Improved potential energy surface for He–CO2

Rotational excitation of CO2 induced by He: New potential energy surface and scattering calculations

The CO2 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, H2, or H2O. The CO2–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 CO2–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 CO2 abundance in astrophysical media.

Theoretical calculation of a full-dimensional ab initio potential energy surface and prediction of infrared spectra for Xe-CS2

An effective four-dimensional (4D) ab initio potential energy surface (PES) for Xe-CS2 which explicitly involves the intramolecular Q 1 symmetric stretching and Q 3 antisymmetric stretching vibrational coordinates of CS2 is constructed. The computations are carried out employing single- and double-excitation coupled-cluster theory with a non-iterative perturbation treatment of triple excitations [CCSD(T)] method with a large basis set. Two vibrationally averaged potentials at the ground and ν 1 + ν 3 (ν 1 = 1, ν 3 = 1) excited states are obtained by integrating the 4D potentials over the Q 1 and Q 3 coordinates. The potentials have a T-shaped global minimum and two equivalent linear local minima. The radial discrete variable representation/angular finite basis representation and the Lanczos algorithm are employed to calculate the rovibrational energy levels for Xe-CS2. The infrared band origin shift associated with the fundamental band of CS2 is predicted, which is red-shifted by -1.996 cm-1 in the ν 1 + ν 3 region. In addition, we further predict the spectroscopic parameters for the ground and the ν 1 + ν 3 excited states of Xe-CS2. Compared with the previous Rg-CS2 (Rg = He, Ne, Ar, Kr) complexes, we found that the complexes of the rare gas atoms with CS2 display obvious regularities.

A new ab initio potential energy surface and infrared spectra for the Ar-CS₂ complex

We report a new three-dimensional potential energy surface for Ar-CS2 involving the Q3 normal mode for the υ3 antisymmetric stretching vibration of the CS2 molecule. The potential energies were calculated using the supermolecular method at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples, using augmented correlation-consistent quadruple-zeta basis set plus midpoint bond functions. Two vibrationally averaged potentials with CS2 at both the ground (υ = 0) and the first excited (υ = 1)υ3 vibrational states were generated from the integration of the three-dimensional potential over the Q3 coordinate. Each potential was found to have a T-shaped global minimum and two equivalent linear local minima. The radial discrete variable representation /angular finite basis representation method and the Lanczos algorithm were applied to calculate the rovibrational energy levels. The calculated band origin shift of the complex (0.0622 cm(-1)) is very close to the observed one (0.0671 cm(-1)). The predicted infrared spectra and spectroscopic parameters based on the two averaged potentials are in excellent agreement with the available experimental data.

Carbondioxide rare-gas systems: sensitivity of basis sets and double-hybrid density functionals

This study emphasizes on the performance of six newly developed double-hybrid density functionals (DHDF) in explaining the potential energy curves of different carbondioxide rare-gas systems. The basis set sensitivity has also been explored with the use of three basis sets. Our results suggest that for lighter He/Ne-CO(2) complexes, proper choice of DHDF and basis set lead to results those matches exactly with earlier calculations and also with the experiment. On the other hand, for heavier Ar/Kr-CO(2) complexes although the equilibrium separation distance matches exactly with earlier observations, the interaction energy values lie far apart. The overall investigation emphasizes on the fact that one has to tune the methods and basis sets properly to achieve good and satisfactory results.

Rotational quenching of CO2 by collision with He atoms

Quantum mechanical scattering calculations are presented for the rotational relaxation of CO(2) in collisions with He atoms with the close-coupling approach and the coupled-states approximation for collision energies between 10(-6) and 10 000 cm(-1). The He-CO(2) interaction potential of Ran and Xie [J. Chem. Phys. 128, 124323 (2008)] was adopted and used to compute state-to-state cross sections for the quenching of the j=2, 4, 6, and 8 rotational levels of CO(2). Numerous resonances, as a consequence of the van der Waals potential, are observed and the cross sections are found to approach the Wigner limit at low energies. Quenching rate coefficients are obtained for temperatures between 10(-5) and 3000 K and applications to astrophysics and cold collisions are briefly discussed.

Infrared spectra of CO2-doped 4He clusters, 4HeN-CO2, with N=1-60

High resolution spectra of (4)He(N)-CO(2) clusters are studied in the region of the CO(2) nu(3) fundamental band (approximately 2300 cm(-1)). The clusters are produced in a pulsed supersonic jet expansion from a cooled nozzle source and probed by direct absorption using a tunable diode laser operating in a rapid-scan mode. Four carbon dioxide isotopes ((16)O(12)C(16)O, (16)O(13)C(16)O, (18)O(13)C(18)O, and (16)O(13)C(18)O) are used to support the analysis, and because additional rotational transitions are allowed for the asymmetric one ((16)O(13)C(18)O). Resolved R(0) (J=1<--0) rotation-vibration transitions are observed for clusters up to N=60. A detailed rotational analysis is possible up to N approximately 20 and, with some assumptions, to N approximately 37 and beyond. The derived rotational constants (B values) vary smoothly with N and show evidence for broad oscillations similar to those already reported for He(N)-OCS and He(N)-N(2)O. Possible indications of a disruption are observed in the J=2 levels of larger clusters (N>22) which could be caused by interactions with a "dark" helium cluster modes.

