Fluorescence lifetimes of alpha- and beta-carotenes.

The fluorescence lifetimes of alpha- and beta-carotenes are both measured by a streak camera technique to be 55 plus or minus 10 ps at a concentration of 6 times 10- minus2 M in chloroform. Carotenes are present in chloroplasts at approximately this concentration.

Picosecond and microsecond pulse laser studies of exciton quenching and exciton distribution in spinach chloroplasts at low temperatures.

Studies of the fluorescence quantum yield and decay times, determined at the emission maxima of 685 and 735 nm, using picosecond laser pulses for excitation, indicate that the pigments which are responsible for the 735 nm emission derive their energy by transfer of singlet excitons from the light-harvesting pigments and not by direct absorption of photons. Microsecond pulse laser studies of the fluorescence quantum yields at these two fluorescence wavelengths indicate that long lived quenchers (most probably triplet states), which quench singlet excitions, accumulate preferentially within the long wavelength pigment system which gives rise to the 735 nm emission band.

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 infrared-continuum generation by three-photon parametric amplification in LiNbO3.

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.

A picosecond pulse train study of exciton dynamics in photosynthetic membranes.

The fluorescence decay time of spinach chloroplasts at 77 degrees K was determined at 735 nm (corresponding to the photosystem I emission) using a train of 10-ps laser pulses spaced 10 ns apart. The fluorescence lifetime is constant at congruent to 1.5 ns for up to the fourth pulse, but then decreases with increasing pulse number within the pulse train. This quenching is attributed to triplet excited states, and it is concluded that triplet excitons exhibit a time lag of about 50 ns in diffusing from light harvesting antenna pigments to photosystem I pigments. The diffusion coefficient of triplet excitons is a least 300--400 times slower than the diffusion coefficient of singlet excitons in chloroplast membranes.

Light collection and harvesting processes in bacterial photosynthesis investigated on a picosecond time scale.

Fluorescence lifetimes have been determined for four strains of Rhodopseudomonas sphaeroides. Chromatophore samples were excited with a single picosecond flash, and the fluorescence was detected with a streak camera. The decay times are 100 psec in strains 2.4.1 and Ga, and 300 psec in the carotenoidless strain R-26. These times are related to the transfer of energy from the light-harvesting antenna pigment molecules to the photochemical reaction center. In strain PM-8 dpl, which lacks reaction centers, the lifetime is 1.1 nsec. In addition, we have obtained curves relating the quantum yield of fluorescence to the photon density of the excitation pulse. These curves can be fit with a simple model that relates excitonic processes to properties of the photosynthetic unit and that qualitatively describes differences between the mutant strains.