257 research outputs found
Collisional excitation of NH(3ς-) by Ar: A new ab initio 3D potential energy surface and scattering calculations
No full textCollisional excitation of light hydrides is important to fully understand the complex chemical and physical processes of atmospheric and astrophysical environments. Here, we focus on the NH(X3ς-)-Ar van der Waals system. First, we have calculated a new three-dimensional Potential Energy Surface (PES), which explicitly includes the NH bond vibration. We have carried out the ab initio calculations of the PES employing the open-shell single- and double-excitation couple cluster method with noniterative perturbational treatment of the triple excitations. To achieve a better accuracy, we have first obtained the energies using the augmented correlation-consistent aug-cc-pVXZ (X = T, Q, 5) basis sets and then we have extrapolated the final values to the complete basis set limit. We have also studied the collisional excitation of NH(X3ς-)-Ar at the close-coupling level, employing our new PES. We calculated collisional excitation cross sections of the fine-structure levels of NH by Ar for energies up to 3000 cm-1. After thermal average of the cross sections, we have then obtained the rate coefficients for temperatures up to 350 K. The propensity rules between the fine-structure levels are in good agreement with those of similar collisional systems, even though they are not as strong and pronounced as for lighter systems, such as NH-He. The final theoretical values are also compared with the few available experimental data
Hyperfine resolved rate coefficients of HC17O+ with H2 (j = 0)
Get PDFThe formyl cation (HCO+) is one of the most abundant ions in molecular clouds and plays a major role in the interstellar chemistry. For this reason, accurate collisional rate coefficients for the rotational excitation of HCO+ and its isotopes due to the most abundant perturbing species in interstellar environments are crucial for non-local thermal equilibrium models and deserve special attention. In this work, we determined the first hyperfine resolved rate coefficients of HC17O+ in collision with H2 (j = 0). Indeed, despite no scattering calculations on its collisional parameters have been performed so far, the HC17O+ isotope assumes a prominent role for astrophysical modelling applications. Computations are based on a new four dimensional (4D) potential energy surface obtained at the CCSD(T)-F12a/aug-cc-pVQZ level of theory. A test on the corresponding cross-section values pointed out that, to a good approximation, the influence of the coupling between rotational levels of H2 can be ignored. For this reason, the H2 collider has been treated as a spherical body and an average of the potential based on five orientations of H2 has been employed for scattering calculations. State-to-state rate coefficients resolved for the HC17O+ hyperfine structure for temperature ranging from 5 to 100K have been computed using recoupling techniques. This study provides the first determination of HC17O+–H2 inelastic rate coefficients directly computed from full quantum close-coupling equations, thus supporting the reliability of future radiative transfer modellings of HC17O+ in interstellar environments
Collisional excitation of PO+ by para-H2: potential energy surface, scattering calculations, and astrophysical applications
Get PDFWe report the derivation of rate coefficients for the rotational (de-)excitation of PO+ induced by collisions with H-2. The calculations were performed on a 4D potential energy surface, obtained on top of highly accurate ab initio energy points. Preliminary tests pointed out the low influence of the coupling between j = 0 and the higher rotational levels of H-2 on the cross-sections values, thus allowing to neglect the rotational structure of H-2. On this basis, state-to-state collisional rate coefficients were derived for temperatures ranging from 5 to 200 K. Radiative transfer calculations have been used to model the recent observation of PO+ in the G+0.693-0.027 molecular cloud, in order to evaluate the possible impact of non-LTE models on the determination of its physical conditions. The derived column density was found to be approximately similar to 3.7 x 10(11) cm(-2), which is 60% (a factor of similar to 1.7) smaller than the previously LTE-derived value. Extensive simulations show that PO+ low-j rotational lines exhibit maser behaviour at densities between 10(4) and 10(6) cm(-3), thus highlighting the importance of a proper treatment of the molecular collisions to accurately model PO+ emissions in the interstellar medium