Infrared spectra of isotopic CO2–He complexes

Infrared spectra of three isotopic forms of the weakly bound CO(2)-He van der Waals complex have been studied in the region of the CO(2) nu(3) fundamental band around 2300 cm(-1), using a tunable diode laser to probe a pulsed supersonic expansion. The complex is a T-shaped near-oblate asymmetric rotor, and it is found that (18)O isotopic substitution is sufficient to interchange the a and b inertial axes. For the symmetric isotopes, such as the normal species and the (16)O(13)C(16)O and (18)O(13)C(18)O forms studied here, half of the normal rotational levels of the complex are missing due to the effects of (16)O (or (18)O) interchange symmetry. However, for asymmetrically substituted ones, such as (16)O(13)C(18)O, all rotational levels are present. Moreover, for the asymmetric isotope, both a- and b-type transitions were observed, so that the spectrum was much richer. The CO(2)-He system is of interest both as a benchmark for intermolecular potential energy surface calculations, and because CO(2) is a valuable probe molecule for helium cluster spectroscopy.

Probing anisotropic interaction potentials of unsaturated hydrocarbons with He*(2 3S) metastable atom: attractive-site preference of sigma-direction in C2H2 and pi-direction in C2H4

State-resolved collision energy dependence of Penning ionization cross sections of acetylene (C2H2) and ethylene (C2H4) with He*(2 3S) metastable atoms was observed in a wide collision energy range from 20 to 350 meV. A recently developed discharge nozzle source with a liquid N2 circulator was employed for the measurements in the low-energy range from 20 to 80 meV. Based on classical trajectory calculations for the energy dependence of the partial ionization cross sections, anisotropic potential energy surfaces for the present systems were obtained by optimizing ab initio model potentials for the chemically related systems Li+C2H2 and C2H4. In the case of C2H2, the global minimum was found to be located around the H atom along the molecular axis with a well depth of 48 meV (ca. 1.1 kcal/mol). On the other hand, a dominant attractive well with a depth of 62 meV (ca. 1.4 kcal/mol) was found in the piCC electron region of C2H4. These findings were discussed in connection with orbital interactions between molecular orbitals of the target molecules and atomic orbitals of the metastable atom. It is concluded that sigma-type unoccupied molecular orbitals of C2H2 and a piCC-type highest occupied molecular orbital of C2H4 play a significant role for the attractive-site preference of sigma direction in C2H2 and pi direction in C2H4, respectively.

High resolution infrared spectra of a carbon dioxide molecule solvated with helium atoms

Infrared spectra of He(N)-CO(2) clusters with N up to about 20 have been studied in the region of the CO(2) nu(3) fundamental band ( approximately 2350 cm(-1)) using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled (>-150 degrees C) pinhole or slit nozzles and high backing pressures (<40 atm). Compared to previous studies of He(N)-OCS and -N(2)O clusters, the higher symmetry of CO(2) results in simpler spectra but less information content. Discrete rotation-vibration transitions have been assigned for N=3-17, and their analysis yields the variation of the vibrational band origin and B rotational constant over this size range. The band origin variation is similar to He(N)-OCS, with an initial blueshift up to N=5, followed by a monotonic redshift, consistent with a model where the first five He atoms fill a ring around the equator of the molecule, forcing subsequent He atom density to locate closer to the ends. The B value initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime, where the CO(2) molecule starts to rotate separately from the He atoms. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets.

EXPERIMENTS ON THE DYNAMICS OF MOLECULAR PROCESSES: A Chronicle of Fifty Years

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▪ Abstract  This paper reviews the way in which, in the Italy of the years immediately after World War II, interest in the dynamics of molecular processes was awakened. The narrative begins with the work of a small number of chemists and physicists who, in the initial stage, interacted closely. In the course of the years, their interests diverged and younger people joined the newly formed groups. Even now, after half a century, a common approach can still to be seen regarding how to attack problems and perform experiments. Experimental work is discussed, bringing out the common viewpoint of fields as diverse as mass spectrometry, isotope effects, chemical kinetics, molecular beams, molecule-molecule interactions, molecule-ion interactions, molecule-surface interactions, and plasma chemistry.