Mutual Neutralization of O

We have measured total absolute cross sections for the mutual neutralization (MN) of O^{-} with O^{+} and N^{+}. A fine resolution (of about 50 meV) in the kinetic energy spectra of the product neutral atoms allows unique identification of the atomic states participating in the mutual neutralization process. Cross sections and branching ratios have also been calculated down to 1 meV center-of-mass collision energy for these two systems, with a multichannel Landau-Zener model and an asymptotic method for the ionic-covalent coupling matrix elements. The importance of two-electron processes in one-electron transfer is demonstrated by the dominant contribution of a core-excited configuration of the nitrogen atom in N^{+}+O^{-} collisions. This effect is partially accounted for by introducing configuration mixing in the evaluation of coupling matrix elements.

Photochemistry of forbidden oxygen lines in the inner coma of 67P/Churyumov-Gerasimenko.

Observations of the green and red-doublet emission lines have previously been realized for several comets. We present here a chemistry-emission coupled model to study the production and loss mechanisms of the O(1S) and O(1D) states, which are responsible for the emission lines of interest for comet 67P/Churyumov-Gerasimenko. The recent discovery of O2 in significant abundance relative to water 3.80 ± 0.85% within the coma of 67P has been taken into consideration for the first time in such models. We evaluate the effect of the presence of O2 on the green to red-doublet emission intensity ratio, which is traditionally used to assess the CO2 abundance within cometary atmospheres. Model simulations, solving the continuity equation with transport, show that not taking O2 into account leads to an underestimation of the CO2 abundance within 67P, with a relative error of about 25%. This strongly suggests that the green to red-doublet emission intensity ratio alone is not a proper tool for determining the CO2 abundance, as previously suggested. Indeed, there is no compelling reason why O2 would not be a common cometary volatile, making revision of earlier assessments regarding the CO2 abundance in cometary atmospheres necessary. The large uncertainties of the CO2 photodissociation cross section imply that more studies are required in order to better constrain the O(1S) and O(1D) production through this mechanism. Space weather phenomena, such as powerful solar flares, could be used as tools for doing so, providing additional information on a good estimation of the O2 abundance within cometary atmospheres.

Photodissociation of the carbon monoxide dication in the (3)Σ(-) manifold: Quantum control simulation towards the C(2+) + O channel.

The photodissociation and laser assisted dissociation of the carbon monoxide dication X(3)Π CO(2+) into the (3)Σ(-) states are investigated. Ab initio electronic structure calculations of the adiabatic potential energy curves, radial nonadiabatic couplings, and dipole moments for the X (3)Π state are performed for 13 excited (3)Σ(-) states of CO(2+). The photodissociation cross section, calculated by time-dependent methods, shows that the C(+) + O(+) channels dominate the process in the studied energy range. The carbon monoxide dication CO(2+) is an interesting candidate for control because it can be produced in a single, long lived, v = 0 vibrational state due to the instability of all the other excited vibrational states of the ground (3)Π electronic state. In a spectral range of about 25 eV, perpendicular transition dipoles couple this (3)Π state to a manifold of (3)Σ(-) excited states leading to numerous C(+) + O(+) channels and a single C(2+) + O channel. This unique channel is used as target for control calculations using local control theory. We illustrate the efficiency of this method in order to find a tailored electric field driving the photodissociation in a manifold of strongly interacting electronic states. The selected local pulses are then concatenated in a sequence inspired by the "laser distillation" strategy. Finally, the local pulse is compared with optimal control theory.

Photodissociation and radiative association of HeH+ in the metastable triplet state.

We investigate the photodissociation of HeH(+) in the metastable triplet state as well as its formation through the inverse process, radiative association. In models of astrophysical plasmas, HeH(+) is assumed to be present only in the ground state, and the influence of the triplet state has not been explored. It may be formed by radiative association during collisions between a proton and metastable helium, which are present in significant concentrations in nebulae. The triplet state can also be formed by association of He(+) and H, although this process is less likely to occur. We compute the cross sections and rate coefficients corresponding to the photodissociation of the triplet state by UV photons from a central star using a wave packet method. We show that the photodissociation cross sections depend strongly on the initial vibrational state and that the effects of excited electronic states and nonadiabatic couplings cannot be neglected. We then calculate the cross section and rate coefficient for the radiative association of HeH(+) in the metastable triplet state.

Radiative lifetime of the a 3Σ+ state of HeH+ from ab initio calculations.

The first metastable triplet state of HeH(+) was found to be present in ion beam experiments, with its lifetime estimated to be between hundreds of milliseconds and thousand of seconds. In this work, we use ab initio methods to evaluate the radiative lifetimes of the six vibrational levels of the a (3)Σ(+) of HeH(+). The transition a (3)Σ(+)→X (1)Σ(+) is spin-forbidden, but acquires intensity through spin-orbit interaction with the singlet and triplet Π states. Large scale CASSCF/MRCI calculations using an adapted basis set were performed to determine the potential energy curves of the relevant states of HeH(+) as well as the matrix elements of the dipole and spin-orbit operators. The wave functions and energies of the vibrational levels of the a (3)Σ(+) and X (1)Σ(+) states are obtained using a B-spline method and compared to previous works. We find that the radiative lifetime of the vibrational levels increases strongly with v, the lifetime of the v=0 state being 150 s. We also analyze the contributions from discrete and continuum parts of the spectrum. With such a long lifetime, the a (3)Σ(+) state could have astrophysical implications.

Nonadiabatic interactions in wave packet dynamics of the bromoacetyl chloride photodissociation.

The competitive photodissociation of bromoacetyl chloride BrCH2COCl in the first 1A" state (S1) by 248 nm photons is investigated by nonadiabatic wave packet simulations. We show that the preferential breaking of the stronger C-Cl bond (alpha to the excited carbonyl) over the weaker C-Br bond (beta) could be explained by a diabatic trapping or nonadiabatic recrossing as previously proposed. Our energy resolved flux analysis agrees fairly well with the experimental branching ratio (C-Cl:C-Br=1.0:0.4). Even if this does not prove the mechanism, this at least prevents to discard it. A reduced dimensionality approach based on constrained Hamiltonian is used. The nonadiabatic dissociation is studied in the two C-O/C-X (X=Br, Cl) subspaces to emphasize the role of the C-O vibration upon [nO-->piCO*] excitation. The internal torsion and wagging dihedral angles are frozen at their Franck-Condon value, according to preliminary dynamical tests. The other inactive coordinates are optimized at the trans and Cs constrained geometry in the first excited state. Corresponding 2D cuts in the potential energy surfaces have been computed at the CASSCF level. The nonadiabatic kinetic couplings are highly peaked along an avoided crossing seam in both cases. A two-state diabatic model with a constant potential coupling is proposed in the two C-O/C-X subspaces. The inclusion of the C-O stretching in the active coordinates improves the value of the branching ratio over our previous 1D computation.