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Title: {sup 4}He adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy

Abstract

This 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 A. 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{sup -16}-10{sup -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 A{sup -1}. For confined helium, significant changes in the values of the single-particle mean kinetic energies <E{sub K}> are found in the bulk phase. These are 32.6{+-}8.7 K for the 24 A and 24.4{+-}5.3 K for the 160 A pore diameters, remarkably higher than <E{sub K}>=16.2{+-}0.4 K, the value of normal liquid {sup 4}He at T=2.5 K and saturated vapor pressure conditions. The results are interpreted in terms of a model where {sup 4}He atoms are arranged in concentric annuli along the cylindrical pore axis, with <E{sub K}> mainly dependent on the ratio between the atomic 'effective' diameter and the pore diameter. The number of solid layersmore » close to pore surface is found to be strongly pore-size dependent with one single solid layer for 24 A diameter pore and three solid layers for 160 A diameter pore.« less

Authors:
;  [1];  [2];  [1]
  1. Dipartimento di Fisica, Universita degli Studi di Roma 'Tor Vergata', Via della Ricerca Scientifica 1, 00133 Rome (Italy)
  2. (Italy)
Publication Date:
OSTI Identifier:
20976693
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 75; Journal Issue: 6; Other Information: DOI: 10.1103/PhysRevB.75.064515; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ATOMS; CYLINDRICAL CONFIGURATION; DEEP INELASTIC SCATTERING; FILMS; HELIUM; HELIUM 4; KINETIC ENERGY; LAYERS; LIQUIDS; NEUTRON DIFFRACTION; NEUTRON SPECTRA; POROSITY; SILICA; SILICON COMPOUNDS; SOLIDS; SURFACES; TIME-OF-FLIGHT METHOD; VAPOR PRESSURE

Citation Formats

Andreani, C., Senesi, R., NAST, Centro di Nanoscienze, Nanotecnologie, Strumentazione, Universita degli Studi di Roma 'Tor Vergata', Via della Ricerca Scientifica 1, 00133 Rome, and Pantalei, C.. {sup 4}He adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy. United States: N. p., 2007. Web. doi:10.1103/PHYSREVB.75.064515.
Andreani, C., Senesi, R., NAST, Centro di Nanoscienze, Nanotecnologie, Strumentazione, Universita degli Studi di Roma 'Tor Vergata', Via della Ricerca Scientifica 1, 00133 Rome, & Pantalei, C.. {sup 4}He adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy. United States. doi:10.1103/PHYSREVB.75.064515.
Andreani, C., Senesi, R., NAST, Centro di Nanoscienze, Nanotecnologie, Strumentazione, Universita degli Studi di Roma 'Tor Vergata', Via della Ricerca Scientifica 1, 00133 Rome, and Pantalei, C.. Thu . "{sup 4}He adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy". United States. doi:10.1103/PHYSREVB.75.064515.
@article{osti_20976693,
title = {{sup 4}He adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy},
author = {Andreani, C. and Senesi, R. and NAST, Centro di Nanoscienze, Nanotecnologie, Strumentazione, Universita degli Studi di Roma 'Tor Vergata', Via della Ricerca Scientifica 1, 00133 Rome and Pantalei, C.},
abstractNote = {This 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 A. 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{sup -16}-10{sup -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 A{sup -1}. For confined helium, significant changes in the values of the single-particle mean kinetic energies <E{sub K}> are found in the bulk phase. These are 32.6{+-}8.7 K for the 24 A and 24.4{+-}5.3 K for the 160 A pore diameters, remarkably higher than <E{sub K}>=16.2{+-}0.4 K, the value of normal liquid {sup 4}He at T=2.5 K and saturated vapor pressure conditions. The results are interpreted in terms of a model where {sup 4}He atoms are arranged in concentric annuli along the cylindrical pore axis, with <E{sub K}> 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 A diameter pore and three solid layers for 160 A diameter pore.},
doi = {10.1103/PHYSREVB.75.064515},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 6,
volume = 75,
place = {United States},
year = {Thu Feb 01 00:00:00 EST 2007},
month = {Thu Feb 01 00:00:00 EST 2007}
}
  • We report the first quantitative study of the size-dependent suprefluid transition in {sup 4}He films adsorbed on porous glasses with pore diameters {ital d} from 50 to 10 000 A. {ital T}{sub {ital c}} for the submonolayer films decreases as {ital d} increases up to {ital d}=2900 A, whereas {ital T}{sub {ital c}} for the full-pore liquid increases with {ital d}. The {ital d} dependence of {ital T}{sub {ital c}} and the dissipation at the transition for the films reveal that the vortex-pair unbinding on length scales smaller than the pore sizes determines the superfluid transition. We obtain the diametermore » of vortex core {ital a}{sub 0}=25{plus minus}12 A from the fit by letting the 2D coherence length {xi}{sub 2}{sup +}({ital T}{sub {ital c}})={pi}{ital d}/2.« less
  • Using an AC calorimetry technique, the authors have performed a systematic specific heat study for {sup 3}He and {sup 4}He films adsorbed in the 0.2 {mu}m cylindrical pores of Anopore membranes. The specific heat submonolayer up to multilayer coverages of {sup 3}He and {sup 4}He films was studied in the temperature range 0.1-1.0 K. For low coverages, common specific heat behavior is found for both {sup 3}He and {sup 4}He films. At thicker coverages, a growing peak, shifting to higher temperatures, is found in {sup 4}He films. It is not present in {sup 3}He film studies. The nature of themore » {sup 4}He peak was probed by adding small doses of {sup 3}He. Mixture films with {sup 3}He concentrations ranging between 0.4 and 530% were thus studied in the same temperature range. It is found that the specific heat peak decreases in size becomes narrower and shifts to lower temperature as a function of {sup 3}He concentration. It eventually disappears at the highest concentration. The authors believe that this peak is related to the superfluid properties of the {sup 4}He films.« less
  • The {sup 13}C NMR spectra of {sup 13}CO adsorbed on a 56% dispersion silica-supported Pd catalyst are found to be quite different from those reported for {sup 13}CO on a 19% dispersion Pd catalyst. The mobility of the {sup 13}CO is greatly reduced on these small Pd particles, which is interpreted as a consequence of the increase in the relative number of low-coordination, electron-deficient Pd sites such as on edges and corners and the concomitant increase in the tendency for CO to bind there in a linear fashion. This is supported by the observation that linear {sup 13}CO on thismore » catalyst has very little Knight shift due to the decrease in the local electron density at the Fermi level for these sites, while bridging {sup 13}CO on terraces is still substantially Knight shifted. Comparison of the {sup 13}C NMR and FTIR spectra shows that the linear CO extinction coefficient is approximately half that for the bridging CO. These NMR results correlate well with previous work on the dispersion dependence of the magnetic susceptibility of silica-supported Pd particles and with the IR spectra for CO adsorbed on such catalysts.« less
  • Deep inelastic neutron scattering measurements have been carried out to measure the kinetic energy and condensate fraction in liquid [sup 3]He-[sup 4]He mixtures. Samples with [sup 3]He concentration on the order of 10% were studied at 1.4 K and 2.3--2.5 K. The condensate fraction was found to be 0.18[plus minus]0.03, much larger than theoretical predictions. The measured kinetic energies of the [sup 3]He and [sup 4]He in the mixture remain the same as their values in the pure phase, in disagreement with current theoretical predictions.