4,626 research outputs found

    Accretion and photodesorption of CO ice as a function of the incident angle of deposition

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    Non-thermal desorption of inter- and circum-stellar ice mantles on dust grains, in particular ultraviolet photon-induced desorption, has gained importance in recent years. These processes may account for the observed gas phase abundances of molecules like CO toward cold interstellar clouds. Ice mantle growth results from gas molecules impinging on the dust from all directions and incidence angles. Nevertheless, the effect of the incident angle for deposition on ice photo-desorption rate has not been studied. This work explores the impact on the accretion and photodesorption rates of the incidence angle of CO gas molecules with the cold surface during deposition of a CO ice layer. Infrared spectroscopy monitored CO ice upon deposition at different angles, ultraviolet-irradiation, and subsequent warm-up. Vacuum-ultraviolet spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp. Molecules ejected from the ice to the gas during irradiation or warm-up were characterized by a quadrupole mass spectrometer. The photodesorption rate of CO ice deposited at 11 K and different incident angles was rather stable between 0 and 45^{\circ}. A maximum in the CO photodesorption rate appeared around 70^{\circ}-incidence deposition angle. The same deposition angle leads to the maximum surface area of water ice. Although this study of the surface area could not be performed for CO ice, the similar angle dependence in the photodesorption and the ice surface area suggests that they are closely related. Further evidence for a dependence of CO ice morphology on deposition angle is provided by thermal desorption of CO ice experiments

    Stable Determination of the Electromagnetic Coefficients by Boundary Measurements

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    The goal of this paper is to prove a stable determination of the coefficients for the time-harmonic Maxwell equations, in a Lipschitz domain, by boundary measurements

    Short-range order and precipitation in Fe-rich Fe-Cr alloys: Atomistic off-lattice Monte Carlo simulations

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    Short-range order (SRO) in Fe-rich Fe-Cr alloys is investigated by means of atomistic off-lattice Monte Carlo simulations in the semi-grand canonical ensemble using classical interatomic potentials. The SRO parameter defined by Cowley [Phys. Rev. B 77, 669 (1950)] is used to quantify the degree of ordering. In agreement with experiments a strong ordering tendency in the Cr distribution at low Cr concentrations (~< 5%) is observed, as manifested in negative values of the SRO parameters. For intermediate Cr concentrations (5% ~< c_Cr ~< 15%) the SRO parameter for the alpha-phase goes through a minimum, but at the solubility limit the alpha-phase still displays a rather strong SRO. In thermodynamic equilibrium for concentrations within the two-phase region the SRO parameter measured over the entire sample therefore comprises the contributions from both the alpha and alpha-prime phases. If both of these contributions are taken into account, it is possible to quantitatively reproduce the experimental results and interpret their physical implications. It is thereby shown that the inversion of the SRO observed experimentally is due to the formation of stable (supercritical) alpha-prime precipitates. It is not related to the loss of SRO in the alpha-phase or to the presence of unstable (subcritical) Cr precipitates in the alpha-phase.Comment: 9 pages, 8 figure

    Phasing diffuse scattering. Application of the SIR2002 algorithm to the non-crystallographic phase problem

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    A new phasing algorithm has been used to determine the phases of diffuse elastic X-ray scattering from a non-periodic array of gold balls of 50 nm diameter. Two-dimensional real-space images, showing the charge-density distribution of the balls, have been reconstructed at 50 nm resolution from transmission diffraction patterns recorded at 550 eV energy. The reconstructed image fits well with scanning electron microscope (SEM) image of the same sample. The algorithm, which uses only the density modification portion of the SIR2002 program, is compared with the results obtained via the Gerchberg-Saxton-Fienup HIO algorithm. In this way the relationship between density modification in crystallography and the HiO algorithm used in signal and image processing is elucidated.Comment: 7 pages, 12 figure

    Prevalence of sexual and physical abuse in patients with obstructed defecation: impact on biofeedback treatment

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    Background: obstructed defecation is one of the most com mon subtypes of constipation, and it is frequently responsive to biofeedback treatment. Aims: since a history of sexual and physical abuse may be pre sent in patients with obstructed defecation, we assessed the inci dence of abuse history in patients with obstructed defecation re ferred to a general gastroenterology practice, and whether such a history may lead to a different outcome of biofeedback training in these patients. Patients and methods: one hundred and twenty-one pa tients (17 men, 104 women, age 53 ± 15 years) with obstructed defecation were studied by retrospective chart review. Their histo ry of sexual, physical and psychological abuse was obtained by a standard interview, and biofeedback training was carried out by means of a three-balloon technique. Results: a history of sexual/physical or psychological abuse was present in 12.4% patients. Biofeedback training yielded a successful improvement of obstructed defecation in 93% patients without abuse and in 100% of patients with abuse; this difference was not statistically different (p = 0.53). Conclusions: the prevalence of sexual/physical or psycholog ical abuse in a population of patients with obstructed defecation referred to a general gastroenterology practice is relatively low; such a history seems not to affect the outcome of biofeedback training in these patients

    On the evaluation of some three-body variational integrals

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    Stable recursive relations are presented for the numerical computation of the integrals dr1dr2r1l1r2m1r12n1exp{αr1βr2γr12}\int d{\bf r}_1 d{\bf r}_2 r_1^{l-1} r_2^{m-1} r_{12}^{n-1} \exp{\{-\alpha r_1 -\beta r_2 -\gamma r_{12}\}} (ll, mm and nn integer, α\alpha, β\beta and γ\gamma real) when the indices ll, mm or nn are negative. Useful formulas are given for particular values of the parameters α\alpha, β\beta and γ\gamma.Comment: 12 pages, 1 figure (PS) and 3 tables. Old figures 2 and 3 replaced by Tables I and III. A further table added. Paper enlarged giving some tips on the convergence of quadrature

    Level Spectrum of Single Gated As Donors

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    We study the electrical transport through single As donors incorporated in the channel of a FinFET, i.e. a donor in a three-terminal geometry. By means of spectroscopic measurements in conjuction with a NEMO-3D model, we can identify the excited states and associate them with either the donors Coulomb potential, a triangular well at the interface or a hybridized combination of the two. The correspondence between the transport measurements, the theoretical model and the local environment provides an atomic understanding of actual gated donors in a nanostructure
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