436 research outputs found
Frequency comb vernier spectroscopy in the near infrared
We perform femtosecond frequency comb vernier spectroscopy in the near
infrared with a femtosecond Er doped fiber laser, a scanning high-finesse
cavity and an InGaAs camera. By utilizing the properties of a frequency comb
and a scanning high-finesse cavity such spectroscopy provides broad spectral
bandwidth, high spectral resolution, and high detection sensitivity on a short
time scale. We achieved an absorption sensitivity of ~8E-8 cm-1Hz-1/2
corresponding to a detection limit of ~70 ppbv for acetylene, with a resolution
of ~1.1 GHz in single images taken in 0.5 seconds and covering a frequency
range of ~5 THz. These measurements have broad applications for sensing other
greenhouse gases in this fingerprint near IR region with a simple apparatus.Comment: 14 pages, 5 figure
Viscosity of andesite melts and its implication for magma mixing prior to Unzen 1991-1995 eruption
The viscosity of an iron-bearing melt with composition similar to Unzen andesite was
determined experimentally in the high (109-1010.5 Pa·s) and low (5-1000 Pa·s) viscosity range
using a parallel plate viscometer and the falling sphere method, respectively. Falling sphere
experiments were carried out in an internally heated argon pressure vessel and in a piston
cylinder apparatus at 1323 to 1573 K and 200 to 2000 MPa. Creep experiments were
performed in the temperature range of 747 - 845 K at 300 MPa. The water content of the melt
varies from nominally dry to 6.2 wt% H2O. The Fe2+/Fetot ratio was determined for each
sample in the quenched glass using a colorimetric method. Pressure has minor influence on
the viscosity compared with the effect of temperature, water content (main compositional
parameter controlling the viscosity) or with the Fe2+/Fetot ratio (especially important at low
water content of the melt). Based on our new viscosity data and literature data with measured
Fe2+/Fetot ratio we propose a new empirical equation to estimate the viscosity η (in Pa·s) of
andesitic melts as a function of temperature T (in K), water content w (in wt%) and Fe2+/Fetot
ratio. The derived relationship reproduces the experimental data (87 in total) in the viscosity
range from 100.5 to 1013 Pa·s with a 1σ standard deviation of 0.17 log units. However,
application of this calculation model is limited to Fe2+/Fetot>0.3 and to temperatures above Tg.
Moreover, in the high viscosity range the variation of viscosity with water content is
constrained only by few experimental data and needs verification by additional
measurements.
The viscosity data are used to interpret mixing processes in the Unzen magma chamber prior
to 1991-1995 eruption. We demonstrate that the viscosities of the rhyolite and andesite melts
from the two end-member magmas are nearly identical prior and during mixing, enabling
efficient magma mixing
Zur Konstruktion von Einstellungsskalen im internationalen Vergleich
Der Bericht ist Teil einer interkulturell vergleichenden Studie, die sich mit der Konstruktion von Meßinstrumenten zur interkulturellen Anwendung befaßt. Dabei handelt es sich um Skalen zur Messung des allgemeinen sozialen Lebensgebühls (Social Life Feeling = SLF-Skalen). Aus einer Vielzahl von ad-hoc-Skalen der über 30jährigen Forschungstradition wurden auf der Basis unterschiedlicher Analysen in der amerikanischen Stichprobe 120 Items, verteilt auf 12 SLF-Skalen und zwei Skalen zur Messung von Antworttendenzen, sowie in der deutschen Studie 80 Items, verteilt auf neun SLF-Strahlen, gewonnen. Die Entwicklung und Kombination dieser Skalen zur Messung der Beziehungen zwischen dem allgemeinen sozialen Lebensgefühl und sozialen Ausgangsbedingungen ist Gegenstand der vorliegenden Studie. In mehreren Schritten wurden die Skalen aus beiden Stichproben so verändert, daß eine Kombination der an der amerikanischen und der deutschen Stichprobe entwickelten Skalen zustande kam (COMB-Skalen), die die statistischen Kriterien der Eindimensionalität in beiden Stichproben erfüllen. (AG
Intrinsic and extrinsic x-ray absorption effects in soft x-ray diffraction from the superstructure in magnetite
We studied the (001/2) diffraction peak in the low-temperature phase of
magnetite (Fe3O4) using resonant soft x-ray diffraction (RSXD) at the Fe-L2,3
and O-K resonance. We studied both molecular-beam-epitaxy (MBE) grown thin
films and in-situ cleaved single crystals. From the comparison we have been
able to determine quantitatively the contribution of intrinsic absorption
effects, thereby arriving at a consistent result for the (001/2) diffraction
peak spectrum. Our data also allow for the identification of extrinsic effects,
e.g. for a detailed modeling of the spectra in case a "dead" surface layer is
present that is only absorbing photons but does not contribute to the
scattering signal.Comment: to appear in Phys. Rev.
On X-ray Optical Depth in the Coronae of Active Stars
We have investigated the optical thickness of the coronal plasma through the
analysis of high-resolution X-ray spectra of a large sample of active stars
observed with the High Energy Transmission Grating Spectrometer on Chandra. In
particular, we probed for the presence of significant resonant scattering in
the strong Lyman series lines arising from hydrogen-like oxygen and neon ions.
The active RS CVn-type binaries II Peg and IM Peg and the single M dwarf EV Lac
show significant optical depth. For these active coronae, the Lya/Lyb ratios
are significantly depleted as compared with theoretical predictions and with
the same ratios observed in similar active stars. Interpreting these decrements
in terms of resonance scattering of line photons out of the line-of-sight, we
are able to derive an estimate for the typical size of coronal structures, and
from these we also derive estimates of coronal filling factors. For all three
sources we find that the both the photon path length as a fraction of the
stellar radius, and the implied surface filling factors are very small and
amount to a few percent at most. The measured Lya/Lyb ratios are in good
agreement with APED theoretical predictions, thus indicating negligible optical
depth, for the other sources in our sample. We discuss the implications for
coronal structuring and heating flux requirements. For the stellar sample as a
whole, the data suggest increasing quenching of Lya relative to Lyb as function
of both L_x/L_bol and the density-sensitive MgXI forbidden to intercombination
line ratio, as might generally be expected.Comment: Accepted for publication on the Astrophysical Journa
Observation of anisotropic fragmentation in methane subjected to femtosecond radiation
We present experimental results on the ionization/dissociation of methane in
femtosecond pulses of radiation. Angular and intensity dependent yields of
singly and doubly charged species were measured using an imaging mass
spectrometer. The measured data shows that all fragments yields exhibit some
degree of anisotropy as a result of them being preferably ejected parallel to
the polarization direction. Additionally, an anomalous perpendicular
fragmentation pattern is found for CH\-(2)\+(2+). We find evidence of multiple
dissociation mechanisms including statistical decay, field assisted
dissociation and Coulomb explosion.Comment: 24 pages, 7 figure
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