arXiv subjects

François Lique

Publications and source records attributed to François Lique.

31 records · Page 2Linked to original sources

arXiv2022

Benchmarking an improved statistical adiabatic channel model for competing inelastic and reactive processes

Inelastic collisions and elementary chemical reactions proceeding through the formation and subsequent decay of an intermediate collision complex, with an associated deep well on the potential energy surface, pose a challenge for accurate fully quantum mechanical approaches, such as the close-coupling method. In this study, we report on the theoretical prediction of temperature-dependent state-to-state rate coefficients for these complex-mode processes, using a statistical quantum method. This statistical adiabatic channel model is benchmarked by a direct comparison using accurate rate coefficients from the literature for a number of systems (H2 + H+, HD + H+, SH+ + H, and CH+ + H) of interest in astrochemistry and astrophysics. For all of the systems considered, an error of less than factor 2 was found, at least for the dominant transitions and at low temperatures, which is sufficiently accurate for applications in the above mentioned disciplines.

physics.chem-ph↗

arXiv2021

High-resolution imaging of C + He collisions using Zeeman deceleration and VUV detection

High-resolution measurements of angular scattering distributions provide a sensitive test for theoretical descriptions of collision processes. Crossed beam experiments employing a decelerator and velocity map imaging have proven successful to probe collision cross sections with extraordinary resolution. However, a prerequisite to exploit these possibilities is the availability of a near-threshold state-selective ionization scheme to detect the collision products, which for many species is either absent or inefficient. We present the first implementation of recoil-free vacuum ultraviolet (VUV) based detection in scattering experiments involving a decelerator and velocity map imaging. This allowed for high-resolution measurements of state-resolved angular scattering distributions for inelastic collisions between Zeeman-decelerated carbon C($^3P_1$) atoms and helium atoms. We fully resolved diffraction oscillations in the angular distributions, which showed excellent agreement with the distributions predicted by quantum scattering calculations. Our approach offers exciting prospects to investigate a large range of scattering processes with unprecedented precision.

physics.atom-ph↗

arXiv2021

Rotational excitation of H$_3$O$^+$ cations by para-H$_2$: improved collisional data at low temperatures

The hydronium cation plays a crucial role in interstellar oxygen and water chemistry. While its spectroscopy was extensively investigated earlier, the collisional excitation of H$_3$O$^+$ is not well studied yet. In this work we present state-to-state collisional data for rotational de-excitation of both ortho- and para-H$_3$O$^+$ due to para-H$_2$ impact. The cross sections are calculated within the close-coupling formalism using our recent, highly accurate rigid-rotor potential energy surface for this collision system. The corresponding thermal rate coefficients are calculated up to 100 K. For para-H$_3$O$^+$ the lowest 20 rotation-inversion states were considered in the calculations, while for ortho-H$_3$O$^+$ the lowest 11 states are involved (up to $j\leq5$), so all levels with rotational energy below 420 K (292 cm$^{-1}$) are studied. In order to analyse the impact of the new collisional rates on the excitation of H$_3$O$^+$ in astrophysical environments radiative transfer calculations are also provided. The most relevant emission lines from an astrophysical point of view are studied, taking into account the transitions at 307, 365, 389 and 396 GHz. We show that our new collisional data have a non-negligible impact (from a few percents up to about a factor of 3) on the brightness and excitation temperatures of H$_3$O$^+$, justifying the revision of the physical conditions in the appropriate astrophysical observations. The calculated rate coefficients allow one to recalculate the column density of hydronium in interstellar clouds, which can lead to a better understanding of interstellar water and oxygen chemistry.

physics.chem-ph↗

arXiv2020

CF$^+$ excitation in the interstellar medium

The detection of CF$^+$ in interstellar clouds potentially allows astronomers to infer the elemental fluorine abundance and the ionization fraction in ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF$^+$ abundance requires one to model its non-LTE excitation via both radiative and collisional processes. Here, we report quantum calculations of rate coefficients for the rotational excitation of CF$^+$ in collisions with para- and ortho-H2 (for temperatures up to 150 K). As an application, we present non-LTE excitation models that reveal population inversion in physical conditions typical of ISM photodissociation regions (PDRs). We successfully applied these models to fit the CF$^+$ emission lines previously observed toward the Orion Bar and Horsehead PDRs. The radiative transfer models achieved with these new rate coefficients allow the use of CF$^+$ as a powerful probe to study molecular clouds exposed to strong stellar radiation fields.