Collisional excitation of PO+ by para-H2: potential energy surface, scattering calculations, and astrophysical applications
Get PDFWe report the derivation of rate coefficients for the rotational (de-)excitation of PO+ induced by collisions with H2. The calculations were performed on a 4D potential energy surface, obtained on top of highly accurate ab initio energy points. Preliminary tests pointed out the low influence of the coupling between j = 0 and the higher rotational levels of H2 on the cross-sections values, thus allowing to neglect the rotational structure of H2. On this basis, state-to-state collisional rate coefficients were derived for temperatures ranging from 5 to 200 K. Radiative transfer calculations have been used to model the recent observation of PO+ in the G+0.693–0.027 molecular cloud, in order to evaluate the possible impact of non-LTE models on the determination of its physical conditions. The derived column density was found to be approximately ∼ 3.7 × 1011 cm−2, which is 60% (a factor of ∼ 1.7) smaller than the previously LTE-derived value. Extensive simulations show that PO+ low-j rotational lines exhibit maser behaviour at densities between 104 and 106 cm−3, thus highlighting the importance of a proper treatment of the molecular collisions to accurately model PO+ emissions in the interstellar medium
An improved study of HCO+ and He system: Interaction potential, collisional relaxation, and pressure broadening
Get PDFIn light of its ubiquitous presence in the interstellar gas, the chemistry and reactivity of the HCO+ ion requires special attention. The availability of up-to-date collisional data between this ion and the most abundant perturbing species in the interstellar medium is a critical resource in order to derive reliable values of its molecular abundance from astronomical observations. This work intends to provide improved scattering parameters for the HCO+ and He collisional system. We have tested the accuracy of explicitly correlated coupled-cluster methods for mapping the short- and long-range multi-dimensional potential energy surface of atom-ion systems. A validation of the methodology employed for the calculation of the potential well has been obtained from the comparison with experimentally derived bound-state spectroscopic parameters. Finally, by solving the close-coupling scattering equations, we have derived the pressure broadening and shift coefficients for the first six rotational transitions of HCO+ as well as inelastic state-to-state transition rates up to j = 5 in the 5-100 K temperature interval
Hyperfine Excitation Of With P- Collisions
No full textThe formyl ion (\ce{HCO+}) is one of the most abundant ions in molecular clouds and represents an excellent candidate to trace dense molecular gas through the evolutionary stages of the interstellar medium (ISM).
For this reason, the accurate rotational rate coefficients of \ce{HCO+} and its isotopes with the most abundant perturbing species in the ISM are crucial in non-local thermal equilibrium (LTE) models and deserve special attention. To this end, many efforts have been made in order to retrieve accurate collisional parameters of \ce{HCO+} interacting with the \ce{He} and \ce{H2} colliders as well as for some of its isotopologues\footnote{Tonolo F., Bizzocchi L., Melosso M., Lique F., Dore L., Barone V., Puzzarini C., 2021, \emph{The Journal of Chemical Physics}, \textbf{155}, 234306.}\footnote{Denis-Alpizar O., Stoecklin T., Dutrey A., Guilloteau S., 2020, \emph{Monthly Notices of the Royal Astronomical Society}, \textbf{497}, 4276.}.
However, in spite of laboratory and observational studies on \ce{HC^{17}O+}\footnote{Plummer G., Herbst E., De Lucia F., 1983, \emph{The Astrophysical Journal}, \textbf{270}, L99.}\footnote{Dore L., Cazzoli G., Caselli P., 2001, \emph{Astronomy \& Astrophysics}, \textbf{368}, 712.}, to the best of our knowledge, an accurate characterization of its collisional parameters has not been carried out yet.
Although rarer, the \ce{HC^{17}O+} isotope assumes a prominent role to avoid problems due to the optical thickness of the parent species emissions.
With the aim of filling this lack, this work reports the first calculations of hyperfine resolved rate coefficients for the excitation of \ce{HC^{17}O+} by -\ce{H2} .\\
We characterized the potential energy surface of the \ce{HCO+} and \ce{H2} collisional system by means of the CCSD(T)-F12a/aug-cc-pVQZ level of theory.
The interaction energy has been averaged over five \ce{H2} orientations and then fitted as an expansion of angular functions.
