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Notes: Objectives 1. To compare the ultrasound biparietal diameter and crown-rump length of fetuses with and without Down's syndrome in the first half of pregnancy; 2. To investigate the effect of estimation of gestational age using either measure on the detection rate of serum screening for Down's syndrome.Design Matched case-control study. Cases were singleton Down's syndrome pregnancies with a biparietal diameter or a crown-rump length recorded. Five controls were matched to each case on: medical centre; the date of the ultrasound scan examination (within two years); gestational age measured as the number of days since the first day of the last menstrual period; and the ultrasound measure used (ie the biparietal diameter (the measure of choice), or the crown-rump length otherwise). If a woman had a serum screening test for Down's syndrome, the biparietal diameter or crown-rump length measurement had to be taken prior to the screening test so that the result of the test could not influence whether a scan was performed.Setting Ten antenatal screening centres in seven countries in Europe and North America.Subjects Two hundred and one women with singleton Down's syndrome pregnancies and 1005 women with unaffected singleton pregnancies.Results The median biparietal diameter of fetuses with Down's syndrome was identical to that among the controls (median difference 0.0mm, 95% confidence intervals (CI)–0.5 to 0.5mm). The estimates of gestational age based on biparietal diameter yielded a median gestational age less than that based on the women's last menstrual period: three days less for cases and two days less for controls; small but statistically significant differences probably reflected a minor systematic difference in the conversion of a biparietal diameter to a gestational age estimate. The median crown-rump length of fetuses with Down's syndrome was also identical to that among controls (median difference 0.0mm, 95% CI–1.5 to 2.0 mm). There was no significant difference between the median gestational age estimate based on crown-rump length and that based on the women's last menstrual period.Conclusion In antenatal screening for Down's syndrome the routine use of an ultrasound biparietal diameter or crown-rump length measurement to estimate gestational age will not adversely affect the detection rate. To avoid differences in gestational age estimates using the last menstrual period and the biparietal diameter influencing screening performance, separate medians should be derived for each serum marker using the two methods of estimating gestational age. The appropriate set of medians can then be used to calculate the multiple of the median value for each woman screened depending on the method used to estimate her gestational age.

Notes: An extract of the whole brain of the frog Rana ridibunda contained high concentrations of substance P-like immunoreactivity, measured with an antiserum directed against the COOH-terminal region of mammalian substance p and neurokinin b-like immurtoreactivity, measured with an antiserum directed against the NH2-terminus of neurokinin B. The primary structure of the substance p-related peptide (ranakinin) was established as: Lys-Pro-Asn-Pro-Glu-Arg-Phe-Tyr-Gly-Leu-Met-NH2. Mammalian substance P was not present in the extract. The primary structure of the neurokinin b-related peptide was established as: Asp-Met-His-Asp-Phe-Phe-Val-Gly-Leu-Met-NH2. This amino acid sequence is the same as that of mammalian neurokinin B.Ranakinin was equipotent with substance p and [Sar9,-Met(O2)11]substance p in inhibiting the binding of 125I-Bolton-Hunter-[Sar9, Met(O2)11]substance p, a selective radio-ligand for the NK1 receptor, to binding sites in rat subman-dibular gland membranes (IC50 1.6 ± 0.3 nM; n = 5). It is concluded that ranakinin is a preferred agonist for the mammalian NK1 tachykinin receptor subtype.

Notes: Abstract We have developed and used a novel technique to investigate the effects of lithium and other psychotropic drugs on the cation-transporting properties of the sodium- and potassium-activated ATPase enzyme (Na+, K+-ATPase) in intact synaptosomes. Rubidium-86 uptake into intact synaptosomes is an active process and is inhibited by ∼75% in the presence of the Na+, K+-ATPase inhibitor acetylstrophanthidin. In vitro addition of lithium to synaptosomes prepared from untreated mice causes a progressive inhibition of acetylstrophanthidin-sensitive 86Rb uptake, but only at concentrations higher than the clinical therapeutic range. However, pretreatment of mice for 14 days in vivo with lithium, carbamazepine, and haloperidol, but not phenytoin, causes a significant stimulation of 86Rb uptake into synaptosomes via Na+, K+-ATPase.

Notes: A model is presented for the steady state operation of polymer light-emitting electrochemical cells (LECs). An LEC consists of a luminescent and ionically conducting polymer, with an ionic salt added to provide ions necessary for p-type and n-type doping, sandwiched between two electrodes. Upon applying a sufficiently large voltage bias, the ions are spatially separated forming an electrical junction. Electrons injected from the n-type side of the junction recombine with holes injected from the p-type side of the junction emitting light. We first describe the LEC at zero bias in which electric fields may occur in charge double layers near the contacts but in which there is a charge neutral, field free region in the device center which has an equal density of anions and cations and essentially no electrons or holes. A threshold voltage for junction formation is found, which depends on the polymer energy gap, the dissociation free energy of the salt, and the added salt density. It is generally somewhat smaller than the polymer energy gap. Below threshold, an applied bias changes the electric fields in the double charge layers near the contacts but the device center remains field free and essentially no current flows. Above threshold, the ions become spatially separated, a junction forms, and current begins to flow. Part of the applied voltage, above threshold, falls in the contact region and is necessary to establish the junction by electrochemical doping and part of the applied voltage falls across the junction. We describe the structure of the junction, which is quite different from that of a conventional p-n junction, including the spatial profiles of the electrons, holes, and ions, and the electrostatic potential. We discuss the current-voltage and capacitance-voltage characteristics of the LECs and show how they depend on the material parameters. © 1997 American Institute of Physics.

