Diamagnetic shift of exciton energy levels in GaAs-Ga1−xAlxAs quantum wells

The results of direct measurements of the diamagnetic shift of axciton levels in narrow quantum wells of a thickness varying between 25 and 150 Å are reported. A perturbation type approach is used to calculate the diamagnetic shift of 1s exciton levels in quantum well structures of Ga_1−xAl_xAs-GaAs-Ga_1−xAl_xAs. The calculations are applicable in the weak field range for which the Coulomb energy dominates over the magnetic one. The experimental results are in satisfactory agreement with the theory throughout the entire well thicknesses range.     

Oxygen adsorption on silver (110): Dispersion,bonding and precursor state

Angle-resolved photoemission, both in the laboratory and with synchrotron radiation, has been used to investigate three states of adsorbed oxygen on Ag(110): chemisorbed atomic oxygen, undissociated chemisorbed dioxygen and physisorbed molecular oxygen. For atomic oxygen, dispersion of adsorbate-induced levels was observed indicating strong oxygen-oxygen interaction in the [001] direction. Polarised light combined with selection rules was used to determine the symmetry of the adsorbate-derived bands. The adsorption geometry and symmetry of the oxygen-induced levels for the chemisorbed dioxygen species were also investigated with the selection rules. The molecule appears to lie parallel to the surface with the OO axis oriented in the [11̄0] direction. The adsorbate-induced feature of lowest ionisation potential at 1.1 eV relative to E_F is 1 π_g-derived. The very low frequency reported for the OO stretch and the analogy with coordination chemistry also suggest that the chemisorbed dioxygen species is lying down on the surface. At sufficiently low temperature, oxygen was found to physisorb on the bare metal also with its OO axis parallel to the surface. We identify physisorbed oxygen as an intrinsic precursor state for chemisorption.     

Incoherent Kondo ground state in CeCu1.54Si1.46

Measurements of the magnetic susceptibility, magnetization, specific heat and electrical resistivity on a new Kondo lattice compound CeCu_1.54Si_1.46 have revealed an antiferromagnetic phase transition at 6.9 K. The analysis of the specific heat demonstrates that this compound is a moderately heavy electron system with strong spin fluctuations. Based on the resistivity result, we maintain that the coherence between Kondo states at Ce sites is hindered by the disorder in the Cu and Si sublattice in this non-stoichiometric compound.     

A 270 MHz 1H-NMR study of three “branched” ribonucleotides A2'p5'U3'p5'g,A2'p5'C3'p5'G and A2'p5'G3'p5'G the importance of being branched with a 2'→5' phosphodiester bond in the pre-mRNA processing reactions (splicing)

Conformational studies with three analogues 3, 4 and 5 of naturally-occurring branched ribonucleotides 1 and 2 have shown that they adopt a secondary structure which is overwhelmingly controlled by a stacking between adenine(2' → 5')nuc1eobase residues while the 3' → 5' linked guanine residue is apart. This observation along with the fact that all four nucleosides can form lariats in nuclear mRNA splicing, but it is guanosine as the 2' → 5' nucleotide that can drive the splicing reaction to completion, suggests a biological significance of the additional 2' → 5' phosphodiester bond formation in the splicing reaction. It is likely that the final implication of formation of such a 2' → 5' linked lariat is that it provides a pathway to assume a free energy minimum conformation through the 2' → 5' stacking, especially in the case of guanosine, to drive the splicing reaction to completion (stacklng-driven-energy-pump).     

Novel friedel - crafts reactions of some arenes

The novel secondary Friedel-Crafts acylation products of the arenes, dimethyl resorcinol, m-xylene, m-methylanisole and m-chlorotoluene using dichloro-acetylchloride and anhydrous AlCl₃ are described. The formation of the unexpected products has been shown to depend on the nucleophilicity of the arene and the electrophilicity of the acyl carbonyl of the initially formed acylated product. Higher molar proportion of the arene and sometimes elevated temperature are necessary for generation of the products. The reaction pathway has been shown to be incluenced by the formation of the possible complex of AlCl₃ and dichloro-acetylchloride with the groups adjacent to the dichloroacetyl function, especially at higher temperature. Dimethylresorcinol yields the unique products 4, 5, 6, 7 and 9 and m-xylene, m-methylanisole and m-chlorotoluene furnish 10, 13 and 15 respectively. Electronic factors responsible for generation of the products and rationalisation of their formation are discussed.