A. Spielfiedel

Inelastic Mg+H collision data for non-LTE applications in stellar atmospheres

Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s(2) S-1, 3s3p P-3, 3s3p P-1, 3s4s S-3, 3s4s S-1, 3s3d D-1, 3s4p P-3)+H(1s) and Mg+(3s S-2)+H-. The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool stellar atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.

Bent valence excited states of CO2

The conical intersection regions on the potential energy functions of the valence excited 1,3Σu−, 1,3Δu, 3Σu+, and 1,3Πg states of CO2 have been investigated by ab initio calculations. Using large scale multireference configuration interaction the ordering of the lowest valence excited states of CO2 has been calculated to be 3B2, 3A2, 1A2 followed by 1B2. All these states have bent equilibrium structures and lie energetically below their dissociation asymptotes. The near equilibrium parts of the potential energy functions have been mapped in three dimensions by multiconfiguration self‐consistent field calculations. The 1,3B2 and 1,3A2 states differ in their equilibrium angles (118° and 127°, respectively), and have much longer equilibrium distances (around 1.26 A) than the electronic ground state. Anomalously low values of ca. 800 cm−1 have been calculated for the wave number of the antisymmetric stretching vibrations of the 1A2, 3B2, and 1B2 states. The crossings between the bent valence excited states i...

Theoretical potential energy function and rovibronic spectrum of CO+2(X 2Πg)

For the electronic ground state of CO+2 the three‐dimensional potential energy, electric dipole, and transition moment functions have been calculated from highly correlated multireference configuration interaction electronic wave functions. Along the antisymmetric stretching displacements the shape of the potential energy functions is found to be very sensitive to the electron correlation effect. Using a modified theoretical potential energy function rovibronic energy levels have been calculated variationally by the method of Carter and Handy. In this approach, anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects have been accounted for. The vibronic band origins agree to within about 10 to 20 cm−1 with the available experimental data, and the rotational levels agree to within 0.01 cm−1 for low J values. Additional vibrational band origins have been predicted for energies up to 3200 cm−1. The anomalously low frequency of the antisymmetric stretching mode and it...

Inelastic Mg+H collision processes at low energies

Full quantum scattering calculations of cross sections for low-energy near-threshold inelastic Mg+H collisions are reported, such processes being of interest for modelling of Mg spectral lines in stellar atmospheres. The calculations are made for three transitions between the ground and two lowest excited Mg states, Mg(3s(2) (1)S(0)), Mg(3s3p (3)P) and Mg(3s3p (1)P). The calculations are based on adiabatic potentials and nonadiabatic couplings for the three low-lying (2)Sigma(+) and the first two (2)Pi states, calculated using large active spaces and basis sets. Non-adiabatic regions associated with radial couplings at avoided ionic crossings in the (2)Sigma(+) molecular potentials are found to be the main mechanism for excitation. Cross sections of similar order of magnitude to those obtained in Li+H and Na+H collisions are found. This, together with the fact that the same mechanism is important, suggests that as has been found earlier for Li and Na, processes such as ion pair production may be important in astrophysical modelling of Mg, and motivates continued study of this system including all states up to and including the ionic limit.

A five-dimensional potential-energy surface for the rotational excitation of SO2 by H2 at low temperatures

The SO(2) molecule is detected in a large variety of astronomical objects, notably molecular clouds and star-forming regions. An accurate modeling of the observations needs a very good knowledge of the collisional excitation rates with H(2) because of competition between collisional and radiative processes that excite and quench the different rotational levels of SO(2). We report here a five-dimensional, rigid-body, interaction potential for SO(2)-H(2). As a first application, we present rate constants for excitation/de-excitation of the 31 first levels of SO(2) by para-H(2) at low temperatures. Propensity rules are discussed.

Ab initio calculation of the ground (1A′) potential energy surface and theoretical rate constant for the Si+O2→SiO+O reaction

The entrance channel of the Si+O2→SiO+O reaction has been investigated for collinear and perpendicular approach of the silicon atom to the O2 molecule by ab initio electronic structure calculations using the multireference configuration interaction (MRCI) method and Davidson correction (MRCI+Q). Results show that the reaction can proceed through the ground singlet (1A′) and first triplet (3A′) electronic states at low temperatures. The ground 1A′ three-dimensional potential energy surface (PES) which correlates the Si(3P)+O2(X 3Σg−) reactants to the SiO(X 1Σ+)+O(1D) products was computed at the MRCI+Q level of theory using the Woon and Dunning cc-pVTZ basis sets. The reaction was found barrierless and three minima have been characterized on the 1A′ PES with energy ordering: linear OSiO(1Σg+)<triangular OSiO(1A1)<linear SiOO(1Σ+). About 2500 ab initio data points have been fitted to a many body expansion using the method of Aguado and Paniagua, with a global root-mean-square of 1.49 kcal/mol. The analytica...

Interactions of Rydberg and valence states in CO2

Abstract The electronically excited singlet and triplet states of CO2 lying below 11.4 eV have been investigated by ab initio MCSCF calculations. It has been found that the valence excited states with the electronic configuration (1π3g2p2π1u form sharp avoided crossings along the symmetric stretching coordinate with the Rydberg states of the same symmetry resulting from the electronic configuration (1π3g3p3π1u). Hence, the Rydberg states can dissociate into the lowest valence asymptotes O(3P, 1D) + CO(X 1Σ+), in accordance with experimental findings. In addition, the photodissociation of CO2 is complicated by the fact that the valence singlet and triplet states 1,3Σ−u and 1,3Δu form conical intersections with the diffuse 1,3Πg states in the Franck-Condon region of the absorption spectrum.

On the Rathenau bands in the absorption spectrum of CO2

High-accuracy multireference configuration interaction calculations for the energies of the lowest two singlet and triplet Πu states of CO2 as a function of the symmetric stretching coordinate are reported. It has been found that these Πu states have ionic cores corresponding to the X 2Πg and A 2Πu states of CO+2. They lie close in energy and form an avoided crossing along the symmetric stretching coordinate. Due to this avoided crossing the lowest 3Πu state exhibits a double minimum in D∞h symmetry. Our results imply that most of the spectral features in the absorption spectrum of CO2 lying between 11.4 and 12.6 eV known as the Rathenau bands originate from the lowest two vibronically coupled Πu states.

The first dipole-allowed electronic transition 1 1Σ+u−X 1Σ+g of CO2

Abstract The potential energy function of the 1 1Σ+u excited state of CO2, which lies around 11 eV above the electronic ground state, has been investigated by ab initio multiconfiguration self-consistent field calculations. It has been found that this state exhibits two minima on one-dimensional cuts of the potential energy function along the bending coordinate. One minimum at colinear geometry has a Rydberg character, whereas the other one located at a bending angle of about 140° has a valence character. Both minima are separated by a small barrier. This seems to be responsible for complicated splittings observed in the absorption spectrum of the first dipole-allowed electronic transition of CO2.

Vibrational motion in the X3∑g− state of N3+

Abstract We have generated an ab initio near-equilibrium three-dimensional potential energy function for the X 3 ∑ g − state of N 3 + using highly correlated multireference configuration interaction wavefunctions. The nuclear motion problem has been solved variationally and the vibrational energy levels up to 3000 cm −1 are given. The antisymmetric stretch wavenumber has been calculated to be 929 cm −1 , and the symmetric stretch wavenumber to be 1190 cm −1 , in opposite order to that calculated previously. The flat shape of the potential along the asymmetric displacement coordinate leads to strong anharmonic resonances.