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A Migus

Publications and source records attributed to A Migus.

At least 19 recordsLinked to original sources
PubMed2000

Photometric observations of a polychromatic laser guide star.

We report the photometric observation of a polychromatic laser guide star (PLGS) using the AVLIS laser at the Lawrence Livermore National Laboratory (LLNL). The process aims at providing a measurement of the tilt of the incoming wave front at a telescope induced by atmospheric turbulence. It relies on the two-photon coherent excitation of the 4D5/2 energy level of sodium atoms in the mesosphere. We used two laser beams at 589 and 569 nm, with a maximum total average output power of approximately 350 W. For the purpose of photometric calibration, a natural star was observed simultaneously through the same instrument as the PLGS at the focus of the LLNL 50-cm telescope. Photometric measurements of the 330-nm return flux confirm our previous theoretical studies that the PLGS process should allow us at a later stage to correct for the tilt at wavelengths as short as approximately 1 microm at good astronomical sites. They show also that, at saturation of two-photon coherent absorption in the mesosphere, the backscattered flux increases by a factor of approximately 2 when the pulse repetition rate decreases by a factor of 3 at constant average power. This unexpected behavior is briefly discussed.

Journal Article↗
PubMed1988

Femtosecond charge separation in organized assemblies: free-radical reactions with pyridine nucleotides in micelles.

Femtosecond laser UV pulse-induced charge separation and electron transfer across a polar interface have been investigated in anionic aqueous micells (sodium lauryl sulfate) containing an aromatic hydrocarbon (phenothiazine). The early events of the photoejection of the electron from the micellized chromophore and subsequent reaction of electron with the aqueous perimicellar phase have been studied by ultrafast infrared and visible absorption spectroscopy. The charge separation (chromophore +...e-) inside the micelle occurs in less than 10(-13) s (100 fs). The subsequent thermalization and localization of the photoelectron in the aqueous phase are reached in 250 fs. This results in the appearance of an infrared band assigned to a nonrelaxed solvated electron (presolvated state). This transient species relaxes toward the fully solvated state of the electron in 270 fs. In anionic aqueous micelles containing pyridine dinucleotides at high concentration (0.025-0.103 M), a single electron transfer can be initiated by femtosecond photoionization of phenothiazine. The one-electron reduction of the oxidized pyridine dinucleotide leads to the formation of a free pyridinyl radical. The bimolecular rate constant of this electron transfer depends on both the pH of the micellar system and the concentration of oxidized acceptor. The free-radical reaction is analyzed in terms of the time dependence of a diffusion-controlled process. In the first 2 ps following the femtosecond photoionization of PTH inside the micelle, an early formation of a free pyridinyl radical is observed. This suggests that an ultrafast free-radical reaction with an oxidized form of pyridine nucleotide can be triggered by a single electron transfer in less than 5 X 10(11) s-1.

Adenosine Monophosphate↗
PubMed1986

Femtosecond spectroscopy of excitation energy transfer and initial charge separation in the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis.

Reaction centers from the photosynthetic bacterium Rhodopseudomonas viridis have been excited within the near-infrared absorption bands of the dimeric primary donor (P), of the "accessory" bacteriochlorophylls (B), and of the bacteriopheophytins (H) by using laser pulses of 150-fsec duration. The transfer of excitation energy between H, B, and P occurs in slightly less than 100 fsec and leads to the ultrafast formation of an excited state of P. This state is characterized by a broad absorption spectrum and exhibits stimulated emission. It decays in 2.8 +/- 0.2 psec with the simultaneous oxidation of the primary donor and reduction of the bacteriopheophytin acceptor, which have been monitored at 545, 675, 815, 830, and 1310 nm. Although a transient bleaching relaxing in 400 +/- 100 fsec is specifically observed upon excitation and observation in the 830-nm absorption band, we have found no indication that an accessory bacteriochlorophyll is involved as a resolvable intermediary acceptor in the primary electron transfer process.

Journal Article↗
PubMed1986

Femtosecond spectroscopy of electron transfer in the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26: Direct electron transfer from the dimeric bacteriochlorophyll primary donor to the bacteriopheophytin acceptor with a time constant of 2.8 +/- 0.2 psec.

The primary light-induced charge separation in reaction centers from Rhodopseudomonas sphaeroides R-26 has been investigated after excitation with laser pulses of 150 fsec duration within the longwave absorption band of the primary donor at 850 nm. An excited state of the primary donor, characterized by a broad absorption spectrum extending over the whole spectral range investigated (545-1240 nm), appeared within 100 fsec and gave rise to stimulated emission in the 870- to 1000-nm region with a 2.8-psec lifetime. The photooxidation of the primary donor, as measured at 1240 nm, and the photoreduction of the bacteriopheophytin acceptor, monitored at 545 nm and 675 nm, have been found to proceed simultaneously with a time constant of 2.8 +/- 0.2 psec. Kinetics of absorbance changes at other probe wavelengths gave no indication that an accessory bacteriochlorophyll is involved as a transient electron acceptor.

Journal Article↗