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The Atacama Large Millimeter/submillimeter Array is allowing us to study the innermost regions of the circumstellar envelopes of evolved stars with unprecedented precision and sensitivity. Key processes in the ejection of matter and dust from these objects occur in their inner zones. In this work, we present sub-arcsecond interferometric maps of transitions of metal-bearing molecules toward the prototypical C-rich evolved star IRC +10216. While Al-bearing molecules seem to be present as a roughly spherical shell, the molecular emission from the salts NaCl and KCl presents an elongation in the inner regions with a central minimum. In order to accurately analyze the emission from the NaCl rotational lines, we present new calculations of the collisional rates for this molecule based on new spectroscopic constants. The most plausible interpretation for the spatial distribution of the salts is a spiral with a NaCl mass of 0.08 . Alternatively, a torus of gas and dust would result in structures similar to those observed. From the torus scenario we derive a mass of ∼1.1 × 10−4. In both cases, the spiral and the torus, the NaCl structure presents an inner minimum of 27 AU. In the case of the torus, the outer radius is 73 AU. The kinematics of both the spiral and the torus suggests that they are slowly expanding and rotating. Alternative explanations for the presence of the elongation are explored. The presence of these features only in KCl and NaCl might be a result of their comparatively high dipole moment with respect to the Al-bearing species. [ABSTRACT FROM AUTHOR]

Modelling of the astrophysical spectra requires accurate radiative and collisional rates for species of astrophysical interest. The present paper focuses on the calculation of ro-vibrational excitation rate coefficients of SiS by He, useful for studies of high-temperature interstellar environments. A new accurate three-dimensional potential energy surface for the SiS-He system, which explicitly takes into account the r-dependence of the SiS vibration, was determined from ab initio calculations. The dynamic calculations were performed using the vibrational close-coupling rotational infinite order sudden (VCC-IOS) method. Cross sections among the first 100 rotational levels of v = 0-4 are calculated for total energies up to 12 000 cm[?]1. Rate coefficients between the ro-vibrational levels are calculated for temperatures from 300 K to 1500 K. [ABSTRACT FROM AUTHOR]

Chloronium (H 2 Cl + ) is an important intermediate of Cl-chemistry in space. The accurate knowledge of its collisional properties allows a better interpretation of the corresponding observations in interstellar clouds and, therefore, a better estimation of its abundance in these environments. While the ro-vibrational spectroscopy of H 2 Cl + is well-known, the studies of its collisional excitation are rather limited and these are available for the interaction with helium atoms only. We provide a new 5-dimensional rigid rotor potential energy surface for the interaction of H 2 Cl + with H 2 , calculated from explicitly correlated coupled cluster ab initio theory, which was fitted then with a set of analytical functions, allowing to perform scattering calculations using accurate quantum theories. We analyze the collision-energy dependence of the rotational state-to-state cross sections and the temperature-dependence of the corresponding thermal rate coefficients, with particular attention on the collisional propensity rules. When comparing our results for collisions with H 2 with those obtained with He as a colliding partner, we found very significant differences with nonlinear scaling trends, which proves again that He is not a suitable proxy for collisions between hydride molecules and molecular hydrogen, the most abundant gas particle in the interstellar medium.

Investigating molecular excitation induced by collisions requires the prior determination of accurate analytical potential energy surfaces for the colliding partners. For carbon-chain molecules, such as cyanopolyynes, this has been a longstanding challenge, resulting in the absence of rate coefficients for HC5N, HC7N, HC9N, and others, induced by collisions with He. To overcome this bottleneck, we introduce a new approach: the Radial Angular Network with Gradual Expansion (RANGE). This method jointly connects the construction of ab initio interaction potentials with the determination of their analytical forms. We use the HC3N-He molecular complex as a reference to assess the reliability of our method, given that its analytical potential has been derived using various methods. Additionally, we apply the RANGE approach to construct the analytical representation of the interaction potential for HC5N-He and HC7N-He. The analysis of the analytical potentials reveals three systematic trends: (i) the anisotropy increases with the length of the carbon chain, (ii) the number of local minima correlates with the number of carbon atoms, and (iii) the shallowest local minimum is consistently located at ∼30 cm-1 below the dissociation limit of the complex. Using the time-independent quantum mechanical close-coupling formalism, we briefly estimate the propensity rules governing the excitation of HC3N, HC5N, and HC7N induced by collisions with He. Consequently, the three collisional systems exhibit the same propensity rule, favoring Δj = 2 transitions., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

