378 research outputs found
Planar Rayleigh scattering results in helium-air mixing experiments in a Mach-6 wind tunnel
Planar Rayleigh scattering measurements with an argon—fluoride excimer laser are performed to investigate helium mixing into air at supersonic speeds. The capability of the Rayleigh scattering technique for flow visualization of a turbulent environment is demonstrated in a large-scale, Mach-6 facility. The detection limit obtained with the present setup indicates that planar, quantitative measurements of density can be made over a large cross-sectional area (5 cm × 10 cm) of the flow field in the absence of clusters
The Impact of Climate Policy on U.S. Aviation
Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).We evaluate the impact of an economy-wide cap-and-trade policy on U.S. aviation taking the American Clean Energy and Security Act of 2009 (H.R.2454) as a representative example. We use an economywide model to estimate the impact of H.R. 2454 on fuel prices and economic activity, and a partial equilibrium model of the aviation industry to estimate changes in aviation carbon dioxide (CO2) emissions and operations. Between 2012 and 2050, with reference demand growth benchmarked to ICAO/GIACC (2009) forecasts, we find that aviation emissions increase by 130%. In our climate policy scenarios, emissions increase by between 97% and 122%. A key finding is that, under the core set of assumptions in our analysis, H.R. 2454 reduces average fleet efficiency, as increased air fares reduce demand and slow the introduction of new aircraft. Assumptions relating to the sensitivity of aviation demand to price changes, and the degree to which higher fuel prices stimulate advances in the fuel efficiency of new aircraft play an important role in this result.U.S. Federal Aviation Administration Office of
Environment and Energy under FAA Award Number: 06HCHNEHMIT, Amendment Nos. 018 and 028. ErichHBecker Foundation. The Joint Program on the Science and
Policy of Global Change is funded by the U.S. Department of Energy and a consortium of
government and industrial sponsors
Interatomic-Coulombic-decay-induced recapture of photoelectrons in helium dimers
We investigate the onset of photoionization shakeup induced interatomic
Coulombic decay (ICD) in He2 at the He+*(n = 2) threshold by detecting two He+
ions in coincidence. We find this threshold to be shifted towards higher
energies compared to the same threshold in the monomer. The shifted onset of
ion pairs created by ICD is attributed to a recapture of the threshold
photoelectron after the emission of the faster ICD electron.Comment: 5 Pages, 2 Figure
Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles
The reverse martensitic ("austenitic") transformation upon heating of
equiatomic nickel-titanium nanoparticles with diameters between 4 and 17 nm is
analyzed by means of molecular-dynamics simulations with a semi-empirical model
potential. After constructing an appropriate order parameter to distinguish
locally between the monoclinic B19' at low and the cubic B2 structure at high
temperatures, the process of the phase transition is visualized. This shows a
heterogeneous nucleation of austenite at the surface of the particles, which
propagates to the interior by plane sliding, explaining a difference in
austenite start and end temperatures. Their absolute values and dependence on
particle diameter are obtained and related to calculations of the surface
induced size dependence of the difference in free energy between austenite and
martensite.Comment: 6 pages, 4 figures, accepted for publication in "The European
Physical Journal B
Vibrationally Resolved Decay Width of Interatomic Coulombic Decay in HeNe
We investigate the ionization of HeNe from below the He 1s3p excitation to
the He ionization threshold. We observe HeNe ions with an enhancement by
more than a factor of 60 when the He side couples resonantly to the radiation
field. These ions are an experimental proof of a two-center resonant
photoionization mechanism predicted by Najjari et al. [Phys. Rev. Lett. 105,
153002 (2010)]. Furthermore, our data provide electronic and vibrational state
resolved decay widths of interatomic Coulombic decay (ICD) in HeNe dimers. We
find that the ICD lifetime strongly increases with increasing vibrational
state.Comment: 7 pages, 5 figure
Ion impact induced Interatomic Coulombic Decay in neon and argon dimers
We investigate the contribution of Interatomic Coulombic Decay induced by ion
impact in neon and argon dimers (Ne and Ar) to the production of low
energy electrons. Our experiments cover a broad range of perturbation strengths
and reaction channels. We use 11.37 MeV/u S, 0.125 MeV/u He,
0.1625 MeV/u He and 0.150 MeV/u He as projectiles and study
ionization, single and double electron transfer to the projectile as well as
projectile electron loss processes. The application of a COLTRIMS reaction
microscope enables us to retrieve the three-dimensional momentum vectors of the
ion pairs of the fragmenting dimer into Ne/Ne and
Ar/Ar (q = 1, 2, 3) in coincidence with at least one emitted
electron
A measurement of the evolution of Interatomic Coulombic Decay in the time domain
During the last 15 years a novel decay mechanism of excited atoms has been
discovered and investigated. This so called ''Interatomic Coulombic Decay''
(ICD) involves the chemical environment of the electronically excited atom: the
excitation energy is transferred (in many cases over long distances) to a
neighbor of the initially excited particle usually ionizing that neighbor. It
turned out that ICD is a very common decay route in nature as it occurs across
van-der-Waals and hydrogen bonds. The time evolution of ICD is predicted to be
highly complex, as its efficiency strongly depends on the distance of the atoms
involved and this distance typically changes during the decay. Here we present
the first direct measurement of the temporal evolution of ICD using a novel
experimental approach.Comment: 6 pages, 4 figures, submitted to PR
Sub-harmonic resonant excitation of confined acoustic modes at GHz frequencies with a high-repetition-rate femtosecond laser
We propose sub-harmonic resonant optical excitation with femtosecond lasers
as a new method for the characterization of phononic and nanomechanical systems
in the gigahertz to terahertz frequency range. This method is applied for the
investigation of confined acoustic modes in a free-standing semiconductor
membrane. By tuning the repetition rate of a femtosecond laser through a
sub-harmonic of a mechanical resonance we amplify the mechanical amplitude,
directly measure the linewidth with megahertz resolution, infer the lifetime of
the coherently excited vibrational states, accurately determine the system's
quality factor, and determine the amplitude of the mechanical motion with
femtometer resolution
Heterogeneous reactions in aircraft gas turbine engines
[1] One-dimensional flow models and unity probability heterogeneous rate parameters are used to estimate the maximum effect of heterogeneous reactions on trace species evolution in aircraft gas turbines. The analysis includes reactions on soot particulates and turbine/nozzle material surfaces. Results for a representative advanced subsonic engine indicate the net change in reactant mixing ratios due to heterogeneous reactions is <10 À6 for O 2 , CO 2 , and H 2 O, and <10 À10 for minor combustion products such as SO 2 and NO 2 . The change in the mixing ratios relative to the initial values is < 0.01%. Since these estimates are based on heterogeneous reaction probabilities of unity, the actual changes will be even lower. Thus, heterogeneous chemistry within the engine cannot explain the high conversion of SO 2 to SO 3 whicsome wake models require to explain the observed levels of vola tile aerosols. Furthermore, turbine heterogeneous processes will not effect exhaust NO x or NO y levels
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