High-brightness ultraviolet excimer lasers

At present the performance of short-pulse rare-gas-halide excimer-laser systems is definitely below that of solid-state systems, as far as the maximum peak-power is concerned. However, short-pulse excimer lasers are expected to be the best candidates to produce the highest focused intensities (I 10²⁰ W/cm²) provided by the shorter wavelength and less optical distortion in the gaseous active medium. This is especially feasible if the present performance of short-wavelength focusing optics is improved, and the problem of the limited extraction efficiency of excimers is solved. In this paper the results of former developments, novel methods, such as spatially-evolving chirped-pulse amplification, off-axis amplification, interferometric multiplexing, and some considerations for the achievable maximum brightness of table-top excimer systems are presented.     

Spectroscopic properties of the photoproducts of pyridoxal-5′-P irradiation: Catalytic site recognition of ribonuclease A

Photoproducts of pyridoxal-5-P, i.e., 4-pyridoxic-5-P and bis-pyridoxal-5-P, have been studied by spectroscopic methods. The spectroscopic properties of bis-pyridoxal-5-P (bis-PLP) resemble those of pyridoxal-5-P (PLP) under similar experimental conditions. The coupling of methylen hydrogens to the phosphorus atom has been shown by NMR spectroscopy. The singlet in the³¹P-NMR spectra and the triplet in¹H-undecoupled experiments confirm the presence of the phosphate group in the 5 position of the structure of the vitamin. The effect of pH and solvent composition on the relative distribution of species of bis-pyridoxine-5-P (bis-PNP) has been investigated by absorption and fluorescence spectroscopy. The acid-base dissociation of the phosphate group is easily detected by emission spectroscopy. Bis-PNP and bis-PLP bind to the enzyme RNase A and they behave as competitive inhibitors with respect to the substrate cytidine-2-3-cyclic phosphate. The natural forms of vitamin B₆, pyridoxine, and pyridoxine-5-P have no effect on the catalytic activity of the protein. Experimental evidence derived from fluorescence and inhibition experiments is consistent with the hypothesis that bis-PNP recognizes the catalytic site of RNase A.     

Propagating fronts,chaos and multistability in a cell replication model

Numerical solutions to a model equation that describes cell population dynamics are presented and analyzed. A distinctive feature of the model equation (a hyperbolic partial differential equation) is the presence of delayed arguments in the time (t) and maturation (x) variables due to the nonzero length of the cell cycle. This transport like equation balances a linear convection with a nonlinear, nonlocal, and delayed reaction term. The linear convection term acts to impress the value of u(t,x=0) on the entire population while the death term acts to drive the population to extinction. The rich phenomenology of solution behaviour presented here arises from the nonlinear, nonlocal birth term. The existence of this kinetic nonlinearity accounts for the existence and propagation of soliton-like or front solutions, while the increasing effect of nonlocality and temporal delays acts to produce a fine periodic structure on the trailing part of the front. This nonlinear, nonlocal, and delayed kinetic term is also shown to be responsible for the existence of a Hopf bifurcation and subsequent period doublings to apparent "chaos" along the characteristics of this hyperbolic partial differential equation. In the time maturation plane, the combined effects of nonlinearity, nonlocality, and delays leads to solution behaviour exhibiting spatial chaos for certain parameter values. Although analytic results are not available for the system we have studied, consistency and validation of the numerical results was achieved by using different numerical methods. A general conclusion of this work, of interest for the understanding of any biological system modeled by a hyperbolic delayed partial differential equation, is that increasing the spatio-temporal delays will often lead to spatial complexity and irregular wave propagation. (c) 1996 American Institute of Physics.     

Pulsed-laser photolysis of Mn2(CO)10: A time-resolved fluorescence study

Mn₂(CO)₁₀ was photolysed in the gas phase by the XeCl-excimer laser with fluence in the range 25–300 mJ/cm² and a dye laser. The UV/VIS emission of the products was probed on a nanosecond time scale. The emission from excited states of metal atoms was detected only. The Mn atoms are predominantly formed in their ground statea ⁶ S _{2 1/2}. The absorption of one photon and the subsequent relaxation process leads to the formation ofz ⁶ P _J ⁰ (J = 11/2, 21/2, 31/2) states and emission of photons at a wavelength of 403 nm. The formation of the excited statese ⁸ D _{5 1/2},z ⁶ F _{4 1/2} ⁰ ande ⁶ D _{4 1/2} and the subsequent emission observed at wavelengths of 357, 383 and 446 nm requires the absorption of two photons by the ground-state Mn atoms. In addition, transition from thea ⁶ D _j (J = 11/2, 21/2) lower states were observed in the wavelength-resolved Laser-Induced Fluorescence (LIF) spectra.     

Production of fast evaporation residues by the reaction20Ne+208Pb at projectile energies of 8.6, 11.4 and 14.9 A.MeV

Velocity distributions and production cross sections of evaporation residues have been measured in the reaction²⁰Ne+²⁰⁸Pb at projectile energies of 8.6, 11.4, 14.9 A.MeV. Essential deviations from statistical model of deexcitation have been observed. Monte Carlo simulations involving emission of non-equilibrium particles have been used in order to reproduce experimental velocity, charge and mass distributions of evaporation residues and to estimate indirectly multiplicities of pre-equilibrium particles. Communicated by V. Metag     

The dilution of silica in a NiZnCo spinel ferrite matrix

Silica may dissolve up to about 0.05 wt% in an iron excess NiZnCo spinel matrix, affecting noticeably the magnetic properties at low and high values of the applied field. The initial permeability is reduced by about 50% with the addition of only 0.1 % SiO₂, while the coercivity almost triples, without noticeable changes in porosity or grain distribution. The vacancy and Fe²⁺ content of the sintered material also show a considerable change. The energy associated to the transformation Fe³⁺ Fe²⁺ for the samples without SiO₂ was found to be 0.41–0.42 eV, while for the samples with SiO₂ additions it decreased to 0.26 eV. The changes in magnetic properties are thought to be caused, more than because of the mentioned changes in ion distribution, by the stresses and defects created in the lattice due to the very small size of the Si⁴⁺ ion, which substitutes Fe³⁺ ions in tetrahedral sites. The transition Fe³⁺ Fe²⁺ seems to take place exclusively in the octahedral sites.     

Influence of oxygen concentration on optical properties of semi-insulating polycrystalline silicon films

Optical properties of semi-insulating polycrystalline silicon (SIPOS) films were investigated over the wide region of wavelengths. The vibrational modes of Si-O-Si and Si-Si bonds were studied by IR and Raman spectroscopy. A pronounced effect of oxygen content on the measured spectra was observed. Optical constants of SIPOS films have been compared with those of amorphous silicon and glassy silicon dioxide and their values have been successfully interpreted in terms of the theory of effective medium. The results have shown that the amorphous character prevails in the as-deposited SIPOS films.     

Isometric embedding for homogeneous compact 3-manifolds

The anisotropic homogeneous ³ is isometrically globally embedded into a nine-dimensional Euclidean or pseudo-Euclidean space. In a special case the Euclidean space is six-dimensional. The space-sections of the anti-Mach metric appear as a submanifold of theE ⁹. We obtain also the isometric embedding ofSO(3) with its natural Killing metric.     

Boson realization of YangiansY(sl(2)) andY(sl(3))

Studying the algebraic structure of the YangiansY(sl(2)), andY(sl(3)) we present their boson realizations. In the case ofY(sl(2)) we give the realization by using 1-canonical boson pair and two parameters and in theY(sl(3)) with 2-canonical boson pairs, two parameters and subalgebrasl(2). Substituting the realization ofU(sl(2)), we can obtain the pure boson realization by 3-canonical pairs and three parameters.Presented at the 4th Colloquium Quantum Groups and Integrable Systems, Prague, 22–24 June 1995.     

Theq-calculus for genericq andq a root of unity

Theq-calculus for genericq is developed and related to the deformed oscillator of parameterq ^1/2. By passing with care to the limit in whichq is a root of unity, one uncovers the full algebraic structure ofZ _n -graded fractional supersymmetry and its natural representation.Presented at the 5th International Colloquium on Quantum Groups: Quantum Groups and Integrable Systems, Prague, 20–22 June 1996.This paper describes research supported in part by E.P.S.R.C and P.P.A.R.C. (UK) and by the C.I.C.Y.T (Spain). J.C.P.B. wishes to acknowledge an FPI grant from the CSIC and the Spanish Ministry of Education and Science.