CH + was the first molecular ion identified in the interstellar medium and is found to be ubiquitous in interstellar clouds. However, its formation and destruction paths are not well understood, especially at low temperatures. A new theoretical approach based on the canonical variational transition state theory was used to study the H + CH + reactive collisions. Rate coefficients for formation of C + ions are calculated as a function of temperature. We considered the participation of a direct path and an indirect path in which the reactants should overcome an entropic barrier to form a van der Waals complex or pass through a CH 2 + intermediate complex, respectively. We show that the contribution of both pathways to the formation of C + has to be taken into account. The new reactive rate coefficients for the title reaction, complemented by reactive data for CH + /CH 2 + in the H/H 2 /He mixture, have been used to simulate the corresponding kinetics experimentally measured using an Atomic Beam 22 Pole Trap apparatus at low temperature. A good agreement with the experimental findings was found at 50 K. At a lower temperature, the model overestimates the formation of C + . This shows that secondary reactions are not responsible for the weak C + production in the experiments at such temperature. Then, we discuss the possible impact of non-adiabatic effects in the study of the H + CH + reactive collisions and we found that such effects can be responsible for the decrease of the H + CH + rate coefficients at low temperature. This study offers an explanation for the disagreement between H + CH + theoretical and experimental rate coefficients which has been going on for 20 years and highlights the need for performing non-adiabatic studies for this simple chemical reaction.

Recent astronomical observations revealed an increasing molecular complexity in the interstellar medium through the detection of a series of large cyclic carbon species. To correctly interpret these detections, a complex analysis is necessary that takes into account the non-local thermodynamic equilibrium (non-LTE) conditions of the emitting media ( e.g. when energy level populations deviate from a Boltzman distribution). This requires proper state-to-state collisional data for the excitation and de-excitation processes of the molecular levels. Cyclopentadiene (c-C 5 H 6 ), which was recently detected in cold interstellar clouds, is extensively studied in many aspects due to its large importance for chemistry in general. At the same time, there are no collisional data available for this species, which are necessary for a more precise interpretation of the corresponding detections. In this work, we first provide an accurate 3D rigid-rotor interaction potential for the [c-C 5 H 6 + He] complex from high-level of ab initio theories, which has been used to study their inelastic collision by the exact close coupling quantum scattering method. To the best of our knowledge, this is the first study where this method is systematically applied to treat the dynamics of molecular collisions involving more than ten atoms. We also analyse the collisional propensity rules and the differences in contrast to calculations, where the approximate coupled states scattering methods is used.

An intriguing phenomenon in molecular collisions is the occurrence of scattering resonances, which originate from bound and quasi-bound states supported by the interaction potential at low collision energies. The resonance effects in the scattering behavior are extraordinarily sensitive to the interaction potential, and their observation provides one of the most stringent tests for theoretical models. We present high-resolution measurements of state-resolved angular scattering distributions for inelastic collisions between Zeeman-decelerated C( 3 P 1 ) atoms and para -H 2 molecules at collision energies ranging from 77 cm -1 down to 0.5 cm -1 . Rapid variations in the angular distributions were observed, which can be attributed to the consecutive reduction of contributing partial waves and effects of scattering resonances. The measurements showed excellent agreement with distributions predicted by ab initio quantum scattering calculations. However, discrepancies were found at specific collision energies, which most likely originate from an incorrectly predicted quasi-bound state. These observations provide exciting prospects for further high-precision and low-energy investigations of scattering processes that involve paramagnetic species.

