Spectral kinetic properties and lasing characteristics of diode-pumped Cr<Superscript>2+</Superscript>:ZnSe single crystals

The temperature and concentration quenching of the luminescence of Cr²⁺ ions in Cr:ZnSe crystals is experimentally investigated and the radiative lifetime of the upper lasing level is determined. The values of the cross sections of absorption from the ground state of Cr²⁺ ions in Cr:ZnSe crystals are refined. It is shown that the absorption from the excited state of Cr²⁺ ions is negligible near 1.5 μm. Lasing in Cr:ZnSe single crystals is implemented with an efficiency of 31% and an output power of 186 mW using laser diode pumping at a wavelength of 1.77 μm.     

Microwave-resonance-induced resistivity: evidence of ultrahot surface-state electrons on liquid 3He

Measurements of the dc resistivity of surface-state electrons on liquid helium exposed to microwave radiation are reported. It is shown that the resonant microwave excitation of surface-state electrons is accompanied by a strong increase in their resistivity, which is opposite to the result expected from the previously used two-level model. We show that even a very small fraction of electrons excited to the first excited state and decaying back due to vapor-atom scattering strongly heat the electron system, causing a population of higher subbands. The calculated resistivity change is in good agreement with the observed data.     

Head-on collision of two coaxial vortex rings in an ideal fluid

The opposing motion of two coaxial vortex rings is classified in relation to the initial parameters.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 60–64, July–August, 1989.The authors are grateful to V. V. Meleshko for discussing their results.     

Pressure distribution on the surface of a spherical segment entering a compressible fluid

The pressure on the progressively expanding wetted surface of a spherical segment entering a compressible fluid has been experimentally determined.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 9–14, March–April, 1986     

A simple and convenient synthesis of 3-[5-(trifluoromethyl)-1,2,3-triazol-4-yl]cinnamic acids from 4-aryl-6-(trifluoromethyl)-2H-pyran-2-ones and sodium azide

4-Aryl-6-(trifluoromethyl)-2H-pyran-2-ones and ethyl 4-aryl-6-(trifluoromethyl)-2-oxo-2H-pyran-3-carboxylates react with sodium azide to produce highly functionalized CF₃-1,2,3-triazoles: 3-[5-(trifluoromethyl)-1,2,3-triazol-4-yl]cinnamic acids and monoethyl esters of [5-(trifluoromethyl)-1,2,3-triazol-4-yl]arylmethylidene malonic acids.     

Synthesis of β-pyrrolic-modified porphyrins and their incorporation into DNA

A synthetic methodology for the synthesis of various β-pyrrolic-functionalised porphyrins and their covalent attachment to 2'-deoxyuridine and DNA is described. Palladium(0)-catalysed Sonogashira and copper(I)-catalysed Huisgen 1,3-dipolar cycloaddition reactions were used to insert porphyrins into the structure of 2'-deoxyuridine and DNA. Insertion of a porphyrin into the middle of single-stranded CT oligonucleotides possessing a 5'-terminal run of four cytosines was shown to trigger the formation of pH- and temperature-dependent i-motif structures. Porphyrin insertion also led to the aggregation of single-stranded purine-pyrimidine sequences, which could be dissociated by heating at 90 °C for 5 min. Parallel triplexes and anti-parallel duplexes were formed in the presence of the appropriate complementary strand(s). Depending on the modification, porphyrins were placed in the major and minor grooves of duplexes and were used as bulged intercalating insertions in duplexes and triplexes. In general, the thermal stabilisation of parallel triplexes possessing porphyrin-modified triplex-forming oligonucleotide (TFO) strands was observed, whereas anti-parallel duplexes were destabilised. These results are compared and discussed on the basis of the results of molecular modelling calculations.     

Conformational properties of <Emphasis Type="Italic">ortho</Emphasis>-nitrobenzenesulfonamide in gas and crystalline phases. Intra- and intermolecular hydrogen bond

A combined gas-phase electron diffraction and quantum chemical (B3LYP/6-311+G**, B3LYP/cc-pvtz, MP2/cc-pvtz) study of molecular structure of 2-nitrobenzenesulfonamide (2-NBSA) was carried out. Quantum chemical calculations showed that 2-NBSA has four conformers, two of which are stabilized by intramolecular hydrogen bond. The latter (with the S–N bond in a close to orthogonal position around the phenyl ring and differing from each other by staggered or eclipsed positions of the N–H and S=O bonds in the SO₂NH₂ group) presented in a saturated vapor over 2-NBSA at T = 433 (3) K in commensurable amounts. Experimental internuclear distances (Ǻ) for the staggered conformer are (?): r _h1(C–H)_av. = 1.071(9), r _h1(C–C)_av. = 1.390(4), r _h1(C–S) = 1.789(8), r _h1(S=O)_av. = 1.427(6), r _h1(S–N) = 1.644(6), r _h1(N–O)_av. = 1.221(4), r _h1(C′–N) = 1.487(8), r _h1(N–H)_av. = 1.014. Calculations at B3LYP/cc-pvtz level were performed to determine the structure and the energies of the transition states between conformers. It was shown that the conformer structures of free molecule differ from those of a molecule stabilized by intermolecular hydrogen bonds in a crystal. Influence of a substituent X (X = –CH₃, –NO₂) on conformational features of the ortho-substituted benzenesulfonamide was established.     

A canonical form for nonderogatory matrices under unitary similarity

A square matrix is nonderogatory if its Jordan blocks have distinct eigenvalues. We give canonical forms for

•

nonderogatory complex matrices up to unitary similarity, and

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pairs of complex matrices up to similarity, in which one matrix has distinct eigenvalues.

The types of these canonical forms are given by undirected and, respectively, directed graphs with no undirected cycles.