First close-coupling study of the excitation of a large cyclic molecule: collision of c-C5H6 with He.

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-C5H6), 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-C5H6 + 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.

Mixed quantum/classical calculations of rotationally inelastic scattering in the CO + CO system: a comparison with fully quantum results.

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.

Low-Energy Collisions of Zeeman-Decelerated NH Radicals with He Atoms.

We report an experimental study of state-to-state inelastic scattering of NH (X 3Σ-, 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 3Π â† X 3Σ- 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.

Collisional excitation of C2H- by H2: New interaction potential and scattering calculations.

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.

H2O-HCN complex: A new potential energy surface and intermolecular rovibrational states from rigorous 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.

Collisional energy transfer in the CO-CO system.

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 ≪ 1010 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.

HD-H+ collisions: statistical and quantum state-to-state studies.

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 scattering in isotopologues of O2-Ar: the effects of mass, symmetry, and density of states.

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.

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.

Collisional excitation of interstellar PN by H2: New interaction potential and scattering calculations.

Rotational excitation of interstellar PN molecules induced by collisions with H2 is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H2 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 H2 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-H2 and para-H2, respectively. Integral cross sections for rotational excitation in PN-H2 collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H2 molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.

Collisional excitation of NH by H2: Potential energy surface and scattering calculations.

Collisional data for the excitation of NH by H2 are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH-H2 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 De = 149.10 cm-1. The calculated dissociation energy D0 is found to be 30.55 and 22.11 cm-1 for ortho-H2 and para-H2 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-H2 and para-H2 for collisional energies up to 500 cm-1. We find strong differences between collisions induced by ortho-H2 and para-H2. 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.

Rotationally inelastic scattering of O3-Ar: state-to-state rates with the multiconfigurational time dependent Hartree method.

The Chapman cycle, proposed in 1930, describes the various steps in the ongoing formation and destruction of stratospheric ozone. A key step in the formation process is the stabilization of metastable ozone molecules through collisions with a third body, usually an inert collider such as N2. The "ozone isotopic anomaly" refers to the observation of larger-than-expected atmospheric concentrations for certain ozone isotopologues. Previous studies point to the formation steps as the origin of this effect. A possibly key aspect of the ozone formation dynamics is that of the relative efficiencies of the collisional cooling of different isotopologues. Although the substitution of low-abundance 18O for 16O in O3 molecules corresponds to a relatively small net change in mass, related to this are some subtleties due to symmetry-breaking and a resulting more than doubling of the density of allowed states governed by nuclear-spin statistics for bosons. Recently, a highly accurate 3D potential energy surface (PES) describing O3-Ar interactions has been constructed and used to benchmark the low-lying rovibrational states of the complex. Here, using this new PES, we have studied the collisional energy-transfer dynamics using the MultiConfiguration Time Dependent Hartree method. A study of the rotationally inelastic scattering was performed for the parent 16O16O16O-Ar system and compared with that of the 16O16O18O-Ar isotopologue. The state-to-state cross-sections and rates from the 00,0 initial state to low lying excited states are reported. Analysis of these results yields insight into the interplay between small changes in the rotational constants of O3 and the reduced mass of the O3-Ar collision system, combined with that of the symmetry-breaking and introduction of a new denser manifold of allowed states.

Possible Formation and Destruction of the OD+ Ions in the Interstellar Medium.

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.

Collisional energy transfer in the HeH+-H reactive system.

The HeH+ molecule is the first to be formed in the Universe. Its recent detection, in the interstellar medium, has increased the interest in the study of the physical and chemical properties of this ion. Here, we report exact quantum time-independent calculations of the collisional cross sections and rate coefficients for the rotational excitation of HeH+ by H. Reactive and exchange channels are taken into account in the scattering calculations. Cross sections are computed for energies of up to 10 000 cm-1, enabling the computation of rate coefficients for temperatures of up to 500 K. The strongest collision-induced rotational HeH+ transitions are those with Δj = 1. Previous results obtained using approximate treatment are compared to the new ones, and significant differences are found. The new rate coefficients are also compared to those for electron-impact rotational excitation, and we found that collisions with H dominate the excitation of HeH+ in media where the electron fraction is less than 10-4. In the light of those results, we recommend the use of the new HeH+-H collisional data in order to accurately model HeH+ excitation in both the interstellar media and early Universe.

Potential energy surface and bound states of the H2O-HF complex.

We present the first global five-dimensional potential energy surface for the H2O-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 De = 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-H2O-HF (D0 = 2089.4 cm-1 or 5.97 kcal/mol) and para-H2O-HF (D0 = 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.

