Fluorescence kinetics of emission from a small finite volume of a biological system

The fluorescence decay, apparent quantum yield and transmission from chromophores constrained to a microscopic volume using a single picosecond laser excitation were measured as a function of incident intensity. The β subunit of phycoeryhthrin aggregate isolated from the photosynthetic antenna system of Nostoc sp. was selected since it contains only four chromophores in a volume of less than 5.6×10⁴ ų. The non-exponential fluorescence decay profiles were intensity independent for the intensity range studied (5 × 10¹³ - 2 × 10¹⁵ photon cm^?2 per pulse). The apparent decrease in the relative fluorescence quantum yield and increase of the relative transmission with increasing excitation intensity is attributed to the combined effects of ground state depletion and upper excited state absorption. Evidence suggests that exciton annihilation is absent within isolated β subunits.     

Direct diode-pumped continuous-wave near-inf rared tunable laser operation of Cr(4+):forsterite and Cr(4+):Ca(2)GeO(4)

Continuous-wave tunable laser operation of direct diode-pumped Cr(4+):Mg(2)SiO(4) (Cr:forsterite) and Cr(4+):Ca(2)GeO(4) (cunyite) crystal were demonstrated. Diode-pumped Cr:forsterite was tunable over the 1236-1300-nm spectral range. The maximum output of 10 mW was measured at 1260 nm for 640 mW of pump power absorbed by the crystal. Diode-pumped laser operation of cunyite was also demonstrated over the 1390-1475-nm range. Free-running 20-mW output was centered at 1410 nm.     

Transient photoconductivity in the ferromagnetic semiconductor CdCr2Se4

The transient photoconductive response time of the ferromagnetic semiconductor CdCr₂Se₄ was measured to be smaller than 90 ps, the response time of the scope. The measurements were performed at room temperature (300 K) for 0.53 and 1.06 m excitations using a mode locked frequency-doubled YAG laser.     

Ultrashort Laguerre-Gaussian pulses with angular and group velocity dispersion compensation

Coherent optical vortices are generated from ultrashort 6.4 fs pulses. Our results demonstrate angular dispersion compensation of ultrashort 6.4 fs Laguerre-Gaussian (LG) pulses as well as what is believed to be the first direct autocorrelation measurement of 80 fs LG amplified pulses. A reflective-mirror-based 4f-compressor is proposed to compensate the angular and group velocity dispersion of the ultrashort LG pulses.     

Gas-phase pyrolysis mechanism and kinetics of 3-chloroquadricyclane

The gas-phase pyrolysis of 3-chloroquadricyclane [1] was investigated over the temperature range 513–550 K at one atm in helium. The initial pyrolysis step is the isomerization of 3-chloroquadricyclane to 7-chloronorbornadiene (E_a=39.63±1.40 kcal/mole, log A=15.18±0.58). 7-chloronorbornadiene rearranges (623–660 K) to exclusively produce benzyl chloride (E_a=48.05±1.10 kcal/mole, log A=15.82±0.38). This two step mechanism affords fewer reactions than the unsubstituted quadricyclane system in the gas phase. The production of a benzene derivative from the chlorinated norbornadiene is a reaction pathway contained in the unsubstituted norbornadiene and other 7-substituted pyrolysis mechanisms. © 1997 John Wiley & Sons, Inc.