1,112 research outputs found
Excitation and characterization of long-lived hydrogenic Rydberg states of nitric oxide
High Rydberg states of nitric oxide (NO) with principal quantum numbers
between 40 and 100 and lifetimes in excess of 10 s have been prepared by
resonance enhanced two-color two-photon laser excitation from the X
ground state through the A intermediate state.
Molecules in these long-lived Rydberg states were detected and characterized
126 s after laser photoexcitation by state-selective pulsed electric field
ionization. The laser excitation and electric field ionization data were
combined to construct two-dimensional spectral maps. These maps were used to
identify the rotational states of the NO ion core to which the observed
series of long-lived hydrogenic Rydberg states converge. The results presented
pave the way for Rydberg-Stark deceleration and electrostatic trapping
experiments with NO, which are expected to shed further light on the decay
dynamics of these long-lived excited states, and are of interest for studies of
ion-molecule reactions at low temperatures.Comment: 12 pages, 10 figure
Service Delivery in Rural Municipalities: Privatize, Cooperate, or Go It Alone?
Choices in production and contracting arrangements for a wide range of services were studied using data from approximately 1,000 small, mostly rural municipalities from Illinois, New Hampshire and Wisconsin. Results suggest the use of both for-profit contractors and cooperative agreements with other governments correlate negatively with population. Small municipalities are less likely to use competitive bidding processes, compare costs between production options, or report that privatization produces savings. Median income, rural geography, and ideology show statistically-significant associations with contracting choices. Respondents generally consider themselves ―satisfied‖ with services provided by contract, although satisfaction levels are lower than those associated with self-provision. Satisfaction associated with services provided by other governments is lower than satisfaction with services provided by private contractors, suggesting no tradeoff in service quality directly attributable to ―for-profit‖ contractors.privatization, municipal services, cooperation, Community/Rural/Urban Development, Public Economics,
On Pulsar Distance Measurements and their Uncertainties
Accurate distances to pulsars can be used for a variety of studies of the
Galaxy and its electron content. However, most distance measures to pulsars
have been derived from the absorption (or lack thereof) of pulsar emission by
Galactic HI gas, which typically implies that only upper or lower limits on the
pulsar distance are available. We present a critical analysis of all measured
HI distance limits to pulsars and other neutron stars, and translate these
limits into actual distance estimates through a likelihood analysis that
simultaneously corrects for statistical biases. We also apply this analysis to
parallax measurements of pulsars in order to obtain accurate distance estimates
and find that the parallax and HI distance measurements are biased in different
ways, because of differences in the sampled populations. Parallax measurements
typically underestimate a pulsar's distance because of the limited distance to
which this technique works and the consequential strong effect of the Galactic
pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance
limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading
to overestimated distances because the bright pulsars on which this technique
is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the
Astrophysical Journa
A trap-based pulsed positron beam optimised for positronium laser spectroscopy
We describe a pulsed positron beam that is optimised for positronium (Ps) laser-spectroscopy experiments. The system is based on a two-stage Surko-type buffer gas trap that produces 4 ns wide pulses containing up to 5 × 105 positrons at a rate of 0.5-10 Hz. By implanting positrons from the trap into a suitable target material, a dilute positronium gas with an initial density of the order of 107 cm−3 is created in vacuum. This is then probed with pulsed (ns) laser systems, where various Ps-laser interactions have been observed via changes in Ps annihilation rates using a fast gamma ray detector. We demonstrate the capabilities of the apparatus and detection methodology via the observation of Rydberg positronium atoms with principal quantum numbers ranging from 11 to 22 and the Stark broadening of the n = 2 → 11 transition in electric fields
Walk, stop, search : the effect of simulated motion on visual attention
Responses in covert orienting of visual attention tasks (COVAT) produce a
biphasic pattern of results. When the stimulus onset asynchrony (SOA) is less than 300
ms, reaction times (RTs) to cued targets are faster than uncued targets, whereas when the
SOA is greater than 300 ms RTs to uncued targets are faster than cued targets. This latter
phenomenon is termed inhibition of return (lOR). lOR is believed to be a mechanism
that promotes efficient search by biasing attention to new locations or objects. To date,
most research on lOR has been restricted to situations in which participants are seated
while viewing stimuli presented on a monitor; however, many real life searches take
place while the searcher is in motion. One way to look at the effect of motion on the
orienting of attention is to stimulate the otoliths of the vestibular system by having people
lie prone with their neck in a flexed position (known as head down neck flexion or
HDNF). We had participants complete a COVAT (with SO As of 100 and 800 ms) while
in three different positions: seated, lying prone, and in HDNF. When in HDNF there
was a significant decrease in the magnitude of responses compared to the other two
positions; both less facilitation and less inhibition were observed. The results are
discussed in terms of the relationship between vestibular activation (i.e., HDNF) and the
orienting of visual attention
VLBI astrometry of PSR J2222-0137: a pulsar distance measured to 0.4% accuracy
The binary pulsar J2222-0137 is an enigmatic system containing a partially
recycled millisecond pulsar and a companion of unknown nature. Whilst the low
eccentricity of the system favors a white dwarf companion, an unusual double
neutron star system is also a possibility, and optical observations will be
able to distinguish between these possibilities. In order to allow the absolute
luminosity (or upper limit) of the companion object to be properly calibrated,
we undertook astrometric observations with the Very Long Baseline Array to
constrain the system distance via a measurement of annual geometric parallax.
