Carlos E. Bielschowsky

Vibronic coupling for H2CO and CO2

Abstract We calculated the optical oscillator strength (OOS) for the sum of the vibronic excitations related to the valence electronic A 1 →A 2 state in H 2 CO and the inner-shell C 1s (2σ g →5σ g ) state of the CO 2 molecule. Electronic transition moments are calculated along the normal coordinates using configuration interaction wave functions. A quantum mechanical sum rule is used, which provides a final expression for the OOS that depends only on the ground-state vibrational function. The present results are in excellent agreement with the experiment. In particular, the calculation for C 1s (2σ g →5σ g ) vibronic coupling is reported for the first time.

Optical and generalized oscillator strengths for inner-shell excitations in Ar and CO2

Abstract We have determined theoretical values for the transition energies, optical oscillator strengths and generalized oscillator strengths (GOSs) of the following inner-shell excitations: (a) 1 P 1 ← X 1 S 0 (2p → 4s) in the Ar atom; (b) 1 Π u ← X 1 Σ g + [C 1s (2σ g ) → 2π u ] in the CO 2 molecule; (c) 1 Π u ← X 1 Σ g + [O 1s (1σ g ) → 2π u ] in the CO 2 molecule; (d) 1 Π g → X 1 Σ g + [O 1s (1σ u ) ← 2π u ] in the CO 2 molecule. The influence of relaxation, correlation and (for the O 1s excitations) hole localization effects on these properties was studied. For this purpose the target wavefunctions for each state were determined at the Hartree—Fock, configuration interaction and (in the case of O 1s excitations in the CO 2 molecule) generalized multistructural levels. The first Born approximation was used in the calculation of the GOSs.

Generalized oscillator strengths for C 1s excitation of acetylene and ethylene

Abstract The generalized oscillator strength profiles for discrete C 1s excited states of C 2 H 2 and C 2 H 4 have been derived from angle-dependent inelastic electron scattering cross-sections measured with 1300 eV final electron energy. The measured GOS profiles for the strong C 1s→π* transition in each species are compared to theoretical calculations computed within the first Born approximation, using ab-initio generalized multi structural wave functions. These wave functions include relaxation, correlation and hole localization effects. Theory predicts large quadrupole contributions to the π* GOS of each species, analogous to those previously reported for computed GOS profiles for O 1s→π* excitation of CO 2 . We find good agreement between experiment and theory as to the shape of the π* GOS but, when the relative GOS extracted from the experimental data is normalized to the optical oscillator strength at K 2 =0, the magnitude is in better agreement with the GOS computed for only the dipole channel than for the sum of the dipole and quadrupole channels.

Intensity of the n→π∗ symmetry-forbidden electronic transition in acetone by direct vibronic coupling mechanism

Abstract Absolute absorption intensities were calculated for the symmetry dipole forbidden n → π ∗ transition in acetone. An analysis of the distribution per normal modes is performed and the results are compared with a recent calculation. Vibronic coupling mechanism is taken into account in a way that is different from the traditional Herzberg–Teller perturbation approach. In the present method the electronic transition moment is directly expanded in power series of the vibration normal coordinates. This approach was recently used for the equivalent n → π ∗ transition in formaldehyde presenting an excellent agreement with the experimental results.

Forbidden transitions in benzene

We have computed the optical oscillator strengths for the symmetry-forbidden transitions 1 1 B2uXand 1 1 B1uXof benzene through vibronic coupling. Electronic transition dipole moments were calculated at the complete active space self consistent field level along the normal coordinates. Optical oscillator strengths for the sum of the total vibronic excitations are compared with available theoretical and experimental results. q 2002 Elsevier Science B.V. All rights reserved.

Theoretical investigations on valence vibronic transitions

This article reviews previously employed methods to study several valence electronic transitions, optically forbidden or not, enhancing intensity through vibronic coupling. Electronic transition dipole moments were calculated using several ab initio methods including electron correlation. In this method the square of the electronic transition dipole moments are directly calculated along the normal coordinates of vibration and then expanded with a polynomial function. Afterwards, analytical vibrational integration using harmonic wave functions, of the square of the transition moments function, allows us to obtain partial (i.e. for each vibrational mode) and total optical oscillator strengths (OOS), for the vibronic transition of interest. We illustrate the accuracy of the method through valence transitions of benzene (C6H6), formaldehyde (H2CO), acetone (C3H6O) and formic acid (HCOOH).

Inner-shell excitations of water molecule

Abstract Theoretical results for the generalized oscillator strength (GOS) and optical oscillator strength (OOS) have been obtained for the excitations from the ground state X1Σ+ to the inner-shell (O1s) 1A1(1a1→4a1), 1B2(1a1→2b2), 1B1(1a1→2b1) electronic states within the first Born approximation (FBA). The target electronic wave functions were determined using the configuration-interaction method, with a Hartree-Fock basis for the occupied molecular orbitals, and improved virtual orbitals for the virtual space. Relaxation and correlation effects were explicitly taken into account. We present, as well, results for the generalized oscillator strength of the first valence A1B1 excited state, in connection with the discussion of the inner-shell problem. The results were compared with the available experimental and theoretical data.

Explicit core-hole localization and relaxation effects in the calculation of inner-shell spectrum of C2H4

Theoretical results for K-shell spectrum (C 1s) of ethylene are presented. Core-hole localization and relaxation effects are explicitly considered by means of a three structures generalized multistructural (GMS) wave function. Values for the generalized (GOS) and optical (OOS) oscillator strengths for the low-lying C 1s states are reported.

Isotopic effects in inner-shell spectrum of methane: a theoretical study

The isotopic effect on the intensity for the inner-shell C 1s (1a1! 3a1 )o r 1s 21 3s (a1) transition in methane (CH4) and deuteromethane (CD4) is calculated. This transition is dipole forbidden and borrows intensity mainly from the C 1s (1a1! 2t2) or 1s 21 3p (t2) state through vibronic coupling with vibrational modes of t2 symmetry. Absolute oscillator strengths are presented for the above transitions and the vibronic coupling mechanism is interpreted. q 2001 Elsevier Science B.V. All rights reserved.

On the origin of 13C and 14N hyperfine interactions in [Co(CN)6]4− and [Rh(CN)6]4− complexes in KCl host lattice

Abstract Ab-initio ROHF, PUHF and PUHF-MP2 calculations of 13 C and 14 N hyperfine interactions for the [Co(CN)6]4− and [Rh(CN)6]4− complexes in the KCl host lattice were performed and compared to experimental results. The host lattice was represented by a set of 80 potentials located at the ion positions, leading to a consistent picture of the complex electronic structure. A detailed analysis of each molecular orbital contribution shows that collective effects are very important and must be considered to achieve a realistic description of this property.