An updated version of the CO + CO potential energy surface from [R. Dawes, X. G. Wang and T. Carrington, J. Phys. Chem. A 2013, 117 , 7612] is presented, that incorporates an improved treatment of the asymptotic behavior. It is found that this new surface is only slightly different from the other popular PES available for this system in the literature [G. W. M. Vissers, P. E. S. Wormer and A. Van Der Avoird, Phys. Chem. Chem. Phys. 2003, 5 , 4767]. The differences are quantified by expanding both surfaces over a set of analytic functions and comparing the behavior of expansion coefficients along the molecule-molecule distance R . It is shown that all expansion coefficients behave similarly, except in the very high energy range at small R where the PES is repulsive. That difference has no effect on low collision-energy dynamics, which is explored via inelastic scattering calculations carried out using the MQCT program which implements the mixed quantum/classical theory for molecular energy exchange processes. The validity of MQCT predictions of state-to-state transition cross sections for CO + CO is also tested by comparison against full-quantum coupled-states calculations. In all cases MQCT gives reliable results, except at very low collision energy where the full-quantum calculations predict strong oscillations of state-to-state transition cross sections due to resonances. For strong transitions with large cross sections, the results of MQCT are reliable, especially at higher collision energy. For weaker transitions, and lower collision energies, the cross sections predicted by MQCT may be up to a factor of 2-3 different from those obtained by full-quantum calculations.

In this work the H2O-HCN complex is quantitatively characterized in two ways. First, we report a new rigid-monomer 5D intermolecular potential energy surface (PES) for this complex, calculated using the symmetry-adapted perturbation theory based on density functional theory method. The PES is based on 2833 ab initio points computed employing the aug-cc-pVQZ basis set, utilizing the autoPES code, which provides a site-site analytical fit with the long-range region given by perturbation theory. Next, we present the results of the quantum 5D calculations of the fully coupled intermolecular rovibrational states of the H2O-HCN complex for the total angular momentum J values of 0, 1, and 2, performed on the new PES. These calculations rely on the quantum bound-state methodology developed by us recently and applied to a variety of noncovalently bound binary molecular complexes. The vibrationally averaged ground-state geometry of H2O-HCN determined from the quantum 5D calculations agrees very well with that from the microwave spectroscopic measurements. In addition, the computed ground-state rotational transition frequencies, as well as the B and C rotational constants calculated for the ground state of the complex, are in excellent agreement with the experimental values. The assignment of the calculated intermolecular vibrational states of the H2O-HCN complex is surprisingly challenging. It turns out that only the excitations of the intermolecular stretch mode can be assigned with confidence. The coupling among the angular degrees of freedom (DOFs) of the complex is unusually strong, and as a result most of the excited intermolecular states are unassigned. On the other hand, the coupling of the radial, intermolecular stretch mode and the angular DOFs is weak, allowing straightforward assignment of the excitation of the former., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Interstellar anions play an important role in astrochemistry as being tracers of the physical and chemical conditions in cold molecular clouds and circumstellar gas. The local thermodynamic equilibrium is generally not fulfilled in media where anions are detected and radiative and collisional data are required to model the observed lines. The C2H- anion has not yet been detected in the interstellar medium; however, collisional data could be used for non-LTE models that would help in identifying the most intense lines. For this purpose, we have computed the first 4D potential energy surface (PES) of the C2H--H2 complex using an explicitly correlated coupled-cluster approach. The PES is characterized by a single deep minimum with a well-depth of 924.96 cm-1. From this interaction potential, we derived excitation cross sections and rate coefficients of C2H- induced by collisions with para- and ortho-H2. The results obtained for collisions with para-H2 are compared to previous calculations performed using a 2D-PES obtained from an average over H2 rotations., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

We report an experimental study of state-to-state inelastic scattering of NH (X 3 Σ - , N = 0, j = 1) radicals with He atoms. Using a crossed molecular beam apparatus that combines a Zeeman decelerator and velocity map imaging, we study both integral and differential cross sections in the N = 0, j = 1 → N = 2, j = 3 inelastic channel. We developed various new REMPI schemes to state-selectively detect NH radicals, and tested their performance in terms of sensitivity and ion recoil velocity. We found a 1 + 2' + 1' REMPI scheme using the A 3 Π ← X 3 Σ - resonant transition, which yields acceptable recoil velocities and is more than an order of magnitude more sensitive than conventional one-color REMPI schemes to detect NH. We used this REMPI scheme to probe state-to-state integral and differential cross sections around the channel opening at 97.7 cm -1 , as well as at higher energies where structure in the scattering images could be resolved. The experimental results are in excellent agreement with the predictions from quantum scattering calculations which are based on an ab initio NH-He potential energy surface.

