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We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction between C$^+(^2P)$ and H$_2(\nu_{\rm H_2}=1, 2)$, 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 H$_2$ 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 $\nu_{\rm H_2}=2$ might be significant in the photon-dominated region of NGC~7027., Comment: 10 pages, 8 figures, accepted 2017 April 7

Full-dimensional semiclassical dynamical calculations are reported for the photofragmentation of isocyanic acid in the S1 state. These calculations, performed for the first time, allow to closely reproduce the key features of high-resolution imaging measurements at photolysis wavelengths of 201 and 210 nm while providing insight into the underlying dissociation mechanism.

For triatomic chemical reactions under single-collision conditions, we propose a new quasi-classical trajectory (QCT) approach to rotational-state distributions of particular interest in the quantum regime where only a few rotational states are available to the products. Our method is directly inspired from the amendments to be introduced in classical phase space theory (PST) in order to make it in exact agreement with quantum PST. The method is applied to the D$^+$ + H$_2$ and H$^+$ + D$_2$ reactions and the population of the rotational ground state is found to be in much closer agreement with the exact quantum one than the same population obtained by means of standard QCT calculations. The impact on the whole distribution is all the stronger as the number of available states is small. Last but not least, the shape of the distribution appears to be controlled to a large extent by three factors, respectively called parity, edge and rotational shift factors.

We present a new theoretical method to treat atom-rigid bender inelastic collisions at the Close Coupling level (RBCC) in the space fixed frame. The coupling between rotation and bending is treated exactly within the rigid bender approximation and we obtain the cross section for the rotational transition between levels belonging to different bending levels. The results of this approach are compared with those obtained when using the rigid bender averaged approximation (RBAA) introduced in our previous work dedicated to this system. We discuss the validity of this approximation and of the previous studies based on rigid linear HCN.

A four-dimensional potential energy surface for the interaction between He and HCN with vibrational bending is presented. Ab initio calculations were carried out at the coupled-cluster level with single and double excitations and a perturbative treatment of triple excitations, using a quadruple-zeta basis set and mid-bond functions. An accurate fit to the Ab-initio data has been obtained with a parametrized functional form. The global minimum is found in the linear He-HCN configuration with the H atom pointing towards Helium at the intermolecular separation of 7.94 a0. The corresponding well depth is 30.35 cm^-1 . The quality of the new potential has been tested by performing two comparisons with previous theoretical and experimental works. First, the rovibrational energy levels of the He-HCN complex for a linear configuration of the HCN molecule have been calculated. The dissociation energy is 8.99 cm^-1, which is slightly smaller than the semi-empirical value of 9.42 cm^-1 . The transitions frequencies are found to be in good agreement with the experimental data. Second, we performed close coupling calculations of the rotational de-excitation of HCN in collisions with He and observed a close similarity with the theoretical data published in a recent study.

We study theoretically and experimentally the electronic relaxation of NO2 molecules excited by absorption of one ~400 nm pump photon. Semi-classical simulations based on trajectory surface hopping calculations are performed. They predict fast oscillations of the electronic character around the intersection of the ground and first excited diabatic states. An experiment based on high-order harmonic transient grating spectroscopy reveals dynamics occuring on the same timescale. A systematic study of the detected transient is conducted to investigate the possible influence of the pump intensity, pump wavelength, and rotational temperature of the molecules. The quantitative agreement between measured and predicted dynamics shows that, in NO2, high harmonic transient grating spectroscopy encodes vibrational dynamics underlying the electronic relaxation., Comment: The following article has been accepted by Journal of Chemical Physics. After it is published, it will be found at http://jcp.aip.org/

The association reaction S + CO {\to} OCS + hnu has been identified as being particularly important for the prediction of gas-phase OCS abundances by chemical models of dark clouds. We performed detailed ab-initio calculations for this process in addition to undertaking an extensive review of the neutral-neutral reactions involving this species which might be important in such environments. The rate constant for this association reaction was estimated to be several orders of magnitude smaller than the one present in current astrochemical databases. The new rate for this reaction and the introduction of other processes, notably OH + CS {\to} OCS + H and C + OCS {\to} CO + CS, dramatically changes the OCS gas-phase abundance predicted by chemical models for dark clouds. The disagreement with observations in TMC-1 (CP) and L134N (N), suggests that OCS may be formed on grain surfaces as is the case for methanol. The observation of solid OCS on interstellar ices supports this hypothesis., Comment: Accepted for publication in MNRAS

We present a theoretical study of the Zeeman relaxation of the magnetically trappable lowest field seeking state of MnH(^7 \Sigma) in collisions with 3He. We analyze the collisional Zeeman transition mechanism as a function of the final diatomic state and its variation as a function of an applied magnetic field. We show that as a result of this mechanism the levels with \Delta Mj>2 give negligible contributions to the Zeemam relaxation cross section. We also compare our results to the experimental cross sections obtained from the buffer gas cooling and magnetic trapping of this molecule and investigate the dependence of the Zeeman relaxation cross section on the accuracy of the three body interaction at ultralow energies.