Excitation Energy Dependent Ultrafast Luminescence Behavior of CdS Nanostructures

Selected semiconductor nanostructures provide extremely localized coherent light sources. Here an ensemble of CdS nanostructures was excited by UV/vis femtosecond laser pulses and their ultrafast luminescence characteristics were investigated as functions of the pulse energy fluence and the photon quantum energy. All optical Kerr gating enabled studies of the emission dynamics with a time resolution of 150 fs avoiding any influence on the CdS emission. The initially observed emission built up after a delay of 0.1–3 ps and decayed rapidly in a biexponential way, strongly dependent on both the laser energy fluence and the quantum energy. The central wavelength of the emission spectrum revealed a significant blue-shift within the first few ps followed by a transient red-shift relative to spontaneous excitonic emission of CdS. All findings are mainly attributed to stimulated radiative carrier recombination in the laser excited electron–hole plasma after its thermalization with the CdS lattice

Triplet-Encoded Prebiotic RNA Aminoacylation

The encoding step of translation involves attachment of amino acids to cognate tRNAs by aminoacyl-tRNA synthetases, themselves the product of coded peptide synthesis. So, the question arisesbefore these enzymes evolved, how were primordial tRNAs selectively aminoacylated? Here, we demonstrate enzyme-free, sequence-dependent, chemoselective aminoacylation of RNA. We investigated two potentially prebiotic routes to aminoacyl-tRNA acceptor stem-overhang mimics and analyzed those oligonucleotides undergoing the most efficient aminoacylation. Overhang sequences do not significantly influence the chemoselectivity of aminoacylation by either route. For aminoacyl-transfer from a mixed anhydride donor strand, the chemoselectivity and stereoselectivity of aminoacylation depend on the terminal three base pairs of the stem. The results support early suggestions of a second genetic code in the acceptor stem

Spectroscopic Characterization of Rocksalt-Type Aluminum Nitride

Starting from nanocrystalline and submicron wurtzite-aluminum nitride (w-AlN) powder rocksalt structure (rs-AlN) samples were synthesized by two different methods of shock wave recovery experiments. The resulting samples contained up to 86% rs-AlN, stable at room temperature, giving for the first time the possibility to comprehensively characterize the material by powder X-ray diffraction, Fourier transform infrared (IR), Raman, and ²⁷Al NMR spectroscopy. Raman and IR modes were calculated by density functional theory, allowing for the interpretation of the respective experimental spectra. By ²⁷Al NMR the chemical shift of rs-AlN was determined, and the quadrupolar coupling constant was estimated