68 research outputs found
Modelling line emission of deuterated H_3^+ from prestellar cores
Context: The depletion of heavy elements in cold cores of interstellar
molecular clouds can lead to a situation where deuterated forms of H_3^+ are
the most useful spectroscopic probes of the physical conditions.
Aims: The aim is to predict the observability of the rotational lines of
H_2D^+ and D_2H^+ from prestellar cores.
Methods: Recently derived rate coefficients for the H_3^+ + H_2 isotopic
system were applied to the "complete depletion" reaction scheme to calculate
abundance profiles in hydrostatic core models. The ground-state lines of
H_2D^+(o) (372 GHz) and D_2H^+(p) (692 GHz) arising from these cores were
simulated. The excitation of the rotational levels of these molecules was
approximated by using the state-to-state coefficients for collisions with H_2.
We also predicted line profiles from cores with a power-law density
distribution advocated in some previous studies.
Results: The new rate coefficients introduce some changes to the complete
depletion model, but do not alter the general tendencies. One of the
modifications with respect to the previous results is the increase of the D_3^+
abundance at the cost of other isotopologues. Furthermore, the present model
predicts a lower H_2D^+ (o/p) ratio, and a slightly higher D_2H^+ (p/o) ratio
in very cold, dense cores, as compared with previous modelling results. These
nuclear spin ratios affect the detectability of the submm lines of H_2D^+(o)
and D_2H^+(p). The previously detected H_2D^+ and D_2H^+ lines towards the core
I16293E, and the H_2D^+ line observed towards Oph D can be reproduced using the
present excitation model and the physical models suggested in the original
papers.Comment: 10 pages, 11 Figures; ver2: updated some of the Figures, added some
references, added an entry to acknowledgement
How can a 22-pole ion trap exhibit 10 local minima in the effective potential?
The column density distribution of trapped OH ions in a 22-pole ion trap
is measured for different trap parameters. The density is obtained from
position-dependent photodetachment rate measurements. Overall, agreement is
found with the effective potential of an ideal 22-pole. However, in addition we
observe 10 distinct minima in the trapping potential, which indicate a breaking
of the 22-fold symmetry. Numerical simulations show that a displacement of a
subset of the radiofrequency electrodes can serve as an explanation for this
symmetry breaking
Discovery of HCCCH in TMC-1
Based on a novel laboratory method, 14 mm-wave lines of the molecular ion
HCCCH have been measured in high resolution, and the spectroscopic
constants of this asymmetric rotor determined with high accuracy. Using the
Yebes 40 m and IRAM 30 m radio telescopes, we detect four lines of
HCCCH towards the cold dense core TMC-1. With a dipole moment of about
0.55 Debye obtained from high-level ab initio calculations, we derive a column
density of 5.4110 cm and
1.60.510 cm for the ortho and para species,
respectively, and an abundance ratio N(HCCC)/N(HCCCH)= 2.80.7.
The chemistry of HCCCH is modelled using the most recent chemical
network for the reactions involving the formation of HCCCH. We find a
reasonable agreement between model predictions and observations, and new
insights into the chemistry of C bearing species in TMC-1 are obtained
Radiofrequency multipole traps: Tools for spectroscopy and dynamics of cold molecular ions
Multipole radiofrequency ion traps are a highly versatile tool to study
molecular ions and their interactions in a well-controllable environment. In
particular the cryogenic 22-pole ion trap configuration is used to study
ion-molecule reactions and complex molecular spectroscopy at temperatures
between few Kelvin and room temperatures. This article presents a tutorial on
radiofrequency ion trapping in multipole electrode configurations. Stable
trapping conditions and buffer gas cooling, as well as important heating
mechanisms, are discussed. In addition, selected experimental studies on cation
and anion-molecule reactions and on spectroscopy of trapped ions are reviewed.
Starting from these studies an outlook on the future of multipole ion trap
research is given
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