primarily from singlet--singlet exciton annihila- tion [2,3,7-91. When the mode of excitation consists of a train of pulses or of a microsecond laser pulse, there exists additional quenching of the fluorescence quantum yield arising from long-lived quenchers, presumably triplet excitons [2,3,10] , which either arise from intersystem crossing from the singlet mani- fold or from random recombination of electrons and holes formed from the autoionization final state channel of singlet fusion [8]. Recently, Geacintov et al. [8] have shown that the quantum yield quenching at emission wavelengths of 685 nm and 735 nm were identical, within experimental error, when single picosecond excitation pulses were employed. This identity in the quenching curves was interpreted in terms of the tripartite fluorescence model with bimolecular singlet fusion occurring exclusively within the light harvesting antenna system. Strong support for this interpretation was provided by the observation of an intensity-independent (for intensities below 10” photons cm-* per pulse) 316 lifetime for the 735 nm emission [l 1] . Presumably, singlet fusion reactions are either inoperative or inefficient within the PS I antenna chlorophyll molecules, at least for intensities below lo’* photons cm-’ per pulse. Within the light harvesting and PS II antenna pigments which give-rise to the 685 nm emission, singlet exciton fusions do give rise to a strong decrease of the fluorescence lifetime with increasing intensity. It was concluded that the pigments which are responsible for the 735 nm emission derive their energy by singlet exciton transfer from the light harvesting system and not by direct photon absorp- tion at 530 nm. In this paper we measure the risetime for the 735 nm emission and identify this time lag with the transfer rate from the light harvesting system to the PS I pigment molecules which give rise to the 735 nm fluorescence band at low temperatures. 2. Materials and methods The experimental arrangement for fluorescence lifetime measurements was similar to that reported previously [3] . A 1060 nm, 30 ps pulse was selected from the pulse train emitted by a modelocked Nd:YAG laser, frequency shifted to 530 nm by passage through a KDP crystal, and then was allowed to excite the spinach samples. The samples, chloroplasts prepared from spinach leaves as described in [ 121 , were

Photosynthetic studies with a 10-psec resolution streak camera.

Abstract The fluorescence decay times for the species Anacystis nidulans and Chlorella pyrenoidosa are measured by means of a streak camera to be 75 ± 10 and 41 ± 5 psec, respectively. Concentrated solutions of chlorophyll a are shown to have fluorescence decay times as short as 10 psec. Fluorescence decay curves similar to that of the living cells can be generated by mixing cellular component molecules at comparable concentrations to that present in living cells.

Excited-state protonation kinetics of coumarin 102

Abstract The rate constant for protonation of the electronically excited state of coumarin 102 in water has been determined to be (1,4 × 0.4) × 10 10 mol −1 s −1 using picosecond spectroscopy. The laser pH jump technique is shown to be suitable for rapidly decreasing, as well as increasing, solution pH.

Exciton transfer in DNA

Abstract Fluorescent risetimes are measured to be less than 20 ps for DNA-acridine orange complexes. This result is consistent with a rapid non-radiative depletion of the excited singlet state of DNA due to exciton-phonon interactions.

Broadly tunable picosecond infrared source.

We report a completely grating tuned (1.9-2.4 microm) picosecond traveling-wave infrared generator capable of controlled spectral-bandwidth operation down to the Fourier-transform limit. Subsequent down-conversion in CdSe extends the tuning range to 10-20 microm.

Picosecond time-resolved spectral shifts in emission: dynamics of excited state interactions in coumarin 102

Abstract The temporal behavior of the spectral emission from coumarin 102 in several liquids is studied as a function of temperature using picosecond techniques. Kinetics of the disappearance of a blue edge emission and the formation of a red edge emission are related. Strong correlations with viscosity suggest that the reorientation time is a key parameter.

Ultraviolet phase conjugation

We report the first known demonstration of UV phase conjugation. By use of a 15-psec, 2660-A pulse, 0.1% conjugate reflectivities were obtained through degenerate four-wave mixing in 1-mm samples of CS(2) mixtures. Although pure CS(2) did not exhibit the effect, dilution of CS(2) in several UV-transmitting solvents opened up a concentration- tunable (2450-2850 A) spectral window, allowing the optical Kerr effect to be utilized. Weaker phase conjugation at 2660 A was also observed in other Kerr media and in saturable absorber media.

Picosecond infrared-continuum generation by three-photon parametric amplification inLiNbO 3

Picosecond infrared continua are produced by propagating an intense ultrashort 1.064-microm pulse through a LiNbO(3) crystal set near the degenerate point. Under our experimental conditions, gain broadening is identified as the principal mechanism leading to the superbroad spectral output.