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A study was conducted relating to research on reaction dynamics of C + CH - C2H - C2 + H in the context of interstellar chemistry. The study aimed to investigate the potential energy surfaces issued from the reactants C + CH in their ground state to obtain an outline of their topology and especially the potential existence of possible energy barriers during the approach of the reactants. The results of the experiment are discussed.

Separate coupled-channel Schrödinger-equation (CSE) models of the interacting 1Πu (b,c,o) and 3Πu (C,C′) states of N2 are combined, through the inclusion of spin-orbit interactions, to produce a five-channel CSE model of the N2 predissociation. Comparison of the model calculations with an experimental database, consisting principally of detailed new measurements of the vibrational and isotopic dependence of the 1Πu linewidths and lifetimes, provides convincing evidence that the predissociation of the lowest 1Πu levels in N2 is primarily an indirect process, involving spin-orbit coupling between the b 1Πu- and C 3Πu-state levels, the latter levels themselves heavily predissociated electrostatically by the C′ 3Πu continuum. The well-known large width of the b(v=3) level in 14N2 is caused by an accidental degeneracy with C(v=9). This CSE model provides the first quantitative explanation of the predissociation mechanism for the dipole-accessible 1Πu states of N2, and is thus likely to prove useful in the construction of realistic radiative-transfer and photochemical models for nitrogen-rich planetary atmospheres. [ABSTRACT FROM AUTHOR]

The chloromethane cation and dication have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction. The 2E ground state of CH3Cl+ presents a weak Jahn–Teller distortion, the 2A′ and 2A″ components being separated by only 3.4 kcal/mol with very similar geometries. Both states are equilibrium structures. Contrary to previous calculations, the lowest singlet and triplet states of CH3Cl++ are calculated to be stable. While the 3A2 state keeps the neutral C3v symmetry, the 1E singlet state shows a large Jahn–Teller effect, with a splitting of 32.7 kcal/mol in favor of the 1A′ state, which is the ground state of the molecule. The planar 1A′ state of CH2ClH++ is found to be the most stable isomer, lying 41.4 kcal/mol below the corresponding state of CH3Cl++. The 3A″ state, which lies 22.59 kcal/mol above the 1A′ state, has a nonplanar Cs-symmetry geometry. Finally, the CH3Cl++ (1A′)→CH2ClH++ (1A′) isomerization takes place via a C1-symmetry transition state, with an energy barrier of 32.85 kcal/mol. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The C2H++2 fragmentation processes have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction, in order to interpret recent charge separation spectroscopy experiments. For two-body processes, the calculated appearance thresholds of the C2H+/H+ and CH+/CH+ fragment pairs are in good agreement with the experimental data. It is shown that the C2H++2→CH++CH+ dissociation occurs with an important rotation of the CH+ ions. The presence of the CH+2 ion is explained by a preliminary isomerization of acetylene to vinylidene dication. This reaction has been studied for the lowest lying states of C2H++2 (3Σ-g and 1Δg) and compared with other acetylenic ions isomerizations (C2H2, C2H+2, C2H-2). For three-body processes, the calculations are consistent with the mechanisms proposed by the experimentalists. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optical potential method initially proposed by Jolicard and Austin in the context of the stabilization method is reviewed here and used with the accurate and the efficient discrete variable representation method to obtain the energies and the widths (respectively, the real and the imaginary part of the resonance energies) of rovibrational predissociated states of diatomic molecules. In this method the resonances for an n coupled states problem are obtained by a direct diagonalization of the Hamiltonian matrix in the diabatic representation. This Hamiltonian matrix is directly evaluated in the discrete variable representation using the Fourier grid Hamiltonian method proposed by Marston and Balint-Kurti. In this approach, two optical potentials are tested and used here to impose the asymptotic behaviors of the boundary conditions which are compatible with the resonance states. The method is exemplified for the B 1Σ+–D’1 Σ+ Rydberg–valence predissociating interaction in the CO molecule. [ABSTRACT FROM AUTHOR]