Protonated molecules have been increasingly detected in the interstellar medium (ISM), and usually astrochemical models fail at reproducing the abundances derived from observational spectra. Rigorous interpretation of the detected interstellar emission lines requires prior calculations of collisional rate coefficients with H 2 and He, i.e., the most abundant species in the ISM. In this work, we focus on the excitation of HCNH + induced by collision with H 2 and He. Therefore, we first calculate ab initio potential energy surfaces (PESs) using the explicitly correlated and standard coupled cluster method with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. Both the HCNH + -H 2 and HCNH + -He potentials are characterized by deep global minima of 1426.60 and 271.72 cm -1 , respectively, and large anisotropies. From these PESs, we derive state-to-state inelastic cross sections for the 16 low-lying rotational energy levels of HCNH + using the quantum mechanical close-coupling approach. The differences between cross sections due to ortho- and para-H 2 impacts turn out to be minor. Using a thermal average of these data, we retrieve downward rate coefficients for kinetic temperatures of up to 100 K. As it could be anticipated, differences of up to two orders of magnitude exist between the rate coefficients induced by H 2 and He collisions. We expect that our new collision data will help to improve the disagreement between abundances retrieved from observational spectra and astrochemical models.

The fine structure excitation of the interstellar CCS radical induced by collisions with He is investigated. The first potential energy surface (PES) for the CCS-He van der Waals complex is presented. It was obtained from a highly correlated spin unrestricted coupled cluster approach with single double and perturbative triple excitations. The PES presents two shallow minima of 31.85 and 37.12 cm -1 for the linear (He facing S) and the nearly T-shaped geometries, respectively. The dissociation energy of the complex was calculated and found to be D 0 = 14.183 cm -1 . Inelastic scattering calculations were performed using the close-coupling approach. Cross-sections for transitions between the 61 first fine structure levels of CCS were obtained for energy up to 600 cm -1 and rate coefficients for the 5-50 K temperature range were derived. This set of collisional data can be used to model CCS emission spectra in dark molecular interstellar clouds and circumstellar envelopes and enable an accurate determination of CCS abundance in these astrophysical media.

HCN and its unstable isomer HNC are widely observed throughout the interstellar medium, with the HNC/HCN abundance ratio correlating strongly with temperature. In very cold environments HNC can even appear more abundant than HCN. Here we use a chirped pulse Fourier transform spectrometer to measure the pressure broadening of HCN and HNC, simultaneously formed in situ by laser photolysis and cooled to low temperatures in uniform supersonic flows of helium. Despite the apparent similarity of these systems, we find the HNC-He cross section to be more than twice as big as the HCN-He cross section at 10 K, confirming earlier quantum calculations. Our experimental results are supported by high-level scattering calculations and are also expected to apply with para-H 2 , demonstrating that HCN and HNC have different collisional excitation properties that strongly influence the derived interstellar abundances., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Due to the lack of specific collisional data, the abundance of NS + in cold dense interstellar clouds was determined using collisional rate coefficients of CS as a substitute. To better understand the chemistry of sulfur in the interstellar medium, further abundance modeling using the actual NS + collisional rate coefficients is needed. For this purpose, we have computed the first full 4D potential energy surface of the NS + -H 2 van der Waals complex using the explicitly correlated coupled cluster approach with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. The potential energy surface exhibits a global minimum of 848.24 cm -1 for a planar configuration of the complex. The long-range interaction energy, described using multipolar moments, is sensitive to the orientation of H 2 up to radial distances of ∼50 a 0 . From this new interaction potential, we derived excitation cross sections, induced by collision with ortho- and para-H 2 , for the 15 low-lying rotational levels of NS + using the quantum mechanical close-coupling approach. By thermally averaging these data, we determined downward rate coefficients for temperatures up to 50 K. By comparing them with the previous NS + -H 2 data, we demonstrated that reduced dimensional approaches are not suited for this system. In addition, we found that the CS collisional data underestimate our results by up to an order of magnitude. The differences clearly indicate that the abundance of NS + , in cold dense clouds retrieved from observational spectra, must be reassessed using these new collisional rate coefficients.

