Stanley L. Shapiro

Picosecond fluorescent kinetics of in vivo chlorophyll

Abstract Fluorescent emission kinetics at 685 nm of in vivo escarole chloroplast chlorophyll has been studied on a picosecond time scale using a frequency-doubled, mode-locked Nd: glass laser (4-ps wide pulses) and an optical Kerr gate. The fluorescent risetime is less than 10 ps and the decay time of the fluorescence envelope is 320 ± 50 ps. An apparent dip in the time dependence of the decay kinetics occurs at about 50 ps after the flash. This may be the result of energy transfer between carotenoids or chlorophyll b and chlorophyll a or an indication that two independent species are fluorescing.

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.

Picosecond measurements of exciton migration in tetracene crystals doped with pentacene

Abstract The kinetics of electronic energy transfer from host to guest have been measured for a tetracene crystal doped with pentacene. With single picosecond pulse excitation and streak camera diagnostics, the fluorescence risetime of the guest is measured to be the same as the decay time of the host. For low pentacene concentrations the exponential decay of the host at 170 K is consistent with a diffusion model for singlet migration from host to guest. At high intensities, no evidence was found for guest saturation because of the dominating effect of bimolecular exciton annihilation in the host.

Soft apertures for reducing damage in high-power laser-amplifier systems

Abstract Soft apertures have been constructed for tailoring the spatial profile of a light beam to minimize harmful diffraction effects that lead to self-focusing and consequent damage while still enabling substantial energy extraction from amplifiers.

Fluorescence risetime of NE 102 scintillator

Abstract NE 102 is excited with picosecond pulses at 0.353 μm, and the onset of fluorescence is detected with a streak camera. The risetime of the fluorescence is measured to be ≤20 ps.