Application of the Π theorem of dimensional analysis to electron scattering in multi-component systems

Summary Monte Carlo simulation of electron scattering in solids is considered and an extremely simplified picture of the process is demonstrated by a semi-empirical application of the Π theorem of dimensional analysis. The case of electron beam lithography is presented as an example. A set of parameters fully characterising the process is derived, and their dependence is investigated on electron energy, substrate thickness and the material physical constants, for a great number of single- and multi-component substrates. A single generalised argument is shown to be able to effectively describe the properties of forward and backward scattering of any substrate, in the elastic regime. The physical laws governing the process are approximated through analytical functions of this argument. These functions are theoretically deduced from very simple models and empirically demonstrated by fitting the numerical data. The efficacy of the proposed method, applicable to all elemental and composite substrate materials, is shown in the 5 to 150 keV energy range, with substrate thickness varying from 0.002 to 8 μm. The leading role of diamond as substrate material, emerging from such an analysis, is finally emphasised.     

Can Copper(II) Mediate Hoogsteen Base‐Pairing in a Left‐Handed DNA Duplex? A Pulse EPR Study

Pulse EPR spectroscopy is used to investigate possible structural features of the copper(II) ion coordinated to poly(dG‐dC)?poly(dG‐dC) in a frozen aqueous solution, and the structural changes of the polynucleotide induced by the presence of the metal ion. Two different copper species were identified and their geometry explained by a molecular model. According to this model, one species is exclusively coordinated to a single guanine with the N7 nitrogen atom forming a coordinative bond with the copper. In the other species, a guanine and a cytosine form a ternary complex together with the copper ion. A copper crosslink between the N7 of guanine and N3 of cytosine is proposed as the most probable coordination site. Moreover, no evidence was found for an interaction of either copper species with a phosphate group or equatorial water molecules. In addition, circular dichroism (CD) spectroscopy showed that the DNA of the Cu^II‐poly(dG‐dC)?poly(dG‐dC) adducts resembles the left‐handed Z‐form. These results suggest that metal‐mediated Hoogsteen base pairing, as previously proposed for a right‐handed DNA duplex, can also occur in a double‐stranded left‐handed DNA.     

Valence and conduction bands in MPS3 layered compounds studied by synchrotron radiation

Summary With the resonant photomeission technique we investigated the valence bands of FePS₃ and NiPS₃. The experimental results, support the ionic picture of the compounds and our previous identification of the valence band structures. The structures rapidly varying in intensity when the excitation energy is scanned across the Fe and NiM _2,3 absorption edge are associated to the transition metal 3d states; the nonresonating features are ascribed to the (P₂P₆)⁴⁻ cluster states. With the yield technique we measured the high-resolution absorption spectra of the phosphorus and sulphur inner-core levels in Mn, Fe and Ni thiophosphates. TheL _2,3(P) andL _2,3(S) spectra are similar to each other in all the compounds and are interpreted in terms of the projected density of states of the conduction bands derived from the (P₂S₆)⁴⁻ cluster states. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

An immunofluorescence technique for staining ciliated protozoans: highlighting cytoplasmic microtubular arrays and stages of micronuclear meiosis.

Ciliated protozoans represent useful organisms for studying the processes involved in the induction and progression of meiosis. In this short report we describe a technique that has allowed us to examine different meiosis phases during the sexual reproduction of Blepharisma japonicum. In order to visualize the phases of meiosis, sexually reproducing pairs were stained by an enhanced technique of anti-tubuline indirect immunofluorescence. Meiotic micronuclei, particularly those showing metaphase spindles, were clearly highlighted. The technique also heavily decorates the main microtubular cytoplasmic arrays in Blepharisma.     

Status of the development of the imaging proportional counters for the Stellar X-Ray Polarimeter

Summary The Stellar X-Ray Polarimeter SXRP will be flown at the focus of the SODART X-ray telescope aboard the Russian satellite SPECTRUM-X-Gamma by the end of 1995. Four imaging proportional counters will detect photons diffracted by a graphite crystal (2.6 and 5.2 keV) and scattered from a lithium rod enclosed in a beryllium case (from 5 to 20 KeV). The counters are position-sensitive by the Wedge and Strip (WS) readout method. The gas mixture is 80% xenon, 10% argon and 10% methane. In this contribution we resume the results of the measurements performed on the engineering models of the counters. Position resolution of about 1.5 mm is attained with an energy resolution of about 25%. The pulse shape discrimination offers more than 98%⁶⁰Co events rejection, with at least 90% X-ray acceptance. Some improvements are foreseen for the flight units in terms of sensitivity and uniformity of response.     

Perspectives in electron scattering by microstructures

Summary In this paper the physical mechanisms of electron scattering in solids are studied, emphasis being placed on the processes relevant in electron spectroscopy and lithography. The essential features of the Monte Carlo approach to electron penetration are reviewed. Lithography applications are briefly discussed. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Smectic Liquid Crystals: Materials with One-dimensional, Periodic Order

Smectic liquid crystals are materials formed by stacking deformable, fluid layers. Although smectics prefer to have flat, uniformly-spaced layers, boundary conditions can impose curvature on the layers. Since the layer spacing and curvature are intertwined, the problem of finding minimal configurations for the layers becomes nontrivial. We discuss various topological and geometrical aspects of these materials and present recent progress on finding some exact layer configurations. We also exhibit connections to the study of certain embedded minimal surfaces and briefly summarize some important open problems.