astro-ph.GA↗

arXiv2020

Hyperfine excitation of SH$^+$ by H

SH$^+$ is a surprisingly widespread molecular ion in diffuse interstellar clouds. There, it plays an important role triggering the sulfur chemistry. In addition, SH$^+$ emission lines have been detected at the UV-illuminated edges of dense molecular clouds, \mbox{so-called} photo-dissociation regions (PDRs), and toward high-mass protostars. An accurate determination of the SH$^+$ abundance and of the physical conditions prevailing in these energetic environments relies on knowing the rate coefficients of inelastic collisions between SH$^+$ molecules and hydrogen atoms, hydrogen molecules, and electrons. In this paper, we derive SH$^+$--H fine and hyperfine-resolved rate coefficients from the recent quantum calculations for the SH$^+$--H collisions, including inelastic, exchange and reactive processes. The method used is based on the infinite order sudden approach. State-to-state rate coefficients between the first 31 fine levels and 61 hyperfine levels of SH$^+$ were obtained for temperatures ranging from 10 to 1000 K. Fine structure-resolved rate coefficients present a strong propensity rule in favour of $Δj = ΔN$ transitions. The $Δj = ΔF$ propensity rule is observed for the hyperfine transitions. {The new rate coefficients will help significantly in the interpretation of SH$^+$ spectra from PDRs and UV-irradiated shocks where the abundance of hydrogen atoms with respect to hydrogen molecules can be significant.

physics.space-ph↗

arXiv2020

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of accurate data: we suggest ways to improve the synergies among users and suppliers of data. We summarize some recent developments in computation of collisional cross sections and rate coefficients. We consider atomic and molecular data that are needed to support astrophysics and astrochemistry with upcoming instruments that operate in the mid- and far-infrared parts of the spectrum.

astro-ph.GA↗

arXiv2020

Physical and chemical modeling of the starless core L1512

The deuterium fractionation in starless cores gives us a clue to estimate their lifetime scales, thus allowing us to distinguish between different dynamical theories of core formation. Cores also seem to be subject to a differential N2 and CO depletion which was not expected from models. We aim to make a survey of 10 cores to estimate their lifetime scales and depletion profiles in detail. After L183, in Serpens, we present the second cloud of the series, L1512 in Auriga. To constrain the lifetime scale, we perform chemical modeling of the deuteration profiles across L1512 based on dust extinction measurements from near-infrared observations and non-local thermal equilibrium radiative transfer with multiple line observations of N2H+, N2D+, DCO+, C18O, and 13CO, plus H2D+ (1$_{10}$--1$_{11}$). We find a peak density of 1.1$\times$10$^5$ cm$^{-3}$ and a central temperature of 7.5$\pm$1 K, which are respectively higher and lower compared with previous dust emission studies. The depletion factors of N2H+ and N2D+ are 27$^{+17}_{-13}$ and 4$^{+2}_{-1}$ in L1512, intermediate between the two other more advanced and denser starless core cases, L183 and L1544. These factors also indicate a similar freeze-out of N2 in L1512, compared to the two others despite a peak density one to two orders of magnitude lower. Retrieving CO and N2 abundance profiles with the chemical model, we find that CO has a depletion factor of $\sim$430-870 and the N2 profile is similar to that of CO unlike towards L183. Therefore, L1512 has probably been living long enough so that N2 chemistry has reached steady state. N2H+ modeling remains compulsory to assess the precise physical conditions in the center of cold starless cores, rather than dust emission. L1512 is presumably older than 1.4 Myr. Therefore, the dominating core formation mechanism should be ambipolar diffusion for this source.

astro-ph.GA↗

arXiv2019

Temperature and density dependent cooling function for H$_2$ with updated H$_2$/H collisional rates

The 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. The present work shows some updated calculations of the H$_2$ cooling function based on novel collisional data which explicitely 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.

astro-ph.GA↗

arXiv2018

Collisional Excitation and Weak Maser Action of Interstellar Methanimine

The inelastic cross sections and rate coefficients for the rotational excitation of methanimine (CH$_2$NH) by cold H$_2$ have been determined quantum mechanically based on a new highly correlated five-dimensional potential energy surface. This surface was fitted to more than 60 000 ab initio points with a root mean square error of $\sim$ 1-2 cm$^{-1}$ in the region of the potential well whose depth is -374.0 cm$^{-1}$. The rotationally inelastic rate coefficients have been combined with spectroscopic data to generate the non-equilibrium spectrum of CH$_2$NH towards the interstellar molecular cloud Sgr~B2(N). The transition $1_{10}\to 1_{11}$ at 5.29 GHz is found to be inverted and the predicted weak maser emission spectrum is in excellent agreement with new observations performed with the 100-meter Green Bank Telescope.

astro-ph.GA↗

arXiv2018

An efficient statistical method to compute molecular collisional rate coefficients

