Study of lasing threshold and efficiency in laser crystal powders

We have analyzed theoretically the dependence of lasing threshold and efficiency of laser crystals powders on the sample thickness, the volume fraction occupied by the particles and the mean particle size. The optimal range of these parameters that minimize the threshold and maximize the efficiency has been determined.     

K5Nd(MoO4)4: a self-tunable laser crystal

We report for the first time to our knowledge an experimental demonstration of wavelength self-tuning in a K5Nd(MoO4)4 broadband-emitting laser crystal (KNM), as well as a theoretical treatment of the system based on its birefringence properties. The self-frequency-tuning of the laser along the full spectral range of the KNM crystal was obtained by rotating a birefringent gain plate in its own plane. The gain plate was placed inside a resonator at Brewster's angle. The tuning characteristics of the spectral filter were obtained by use of the Jones vector formalism. The calculated wavelength-selective tuning precisely matches the experimental observations.     

The stereochemistry of asteriscunolides: An x-ray based correction

The application of X-ray diffraction methods has shown that the configurations of Asteriscunolides A and B at the Δ⁹ double bond are Z, rather than those previously proposed E, while the stereochemistries of Asteriscunolides C and D were correctly established. Some unexpected NMR data and the new assignment of the spectral signals, based on 2D-NMR $\text{C}\text{H}$ correlations and NOE-difference experiments are reported.     

Optical spectroscopy of (C2H5NH3)2CdCl4:Cu2+ under pressure: Study of Cu2+ local structure from theoretical calculations

The variations experienced by the energy E_u(π) of the e_u(π)→b_1g (～x²–y²) charge‐transfer transition of (C₂H₅NH₃)₂CdCl₄:Cu²⁺ upon pressure in the 0‐ to 40‐kbar range have been measured at room temperature by means of a sapphire anvil cell. These data reveal that E_u(π) undergoes a red shift of 1400 cm^?1 on passing from ambient pressure to 40 kbars. To understand this puzzling result theoretical calculations of ?E_u(π)/?R_eq and ?E_u(π)/?R_ax have been performed where R_eq and R_ax mean the equatorial and axial Cu²⁺–Cl^? distances of the elongated CuCl₆^4? complex, respectively. All results indicate that ?E_u(π)/?R_eq and ?E_u(π)/?R_ax for R_eq=228 pm and R_ax=297 pm are indeed negative. Moreover ab initio complete active space self‐consistent field (CASSCF/CASPT2) and density functional calculations lead to ?E_u(π)/?R_ax values, which are about 10 times smaller than those of ?E_u(π)/?R_eq. From the ensemble of experimental and theoretical results, it is concluded that a pressure of 40 kbars gives rise to a decrement of ≈25 pm of the axial distance and at the same time to an increase of ≈7 pm of the equatorial one. It is stressed that the present study on a diluted Jahn–Teller impurity lies far beyond the current possibilities of X‐ray absorption structure techniques. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful.