6450 nm wavelength tissue ablation using a nanosecond laser based on difference frequency mixing and stimulated Raman scattering

A four-stage laser system was developed, emitting at a wavelength of 6450 nm with a 3-5 ns pulse duration, < or = 2 mJ pulse energy, and 1/2 Hz pulse repetition rate. The laser system successfully ablated rat brain tissue, where both the collateral damage and the ablation rate compare favorably with that previously observed with a Mark-III Free-Electron Laser.     

Variations of mixed Hodge structure, Higgs fields, and quantum cohomology

Following C. Simpson, we show that every variation of graded-polarized mixed Hodge structure defined over ℚ carries a natural Higgs bundle structure which is invariant under the ℂ^* action studied in [20]. We then specialize our construction to the context of [6], and show that the resulting Higgs field θ determines (and is determined by) the Gromov–Witten potential of the underlying family of Calabi–Yau threefolds. Received: 14 February 2000     

The locus of Hodge classes in an admissible variation of mixed Hodge structure

We generalize the theorem of E. Cattani, P. Deligne, and A. Kaplan to admissible variations of mixed Hodge structure.     

Computer-aided molecular modeling of polymers. III. enthalpy of polymerization as a measure of stability

A formalism of computational chemistry methods is presented to estimate the stability of vinyl polymers. This approach takes into account changes in electronic energy upon polymerization using quantum mechanical methods and contributions of the conformational energetics of the polymerized state using a molecular mechanics force field. A work term, ΔV, based upon the molecular volume difference between the monomer and the reactant, is shown to be negligible. For 10 structurally diverse vinyl polymers, the sum of the electronic and conformational energy differences between reactant and monomer states, AEp, has a high linear correlation with corresponding measured enthalpies of polymerization, ΔHp. The linear regression least square fit is ΔHp = 0.89 AEp-13.39. Errors due to possible contributions to AHp not included in the formalism reported here are probably small and/or relatively constant over the set of polymers studied. If this were not the case, a linear correlation between AHp and AEp, with a slope near 1, would not be observed. Most likely the intercept of-13.39 kcal/mol is due to the choice of the quantum mechanical method used, MNDO. Overall, the formalism presented here seems a reliable means of predicting relative polymerization stability, in advance of synthesis, for a structurally congeneric set of polymers.     

PHOSPHORESCENCE QUENCHING OF POLY rA IN GLUCOSE-SODIUM ACETATE SOLUTION AT 77 K*

Abstract. The phosphorescence of poly rA at 77 K in an aqueous medium containing glucose is half-quenched by bound ethidium bromide at a fractional concentration, r_1/2, of 0.005, and by bound proflavine at r_1/2= 0.002: r_1/2= 0.024 for Co²⁺ and r_1/2= 0.039 for Mn²⁺. The decay of the dye-quenched poly rA phosphorescence is markedly nonexponential and decays more rapidly with increasing dye concentration, while the decay of the metal-ion quenched poly rA phosphorescence parallels that of the unquenched poly rA, independent of metal-ion concentration. Förster overlap integrals and critical distances for transfer of both poly rA singlet and triplet excitation to dye singlet states are calculated, and used to explain a consistent interpretation of the experimental results in terms of one-step direct excitation transfer from base to dye or metal ion in a highly folded polymer conformation.     

On the properties of the extended ground rotational bands in actinide even-even nuclei

The energies of the yrast levels extending up to I = 28 in even-even actinide nuclei were analyzed in terms of an angular velocity expansion E_rot(I) = ∑_i=1ⁿα_iω_I²ⁱ with n = 2?7 to test the application of this expansion to high spin. A strong ω²-dependence is observed for the Variable Moment of Inertia model (VMI) with two parameters α_i(ω²), particularly at the higher spins. There is a marked difference in the ω²-dependence, of these parameters between the N = 142 and other nuclei studied in this region to indicate structure effects are present at the higher spin.     

SIMULATIONS OF Prosthecochloris BACTERIOCHLOROPHYLL a-PROTEIN OPTICAL SPECTRA IMPROVED BY PARAMETRIC COMPUTER SEARCH

Recently, reasonable spectral simulations of the experimental low-temperature absorption and circular dichroism (CD) spectra of the bacteriochlorophyll (BChl) a protein from Prosthecochloris aestuarii were produced for the unaggregated protein trimer based on standard assumptions regarding Q_Y transition moment directions. We report here significant improvements of these simulations, obtained by computer search of unknown parameters including the site wavelengths of the individual BChl and exciton-transition line widths. Absorption and CD spectra are fit with a common set of search parameter values. As before, calculated exciton-transition wavelengths and line widths also compare favorably with values deduced from laser hole-burning experiments. However, the improved simulations are only obtained when comparison is made to 77 K data obtained with a cryosolvent containing glycerol but not glycerophosphate. Simulations of 77 K data obtained with a mixture of glycerol and glycerophosphate are poorer than those recently reported. (The latter are based on 5 K absorption and 77 K CD data, with transition line widths independently selected for each simulation.) Computer tests of the search routine demonstrate that these results are independent of the routine itself. A possible explanation is that the glycerophosphate-containing cryosolvent perturbs the protein structure enough to alter slightly the inter-BChl geometry.     

Effect of steady ampoule rotation on radial dopant segregation in vertical Bridgman growth of GaSe

For vertical Bridgman growth of the nonlinear optical material GaSe in an ampoule sufficiently long that flow and dopant transport are not significantly influenced by the upper free surface, we show computationally that steady rotation about the ampoule axis strongly affects the flow and radial solid-phase dopant segregation. Radial segregation depends strongly on both growth rate U and rotation rate Ω over the ranges 0.25 μms⁻¹U3.0 μms⁻¹ and 0Ω270 rpm. For each growth rate considered, the overall radial segregation passes through two local maxima as Ω increases, before ultimately decreasing at large Ω. Rotation has only modest effects on interface deflection. Radial segregation computed using a model with isotropic conductivity (one-third the trace of the conductivity tensor) predicts much less radial segregation than the “correct” model using the anisotropic conductivity, with the segregation decreasing monotonically with Ω. Consideration of a model in which centrifugal acceleration is deliberately omitted shows that, as Ω increases, diminution and ultimately disappearance of the “secondary” vortex lying immediately above the interface is due to centrifugal buoyancy, while axial distension of the larger “primary” vortex above is due to Coriolis effects. These results, which are qualitatively different from those accounting for centrifugal buoyancy, suggest that several earlier computational and analytical predictions of rotating vertical Bridgman growth are either limited to rotation rates sufficiently low that centrifugal buoyancy is unimportant, or are artifacts associated with its neglect. The overall radial segregation depends approximately linearly on the product of and the growth rate U for the conditions considered, where is the segregation coefficient.