With these observations, we measure the parallax of the J2222-0137 system to be
3.742 +0.013 -0.016 milliarcseconds, yielding a distance of 267.3 +1.2 -0.9 pc,
and measure the transverse velocity to be 57.1 +0.3 -0.2 km/s. Fixing these
parameters in the pulsar timing model made it possible to obtain a measurement
of Shapiro delay and hence the system inclination, which shows that the system
is nearly edge-on (sin i = 0.9985 +/- 0.0005). Furthermore, we were able to
detect the orbital motion of J2222-0137 in our VLBI observations and measure
the longitude of ascending node. The VLBI astrometry yields the most accurate
distance obtained for a radio pulsar to date, and is furthermore the most
accurate parallax for any radio source obtained at "low" radio frequencies
(below ~5 GHz, where the ionosphere dominates the error budget). Using the
astrometric results, we show the companion to J2222-0137 will be easily
detectable in deep optical observations if it is a white dwarf. Finally, we
discuss the implications of this measurement for future ultra-high-precision
astrometry, in particular in support of pulsar timing arrays.Comment: 22 pages, 7 figures, accepted for publication in Ap
LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
We present the first LOFAR observations of the radio jet in the quasar
4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved
resolution is very well matched to that of archival Jansky Very Large Array
(JVLA) observations at higher radio frequencies as well as the archival X-ray
images obtained with {\it Chandra}. We found that, for several knots along the
jet, the radio flux densities measured at hundreds of MHz lie well below the
values estimated by extrapolating the GHz spectra. This clearly indicates the
presence of spectral curvature. Radio spectral curvature has been already
observed in different source classes and/or extended radio structures and it
has been often interpreted as due to intrinsic processes, as a curved particle
energy distribution, rather than absorption mechanisms ({ Razin-Tsytovich}
effect, free-free or synchrotron self absorption to name a few). Here we
discuss our results according to the scenario where particles undergo
stochastic acceleration mechanisms also in quasar jet knots.Comment: 13 pages, 4 tables, 4 figures, pre-proof version, published on the
Astrophysical Journal (Harris, et al. 2019 ApJ, 873, 21
Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms
Predicted twenty years ago, positron binding to neutral atoms has not yet
been observed experimentally. A new scheme is proposed to detect positron-atom
bound states by colliding Rydberg positronium (Ps) with neutral atoms.
Estimates of the charge-transfer reaction cross section are obtained using the
first Born approximation for a selection of neutral atom targets and a wide
range of incident Ps energies and principal quantum numbers. We also estimate
the corresponding Ps ionization cross section. The accuracy of the calculations
is tested by comparison with earlier predictions for Ps charge transfer in
collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps
beam suitable for producing positron-atom bound states and estimate signal
rates based on the calculated cross sections and realistic experimental
parameters. We conclude that the proposed methodology is capable of producing
such states and of testing theoretical predictions of their binding energies.Comment: 19 pages, 11 figures, submitted to Phys. Rev.
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