State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions.

We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and nonreactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2 = 1, 2), while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ toward the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2 = 2 might be significant in the photon-dominated region of NGC 7027.

Comparative experimental and theoretical study of the rotational excitation of CO by collision with ortho- and para-D2 molecules.

A joint crossed beam and quantum mechanical investigation of the rotationally inelastic collisions of CO with ortho- and para-D2 molecules is reported. A new 4D potential energy surface (PES) averaged over the ground vibrational states of D2 and CO is used to calculate the rovibrational bound states of the ortho-D2-CO complexes. Close coupling calculations are then performed in the rigid rotor approximation for ortho- and para-D2 colliding with CO for the experimentally investigated transition of CO (j = 0 → 1) and for collision energies ranging from 0.1 to 25 cm-1. The agreement between theory and experiment is found to be very good for both the bound state energies of the ortho-D2-CO complexes and for the inelastic scattering cross-sections showing that the free rotation of two rigid rotors is a very good model of the D2-CO system in this low collision energy domain.

Rotational Excitation of the OH(+) Radical by Collision with H at Low Temperature.

A ro-vibrationally inelastic close coupling study of the rotational excitation of OH(+)(X(3)Σ(-)) by collisions with H((2)S) is presented. The two lowest potential energy surfaces of doublet and quadruplet spin multiplicity are involved. The former is the one we developed recently, and the latter is a modified version of the quadruplet surface of Martinez et al. to include the long-range charge-induced-dipole potential. The details of the modification of this surface are presented as well as the comparison of the rotational excitation resulting from collisions with hydrogen on these two surfaces. The effect of the coupling between vibration and rotation on the rotational excitation rate is also discussed, as the potential well depth of the doublet surface is quite large and allows the coupling between many vibrational channels of OH(+). As the hydrogen exchange reaction can occur for both potential energy surfaces, we discuss the reliability of the approximation made by the calculation of the cross sections with a quantum dynamics limited to the inelastic process. The relative importance of the collisions on the doublet or quadruplet surface within a given rotational transition is also discussed.

Theoretical spectroscopic characterization of the ArBeO complex.

Using the recently developed explicitly correlated coupled cluster method in connection with the aug-cc-pVTZ basis set, we generated the three-dimensional potential energy surface (3D-PES) of the ground state of the Ar-BeO complex. This PES covers the regions of the global and local minima, the saddle point, and the dissociation of the complex. The PES is also used for the calculation of the rovibrational spectrum up to the dissociation limit. The high density of levels which is observed favors the mixing of the states and hence the occurrence of anharmonic resonances. The wavefunctions of the high rovibrational levels exhibit large amplitude motions in addition to strong anharmonic resonances. Our theoretical spectrum should be helpful in identifying the van der Waals modes of this complex in laboratory.

On the use of explicitly correlated treatment methods for the generation of accurate polyatomic -He/H2 interaction potential energy surfaces: The case of C3-He complex and generalization.

Through the study of the C3(X1Σg (+)) (1)Σg (+)) + He((1)S) astrophysical relevant system using standard (CCSD(T)) and explicitly correlated (CCSD(T)-F12) coupled cluster approaches, we show that the CCSD(T)-F12/aug-cc-pVTZ level represents a good compromise between accuracy and low computational cost for the generation of multi-dimensional potential energy surfaces (PESs) over both intra- and inter-monomer degrees of freedom. Indeed, the CCSD(T)-F12/aug-cc-pVTZ 2D-PES for linear C3 and the CCSD(T)-F12/aug-cc-pVTZ 4D-PES for bent C3 configurations gently approach those mapped at the CCSD(T)/aug-cc-pVXZ (X = T,Q) + bond functions level, whereas a strong reduction of computational effort is observed. After exact dynamical computations, the pattern of the rovibrational levels of the intermediate C3-He complex and the rotational and rovibrational (de-) excitation of C3 by He derived using both sets of PESs agree quite well. Since C3 shows a floppy character, the interaction PES is defined in four dimensions to obtain realistic collisional parameters. The C-C-C bending mode, which fundamental lies at 63 cm(-1) and can be excited at very low temperatures is explicitly considered as independent coordinate. Our work suggests hence that CCSD(T)-F12/aug-cc-pVTZ methodology is the key method for the generation of accurate polyatomic - He/H2 multi-dimensional PESs.

Accurate global potential energy surface for the H + OH+ collision.

We mapped the global three-dimensional potential energy surface (3D-PES) of the water cation at the MRCI/aug-cc-pV5Z including the basis set superposition (BSSE) correction. This PES covers the molecular region and the long ranges close to the H + OH(+)(X(3)Σ(-)), the O + H2(+)(X(2)Σg(+)), and the hydrogen exchange channels. The quality of the PES is checked after comparison to previous experimental and theoretical results of the spectroscopic constants of H2O(+)(X(2)B1) and of the diatomic fragments, the vibronic spectrum, the dissociation energy, and the barrier to linearity for H2O(+)(X(2)B1). Our data nicely approach those measured and computed previously. The long range parts reproduce quite well the diatomic potentials. In whole, a good agreement is found, which validates our 3D-PES.

A new theoretical method for calculating the radiative association cross section of a triatomic molecule: application to N2-H-.

We present a new theoretical method to treat the atom-diatom radiative association within a time independent approach. This method is an adaptation of the driven equations method developed for photodissociation. The bound state energies and wave functions of the molecule are calculated exactly and used to propagate the overlap with the initial scattering wave function. In the second part of this paper, this approach is applied to the radiative association of the N2H(-) anion. The main features of the radiative association cross sections are analysed and the magnitude of the calculated rate coefficient at 10 K is used to discuss the existence of the N2H(-) in the interstellar medium which could be used as a tracer of both N2 and H(-).

