169,738 research outputs found
Mean kinetic energy of helium atoms in fluid He-3 and He-3-He-4 mixtures
No full textMomentum distributions and mean kinetic energies of helium atoms, in pure
fluid 3He and 3He–4He mixtures, at T = 2 K and 3He concentrations of
x = 0.20 and x = 1.00, are presented. The experimental technique employed
is deep inelastic neutron scattering measurements in the eV energy range, with
wavevector transfer of typically 100 °A−1
< q < 250 °A−1. Single-particle
dynamical properties of 3He–4He mixtures are discussed in the context of
previous results on mixtures at different concentrations and pure 3He and 4He. In the pure fluids, the kinetic energy of 3He and 4He are remarkably similar
for molar volumes above 25 cm3 mol−1, while for smaller molar volumes, upon approaching the liquid–solid transition, the kinetic energy is larger in 3He
than 4He. On the other hand, the short-time dynamics of the helium mixture reveal quite a different picture with respect to the pure 3He and 4He: the momentum distribution and mean kinetic energy of the light helium component are independent of the molar volume and concentration
He-4 adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy
No full textThis paper reports a study of the short-time dynamics of helium confined in silica nanopores (xerogel powder), with average pore diameters of 24 and 160 Å. The longitudinal momentum distribution of helium adsorbed in xerogels has been determined via deep inelastic neutron scattering (DINS) measurements performed on the VESUVIO spectrometer at the ISIS spallation source. DINS measurements, in the attosecond time scale, (i.e., 10−16–10−15 s), were performed at a temperature of T=2.5 K and saturated vapor pressure conditions, with 95% pore volume filling. The average wave-vector transfer q was about 130 Å−1. For confined helium, significant changes in the values of the single-particle mean kinetic energies ⟨EK⟩ are found in the bulk phase. These are 32.6±8.7 K for the 24 Å and 24.4±5.3 K for the 160 Å pore diameters, remarkably higher than ⟨EK⟩=16.2±0.4 K, the value of normal liquid 4He at T=2.5 K and saturated vapor pressure conditions. The results are interpreted in terms of a model where 4He atoms are arranged in concentric annuli along the cylindrical pore axis, with ⟨EK⟩ mainly dependent on the ratio between the atomic “effective” diameter and the pore diameter. The number of solid layers close to pore surface is found to be strongly pore-size dependent with one single solid layer for 24 Å diameter pore and three solid layers for 160 Å diameter pore
Single-particle dynamics of helium mixtures and 4He in nanometric confinement
No full textScopo di questa tesi e' lo studio, tramite Deep Inelastic Neutron Scattering, della
dinamica microscopica di due differenti sistemi di elio, a bassa temperatura (circa 2
K): una miscela isotopica (nella fase fluida e vicino al punto di melting) e 4He in in
confinamento nanometrico.
L'interesse per l'elio, gia' dai primi decenni del `900, nasce dalla sua unica proprieta':
e' l'unico elemento in natura a non avere una fase solida allo zero assoluto. A basse
temperature, quindi, presenta effetti quantistici, normalmente trascurabili in altri
sistemi fisici, che nelle stesse condizioni solidificano. l'elio e' quindi l'unico banco di
prova per i modelli teorici quantistici, in particolare per lo studio di bosoni e fermioni
interagenti.
In questo contesto, molti esperimenti sono stati effettuati sull'elio, sia nella fase liquida che solida. Misure su 3He e 4He hanno mostrato che l'energia cinetica dei liquidi
puri dipende dalla densita' del sistema, crescendo con una diminuzione del volume
molare. D'altra parte, la dinamica microscopica delle miscele mostra un differente
comportamento rispetto al 3He e 4He puri: l'energia cinetica media dell'isotopo piu'
leggero, a volumi molari maggiori di 25cm3/mole, sembra essere indipendente dal
volume molare e dalla concentrazione. Questo andamento potrebbe essere spiegato
da effetti quantistici, come gli effetti di scambio. Nella prima parte del presente
lavoro si e' investigata la dinamica delle miscele per volumi molari tra 22cm3/mole
e 25cm3/mole, e gli esperimenti compiuti hanno mostrato che in questo range di
volumi molari l'energia cinetica media degli atomi di 3He risulta dipendente dal volume molare, aumentando fino ad avere un valore corrispondente a quello del 3He
puro.
Recentemente sono state compiute anche misure per studiare l'influenza di un confinamento sull'elio. Esperimenti su 4He, adsorbito in superfici piane o substrati
porosi, hanno rivelato un elevato aumento nel valore dell'energia cinetica. Questo
comportamento e' stato attribuito alla localizzazione degli atomi di He dovuta al
potenziale di interazione He-substrato, che influenza fortemente i primi due o tre layers. Questi tipi di effetti possono essere studiati confinando 4He in pori cilindrici con
differenti diametri dei pori. Scopo della seconda parte di questa tesi e' stata appunto
quella di determinare l'energia cinetica media degli atomi di 4He adsorbiti in sistemi
nanoporosi a geometria cilindrica (Xerogel) con due diametri medi dei pori, di 24ºA
160ºA, per valutare la dipendenza della dinamica a singola particella con la dimensione dei pori e con il tipo di geometria. Le misure sono state effettuate a T=2.5K, a
pressione di vapor saturo e con un riempimento dei pori del 95%. L'esperimento ha
mostrato che l'energia cinetica del 4He nei pori e' maggiore rispetto a quella del 4He
liquido nelle stesse condizioni. I risultati sono stati interpretati tramite un modello
teorico, secondo il quale gli atomi si posizionano in anelli concentrici, stratificando
layer per layer, e con un'energia cinetica dipendente dal rapporto tra il diametro del
poro e quello dell'atomo di elio.The aim of this thesis work is the study, by means of Deep Inelastic Neutron Scatter-
ing, of the microscopic dynamics of two different helium systems at low temperature
(T=2K): an isotopic helium mixture (in the fluid phase and near the melting point)
and a system of 4He in nanometric confinement.