Our knowledge about the "cold" Universe often relies on molecular spectra. A general property of such spectra is that the energy level populations are rarely at local thermodynamic equilibrium. Solving the radiative transfer thus requires the availability of collisional rate coefficients with the main colliding partners over the temperature range 10-1000 K. These rate coefficients are notoriously difficult to measure and expensive to compute. In particular, very few reliable collisional data exist for collisions involving reactive radicals or ions. Here we explore the use of a fast quantum statistical method to determine molecular collisional excitation rate coefficients. The method is benchmarked against accurate (but costly) close-coupling calculations. For collisions proceeding through the formation of a strongly-bound complex, the method is found to be highly satisfactory up to room temperature. Its accuracy decreases with the potential well depth and with increasing temperature, as expected. This new method opens the way to the determination of accurate inelastic collisional data involving key reactive species such as H3+, H2O+, and H3O+ for which exact quantum calculations are currently not feasible.

astro-ph.IM↗

arXiv2014

Hyperfine excitation of N$_2$H$^+$ by H$_2$: Toward a revision of N$_2$H$^+$ abundance in cold molecular clouds

The modelling of emission spectra of molecules seen in interstellar clouds requires the knowledge of collisional rate coefficients. Among the commonly observed species, N$_2$H$^+$ is of particular interest since it was shown to be a good probe of the physical conditions of cold molecular clouds. Thus, we have calculated hyperfine-structure resolved excitation rate coefficients of N$_2$H$^+$(X$^1Σ^+$) by H$_2(j=0)$, the most abundant collisional partner in the cold interstellar medium. The calculations are based on a new potential energy surface, obtained from highly correlated {\it ab initio} calculations. State-to-state rate coefficients between the first hyperfine levels were calculated, for temperatures ranging from 5 K to 70 K. By comparison with previously published N$_2$H$^+$-He rate coefficients, we found significant differences which cannot be reproduced by a simple scaling relationship. As a first application, we also performed radiative transfer calculations, for physical conditions typical of cold molecular clouds. We found that the simulated line intensities significantly increase when using the new H$_2$ rate coefficients, by comparison with the predictions based on the He rate coefficients. In particular, we revisited the modelling of the N$_2$H$^+$ emission in the LDN 183 core, using the new collisional data, and found that all three of the density, gas kinetic temperature and N$_2$H$^+$ abundance had to be revised.

astro-ph.GA↗

arXiv2014

Ortho-para-H$_2$ conversion processes in astrophysical media

We report in this review recent fully-quantum time-independent calculations of cross sections and rate constants for the gas phase ortho-to-para conversion of H$_2$ by H and H$^+$. Such processes are of crucial interest and importance in various astrophysical environments. The investigated temperature ranges was 10$-$1500 K for H+H$_2$ and 10$-$100 K for H$^+$+H$_2$. Calculations were based on highly accurate H$_3$ and H$_3^+$ global potential energy surfaces. Comparisons with previous calculations and with available measurements are presented and discussed. It is shown that the existence of a long-lived intermediate complex H$_3^+$ in the (barrierless) H$^+$+H$_2$ reaction give rise to a pronounced resonance structure and a statistical behaviour, in contrast to H+H$_2$ which proceeds through a barrier of $\sim 5000$ K. In the cold interstellar medium ($T\leq 100$ K), the ortho-to-para conversion is thus driven by proton exchange while above $\sim$300 K, the contribution of hydrogen atoms become significant or even dominant. Astrophysical applications are briefly discussed by comparing, in particular, the relative role of the conversion processes in the gas phase ({\it via} H, H$^+$, H$_2$ and H$_3^+$) and on the surface of dust particles. Perspectives concerning future calculations at higher temperatures are outlined.

physics.chem-ph↗

arXiv2010

Low-energy inelastic collisions of OH radicals with He atoms and D$_2$ molecules

We present an experimental study on the rotational inelastic scattering of OH ($X^2Π_{3/2}, J=3/2, f$) radicals with He and D$_2$ at collision energies between 100 and 500 cm$^{-1}$ in a crossed beam experiment. The OH radicals are state selected and velocity tuned using a Stark decelerator. Relative parity-resolved state-to-state inelastic scattering cross sections are accurately determined. These experiments complement recent low-energy collision studies between trapped OH radicals and beams of He and D$_2$ that are sensitive to the total (elastic and inelastic) cross sections (Sawyer \emph{et al.}, \emph{Phys. Rev. Lett.} \textbf{2008}, \emph{101}, 203203), but for which the measured cross sections could not be reproduced by theoretical calculations (Pavlovic \emph{et al.}, \emph{J. Phys. Chem. A} \textbf{2009}, \emph{113}, 14670). For the OH-He system, our experiments validate the inelastic cross sections determined from rigorous quantum calculations.

physics.atom-ph↗