Collisional relaxation of MnH (X7Σ+) in a magnetic field: effect of the nuclear spin of Mn.

In the present study we investigate the role played by the hyperfine structure of manganese in the cooling and magnetic trapping of MnH((7)Σ(+)). The effect of the hyperfine structure of Mn on the relaxation of the magnetically trappable maximally stretched low-field seeking state of MnH((7)Σ(+)) in collisions with (3)He is deduced from comparison between the results of the present approach and our previous nuclear spin free calculations. We show that our previous results are unchanged at the temperature of the buffer gas cooling experiment but find a new resonance at very low collision energy. The role played by the different contributions to the hyperfine diatomic Hamiltonian considered in this work as well as the effect of an applied magnetic field on this resonance are also analyzed.

Vibrational and rotational cooling of NO+ in collisions with He.

A quantum mechanical investigation of the vibrational and rotational deactivation of NO(+) in collisions with He atoms in the cold and ultracold regime is presented. Ab initio potential energy calculations are carried out at BCCD(T) level and a new global 3D potential energy surface (PES) is obtained by fitting ab initio points within the reproducing kernel Hilbert space method. As a first test of this PES the bound state energies of the (3)He-NO(+) and (4)He-NO(+) complexes are calculated and compared to previous rigid rotor calculations. The efficiency of the vibrational and the rotational cooling of this molecular ion using a buffer gas of helium is then investigated by performing close coupling scattering calculations for collision energy ranging from 10(-6) to 2000 cm(-1). The calculations are performed for the two isotopes (3)He and (4)He and the results are compared to the available experimental data.

Analytical calculation of the Smith lifetime Q matrix using a Magnus propagator: applications to the study of resonances occurring in ultracold inelastic collisions with and without an applied magnetic field.

We take advantage of the simple expression of the sector adiabatic wave functions of the Magnus propagator to obtain accurate values of the energy derivative of the S matrix which, in turn, is used to get the Smith lifetime Q matrix. The procedure involves the simultaneous generation of both the R matrix and its energy derivative dR/dE which are propagated along the scattering coordinate. We present a few examples of application to the field free He-N(2)(+) inelastic collisions which we previously studied. This method is then applied to the calculation of the lifetime of tuned zero energy Feshbach resonances using a magnetic field. We give and discuss the law of variation as a function of the magnetic field of the Q matrix eigenvalues across such resonance. Some examples of application are given for the He-N(2)(+) collisions in a magnetic field.

Rotational relaxation of HF by collision with ortho- and para-H2 molecules.

The first quantum mechanical investigation of the rotational deactivation of HF induced by collisions with ortho- and para-H(2) molecules is reported. Ab initio potential energy calculations are carried out at the coupled cluster level with single and double excitations, using a quadruple-zeta basis set. The global rigid rotor four-dimensional potential energy surface is obtained by fitting ab initio points with a least squares procedure for the angular terms and interpolating the radial coefficients with cubic splines. It is shown that the equilibrium structure of the H(2)-HF complex is T-shaped and the well depth is found to be 359 cm(-1). Close coupling scattering calculations are performed at collision energy ranging from 10(-2) to 1600 cm(-1). A comparison of the rotational quenching of HF with para-H(2) and (4)He is used to validate our potential energy surface. The rotational quenching cross sections of HF by ortho- and para-H(2) are also compared and found to be very different. An explanation of these differences based on a resonance mechanism is proposed.

Three dimensional atom-diatom quantum reactive scattering calculations using absorbing potential: speed up of the propagation scheme.

In this paper a new propagation scheme is proposed for atom-diatom reactive calculations using a negative imaginary potential (NIP) within a time independent approach. It is based on the calculation of a rotationally adiabatic basis set, the neglected coupling terms being re-added in the following step of the propagation. The results of this approach, which we call two steps rotationally adiabatic coupled states calculations (2-RACS), are compared to those obtained using the adiabatic DVR method (J. C. Light and Z. Bazic, J. Chem. Phys., 1987, 87, 4008; C. Leforestier, J. Chem. Phys., 1991, 94, 6388), to the NIP coupled states results of the team of Baer (D. M. Charutz, I. Last and M. Baer, J. Chem. Phys., 1997, 106, 7654) and to the exact results obtained by Zhang (J. Z. H. Zhang and W. H. Miller, J. Chem. Phys., 1989, 91, 1528) for the D + H(2) reaction. The example of implementation of our method of computation of the adiabatic basis will be given here in the coupled states approximation, as this method has proved to be very efficient in many cases and is quite fast.

Exact, Born-Oppenheimer, and quantum-chemistry-like calculations in helium clusters doped with light molecules: The He2N2(X) system.

Helium clusters doped with diatomic molecules, He(N)-BC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as "electrons" and dopants as "nuclei" in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and Born-Oppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the (4)He(2)-Br(2)(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N(2)(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced.

Low temperature quantum rate coefficient of the H + CH+ reaction.

In this paper we report the first theoretical study of the title reaction. A global, single-valued model of the ground-state potential energy surface has been obtained by fitting to an extensive set of high-level ab initio calculations. The surface is found to be attractive apart from linear geometries where energy barriers appear due to conical intersections. This model was then used to calculate the reactive reactant state selected cross sections for collision energies ranging from threshold up to 4000 cm(-1). These calculations were performed using our version of the Baer's approach of the RIOSA-NIP method which is based on the use of a negative imaginary potential. We find that the reaction probability is extremely oscillatory as a function of kinetic energy as it is a case for insertion reactions with a low exoergicity. The resulting reaction rate coefficient is found to first increase slowly as a function of temperature up to a broad maximum around 20 K and then to decrease slowly when temperature keeps increasing.