Acquisition of high-precision images for non-contact atomic force microscopy

The atomic force microscope (AFM) system has evolved into a useful tool for direct measurements of intermolecular forces with atomic-resolution characterization that can be employed in a broad spectrum of applications. The distance between cantilever tip and sample surface in non-contact AFM is a time-varying parameter even for a fixed sample height, and typically difficult to identify. A remedy to this problem is to directly identify the sample height in order to generate high-precision atomic-resolution images. For this, the microcantilever (which forms the basis for the operation of AFM) is modeled as a single mode approximation and the interaction between the sample and cantilever is derived from a van der Waals potential. Since in most practical applications only the microcantilever deflection is accessible, we will use merely this measurement to identify the sample height. In most non-contact AFMs, cantilevers with high-quality factors are employed essentially for acquiring high-resolution images. However, due to high-quality factor, the settling time is relatively large and the required time to achieve a periodic motion is long. As a result, identification methods based on amplitude and phase measurements cannot be efficiently utilized. The proposed method overcomes this shortfall by using a small fraction of the transient motion for parameter identification, so the scanning speed can be increased significantly. Furthermore, for acquiring atomic-scale images of atomically flat samples, the need for feedback loop to achieve setpoint amplitude is basically eliminated. On the other hand, for acquiring atomic-scale images of highly uneven samples, a simple PI controller is designed to track the desired constant sample height. Simulation results are provided to demonstrate the feasibility of the approach for both sample height identification and tracking the desired sample height.     

Novel cycloartane saponins from Corchorus depressus L

Two novel bidesmosidic cycloartane-type glycosides, depressosides C and D were isolated from Corchorus depressus L. Their structures were elucidated as (22R)-16beta,22-epoxy-3beta,26-dihydroxy-9,19-cyclolanost-++ +24E-ene 3, 26-di-O-beta-D-glucopyranoside and (22R,24S)-22,25-epoxy-3beta,16beta,24-trihydroxy-9,19-cyclolano stane 3, 24-di-O-beta-glucopyranoside, respectively on the basis of chemical evidence and detailed spectroscopic studies.     

Pyridinium hydrobromide perbromide: a versatile catalyst for aziridination of olefins using Chloramine-T

[reaction: see text] Pyridinium hydrobromide perbromide (Py x HBr3) catalyzes effectively the aziridination of electron-deficient as well as electron-rich olefins using Chloramine-T (N-chloro-N-sodio-p-toluenesulfonamide) as a nitrogen source to afford the corresponding aziridines in moderate to good yields.     

Second-order subgradients of convex integral functionals

The purpose of this work is twofold: on the one hand, we study the second-order behaviour of a nonsmooth convex function defined over a reflexive Banach space . We establish several equivalent characterizations of the set , known as the second-order subdifferential of at relative to . On the other hand, we examine the case in which is the functional integral associated to a normal convex integrand . We extend a result of Chi Ngoc Do from the space to a possible nonreflexive Banach space . We also establish a formula for computing the second-order subdifferential .

     

On the local stability of limit cycles

Orbital stability of limit cycles is the result of the competing local tendencies of perturbations from the cycle to decay (during phases of local stability) and to grow (during phases of local instability), averaged over a cycle. We examine this coexistence of attractive and repulsive phases on limit cycles, including the local rates of expansion and contraction of phase space volumes. This is done in a frame of reference that moves along the orbit, to partially decouple motions tangential and perpendicular to the cycle. Dynamical systems used for illustration are the generalized Bonhoeffer-van-der-Pol and Rossler models, both far from and near to different types of bifurcations. Finally, it is shown that the nonuniformity of local stability in phase space affects the response of limit cycle oscillators to perturbations and gives rise to their phase-dependent response. (c) 1999 American Institute of Physics.     

Study of the kinetics and activation parameters of reduction of Mn(III) to Mn(II) by SO3 2− ion in (MnSiW11O40H2)5- heteropoly ion

In order to gain an insight into the mechanism of reduction of Mn(III) heteropoly ions, and also to establish the conditions for use of some of these ions as oxidizing agent, following measurements have been made. The pseudo-first order rate constants, k_obs, have been determined and specific rate constants, k, were calculated from the plots of k_obs against SO₃ ^2? concentrations. A plot of ln(k₂/T) against inverse temperature gives enthalpy of activation as 10.67 kJ mol^?1 and entropy of activation as ?237.90 J K^?1 mol^?1. Effects of ionic strength and pH have also been studied over a limited range.     

Separation of radioindium (N.C.A.) from neutron irradiated tin and cadmium on hydrated antimony pentoxide

Hydrated antimony pentoxide (HAP) as an absorbent for column operation has been prepared by hydrolysis of SbCl₅ with deionized water. Sorption behavior of Sn, Cd, Sb and In was studied on HAP in HCl medium. Radiochemical separation of no-carrier added^113mIn from¹¹³Sn/¹²⁵Sb and^115mIn from¹¹⁵Cd wa achieved over a HAPO column. The separated products were of high radionuclidic purity.     

Effect of steam explosion on the physicochemical properties and enzymatic saccharification of rice straw

Applied Biochemistry and Biotechnology - The effects of steam explosion pretreatment on the physical and chemical properties of rice straw and on the enzymatic saccharification of the straw were...     

Kinetic studies on the reduction of a nickel (III) oxime-imine complex by sulphur(IV) and selenium(IV)

The kinetics of the reduction of [Ni^III(L¹)]²⁺ (where HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) by sulphur(IV) and selenium (IV) over the regions pH 2.50–8.02 and 2.01–4.00 respectively have been investigated at 30°C. Attempts were made to evaluate the reactivity of all the reacting species, of sulphur(IV) and selenium(IV) by considering suitable pH ranges. The oxidation of SC₂·H₂O and HSO ₃ ⁻ is proposed to proceed through the formation of a hydrogen-bonded adduct. The reaction with SO ₃ ²⁻ seems to follow a direct outer-sphere route which is well supported by Marcus crossrelation calculation. The oxidation of HSeO _l3 ⁻ is ≈ 10³ times slower than that of H₂SeO₃. The kinetic data indicate that the oxidation of sulphur(IV) by [Ni^IIIL¹)]²⁺ is much more favourable as compared to the corresponding oxidation of selenium(IV).