We present the first unambiguous detection of helium emission lines in spectra of Type Ic supernovae (SNe Ic). The presence of He I lines, with full width at half maximum ~ 2000 km/s, and the distinct absence of any other intermediate-width emission (e.g., Halpha), implies that the ejecta of SN Ic 1999cq are interacting with dense circumstellar material composed of almost pure helium. This strengthens the argument that the progenitors of SNe Ic are core-collapse events in stars that have lost both their hydrogen and helium envelopes, either through a dense wind or mass-transfer to a companion. In this way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that helped firmly establish a physical connection between Type Ib and Type II supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid rise followed by a quick decline. SN 1999cq is also found to exhibit a high level of emission at blue wavelengths (< 5500 A), likely resulting from either an unusually large amount of iron and iron-group element emission or uncharacteristically low reddening compared with other SNe Ic.
Since initial reports, cross-coupling technologies employing photoredox catalysts to access novel reactivity have developed with increasing pace. In this Outlook, prominent examples from the recent literature are organized on the basis of the elementary transformation enabled by photoredox catalysis and are discussed in the context of relevant historical precedent in stoichiometric organometallic chemistry. This treatment allows mechanistic similarities inherent to odd-electron transition metal reactivity to be generalized to a set of lessons for future reaction development.
The behavior of physicians in referring patients to other physicians is considered from the standpoint of social costs and rewards to the interacting physicians, and the implications of referral for quality of care and cost to the patient are discussed. Exchange theory is offered as a framework allowing conceptualization and analysis of the various aspects of the referral interaction. Relevant variables for investigation are identified, and suggestions are made for future research into this aspect of the health care delivery system.
Well ordered epitaxial iron oxide layers with FeO stoichiometry were grown onto Pt(111) single crystal substrates and characterized afterwards with low-energy electron diffraction (LEED) and Auger electron spectroscopy. They were prepared by first depositing iron from the vapor onto the substrate and then oxidizing it in 10−6 Torr of oxygen. At 1 monolayer coverage, a very stable structure with a hexagonal unit cell 15% larger than the platinum (1×1) substrate unit cell is formed, exhibiting a threefold symmetric LEED pattern. This pattern is consistent with a (8×8) superstructure, as 8 FeO(111)-(1×1) lattice spacings fit onto nine lattice spacings on the Pt(111)-(1×1) substrate surface due to the lattice mismatch. Above 2 monolayer coverage a hexagonal (2×2) structure with respect to the FeO(111)-(1×1) unit cell forms, which shows a sixfold symmetric LEED pattern revealing a lattice constant which is in agreement with that of the sodium chloride structure of bulk FeO. LEED beam intensity spectra as a function of the electron energy of these two LEED structures have been measured and are discussed with respect to their symmetry and their changes as a function of the overlayer thickness. The step structure on the surface of an 8 monolayer thick iron oxide film with a FeO(111)-(2×2) LEED pattern was studied by analyzing the LEED beam profiles. We observe steps one iron–oxygen bilayer high as well as one Pt(111) layer distance high. This indicates that only one type of atom termination exists on the FeO(111)-(2×2) surface.
A nonlinear s.c.r. circuit model made up of 6 lumped-circuit elements (3 nonlinear capacitors and 3 nonlinear voltage-controlled current sources) is presented. The model can be used for simulating arbitrary s.c.r. circuits under all operating conditions. In particular, it is capable of predicting all important dynamic effects such as turn-on and turn-off transients, dV/dt triggering, and the minimum commutation-time phenomenon. Computer-simulation results show that the model will correctly simulate all well-known triggering modes and turn-off mechanisms. The model is based entirely upon the device's physical operating principles. Each element in the circuit model corresponds to an actual current component. In particular, carrier currents due to both diffusion and generation-recombination are included, in addition to the usual displacementcurrent components across the 3 depletion layers
