385 research outputs found
Internal-state thermometry by depletion spectroscopy in a cold guided beam of formaldehyde
We present measurements of the internal state distribution of
electrostatically guided formaldehyde. Upon excitation with continuous tunable
ultraviolet laser light the molecules dissociate, leading to a decrease in the
molecular flux. The population of individual guided states is measured by
addressing transitions originating from them. The measured populations of
selected states show good agreement with theoretical calculations for different
temperatures of the molecule source. The purity of the guided beam as deduced
from the entropy of the guided sample using a source temperature of 150K
corresponds to that of a thermal ensemble with a temperature of about 30 K
Ionenfallen- und Speicherringtechnik zur Untersuchung kalter Wasserstoffmolekülionen
An important participant in interstellar chemistry is the H+3 molecular ion. Its concentration depends on the rate coefficient of dissociative recombination with electrons at low temperatures. Experiments aiming at a determination of this rate coefficient scatter by several orders of magnitude. This might by attributed to vibrational or rotational excitations of the H+3 molecular ions. In particular previous storage ring experiments revealed long lifetimes of certain rotational excitations. Hence a source for rotationally cold H+3 molecular ions was installed at the Max-Planck-Institut fur Kernphysik in Heidelberg. The setup is based on a cryogenic 22-pole radiofrequency ion trap which allows collisional cooling of the H+3 molecular ions by the use of helium buffergas. The molecular ions can be extracted from the trap and transferred to the storage ring (TSR) for recombination experiments. First results suggest that by the use of this setup a spectrum of the dissociative recombination of cold H+3 molecular ions could be measured. Moreover the setup can be used to probe gas phase chemical reactions at low temperatures. The formation of deuterium cluster ions at 13K was studied and rate coefficients for association and dissociation reactions of these clusters ions could be extracted
Continuum limit of self-driven particles with orientation interaction
We consider the discrete Couzin-Vicsek algorithm (CVA), which describes the
interactions of individuals among animal societies such as fish schools. In
this article, we propose a kinetic (mean-field) version of the CVA model and
provide its formal macroscopic limit. The final macroscopic model involves a
conservation equation for the density of the individuals and a non conservative
equation for the director of the mean velocity and is proved to be hyperbolic.
The derivation is based on the introduction of a non-conventional concept of a
collisional invariant of a collision operator
Cold guided beams of water isotopologs
Electrostatic velocity filtering and guiding is an established technique to
produce high fluxes of cold polar molecules. In this paper we clarify different
aspects of this technique by comparing experiments to detailed calculations. In
the experiment, we produce cold guided beams of the three water isotopologs
H2O, D2O and HDO. Their different rotational constants and orientations of
electric dipole moments lead to remarkably different Stark shift properties,
despite the molecules being very similar in a chemical sense. Therefore, the
signals of the guided water isotopologs differ on an absolute scale and also
exhibit characteristic electrode voltage dependencies. We find excellent
agreement between the relative guided fractions and voltage dependencies of the
investigated isotopologs and predictions made by our theoretical model of
electrostatic velocity filtering.Comment: 14 pages, 13 figures; small changes to the text, updated reference
Electrostatic extraction of cold molecules from a cryogenic reservoir
We present a method which delivers a continuous, high-density beam of slow
and internally cold polar molecules. In our source, warm molecules are first
cooled by collisions with a cryogenic helium buffer gas. Cold molecules are
then extracted by means of an electrostatic quadrupole guide. For ND the
source produces fluxes up to molecules/s with
peak densities up to molecules/cm. For
HCO the population of rovibrational states is monitored by depletion
spectroscopy, resulting in single-state populations up to .Comment: 4 pages, 4 figures, changes to the text, updated figures and
reference
Velocity-selected molecular pulses produced by an electric guide
Electrostatic velocity filtering is a technique for the production of
continuous guided beams of slow polar molecules from a thermal gas. We extended
this technique to produce pulses of slow molecules with a narrow velocity
distribution around a tunable velocity. The pulses are generated by
sequentially switching the voltages on adjacent segments of an electric
quadrupole guide synchronously with the molecules propagating at the desired
velocity. This technique is demonstrated for deuterated ammonia (ND),
delivering pulses with a velocity in the range of and a
relative velocity spread of at FWHM. At velocities around
, the pulses contain up to molecules each. The data are
well reproduced by Monte-Carlo simulations, which provide useful insight into
the mechanisms of velocity selection.Comment: 8 pages, 6 figure
Opto-Electrical Cooling of Polar Molecules
We present an opto-electrical cooling scheme for polar molecules based on a
Sisyphus-type cooling cycle in suitably tailored electric trapping fields.
Dissipation is provided by spontaneous vibrational decay in a closed level
scheme found in symmetric-top rotors comprising six low-field-seeking
rovibrational states. A generic trap design is presented. Suitable molecules
are identified with vibrational decay rates on the order of 100Hz. A simulation
of the cooling process shows that the molecular temperature can be reduced from
1K to 1mK in approximately 10s. The molecules remain electrically trapped
during this time, indicating that the ultracold regime can be reached in an
experimentally feasible scheme
Solutions to Maxwell's Equations using Spheroidal Coordinates
Analytical solutions to the wave equation in spheroidal coordinates in the
short wavelength limit are considered. The asymptotic solutions for the radial
function are significantly simplified, allowing scalar spheroidal wave
functions to be defined in a form which is directly reminiscent of the
Laguerre-Gaussian solutions to the paraxial wave equation in optics.
Expressions for the Cartesian derivatives of the scalar spheroidal wave
functions are derived, leading to a new set of vector solutions to Maxwell's
equations. The results are an ideal starting point for calculations of
corrections to the paraxial approximation
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