Finally, state-to-state rate coefficients between the lower hyperfine levels have been computed using recoupling techniques for temperature ranging from 5 to 100 K
Temperature and density dependent cooling function for H2 with updated H2/H collisional rates
Get PDFThe energy transfer among the components in a gas determines its fate. Especially at low temperatures, inelastic collisions drive the cooling and the heating mechanisms. In the early Universe as well as in zero- or low-metallicity environments the major contribution comes from the collisions among atomic and molecular hydrogen, also in its deuterated version. This work shows some updated calculations of the H2 cooling function based on novel collisional data which explicitly take into account the reactive pathway at low temperatures. Deviations from previous calculations are discussed and a multivariate data analysis is performed to provide a fit depending on both the gas temperature and the density of the gas
The Chemistry of the Early Universe
No full textGas-Phase Chemistry in Space: From elementary particles to complex organic molecules is written by a collection of experts in the field of astrochemistry. The book introduces essential concepts that govern the formation, excitation and destruction of molecules at a postgraduate and research level. A broad range of topics are covered; from early universe chemistry and stellar nucleosynthesis, to the study of bimolecular reaction kinetics. Detailed description of the gas-phase process is provided and recent examples of the interplay between observational and laboratory astrophysics are examined. Using more than 100 figures, as well as examples, this work reveals, in detail, both theoretical and experimental perspectives that can be implemented in future discoveries
Phys. Chem. Chem. Phys. 17, 21583 (2015): Supplementary download of He-CH(X) PES Fortran Routine
No full text<p>This is Fortran routine for He-CH(X) RCCSD(T)/aug-cc-pvqz+BF PES published by:</p>
<p>S. Marinakis, I. L. Dean, J. Klos and F. Lique Phys. Chem. Chem. Phys. 17, 21583 (2015)</p>
<p>Files:</p>
<p>(1) hechx_lique_klos.f : He-CH(X) PES routine in Jacobi (R,theta) coordinates (with respect to c.o.m of CH) for CH(r=re) equilibrium distance</p>
<p> Input: R in bohr (a0)</p>
<p> Theta in degrees</p>
<p> Output: Energy in cm-1, Asymptote for R=infinity has E=0 for He---CH(r=re) limit.</p>
<p> Theta=0 degrees corresponds to He---C-H collinear arrangement.</p>
<p> </p>
<p>(2) HeCHX.grid : Example of the output. Columns in this file are: R/a0, Theta/deg, Vsum, Vdif, A", A' (all in cm-1)</p>
<p> </p>
<p>(3) compile: example of compilation with Intel Fortran compiler. Caution: gfortran compiler has not been tested.</p>
From Gap-Exponential Time Hypothesis to Fixed Parameter Tractable Inapproximability: {C}lique, Dominating Set, and More
No full textWe consider questions that arise from the intersection between the areas of polynomial-time approximation algorithms, subexponential-time algorithms, and fixed-parameter tractable (FPT) algorithms. The questions, which have been asked several times, are whether there is a nontrivial FPT-approximation algorithm for the Maximum Clique (Clique) and Minimum Dominating Set (DomSet) problems parameterized by the size of the optimal solution. In particular, letting OPT be the optimum and N be the size of the input, is there an algorithm that runs in t(OPT)poly(N) time and outputs a solution of size f(OPT) for any computable functions t and f that are independent of N (for Clique, we want f (OPT) = omega(1))? In this paper, we show that both Clique and DomSet admit no nontrivial FPT-approximation algorithm, i.e., there is no o(OPT)-FPT-approximation algorithm for Clique and no f (OPT)-FPT-approximation algorithm for DomSet for any function f. In fact, our results imply something even stronger: The best way to solve Clique and DomSet, even approximately, is to essentially enumerate all possibilities. Our results hold under the Gap Exponential Time Hypothesis [I. Dinur. ECCC, TR16-128, 2016; P. Manurangsi and P. Raghavendra, preprint, arXiv:1607.02986, 2016], which states that no 2(o(n))-time algorithm can distinguish between a satisfiable 3 SAT formula and one which is not even (1 - epsilon)-satisfiable for some constant epsilon > 0. Besides Clique and DomSet, we also rule out nontrivial FPT-approximation for the Maximum Biclique problem, the problem of finding maximum subgraphs with hereditary properties (e.g., Maximum Induced Planar Subgraph), and Maximum Induced Matching in bipartite graphs, and we rule out the k(o(1))-FPT-approximation algorithm for the Densest k-Subgraph problem.Peer reviewe
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