Low-divergence operation of a long-pulse excimer laser using a SBS phase-conjugate cavity

The use of an optical cavity containing a stimulated Brillouin scattering (SBS) phase-conjugate reflector to control the spatial and spectral output of a long pulse (>100 ns) X-ray preionized XeCl laser is reported. The divergence of the 3×4 cm² output beam has been improved by a factor of 20 to <150 rad. The temporal form of the output depends on the acoustic decay times of the liquid and gaseous SBS media used. Four-wave mixing via gain saturation has been observed within the discharge volume, leading to depolarisation of the output beam.     

Non-aqueous sers: Pyridine in N,N'-dimethylformamide solution at a silver electrode

Surface-enhanced Raman spectra are reported from pyridine in N,N′-dimethylformamide at a silver electrode. Potential and time-dependent results are presented and discussed in terms of the electrode surface morphology.     

Photoisomerization of cis‐1,2‐di(1‐Methyl‐2‐naphthyl)ethene at 77 K in Glassy Media

cis‐1,2‐Di(1‐methyl‐2‐naphthyl)ethene, c‐ 1,1 , undergoes photoisomerization in methylcyclohexane, isopentane and diethyl ether/isopentane/ethanol glasses at 77 K. On 313 nm excitation the fluorescence of c‐ 1,1 is replaced by fluorescence from t‐ 1,1 . Singular value decomposition reveals that the spectral matrices behave as two component systems suggesting conversion of a stable c‐ 1,1 conformer to a stable t‐ 1,1 conformer. However, the fluorescence spectra are λ_exc dependent. Analysis of global spectral matrices shows that c‐ 1,1 is a mixture of two conformers, each of which gives one of four known t‐ 1,1 conformers. The λ_exc dependence of the c‐ 1,1 fluorescence spectrum is barely discernible. Structure assignments to the resolved fluorescence spectra are based on the principle of least motion and on calculated geometries, energy differences and spectra of the conformers. The relative shift of the c‐ 1,1 conformer spectra is consistent with the shift of the calculated absorption spectra. The calculated structure of the most stable conformer of c‐ 1,1 agrees well with the X‐ray crystal structure. Due to large deviations of the naphthyl groups from the ethenic plane in the conformers of both c‐ and t‐ 1,1 isomers, minimal motion of these bulky substituents accomplishes cis → trans interconversion by rotation about the central bond.     

Studies of higher temperature polymerization of n-butyl methacrylate and n-butyl acrylate

Free-radical acrylic polymerizations of n-butyl methacrylate and n-butyl acrylate at temperatures above 120°C show significant departure from classic free-radical kinetics. An extended model of depropagation, where the equilibrium monomer concentration varies with temperature and polymer content, is postulated and shown to adequately explain the data for n-butyl methacrylate. Intramolecular chain transfer and scission is postulated to explain the apparent reduction in molecular weight and rate of polymerization seen in n-butyl acrylate polymerization, with supporting experimental evidence found via electrospray-ionization mass spectrometry.     

A module as a torsion-free cover

LetR be a ring, and let (ℐ, ℱ) be an hereditary torsion theory of leftR-modules. An epimorphism ψ:M→X is called a torsion-free cover ofX if (1)M∈ ℱ, (2) every homomorphism from a torsion-free module intoX can be factored throughM, and (3) ker ψ contains no nonzero ℐ -closed submodules ofM. Conditions onM andN are studied to determine when the natural mapsM→M/N andQ(M)→Q(M)/N are torsion-free covers, whenQ(M) is the localization ofM with respect to (ℐ, ℱ). IfM→M/N is a torsion-free cover andM is projective, thenN⊆radM. Consequently, the concepts of projective cover and torsion-free cover coincide in some interesting cases.