The C2H photodissociation processes have been studied using ab initio multireference perturbative configuration interaction methods. The calculated structural parameters of the two linear lowest lying states X 2Σ+ and A 2Π were found to be in good agreement with previous theoretical and experimental works. Construction of the correlation diagram indicates that C2H cannot photodissociate into C+CH. Therefore, two-dimensional potential energy surfaces of the six lowest lying A’ and A‘ doublet states have been calculated as functions of the bond angle and the C–H bond distance, keeping the C–C distance at the equilibrium C2H value. It appears that the 1 2A’, 2 2A’, and 1 2A‘ states correlating to the X 2Σ+ and A 2Π states in linear geometry remain bound at all angles, whereas the 3 2A’, 4 2A’, and 4 2A‘ are dissociative. However, the structure of the energy surfaces is complicated by the presence of numerous avoided crossings between these states. Transition dipole moments connecting the excited states with the X 2Σ+ and A 2Π states have been also obtained in linear geometry. They were calculated to be very weak, leading to the conclusion that the photodissociation rate of C2H will be negligible below 7 eV. Higher lying states, including Rydberg states, were not considered here, since they cannot dissociate directly into C2+H. [ABSTRACT FROM AUTHOR]

The present article describes fully quantum calculations of spin changing collisions of Ca(4s5p 1P)⇄Ca(4s5p 3P) in full collisions with all the noble gases (He, Ne, Ar, Kr, and Xe). The approach and scattering formalism follow our earlier article [J. Chem. Phys. 86, 4790 (1987)]. Flexible Morese-Spline–van der Waals functions are used to describe the relevant 1,3Σ and 1,3Π Ca*+M potential curves. The interpretation of the dependence of the cross section on initial polarization will be guided by diatomic spectroscopy and a consideration of the crossings between thse potential curves. The interpretation of the dependence of the cross section on initial polarization will be guided by diatomic spectroscopy and a consideration of the crossings between these potential curves. A new simple statistical model, based on the consideration of the reflection symmetry of the initially prepared state, will provide the point of departure for the prediction of the polarization dependence. The exact quantum cross sections agree well both with the predictions of the statistical model and with the recent experimental results from Leone’s laboratory at JILA. In addition the calculated 1P→3P cross sections increase with decreasing collision energy, since the crucial 1Π→3Σ crossing point is reached by the attractive branch of the 1Π potential. [ABSTRACT FROM AUTHOR]

Dissociations of the ethyne dication following its production by photoionization in the photon energy range of 35–65 eV have been investigated by the photoelectron–ion–ion coincidence technique using both synchrotron radiation and laboratory light sources. New quantum mechanical calculations identify and locate the electronic states of the molecular dication in this energy range and show that the dissociation products are formed in their ground states by heterogeneous processes. Five reaction channels leading to three molecular fragments have been identified and are interpreted as sequential processes, several faster than fragment rotation and one possibly involving dissociation of CH+ to H+ with a lifetime of the order of 25 fs. [ABSTRACT FROM AUTHOR]

The strong interaction between the B 3sσ 1Σ+ Rydberg state and the D’ 1Σ+ valence state of the CO molecule is shown to cause large changes in the vibrational and rotational constants of the B state, as well as predissociation of all rotational levels of B (v’=2) and a breaking off in the emission of B (v’=1) levels at J=36 in 12C 16O and J=37 in 13C 16O. A two-state diabatic model of the Rydberg–valence interaction is constructed and vibrational term values, widths, and intensities are calculated by close coupling in order to account for the strong mixing. The model separates the differences between the spectroscopic constants of the B state and those of the ground state molecular ion into two components, one due to the R-dependent quantum defect of the B state and another due to the strong Rydberg–valence perturbation. The perturbation is characterized by a constant coupling matrix element of 2900 cm-1 inside the crossing point of the two diabatic potentials, decaying to zero at long internuclear distances. Basically good agreement is found between the model and experiment for shifts in vibrational and rotational terms and for predissociation widths and relative band intensities. The second breaking off in emission in the B (v’=1) rotational series is used to estimate the height of the long-range barrier maximum in the D’ 1Σ+ state to be about 1048±19 cm-1 above the ground state dissociation limit. Comparison of predicted widths from the two channel close coupled model with those from a single channel adiabatic model shows differences on the order of a factor of 2. [ABSTRACT FROM AUTHOR]

The energy levels of triplet states of doubly charge oxygen, obtained using double charge transfer spectroscopy, have been examined in the light of ab initio calculations. Four peaks were found in the double charge transfer spectrum with energies of 41.1±0.2, 43.2±0.2, 48.2±0.3, and 51.3±0.3 eV, respectively. The first and third peaks have been assigned to the 3Σ+u and the 3Πu excited states, respectively. The second peak has been assigned to the population of the 3Πg states. The configuration of the state at 51.3 eV is unknown. The effect of the symmetry rule Σ+\Σ- is discussed and the validity of the spin conservation rule is demonstrated when a proton is used as a projectile in this experimental technique. [ABSTRACT FROM AUTHOR]