U. T. Lamanna

Electronic Properties of Sulphur Hexafluoride. II. Molecular Orbital Interpretation of Its X‐Ray Absorption Spectra

The unusual behavior of the sulphur hexafluoride spectra in the x‐ray region is discussed and interpreted within the framework of a MO‐LCAO description of the molecular wavefunction. Single‐ and double‐excitation processes from inner and outer orbitals are suggested to take place during the x‐ray absorption and calculated energies are compared with experimentally observed peaks. A qualitative general agreement of the fitting with the measured data seems to indicate that such processes are very likely to happen in large, multielectron, molecular systems.

Electric and magnetic properties of N2 and H2O by the equations‐of‐motion method

Dynamic multipole polarizabilities and the magnetic dipole susceptibility of N2 and H2O are evaluated in the framework of the equation‐of‐motion method corresponding to the random phase (RPA) and Tamm–Dancoff approximations (TDA). The results are presented in the form of a Cauchy expansion truncated after the ω4 terms. Nuclear magnetic shielding constants of N, H, and O are obtained as by products of the main calculations. While the RPA results for the magnetic properties compare very well with available experimental data, the computed values for the electric observables are only in moderately good agreement with the experiments, suggesting an inadequacy of the employed basis sets and a different relevance of the correlation effects as well. The dipole–dipole van der Waals dispersion coefficients for the pairs N2–N2, H2O–H2O, and N2–H2O obtained by RPA values of the dipole polarizability at imaginary arguments compare only approximately well with semiempirical estimates.

He-HF vibrational inelasticity at low and intermediate energies from quantum collision theory

Abstract The rotational-sudden approximation (IOSA) has been applied to He-HF scattering at intermediate energies by using an ab initio computed potential-energy surface which explicitly included the dependence on the internal molecular coordinate. The vibrational degree of freedom has therefore been treated exactly by solving the coupled radial equations that were obtained after expanding the target vibrations over a large number of HO wavefunctions. The radial behaviour of the computed coupling matrix elements between lower-lying target states is analyzed at various relative orientations of the atom-to-target vector with respect to the molecular bond. The present results clearly show that the coupling strength and range can be directly related to the specific anisotropic behaviour of the potential surface and to the varying efficiency of (V, T) coupling as the He atom probes different molecular regions. The computed relaxation times are compared with experiments at various temperatures and with previous calculations performed via different surfaces and dynamical models and show satisfactory agreement with observations, thus markedly improving the quality of earlier computed cross sections.

The gerade-ungerade splitting of N2+ potentials: effects on the resonant charge transfer cross sections of nitrogen atoms

Abstract Ab initio N 2 + ( 2,4,6 Σ g,u , 2,4,6 Π g,u ) potential splittings have been calculated in the internuclear distance range 2–5 A. The results for the Σ potentials have been obtained in the Heitler-London formulation either retaining all relevant integrals (method 1), or by neglecting some terms (method 2), while the Π splittings have been calculated only within the more approximate method 2. The results have been used for calculating the total charge transfer sections Q tr of the process N( 4 S) + (N + ( 3 P) = N 3 P) + N( 4 S). A reasonably satisfactory agreement with the corresponding Q tr values obtained from previous potentials can be pointed out, with the exception of the results based on the extrapolation of spectroscopic potential curves. A good agreement has also been found with the experimental Q tr values of Belyaev as well as with the Q tr cross sections derived by Duman and Smirnov by means of a completely different theoretical approach.

Refractivity of He Gas: An estimate of its second virial coefficient

Abstract The second virial coefficient of refractivity of He gas at room temperature is calculated by approximating the incremental polarizability of the diatom (He) 2 , at various internuclear distances, by its RPA value. The virial coefficient of refractivity can be expressed in the Cauchy form, at sufficiently low frequencies. The values evaluated for the first three Cauchy moments at T = 322 K are B 2,0 = −0.040 cm 6 /mole 2 , B 2,1 = −0.010 × 10 −32 cm 6 /mole 2 s −4 , respectively. As far as our value for the dielectric virial coefficient B 2,0 is concerned, it compares favourably with the experimental one. B cxpt 2,0 = −0.06 = 0.04 cm 6 /mole 2 , and other previous theoretical estimates as well, while the other two moments, for which no estimates are presently available, are argued to be reasonably reliable.

Interaction potentials and collisional (V, T) transfer in HeN2 and ArN2 gaseous mixtures

Abstract The angle-averaged interaction potentials for He and Ar atoms with N 2 molecules are obtained from the analysis of several scattering experiments and their general features are discussed in comparison with previous findings. A model coupling potential is then derived in order to include target vibrational motion during encounters with both projectiles at intermediate collision energies. An extensive close-coupling calculation for individual state-to-state integral cross sections was thus carried out over the whole range of vibrational excitation and/or de-excitation (V,T) processes which involved single and multiple quantum jumps between target vibrational levels. The corresponding rate constants were finally obtained and compared with available experiments. Results indicate for these systems short-range dominated vibrational coupling and rather weak effects from (R,V) interactions.

Vibrational excitation of H2(X1Σ+g, v)/D2(X1Σ+g, v) through excitation of electronically excited singlet states and radiative cascade

Abstract A complete set of cross sections for the process e+H 2 (X 1 Σ + g , ν 1 )/D 2 (X 1 Σ + g , ν i )→e+H 2 (B 1 Σ + u , C 1 (Π u )/ D 2 (B 1 Σ + u , C 1 Π u )→e+H 2 (X 1 Σ + g , ν f )/D 2 (X 1 Σ + g , ν f )+ h ν has been obtained by using recent calculated excitation cross sections in combination with Einstein probabilities linking vibrational excited states of electronically excited states and the corresponding ones of ground state. An isotopic effect is found into the cross sections: in particular the H 2 cross sections are up to a factor 1.5 larger than the corresponding D 2 cross sections for ν i =O and ν f

Ab initio calculations for ground states of CH2Li− and CH2Be

The ground state of CH2Li− and CH2Be molecules has been investigated by an SCF calculation using a contracted Gaussian basis set. Only for the second system a bound state with respect to the ground states of the molecular fragments has been found.

Generalized oscillator strengths of polyatomic molecules

SummaryGeneralized oscillator strengths for a number of singlet transitions of the H2O molecule, evaluated according to the Random Phase Approximation approach, are presented and discussed so as to offer a partial characterization of the Bethe surface of the molecule.

The COHe system revisited: V—T collisional relaxation from AB initio potentials

Abstract A new potential energy surface (PES), recently computed within an ab initio scheme for the CO + He system, is here used to calculate vibrationally inelastic cross sections via a coupled quantum treatment of the dynamics. Results are obtained with different decoupling approximations (BS and IOS) and compared with those yielded by a previously available PES [3] that showed too strong an attractive behaviour. The effect of anisotropic components in the potential is also analysed and discussed for both PES and appears to play a noticeable role only at low collision energies. Relaxation rates are also computed and used to obtain relaxation times within the Landau—Teller (LT) scheme. Comparison with experiments is very satisfactory at high temperatures and medium temperatures (3000 ⩾ T ⩾ 200 K) while low T behaviour is overestimated by the present calculations.