Change in elemental composition and structural-phase transformations in molybdenum with the high-dose implantation of metal ions

A study is made of features of structural-phase transformations that occur in molybdenum during the high-dose implantation of metal ions in a vacuum ∼2·10⁻⁴ torr. It is established that ion-stimulated interaction of the surface of the target with elements in the residual gases plays an important role in the modification of the microstructure of the ion-alloyed layer under these conditions. It is shown that such interaction leads to the formation of thin continuous layers of a carbide phase in the surface layer. The continuous layers are in highly nonequilibrium structural-phase states. Experimental data is used as a basis for discussion of the mechanism of this interaction as a function of implantation conditions. V. D. Kuznetsov Siberian Physico-Technical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 65–74, September, 1996.     

Pair diffusion in quasi-one- and quasi-two-dimensional binary colloid suspensions

The authors report the results of measurements of the center of mass and relative pair diffusion coefficients in quasi-one-dimensional (q1D) and quasi-two-dimensional (q2D) binary colloid suspensions. The new results extend the findings of similar studies of one-component quasi-one-dimensional and quasi-two-dimensional colloid suspensions. Our principal new finding is that the presence of the smaller diameter component can destroy the oscillatory structure of the separation dependence of the q2D relative pair diffusion coefficient of the large particles even though the oscillatory character of the large particle equilibrium pair correlation function remains prominent, and that no such effect occurs with the q1D suspension. An interpretation of these results is proposed.     

Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases

We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.25 M or higher the proton transfer rates to the fluoride, dihdyrogen phosphate or acetate bases are fast and comparable. The fluorescence signals are nonexponential and complex. We suggest that the fastest decay component arises from a direct proton transfer process from the hydroxyl group of D-luciferin to the mild base. The proton donor and acceptor molecules form an ion pair prior to photoexcitation. Upon photoexcitation solvent rearrangement occurs on a 1 ps time-scale. The PT reaction time constant is ～2 ps for all three bases. A second decay component of about 10 ps is attributed to the proton transfer in a contact pair bridged by one water molecule. The longest decay component is due to both the excited-state proton transfer (ESPT) to the solvent and the diffusion-assisted PT process between a photoacid and a base pair positioned remotely from each other prior to photoexcitation.     

Excited-state intermolecular proton transfer of firefly luciferin IV. Temperature and pH dependence

Time-resolved emission as well as steady-state UV-vis techniques were employed to study the photoprotolytic processes that d-luciferin, the natural substrate of the firefly luciferase, undergoes in both acidic aqueous solutions and ice. The emission spectrum of D-luciferin in a 20 mM HCl aqueous solution or higher has an additional emission band at 590 nm red-shifted with respect to the strongest emission band positioned at 530 nm of the deprotonated NRO(-*) form in a pH-neutral aqueous solution. We attribute this emission band to the zwitterion form designated as (+)HNRO(-). The time-resolved emission signals show that the NRO(-*) emission band at 530 nm and the zwitterion emission band at 590 are strongly quenched by a recombination process with a proton in an acidic solution and in ice. In ice, the quenching rate is 10 times faster than in the liquid state. We attribute the fast quenching rate to the high value of the proton diffusion constant in ice.     

Compression induced folding of a sheet: an integrable system

The apparently intractable shape of a fold in a compressed elastic film lying on a fluid substrate is found to have an exact solution. Such systems buckle at a nonzero wave vector set by the bending stiffness of the film and the weight of the substrate fluid. Our solution describes the entire progression from a weakly displaced sinusoidal buckling to a single large fold that contacts itself. The pressure decrease is exactly quadratic in the lateral displacement. We identify a complex wave vector whose magnitude remains invariant with compression.     

Tuning the critical temperature of cuprate superconductor films with self-assembled organic layers

Control over the T(c) value of high-T(c) superconductors by self-assembled monolayers is demonstrated (T(c) = critical temperature). Molecular control was achieved by adsorption of polar molecules on the superconductor surface (see scheme) that change its carrier concentration through charge transport or light-induced polarization.     

Comparative study of the photoprotolytic reactions of D-luciferin and oxyluciferin

Optical steady-state and time-resolved spectroscopic methods were used to study the photoprotolytic reaction of oxyluciferin, the active bioluminescence chromophore of the firefly's luciferase-catalyzed reaction. We found that like D-luciferin, the substrate of the firefly bioluminescence reaction, oxyluciferin is a photoacid with pK(a)* value of ～0.5, whereas the excited-state proton transfer (ESPT) rate coefficient is 2.2 × 10(10) s(-1), which is somewhat slower than that of D-luciferin. The kinetic isotope effect (KIE) on the fluorescence decay of oxyluciferin is 2.5 ± 0.1, the same value as that of D-luciferin. Both chromophores undergo fluorescence quenching in solutions with a pH value below 3.     

Time-invariant person-specific frequency templates in human brain activity

The various human brain tasks are performed at different locations and time scales. Yet, we discovered the existence of time-invariant (above an essential time scale) partitioning of the brain activity into personal state-specific frequency bands. For that, we perform temporal and ensemble averaging of best wavelet packet bases from multielectrode electroencephalogram recordings. These personal frequency bands provide new templates for quantitative analyses of brain function, e.g., normal versus epileptic activity.     

Microscopic folds and macroscopic jerks in compressed lipid monolayers

We report hitherto unrecognized cooperative behavior in the stochastic collapse of certain compressed lipid monolayers implicated in pulmonary function. The cooperativity emerges from a statistical analysis of the collapse events captured using fluorescence microscopy and digital image analysis. The collapse events involve folding of the monolayer on a micron scale, yet each event produces a macroscopic jerk of the layer. The cooperative collapse is striking for its temporal sharpness and large spatial extent.