Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.

Optical spectroscopy of extrinsic recombinations in gallium selenide

Abstract: Luminescence at energy lower than the absorption edge has been investigated in crystals of GaSe, containing different degrees of lattice disorder, as a function of temperature, of photoexcitation intensity, and of excitation energy. At low excitation intensity, the extrinsic luminescence is composed of broad overlapping bands that present a blue shift when the temperature increases. Their shape and intensity is strongly dependent on the laser excitation frequency and on the degree of lattice disorder. The results are discussed in terms of a model involving the recombination of shallow donors and free electrons with deep acceptors. These two recombination mechanisms and those involving free and bound excitons are found to be competitive. Their relative importance is strongly dependent on the concentration of structural defects, on the temperature, on the excitation intensity, and on the excitation frequency.

Interplay between the hot phonon effect and intervalley scattering on the cooling rate of hot carriers in GaAs and InP

Abstract: The influence of hot phonon effect and intervalley scattering on the hot carrier cooling rate was investigated using femtosecond time-resolved photoluminescence spectroscopy in bulk GaAs and InP, two electronically similar but vibrationally distinct semiconductors. In both materials, a broad photoluminescence signal that extends from the band gap energy to values larger than the pump pulse energy was observed during the first few picoseconds after photoexcitation, for different excitation energies (1.7, 1.88, and 2.4 eV) at high carrier densities (>1019 cm−3). Different hot carrier relaxation times were observed in GaAs and InP for different excitation energies, demonstrating the influence of intervalley scattering phenomena in GaAs. When electrons were not energetic enough to access satellite valleys, longer decay transients were observed for InP compared with GaAs. This provides experimental evidence of the hot phonon effect in InP. Temperature transients were calculated by analyzing the topography of the two-dimensional spectra. Copyright © 2011 John Wiley & Sons, Ltd.

Decarboxylative cross-nucleophile coupling via ligand-to-metal charge transfer photoexcitation of Cu(ii) carboxylates

Abstract: Reactions that enable carbon-nitrogen, carbon-oxygen and carbon-carbon bond formation lie at the heart of synthetic chemistry. However, substrate prefunctionalization is often needed to effect such transformations without forcing reaction conditions. The development of direct coupling methods for abundant feedstock chemicals is therefore highly desirable for the rapid construction of complex molecular scaffolds. Here we report a copper-mediated, net-oxidative decarboxylative coupling of carboxylic acids with diverse nucleophiles under visible-light irradiation. Preliminary mechanistic studies suggest that the relevant chromophore in this reaction is a Cu(II) carboxylate species assembled in situ. We propose that visible-light excitation to a ligand-to-metal charge transfer (LMCT) state results in a radical decarboxylation process that initiates the oxidative cross-coupling. The reaction is applicable to a wide variety of coupling partners, including complex drug molecules, suggesting that this strategy for cross-nucleophile coupling would facilitate rapid compound library synthesis for the discovery of new pharmaceutical agents.