183 research outputs found
Vibrational effects in the quantum dynamics of the H + D_2^+ charge transfer reaction
The H + D_2^+(v=0,1 and 2) charge transfer reaction is studied using an
accurate wave packet method, using recently proposed coupled diabatic potential
energy surfaces. The state-to-state cross section is obtained for three
different channels: non-reactive charge transfer, reactive charge transfer, and
exchange reaction. The three processes proceed via the electronic transition
from the first excited to the ground electronic state. The cross section for
the three processes increases with the initial vibrational excitation. The
non-reactive charge transfer process is the dominant channel, whose branching
ratio increases with collision energy, and it compares well with experimental
measurements at collision energies around 0.5 eV. For lower energies the
experimental cross section is considerably higher, suggesting that it
corresponds to higher vibrational excitation of D_2^+(v) reactants. Further
experimental studies of this reaction and isotopic variants are needed, where
conditions are controlled to obtain a better analysis of the vibrational
effects of the D_2^+ reagents.Comment: 15 pages, 7 figure
Quantum Zeno-based control mechanism for molecular fragmentation
A quantum control mechanism is proposed for molecular fragmentation processes
within a scenario grounded on the quantum Zeno effect. In particular, we focus
on the van der Waals Ne-Br complex, which displays two competing
dissociation channels via vibrational and electronic predissociation.
Accordingly, realistic three dimensional wave packet simulations are carried
out by using ab initio interaction potentials recently obtained to reproduce
available experimental data. Two numerical models to simulate the repeated
measurements are reported and analyzed. It is found that the otherwise fast
vibrational predissociation is slowed down in favor of the slow electronic
(double fragmentation) predissociation, which is enhanced by several orders of
magnitude. Based on these theoretical predictions, some hints to
experimentalists to confirm their validity are also proposed.Comment: 4 pages, 3 figure
Ionization fraction and the enhanced sulfur chemistry in Barnard 1
Barnard B1b has revealed as one of the most interesting globules from the
chemical and dynamical point of view. It presents a rich molecular chemistry
characterized by large abundances of deuterated and complex molecules.
Furthermore, it hosts an extremely young Class 0 object and one candidate to
First Hydrostatic Core (FHSC). Our aim was to determine the cosmic ray
ionization rate and the depletion factors in this extremely young star forming
region. We carried out a spectral survey towards Barnard 1b as part of the IRAM
Large program ASAI using the IRAM 30-m telescope at Pico Veleta (Spain). This
provided a very complete inventory of neutral and ionic C-, N- and S- bearing
species with, up to our knowledge, the first secure detections of the
deuterated ions DCS+ and DOCO+. We used a state-of-the-art
pseudo-time-dependent gas-phase chemical model to determine the value of the
cosmic ray ionization rate and the depletion factors. The observational data
were well fitted with between 3E-17 s and 1E-16 s.
Elemental depletions were estimated to be ~10 for C and O, ~1 for N and ~25 for
S. Barnard B1b presents similar depletions of C and O than those measured in
pre-stellar cores. The depletion of sulfur is higher than that of C and O but
not as extreme as in cold cores. In fact, it is similar to the values found in
some bipolar outflows, hot cores and photon-dominated regions. Several
scenarios are discussed to account for these peculiar abundances. We propose
that it is the consequence of the initial conditions (important outflows and
enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that
permits to maintain most S- and N-bearing species in gas phase to great optical
depths. The interaction of the compact outflow associated with B1b-S with the
surrounding material could enhance the abundances of S-bearing molecules, as
well.Comment: Paper accepted in Astronomy and Astrophysics; 28 pags, 21 figure
The chemistry of H2NC in the interstellar medium and the role of the C + NH3 reaction
We carried out an observational search for the recently discovered molecule
H2NC, and its more stable isomer H2CN, toward eight cold dense clouds (L1544,
L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH3, L1498, and L1641N) and two diffuse
clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in
different types of interstellar regions and shed light on its formation
mechanism. We detected H2NC in most of the cold dense clouds targeted, 7 out of
8, while H2CN was only detected in 5 out of 8 clouds. The column densities
derived for both H2NC and H2CN are in the range 1e11-1e12 cm-2 and the
abundance ratio H2NC/H2CN varies between 0.51 and >2.7. The metastable isomer
H2NC is therefore widespread in cold dense clouds where it is present with an
abundance similar to that of H2CN. We did not detect either H2NC or H2CN in any
of the two diffuse clouds targeted, which does not allow to shed light on how
the chemistry of H2NC and H2CN varies between dense and diffuse clouds. We
found that the column density of H2NC is correlated with that of NH3, which
strongly suggests that these two molecules are chemically linked, most likely
ammonia being a precursor of H2NC through the C + NH3 reaction. We performed
electronic structure and statistical calculations which show that both H2CN and
H2NC can be formed in the C + NH3 reaction through two different channels
involving two different transition states which lie very close in energy. The
predicted product branching ratio H2NC/H2CN is very method dependent but values
between 0.5 and 0.8 are the most likely ones. Therefore, both the astronomical
observations and the theoretical calculations support that the reaction C + NH3
is the main source of H2NC in interstellar clouds.Comment: Accepted for publication in A&
Quantum Zeno effect: Quantum shuffling and Markovianity
The behavior displayed by a quantum system when it is perturbed by a series
of von Neumann measurements along time is analyzed. Because of the similarity
between this general process with giving a deck of playing cards a shuffle,
here it is referred to as quantum shuffling, showing that the quantum Zeno and
anti-Zeno effects emerge naturally as two time limits. Within this framework, a
connection between the gradual transition from anti-Zeno to Zeno behavior and
the appearance of an underlying Markovian dynamics is found. Accordingly,
although a priori it might result counterintuitive, the quantum Zeno effect
corresponds to a dynamical regime where any trace of knowledge on how the
unperturbed system should evolve initially is wiped out (very rapid shuffling).