An accurate determination of the physical conditions in astrophysical environments relies on the modeling of molecular spectra. In such environments, densities can be so low ( n ≪ 10 10 cm -3 ) that local thermodynamical equilibrium conditions cannot be maintained. Hence, radiative and collisional properties of molecules are needed to correctly model molecular spectra. For comets at large heliocentric distances, the production of carbon monoxide (CO) gas is found to be larger than the production of water, so that molecular excitation will be induced by collisions with CO molecules. This paper presents new scattering calculations for the collisional energy transfer in CO-CO collisions. Using the quantum coupled states approach, cross sections and rate coefficients are provided between the first 37 rotational states of the CO-CO system. Cross sections were calculated for energies up to 800 cm -1 , and excitation rate coefficients were derived for temperatures up to 100 K. In comparison with data available in the literature, significant differences were found, especially for the dominant transitions. Due to the high cost of the calculations, we also investigated the possibility of using an alternative statistical approach to extend our calculations both in terms of rotational states and temperatures considered. The use of these new collisional data should help in accurately deriving the physical conditions in CO-dominated comets.

The CO 2 molecule is of great interest for astrophysical studies since it can be found in a large variety of astrophysical media where it interacts with the dominant neutral species, such as He, H 2 , or H 2 O. The CO 2 -He collisional system was intensively studied over the last two decades. However, collisional data appear to be very sensitive to the potential energy surface (PES) quality. Thus, we provide, in this study, a new PES of the CO 2 -He van der Waals complex calculated with the coupled-cluster method and a complete basis set extrapolation in order to provide rotational rate coefficients that are as accurate as possible. The PES accuracy was tested through the calculations of bound state transition frequencies and pressure broadening coefficients that were compared to experimental data. An excellent agreement was globally found. Then, revised collisional data were provided for the 10-300 K temperature range. Rate coefficients were compared to previously computed ones and are found to be up to 50% greater than previously provided ones. These differences can induce non-negligible consequences for the modeling of CO 2 abundance in astrophysical media.

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( 3 P 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.

In 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.

Collisional data for the excitation of NH by H 2 are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH-H 2 van der Waals complex. The ab initio calculations of the PES were carried out using the explicitly correlated partially spin-restricted coupled cluster method with single, double, and perturbative triple excitations [RCCSD(T)-F12a] with the augmented correlation-consistent polarized valence triple zeta basis set. The PES was represented by an angular expansion in terms of coupled spherical harmonics. The global minimum corresponds to the linear structure with a well depth D e = 149.10 cm -1 . The calculated dissociation energy D 0 is found to be 30.55 and 22.11 cm -1 for ortho-H 2 and para-H 2 complexes, respectively. These results are in agreement with the experimental values. Then, we perform quantum close-coupling calculations of the fine structure resolved excitation cross sections of NH induced by collisions with ortho-H 2 and para-H 2 for collisional energies up to 500 cm -1 . We find strong differences between collisions induced by ortho-H 2 and para-H 2 . Propensity rules are discussed. The cross sections are larger for fine structure conserving transitions than for fine structure changing ones, as predicted by theory. These new results should help in interpreting NH interstellar spectra and better constrain the abundance of NH in interstellar molecular clouds.

In the early Universe, the cooling mechanisms of the gas significantly rely on the HD abundance and excitation conditions. A proper modeling of its formation and destruction paths as well as its excitation by both radiative and collisional processes is then required to accurately describe the cooling mechanisms of the pristine gas. In such media, ion-molecule reactions are dominant. Their theoretical study is challenging and state-of-the-art quantum time-independent methods are computationally limited to collisions involving light molecules. Here, we report a state-to-state scattering study of the HD-H + collisional system using two different methods: an exact quantum time-independent approach and a recently developed fast and efficient statistical method. Reactive and inelastic rate coefficients were obtained for temperatures up to 300 K. The statistical method is able to reproduce exact calculations with an accuracy reaching the astrophysical needs while drastically reducing the computational resources requirements. Such results suggest that this new statistical method should be considered to provide the astrophysical community collisional data for which quantum calculations are impossible.

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 (H 2 + 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.