Collisional Excitation of CF+ by H2: Potential Energy Surface and Rotational Cross Sections.

The CF+ molecule is considered one of the key species for the study of fluorine chemistry in the interstellar medium (ISM). Its recent detection, as well as its potential use as a tracer for atomic fluorine in the ISM, has increased the interest in the study of the physical and chemical properties of this cation. Accurate determination of the CF+ abundance in the ISM requires detailed modeling of its excitation from both radiation and collisions with the most dominant species, which are usually atomic and molecular hydrogen. Here, we report a new highly accurate potential energy surface (PES) describing the interaction between CF+ and the H2 molecule. Exact quantum time-independent calculations of the rotational excitation cross sections for collisions of CF+ with both para- and ortho-H2 are reported. Results obtained for collisions with para-H2 are compared to previous calculations performed using an approximate PES averaged over H2 rotation. Excitation data for collisions with ortho-H2 are provided for the first time.

Collisional excitation of interstellar PO(X2Π) by He: new ab initio potential energy surfaces and scattering calculations.

We present the first ab initio potential energy surfaces (PESs) for the PO(X2Π)-He van der Waals system. The PESs were obtained using the open-shell partially spin-restricted coupled cluster approach with single, double and perturbative triple excitations [UCCSD(T)]. The augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set was employed supplemented by mid-bond functions. Integral and differential cross sections for the rotational excitation in PO-He collisions were calculated using the new PES and compared with results in similar systems. Finally, our work presents the first hyperfine-resolved cross sections for this system that are needed for accurate modelling in astrophysical environments.

Rotational Excitation of HD by Hydrogen Revisited.

The HD molecules are key species for the cooling of pristine gas at temperatures below 100 K. They are also known to be key tracers of H2 in protoplanetary disks and thus, they can be used as a measure of protoplanetary disks mass. Accurate modeling of the cooling mechanism and of HD abundance in astrophysical media requires a proper modeling for its excitation by both radiative and collisional processes. Here, we report quantum time-independent calculations of collisional rate coefficients for the rotational excitation of HD by H for temperatures ranging from 10 to 1000 K. The reactive and hydrogen exchange channels are taken into account in the scattering calculations. New exact quantum results are compared to previous calculations performed neglecting reactive and exchange channels. We found that for temperatures higher than ∼300 K, the impact of these channels on the rate coefficients cannot be neglected. Such results suggest that the new HD-H collisional data have to be used for properly modeling HD cooling function and HD abundance in all the astrophysical environments where HD plays a role, e.g. in photon-dominated regions, protoplanetary disks, early Universe chemistry, and primordial star forming regions.

Communication: State-to-state inelastic scattering of interstellar O2 with H2.

Molecular oxygen (O2) is predicted to be a major reservoir of elemental oxygen in dense interstellar molecular clouds. However, the abundance of O2 derived from astronomical observations is much lower than expected. Solving the discrepancies between models and observations requires a review of the chemistry and collisional excitation of O2 in space. In particular, O2-H2 collisions are crucial to derive O2 abundance in space from the interstellar spectra. A crossed molecular beam experiment to probe the rotational excitation of O2 due to H2 collisions at energies of 650 cm-1 is reported. Velocity map imaging was combined with state-selective detection of O2( X 3 Σ g - ) by (2 + 1) resonance-enhanced multiphoton ionization. The obtained raw O 2 + images were corrected from density to flux and the differential cross sections (DCSs) were then extracted. Exact quantum mechanical calculations were also performed. Very good agreement between experimental and theoretical DCSs was found. The agreement demonstrates our ability to determine inelastic processes between O2 molecules and H2 both theoretically and experimentally and that the excitation of O2 in the interstellar medium can be correctly modeled. Consequences on the astrophysical modeling are briefly evaluated.

Rotational excitation of the interstellar NH2 radical by H2.

We present quantum close-coupling calculations for the rotational excitation of the interstellar amidogen radical NH2 due to collisions with H2 molecules. The calculations are based on a recent, high-accuracy full-dimensional NH4 potential energy surface adapted for rigid-rotor scattering calculations. The collisional cross section calculations are performed for all transitions among the first 15 energy levels of both ortho- and para-NH2 and for total energies up to 1500 cm-1. Both para- and ortho-H2 colliding partners are considered. The cross sections for collision with para- and ortho-H2 are found to differ significantly, the magnitude of the ortho-H2 ones being dominant. No strong propensity rules are observed but transitions with Δkc=0 are slightly favored.