Notes: Calculations of the effects of external stress on the current–voltage characteristics of double-barrier (001)- and (111)-oriented resonant tunneling devices are presented. Crystal strains arising from the application of external pressure and, in pseudomorphic structures, lattice mismatch cause shifts in the conduction and valence bands of the well and barrier layers with respect to the unstrained alignment. For certain stress orientations piezoelectric effects give rise to internal electric fields parallel to the current direction. The combined piezoelectric and band-structure effects modulate the transmission resonances which control the shape of the current versus voltage characteristics of the structures. © 1996 American Institute of Physics.

Notes: We present a device model to describe polymer light-emitting diodes (PLEDs) under bias conditions for which strong electrical injection does not occur (i.e., reverse, zero, and weak forward bias). The model is useful to interpret: capacitance–voltage measurements, which probe the charged trap density in the PLEDs; electroabsorption measurements on PLEDs, which probe the built-in electric field in the device; and internal photoemission measurements, which probe the effective Schottky barriers at the contacts of the PLED. The device model is based on the low-density nondegenerate continuum model for the electronic structure of polymers. Polarons and bipolarons are the principal charged excitations in this model. Polarons are singly charged excitations which play the primary role in charge injection and in experiments such as internal photoemission which probe single particle interface properties. Bipolarons are doubly charged excitations which can play an important role in establishing Schottky barriers at metal/polymer interfaces. In the device model, the region of the polymer near each contact is assumed to be in quasiequilibrium with that contact. The charge density as a function of position is found from the electrostatic potential and equilibrium statistics. Poisson's equation is integrated to determine the electrostatic potential. We find that a large charge density is transferred into the polymer if the chemical potential of a contact is higher than the negative bipolaron formation energy per particle or lower than the positive bipolaron formation energy per particle. The transferred charge pins the Fermi level and establishes the effective Schottky barrier. If the contact chemical potential is between the formation energy per particle of the two types of charged bipolarons, there is little charge transfer into the polymer and the Fermi level is not pinned. The electric field in the device is found for different contacts and bias conditions. Capacitance as a function of voltage is calculated for various trap binding energies and densities. The calculated results are used to interpret recent measurements on PLEDs. © 1995 American Institute of Physics.

Notes: In this paper we discuss a novel material which has nearly ideal properties at optical frequencies. It combines the low dissipation of a dielectric with the reflectivity of a metal. This material employs a two-dimensional photonic band gap structure to achieve in-plane confinement of light and uses index contrast to achieve vertical confinement. We discuss how this material can be used to create microcavities for the production of low threshold lasers and waveguides capable of low-loss bends.

Notes: We report electroabsorption measurements of the built-in electrostatic potential in metal/C60-doped polymer/metal structures to investigate chemical potential pinning due to equilibrium electron transfer from a metal contact to the electron acceptor energy level of C60 molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and C60-doped poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were measured. For undoped MEH-PPV, which has an energy gap of about 2.4 eV, the maximum built-in potential is about 2.1 eV, whereas for C60-doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the C60 molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of C60 and decreases the built-in potential of the structure. From the systematic dependence of the built-in potential on the metal work function we find that the electron acceptor energy level of C60 in MEH-PPV is about 1.7 eV above the hole polaron energy level of MEH-PPV. © 1997 American Institute of Physics.

Notes: The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 a˜ 3B1–X˜ 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 A˜ 1B1–X˜ 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D˜ 2A1 state of SiF4+ predominantly in the range 280–350 nm for photon energies greater than 21.5 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection at a specific (dispersed) wavelength. Using the single-bunch mode of the synchrotron, lifetimes of all the emitting states have been measured. The lifetimes of the unassigned emitting state in SiF3, the A˜ 1B1 state of SiF2, and the D˜ 2A1 state of SiF4+ are 3.9±0.7, 11.2±1.5, and 9.16±0.02 ns, respectively. This is the first measurement of the lifetimes of these excited states in SiF3 and SiF2. The decay from the a˜ 3B1 state of SiF2 has a fast component of 2.6±0.4 ns. We conclude that the lifetime of the a˜ 3B1 state of SiF2 is either as low as 2.6 ns or too high (τ〉∼200 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for a spin-forbidden phosphorescence to the 1A1 ground state, the 2.6 ns component could be the lifetime of intersystem crossing from higher vibrational levels of the a˜ 3B1 state of SiF2 into its ground state. © 1997 American Institute of Physics.