Filtering the histories of a partially observed marked point process

We consider a situation in which the evolution of an ‘underlying’ marked point process is of interest, but where this process is not directly observable. Instead, we assume that another marked point process, which is fully determined by the underlying process, can be observed. The problem is then the estimation, at any given time t, of the underlying development so far, given the corresponding observations. The solution, in the sense of a conditional distribution of the underlying pre-t history, is shown to satisfy a recursive filter formula. Sufficient conditions for the uniqueness of the solution are given. Two non-trivial examples are considered in detail.     

A new synthesis of methoxalen

A new seven step synthesis of methoxalen, 9-methoxy-7H-furo[3,2-g][1]benzopyran-7-one, starting from 2-chloro-2′-hydroxy-3′,4′-dimethoxyacetophenone is described. The yields in every step are good (60-100%).     

Current-density imaging using ultra-low-field MRI with adiabatic pulses

Magnetic resonance imaging (MRI) allows measurement of electric current density in an object. The measurement is based on observing how the magnetic field of the current density affects the associated spins. However, as high-field MRI is sensitive to static magnetic field variations of only the field component along the main field direction, object rotations are typically needed to image three-dimensional current densities. Ultra-low-field (ULF) MRI, on the other hand, with B₀ on the order of 10–100 μT, allows novel MRI sequences. We present a rotation-free method for imaging static magnetic fields and current densities using ULF MRI. The method utilizes prepolarization pulses with adiabatic switch-off ramps. The technique is designed to reveal complete field and current-density information without the need to rotate the object. The method may find applications, e.g., in conductivity imaging. We present simulation results showing the feasibility of the sequence.     

Alpha-decay studies of the new isotopes 191At and 193At

Detailed alpha-decay studies have been performed for the neutron-deficient isotopes ¹⁹¹At and ¹⁹³At. The nuclei were produced in fusion-evaporation reactions of ⁵⁴Fe and ⁵⁶Fe ions with a ¹⁴¹Pr target. The fusion products were separated in-flight using a gas-filled recoil separator and implanted into a position-sensitive silicon detector. The isotopes were identified using position, time and energy correlations between the implants and subsequent alpha-decays. Three alpha-decaying states were identified for ¹⁹³At and two for ¹⁹¹At. The spin and parity of the initial states in the astatine isotopes were deduced based on unhindered alpha-decays to states in the bismuth daughter nuclei. In both astatine isotopes the 1/2⁺ intruder state was determined to be the ground state and a 7/2^- state to be the first-excited state. In ¹⁹³At the alpha-decay of the 13/2⁺ state was observed in coincidence with a previously known gamma-ray transition from the 13/2⁺ state in the corresponding daughter nucleus ¹⁸⁹Bi. In ¹⁸⁷Bi and ¹⁸⁹Bi low-lying 7/2^- states were observed for the first time via alpha-decay of the mother nuclei.Received: 16 December 2002, Revised: 26 March 2003, Published online: 4 August 2003PACS: 23.60.+e Alpha decay - 27.80.+w - 23.20.Lv transitions and level energies - 21.10.Dr Binding energies and masses     

Positron detrapping from defects: A thermodynamic approach

The rate of positron detrapping in thermal equilibrium from lattice defects has been calculated by relating it to the specific trapping rate. The results for vacancies, dislocations and surfaces each show a different temperature dependence for the escape rate. For vacancies a measure of the importance of the detrapping can be obtained from the ratio of the vacancy formation energy to the positron binding energy in the defect. The positronium desorption rate from a surface is also calculated and agreement with experimental results is found.     

Density-functional approach to charge-transfer insulators

Self-consistent density-functional calculations have been carried out for Ag-row impurities in Cs within the spherical solid model. The results for the electronic structure provide an account of the observed resistivity, susceptibility and spin-flip scattering behaviour.     

Adsorption of acetic and trifluoroacetic acid on the TiO2(110) surface

We use the first-principles static and dynamic simulations to study the adsorption of acetic (CH(3)COOH) and trifluoroacetic (CF(3)COOH) acid on the TiO(2)(110) surface. The most favorable adsorption for both molecules is a dissociative process, which results in the two oxygens of the carboxylate ion bonding to in-plane titanium atoms in the surface. The remaining proton then bonds to a bridging oxygen site, forming a hydroxyl group. We further show that, by comparing the calculated dipoles of the molecules on the surface, it is possible to understand the difference in contrast over the acetate and trifluoroacetate molecules in the atomically resolved noncontact atomic force microscopy images.     

Imaging water on Ag(111): field induced reorientation and contrast inversion

Water adsorbed on Ag(111) at 70 K forms circular clusters that consist of six molecules. In scanning tunneling microscopy, this cyclic hexamer is imaged as a protrusion for voltages below V(SS)=-93 meV and as a depression for voltages above V(SS). The electronic density of states, however, increases around V(SS). We explain this counterintuitive result with the aid of calculated images by a change from constructive to destructive interference between different tunneling channels due to a field induced reorientation of the molecule under the tunneling tip.     

Interaction of silicon dangling bonds with insulating surfaces

We use first principles density functional theory calculations to study the interaction of a model dangling bond silicon tip with the surfaces of CaF2, Al2O3, TiO2, and MgO. In each case the strongest interaction is with the highest anions in the surface. We show that this is due to the onset of chemical bonding with the surface anions, which can be controlled by an electric field across the system. Combining our results and previous studies on semiconductor surfaces suggests that using dangling bond Si tips can provide immediate identification of surface species in atomically resolved noncontact atomic force microscopy and facilitate selective measurements of short-range interactions with surface sites.