This would explain why the system apparently does not evolve or decay for a
relatively long time, although it eventually undergoes an exponential decay. By
means of a simple working model, conditions characterizing the shuffling
dynamics have been determined, which can be of help to understand and to devise
quantum control mechanisms in a number of processes from the atomic, molecular
and optical physics.Comment: 12 pages, 2 figure
OH+ in astrophysical media: state-to-state formation rates, Einstein coefficients and inelastic collision rates with He
The rate constants required to model the OH observations in different
regions of the interstellar medium have been determined using state of the art
quantum methods.
First, state-to-state rate constants for the H+ O()
H + OH reaction have been obtained using
a quantum wave packet method. The calculations have been compared with
time-independent results to asses the accuracy of reaction probabilities at
collision energies of about 1 meV. The good agreement between the simulations
and the existing experimental cross sections in the 1 eV energy range
shows the quality of the results.
The calculated state-to-state rate constants have been fitted to an
analytical form. Second, the Einstein coefficients of OH have been obtained
for all astronomically significant ro-vibrational bands involving the
and/or electronic states.
For this purpose the potential energy curves and electric dipole transition
moments for seven electronic states of OH are calculated with {\it ab
initio} methods at the highest level and including spin-orbit terms, and the
rovibrational levels have been calculated including the empirical spin-rotation
and spin-spin terms. Third, the state-to-state rate constants for inelastic
collisions between He and OH have been calculated using a
time-independent close coupling method on a new potential energy surface. All
these rates have been implemented in detailed chemical and radiative transfer
models. Applications of these models to various astronomical sources show that
inelastic collisions dominate the excitation of the rotational levels of
OH. In the models considered the excitation resulting from the chemical
formation of OH increases the line fluxes by about 10 % or less depending
on the density of the gas
Vibrational, non-adiabatic and isotopic effects in the dynamics of the H2 + H2+ → H3+ + H reaction: application to plasma modelling
The title reaction is studied using a quasi-classical trajectory method for collision energies between 0.1 meV and 10 eV, considering the vibrational excitation of (Formula presented.) reactant. A new potential energy surface is developed based on a Neural Network many body correction of a triatomics-in-molecules potential, which significantly improves the accuracy of the potential up to energies of 17 eV, higher than in other previous fits. The effect of the fit accuracy and the non-adiabatic transitions on the dynamics are analysed in detail. The reaction cross section for collision energies above 1 eV increases significantly with the increasing of the vibrational excitation of (Formula presented.) ((Formula presented.)), for values up to (Formula presented.) =6. The total reaction cross section (including the double fragmentation channel) obtained for (Formula presented.) =6 matches the new experimental results obtained by Savic, Schlemmer and Gerlich [Chem. Phys. Chem. 21 (13), 1429.1435 (2020). doi:10.1002/cphc.v21.13]. The differences among several experimental setups, for collision energies above 1 eV, showing cross sections scattered/dispersed over a rather wide interval, can be explained by the differences in the vibrational excitations obtained in the formation of (Formula presented.) reactants. On the contrary, for collision energies below 1 eV, the cross section is determined by the long range behaviour of the potential and do not depend strongly on the vibrational state of (Formula presented.). In addition in this study, the calculated reaction cross sections are used in a plasma model and compared with previous results. We conclude that the efficiency of the formation of (Formula presented.) in the plasma is affected by the potential energy surface use
Vibrational, non-adiabatic and isotopic effects in the dynamics of the H2 + H2+ → H3+ + H reaction: application to plasma modelling
The title reaction is studied using a quasi-classical trajectory method for collision energies between 0.1 meV and 10 eV, considering the vibrational excitation of H+2 reactant. A new potential energy surface is developed based on a Neural Network many body correction of a triatomics-in-molecules potential, which significantly improves the accuracy of the potential up to energies of 17 eV, higher than in other previous fits. The effect of the fit accuracy and the non-adiabatic transitions on the dynamics are analysed in detail. The reaction cross section for collision energies above 1 eV increases significantly with the increasing of the vibrational excitation of H+2(v'), for values up to v'=6. The total reaction cross section (including the double fragmentation channel) obtained for v'=6 matches the new experimental results obtained by Savic, Schlemmer and Gerlich [Chem. Phys. Chem. 21 (13), 1429.1435 (2020). doi:10.1002/cphc.v21.13]. The differences among several experimental setups, for collision energies above 1 eV, showing cross sections scattered/dispersed over a rather wide interval, can be explained by the differences in the vibrational excitations obtained in the formation of H+2 reactants. On the contrary, for collision energies below 1 eV, the cross section is determined by the long range behaviour of the potential and do not depend strongly on the vibrational state of H+2. In addition in this study, the calculated reaction cross sections are used in a plasma model and compared with previous results. We conclude that the efficiency of the formation of H+3 in the plasma is affected by the potential energy surface used
Validation of the Spanish version of the Fear of Self Questionnaire
Cognitive models, from both the appraisal and inferential confusion perspectives, propose that the self is a relevant variable in the development and maintenance of obsessive-compulsive (OC) disorder. In this study, we examined the psychometric properties of the Spanish version of the Fear of Self Questionnaire (FSQ) and analyzed the role of the fear of self (the sort of person we are afraid of becoming) as a predictor of OC beliefs and symptoms. A sample of 359 non-clinical participants completed a set of questionnaires, including the FSQ. Confirmatory factor analysis replicated the original one-factor solution for both the FSQ-8- and 20-item versions. The FSQ demonstrated excellent reliability, and fear of self predicted OC symptoms and cognitions, especially unacceptable obsessions
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