Rotational Excitation Cross Sections for Chloronium Based on a New 5D Interaction Potential with Molecular Hydrogen.

Chloronium (HCl) 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 HCl is well-known, the studies of its collisional excitation are rather limited and these are available for the interaction with helium atoms only.

The temperature variation of the CH + H reaction rate coefficients: a puzzle finally understood?

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.

First close-coupling study of the excitation of a large cyclic molecule: collision of c-CH 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 ( 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.

Imaging Resonance Effects in C + H Collisions Using a Zeeman Decelerator.

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() atoms and -H molecules at collision energies ranging from 77 cm down to 0.

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.

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.

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.

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 Σ, = 0, = 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 = 0, = 1 → = 2, = 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.

Collisional excitation of HNC by He found to be stronger than for structural isomer HCN in experiments at the low temperatures of interstellar space.

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.

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 ( ≪ 10 cm) that local thermodynamical equilibrium conditions cannot be maintained. Hence, radiative and collisional properties of molecules are needed to correctly model molecular spectra.

High-Resolution Imaging of C + He Collisions using Zeeman Deceleration and Vacuum-Ultraviolet 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.

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

Collisional data for the excitation of NH by H 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 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.

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 (H + H, HD + H, SH + H, and CH + H) of interest in astrochemistry and astrophysics.

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.

Inelastic scattering in isotopologues of O-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.

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

We present the first global five-dimensional potential energy surface for the HO-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.

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.

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.

Rotationally inelastic scattering of O-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 N. The "ozone isotopic anomaly" refers to the observation of larger-than-expected atmospheric concentrations for certain ozone isotopologues.

Collisional Excitation of CF by H: 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.

Enhanced reactivity of fluorine with para-hydrogen in cold interstellar clouds by resonance-induced quantum tunnelling.

Chemical reactions are important in the evolution of low-temperature interstellar clouds, where the quantum tunnelling effect becomes significant. The F + para-H → HF + H reaction, which has a significant barrier of 1.8 kcal mol, is an important source of HF in interstellar clouds; however, the dynamics of this quantum-tunnelling-induced reactivity at low temperature is unknown.

Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D at Low Energy.

Fine-structure populations and collision-induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state P spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H, the most abundant molecule in the interstellar medium.

Probing Nonadiabatic Effects in Low-Energy C( P ) + H Collisions.

Nonadiabatic effects are of fundamental interest in collision dynamics. In particular, inelastic collisions between open-shell atoms and molecules, such as the collisional excitation of C( P ) by H, are governed by nonadiabatic and spin-orbit couplings that are the sole responsible of collisional energy transfer. Here, we study collisions between carbon in its ground state C( P ) and molecular hydrogen (H) at low collision energies that result in spin-orbit excitation to C( P ) and C( P ).