178 research outputs found
Interaction of oxygen with TiN (001): N↔O exchange and oxidation process
This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450K. At the lowest temperature, there was chemisorption of oxygen (O2,gas→2Oads) without significant surface oxidation. In contrast, at 450K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (ΔE=2.84eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2→TiOx+NO reaction is an important exit channel for nitrogen in the oxidation process.Ministerio de Educación y Ciencia de España, MEC. MAT2005-01872Junta de AndalucÃa. FQM-132División de Ciencias QuÃmicas del Departamento de EnergÃa de EE. UU. DE-AC02-98CH10086Fundación japonesa para la Ciencia de los Materiale
Ionization dynamics in intense pulsed laser radiation. Effects of frequency chirping
Via a non-perturbative method we study the population dynamics and
photoelectron spectra of Cs atoms subject to intense chirped laser pulses, with
gaussian beams. We include above threshold ionization spectral peaks. The
frequency of the laser is near resonance with the 6s-7p transition. Dominant
couplings are included exactly, weaker ones accounted for perturbatively. We
calculate the relevant transition matrix elements, including spin-orbit
coupling. The pulse is taken to be a hyperbolic secant in time and the chirping
a hyperbolic tangent. This choice allows the equations of motions for the
probability amplitudes to be solved analytically as a series expansion in the
variable u=(tanh(pi t/tau)+1)/2, where tau is a measure of the pulse length. We
find that the chirping changes the ionization dynamics and the photoelectron
spectra noticeably, especially for longer pulses of the order of 10^4 a.u. The
peaks shift and change in height, and interference effects between the 7p
levels are enhanced or diminished according to the amount of chirping and its
sign. The integrated ionization probability is not strongly affected.Comment: Accepted by J. Phys. B; 18 pages, 17 figures. Latex, uses
ioplppt.sty, iopl10.sty and psfig.st
Galactic Parameters from Masers with Trigonometric Parallaxes
Spatial velocities of all currently known 28 masers having trigonometric
parallaxes, proper motion and line-of-site velocities are reanalyzed using
Bottlinger's equations. These masers are associated with 25 active star-forming
regions and are located in the range of galactocentric distances 3<R<14 kpc. To
determine the Galactic rotation parameters, we used the first three Taylor
expansion terms of angular rotation velocity {\Omega} at the galactocentric
distance of the Sun R0=8 kpc. We obtained the following solutions:
{\Omega}o=-31.0 +/- 1.2 km/s/kpc, {\Omega}o'=4.46 +/- 0.21 km/s/kpc^2,
{\Omega}o"=-0.876 +/- 0.067 km/s/kpc^3, Oort constants: A=17.8 +/- 0.8
km/s/kpc, B=-13.2 +/- 1.5 km/s/kpc and circular velocity of the Solar
neighborhood rotation Vo=248 +/- 14 km/s. Fourier analysis of galactocentric
radial velocities of masers VR allowed us to estimate the wavelength
{\lambda}=2.0 +/- 0.2 kpc and peak velocity f_R=6.5 +/- 2 km/s of periodic
perturbations from the density wave and velocity of the perturbations 4 +/- 1
km/s near the location of the Sun. Phase of the Sun in the density wave is
estimated as {\chi}o ~ -130^o +/- 10^o. Taking into account perturbations
evoked by spiral density wave we obtained the following non-perturbed
components of the peculiar Solar velocity with respect to the local standard of
rest (LSR) (Uo,Vo,Wo)LSR=(5.5,11,8.5) +/- (2.2,1.7,1.2) km/s.Comment: 8 pages, 1table, 9 figures, accepte
ALMA High-frequency Long Baseline Campaign in 2021: Highest Angular Resolution Submillimeter Wave Images for the Carbon-rich Star R Lep
The Atacama Large Millimeter/submillimeter Array (ALMA) was used in 2021 to
image the carbon-rich evolved star R Lep in Bands 8-10 (397-908 GHz) with
baselines up to 16 km. The goal was to validate the calibration, using
band-to-band (B2B) phase referencing with a close phase calibrator J0504-1512,
1.2 deg from R Lep in this case, and the imaging procedures required to obtain
the maximum angular resolution achievable with ALMA. Images of the continuum
emission and the hydrogen cyanide (HCN) maser line at 890.8 GHz, from the
J=10-9 transition between the (1110) and (0400) vibrationally excited states,
achieved angular resolutions of 13, 6, and 5 mas in Bands 8-10, respectively.