The two species considered here, O2 (oxygen molecule) and Ar (argon-atom), are both abundant components of Earth's atmosphere and hence familiar collision partners in this medium. O2 is quite reactive and extensively involved in atmospheric chemistry, including Chapman's cycle of the formation and destruction of ozone; while Ar, like N2, typically plays the nevertheless crucial role of inert collider. Inert species can provide stabilization to metastable encounter-complexes through the energy transfer associated with inelastic collisions. The interplay of collision frequency and energy transfer efficiency, with state lifetimes and species concentrations, contributes to the rich and varied chemistry and dynamics found in diverse environments ranging from planetary atmospheres to the interstellar and circumstellar media. The nature and density of bound and resonance states, coupled electronic states, symmetry, and nuclear spin-statistics can all play a role. Here, we systematically investigate some of those factors by looking at the O2-Ar system, comparing rigorous quantum-scattering calculations for the 16O16O-40Ar, 18O16O-40Ar, and 18O18O-40Ar isotope combinations. A new accurate potential energy surface was constructed for this purpose holding the O2 bond distance at its vibrationally averaged distance.

Rotational excitation of interstellar PN molecules induced by collisions with H 2 is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H 2 van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b]. The method of interpolating moving least squares was used to construct an analytical PES from these data. The equilibrium structure of the complex was found to be linear, with H 2 aligned at the N end of the PN molecule, at an intermolecular separation of 4.2 Å. The corresponding well-depth is 224.3 cm -1 . The dissociation energies were found to be 40.19 cm -1 and 75.05 cm -1 for complexes of PN with ortho-H 2 and para-H 2 , respectively. Integral cross sections for rotational excitation in PN-H 2 collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H 2 molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.

We present the first global five-dimensional potential energy surface for the H 2 O-HF dimer, a prototypical hydrogen bonded complex. Large scale ab initio calculations were carried out using the explicitly correlated coupled cluster approach with single- and double-excitations together with non-iterative perturbative treatment of triple excitations with the augmented correlation-consistent triple zeta basis sets, in which the water and hydrogen fluoride monomers were frozen at their vibrationally averaged geometries. The ab initio data points were fitted to obtain a global potential energy surface for the complex. The equilibrium geometry of the complex corresponds to the formation of a hydrogen bond with water acting as a proton acceptor and a binding energy of D e = 3059 cm -1 (8.75 kcal/mol). The energies and wavefunctions of the lowest bound states of the complex were computed using a variational approach, and the dissociation energies of both ortho-H 2 O-HF (D 0 = 2089.4 cm -1 or 5.97 kcal/mol) and para-H 2 O-HF (D 0 = 2079.6 cm -1 or 5.95 kcal/mol) were obtained. The rotational constant of the complex was found to be in good agreement with the available experimental data.

Modeling of the observational spectra of H 3 O + allows for a detailed understanding of the interstellar oxygen chemistry. While its spectroscopy was intensively studied earlier, our knowledge about the collision of H 3 O + with the abundant colliders in the interstellar medium is rather limited. In order to treat these collisional excitation processes, it is first necessary to calculate the potential energy surface (PES) of the interacting species. We have computed the five-dimensional rigid-rotor PES of the H 3 O + -H 2 system from the explicitly correlated coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)-F12] with the moderate-size augmented correlation-consistent valence triple zeta (aug-cc-pVTZ) basis set. The well depth of the PES is found to be rather large, about 1887.2 cm -1 . The ab initio potential was fitted over an angular expansion in order to effectively use it in quantum scattering codes. As a first application, we computed dissociation energies for the different nuclear spin isomers of the H 3 O + -H 2 complex.

The OH + ion is an important constituent of the interstellar medium (ISM). It can be used as a probe of cosmic ray and X-ray ionization rates in molecular clouds as well as a tracer of oxygen chemistry. The deuterated variant of OH + , the OD + ion, may also be present in the ISM despite the fact that it has not been detected yet. In this paper, we aim at providing quantitative insight into the OD + chemistry and at accurately studying the possible formation and destruction processes of OD + in the ISM. We study the formation and destruction of OD + through the O + + HD → OD + + H and OH + + D ↔ OD + + H reactions that can occur in diffuse ISM. Reactive rate constants have been obtained from exact state-to-state quantum wave packet calculations for temperatures ranging from 10 to 1000 K. The new theoretical data are validated through a detailed comparison with available experimental data. The formation of OD + is found to be less efficient than that of OH + . As a first application, the OD + /OH + abundance ratio in ISM has been evaluated from a simple astrochemical model, and we found that this ratio can be larger than D/H abundance ratio only at low temperatures. These calculations may help in an astrochemical search of OD + in cold ISM.