The interest in the helium, from the first decades of 1900, is due to its unique features: it is the only element in nature that doesn't have a solid phase at absolute
zero. Thus, at low temperatures it presents quantum effects, usually negligible in
other physical systems that in this condition crystallise. The helium is thus the
unique test-bed for theoretical quantum models, in particular for studying the interacting boson (4He) and fermion (3He) systems. Moreover, if in 4He are added some
atoms of 3He it is possible to derive important information about the interplay of
these two statistics.
In this context, several experiments on liquid and solid helium have been performed.
Measurements on pure 3He and 4He have shown that the mean kinetic energy of
pure liquids depends on the density of the system and increases decreasing the molar volume. On the other hand, the microscopic dynamics of helium mixtures reveals
quite a different picture with respect to pure 3He and 4He: the mean kinetic energy
of the light isotope, above a molar volume of 25cm3/mole, shows a remarkable independence from molar volume and concentration. This behaviour could be explained
by quantum effects, such as exchange effects. The first part of the present work deals
with the experiments performed to investigate the dynamics of the mixtures from
22cm3/mole to 25cm3/mole and shows how, at these low molar volumes, the mean
kinetic energy of 3He starts again to be strongly dependent on the molar volume,
increasing until reaching, at 22.7cm3/mole, the corresponding value of pure helium.
Recent measurements have been also performed to investigate the influence of confinement on helium. Experiments on 4He, adsorbed in flat surface or slit geometry
porous substrates, have shown a large increase in helium mean kinetic energy. This
has been attributed to the strong localisation effects induced by the helium-substrate
interaction potential, which mainly influence the firsts two or three adsorbed layers.
Such effects can be also investigated by confining 4He atoms in cylindrical pore geometries and by studying their dynamics as function of pore size. Aim of the second
part of the thesis has been the determination of the single particle mean kinetic
energy of 4He adsorbed in cylindrical silica nanopores (Xerogel) having two different
pore diameters, namely, 24 ºAand 160 ºA, and to evaluate the dependence of single-
particle dynamics on pore sizes, layer coverage, and confining system geometry. The
measurements have been performed at a temperature of T=2.5K, saturated vapour
pressure, and 95% volume filling. Significant changes in the values of the single particle mean kinetic energy are found: they are remarkably higher than the value of
normal liquid 4He at the same conditions. The results are interpreted in terms of
a model in which 4He atoms are arranged in concentric annuli along the cylindrical
pore axis, growing layer-by-layer and with the mean kinetic energy mainly dependent
on the ratio between the atomic diameter and the pore diameter
Proton momentum distribution of liquid water from room temperature to the supercritical phase
No full textMeasurements of the proton momentum distribution n(p) in water from
ambient conditions to above the supercritical point are compared with
theoretical calculations based on a recently developed polarizable water
model. The n(p) along the H-bond direction is narrower in the dense
phases, and approaches that of the isolated molecule in the more dilute
phases. The theoretical model, which includes only electrostatic
interactions, is unable to explain the softening of the local potential
experienced by the proton in the dense phases, but it accurately
predicts the n(p) for the dilute phases
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Potentials and limitations of a porphyrin-based AT-cut resonator for sensing applications
No full textAcoustic sensors are generally known as high-resolution mass-sensitive transducers. They are composed of piezoelectric crystal plus at least one layer of organic matter to improve chemical sensitivity. The acoustic properties, resumed in the acoustic load impedance, can be exploited by measuring the electric parameters of an equivalent circuit. Some sensing results of a thickness shear mode resonator coated by increasing amounts of a porphyrin solid film have been studied and compared with the performances of the device (quality factor, impedance, motional parameters) to exploit the capabilities and the limitations of a porphyrin-based acoustic chemical sensor. (C) 2007 Elsevier B.V. All rights reserved
Direct Measurement of Competing Quantum Effects on the Kinetic Energy of Heavy Water upon Melting
No full textEven at room temperature, quantum mechanics plays a major role in determining the quantitative behaviour of light nuclei, changing significantly the values of physical properties such as the heat capacity. However, other observables appear to be only weakly affected by nuclear quantum effects (NQEs): for instance, the melting temperatures of light and heavy water differ by less than 4 K. Recent theoretical work has attributed this to a competition between intra and inter molecular NQEs, which can be separated by computing the anisotropy of the quantum kinetic energy tensor. The principal values of this tensor change in opposite directions when ice melts, leading to a very small net quantum mechanical effect on the melting point. This paper presents the first direct experimental observation of this phenomenon, achieved by measuring the deuterium momentum distributions n(p) in heavy water and ice using Deep Inelastic Neutron Scattering (DINS), and resolving their anisotropy. Results from the experiments, supplemented by a theoretical analysis, show that the anisotropy of the quantum kinetic energy tensor can also be captured for heavier atoms such as oxygen
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
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