Self-calibration (self-cal) was used to produce ideal images as to compare with
the B2B phase referencing technique. The continuum emission was resolved in
Bands 9 and 10, leaving too little flux for self-cal of the longest baselines,
so these comparisons are made at coarser resolution. Comparisons showed that
B2B phase referencing provided phase corrections sufficient to recover 92%,
83%, and 77% of the ideal image continuum flux densities. The HCN maser was
sufficiently compact to obtain self-cal solutions in Band 10 for all baselines
(up to 16 km). In Band 10, B2B phase referencing as compared to the ideal
images recovered 61% and 70% of the flux density for the HCN maser and
continuum, respectively.Comment: 37 pages, 12 figures, 9 tables, accepted by ApJ (Aug 30, 2023
ALMA Observations of Asteroid 3 Juno at 60 Kilometer Resolution
We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm
continuum images of the asteroid 3 Juno obtained with an angular resolution of
0.042 arcseconds (60 km at 1.97 AU). The data were obtained over a single 4.4
hr interval, which covers 60% of the 7.2 hr rotation period, approximately
centered on local transit. A sequence of ten consecutive images reveals
continuous changes in the asteroid's profile and apparent shape, in good
agreement with the sky projection of the three-dimensional model of the
Database of Asteroid Models from Inversion Techniques. We measure a geometric
mean diameter of 259pm4 km, in good agreement with past estimates from a
variety of techniques and wavelengths. Due to the viewing angle and inclination
of the rotational pole, the southern hemisphere dominates all of the images.
The median peak brightness temperature is 215pm13 K, while the median over the
whole surface is 197pm15 K. With the unprecedented resolution of ALMA, we find
that the brightness temperature varies across the surface with higher values
correlated to the subsolar point and afternoon areas, and lower values beyond
the evening terminator. The dominance of the subsolar point is accentuated in
the final four images, suggesting a reduction in the thermal inertia of the
regolith at the corresponding longitudes, which are possibly correlated to the
location of the putative large impact crater. These results demonstrate ALMA's
potential to resolve thermal emission from the surface of main belt asteroids,
and to measure accurately their position, geometric shape, rotational period,
and soil characteristics.Comment: 8 pages, 3 figures, 2 tables, accepted for publication in the
Astrophysical Journal Letter
ALMA High-frequency Long Baseline Campaign in 2017:Band-to-band Phase Referencing in Submillimeter Waves
In 2017, an Atacama Large Millimeter/submillimeter Array (ALMA)
high-frequency long baseline campaign was organized to test image capabilities
with baselines up to 16 km at submillimeter (submm) wavelengths. We
investigated image qualities using ALMA receiver Bands 7, 8, 9, and 10 (285-875
GHz) by adopting band-to-band (B2B) phase referencing in which a phase
calibrator is tracked at a lower frequency. For B2B phase referencing, it is
expected that a closer phase calibrator to a target can be used, comparing to
standard in-band phase referencing. In the first step, it is ensured that an
instrumental phase offset difference between low- and high-frequency Bands can
be removed using a differential gain calibration in which a phase calibrator is
certainly detected while frequency switching. In the next step, comparative
experiments are arranged to investigate the image quality between B2B and
in-band phase referencing with phase calibrators at various separation angles.
In the final step, we conducted long baseline imaging tests for a quasar at 289
GHz in Band 7 and 405 GHz in Band 8 and complex structure sources of HL Tau and
VY CMa at ~670 GHz in Band 9. The B2B phase referencing was successfully
applied, allowing us to achieve an angular resolution of 14x11 and 10x8 mas for
HL Tau and VY CMa, respectively. There is a high probability of finding a
low-frequency calibrator within 5.4 deg in B2B phase referencing, bright enough
to use an 8 s scan length combined with a 7.5 GHz bandwidth.Comment: 61 pages, 17 figures, 8 table
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