Miljenko Perić

AB initio vibrational analysis of the Schumann—Runge bands and the neighboring absorption region of molecular oxygen

Abstract Vibrational wavefunctions are calculated for a number of electronically excited states of molecular oxygen whose potential curves are obtained by large-scale configuration interaction calculation. The calculated oscillator strengths for the transitions to the first eleven vibrational levels corresponding to the discrete portion of the Schumann—Runge band systems are in quite good agreement with experimental data. The theoretical crossing of the 3 Π u , 1 Π u and 5 Π u potential curves leading to predissociation of O 2 in its 3 Σ − u state is also determined. The three relatively strong hitherto unidentified bands at the high-energy side of the Schumann—Rudge continuum at 9.96, 10.58 and 10.58 eV respectively are shown to result from transitions to the first three vibrational levels of a second mixed valence—Rydberg state of 3 Σ − u symmetry determined earlier, calculated at 9.93, 10.29 and 10.63 eV respectively; the observed f values for these lines cannot be explained in the usual way on the basis of Franck—Condon overlaps alone, but rather are only reproduced in the calculations if account is taken of the strong variation of the electronic transition moment with internuclear distance caused by the rapid change from Rydberg to valence character in the region of the 3 Σ − u curve crossing. Higher-lying less intense and previously unidentified bands are similarly indicated to result from transitions to a mixed Rydberg—valence 3 Π u species.

Calculation of the electron affinity and 1A1-3B1T0 value of methylene using the ab initio MRD CI method for a large AO basis

Abstract Ab initio CI calculations for CH 2 and CH − 2 predict 1 A 1 - 2 B 1 and 1 A 1 - 3 B 1 T 0 values of 1.01 eV (after applying a 0.20 eV correction based on carbon atom EA results) and 0.50 eV respectively. A reinterpretation of recent photodetachment experiments is suggested which places the corresponding observed findings at 1.05 ± 0.03 and 0.51 ± 0.03 eV respectively, whereby two low-intensity features seen in the spectrum are reassigned as either 3 B 1 - 2 B 1 hot bands or as transitions originating from the 2 A 1 excited state of the negative ion.

Use of the vibronic CI method in accurate calculations of the Renner-Teller effect

Calculations of the Renner-Teller effect in various triatomic systems are carried out employing the vibronic CI method, according to which a matrix representation of the pertinent global hamiltonian is effected and the corresponding eigenvalues and eigenvectors are obtained by a single diagonalization. Conventional least squares polynomial fitting procedures are employed to represent all kinetic and potential energy terms, thereby greatly simplifying the computation of the required matrix elements. Both the adiabatic (bent) and diabatic (linear) transformations of the electronic wavefunctions are employed and the numerical results are compared with one another as well as with data resulting from earlier accurate treatments of the Renner-Teller effect in which analytical expressions for the various hamiltonian terms are treated by means of numerical procedures. Special attention is given to the effect of considering the bending angle dependence of the non-adiabatic coupling terms explicitly. It is found th...

Ab initio treatment of the Renner-Teller effect for the X 2 B 1 and A 2 A 1 electronic states of NH2

A large AO basis including atom-centred f- and s- and p-type bond functions is employed to obtain potential curves for the X 2 B 1 and A 2 A 1 states of NH2 at close to the full CI level of treatment. Two different matrix methods are then applied to describe the Renner-Teller coupling between the bending vibrational states of the two electronic species; excellent agreement is found between the two sets of theoretical results. Use of a simple kinetic energy operator with a constant reduced mass leads to a term value spectrum which agrees to within 100 cm-1 for the measured locations of all low lying vibronic levels in this system and to within 300 cm-1 of the highest lying such species observed to date. Similarly good agreement is found for the ND2 and NHD isotopic species, whereby in these cases the amount of vibronic coupling is, not surprisingly, found to be markedly less than in NH2 itself. Equilibrium bond lengths and angles of this system are calculated to agree to within 0·01 A and 1° of their obser...

Calculation of wavefunctions and frequencies for noninfinitesimal vibrations: Comparison of various methods using ab initio CI potential curves

Abstract Three computational methods employing different means of representing molecular potentials are used to obtain wavefunctions and energy levels for noninfinitesimal vibrations. An interpolation scheme based on a cubic spline fitting procedure is introduced to supplement the CI energy results obtained explicitly in actual calculations. As long as a representative set of potential points is available it is found that the results of all three methods (two of variational and one of numerical integration type) are quite consistent, for both bending and stretching vibrations of ground and electronically excited states (examples for HCN and O 2 are considered). In addition a comparison of one- and two-dimensional bending vibrational treatments is made.

AB INITIO INVESTIGATION OF THE RENNER-TELLER EFFECT IN TETRA-ATOMIC MOLECULES

Abstract A method for theoretical ab initio treatment of the Renner-Teller effect in tetra-atomic molecules is described. It is based on the model developed in 1972 by Petelin and Kiselev, but instead perturbationally, as in the original work, the vibronic problem is solved by a variational approach. The reliability of the approximations on which the model is based is discussed in detail and checked by the explicit ab initio computations carried out at various levels of sophistication. The model is extended to take into account the interplay between the vibronic, spin-orbit and magnetic hyperfine couplings. The results of ab initio investigations of the structure of spectra involving the ground states, X 2 Π u of C 2 H + 2 and B 2 H + 2 are reviewed.

Ab initio calculation of the potential surfaces for low-lying valence electronic states of the C2H radical

The results of an extensive CI treatment for the three lowest-lying electronic states 12A′, 22A′ and 12A″ of C2H are reported. Two-dimensional C-C stretching/bending potential surfaces for these species are calculated. Electronic dipole and transition moments are computed as a function of the bond angle and the C-C bond length. The results serve as a starting point in a theoretical analysis of the rovibronic structure of the long-wavelength spectra of C2H.

Ab initio investigation of the Renner–Teller effect in the X 2Πu electronic state of C2H+2

Results of an ab initio investigation of the vibronic structure of the X 2Πu electronic state of C2H+2 are presented. Calculations are performed using a variational approach for handling the Renner–Teller effect in tetra‐atomic molecules [Peric et al., Mol. Phys. 55, 1129 (1985)]. In these computations both the ab initio potential surfaces and those derived on the basis of experimental findings are employed. The results of the calculations strongly support the recent analysis of the C2H+2 spectrum [Pratt et al., J. Chem. Phys. 99, 6233 (1933)] and predict a number of yet unobserved features in the energy range between 0 and 3000 cm−1.

Ab initio study of the vibronic spectrum in the X 2Π electronic state of HCCS

Potential energy surfaces for the electronic states of the HCCS radical correlating at linear nuclear arrangement with the X 2Π state are calculated by means of an extensive ab initio approach. Particular attention is paid to calculating accurate three-dimensional potential surfaces involving variations of two bending and torsional coordinates, which play the central role in vibronic interactions (Renner–Teller effect), determining the structure of spectra of this radical. In the second part of this paper we use these potential surfaces and the ab initio computed spin–orbit coupling constant to calculate vibronic spectra of HCCS and DCCS in the framework of a theoretical model developed in our laboratory. The results of the present study are in excellent agreement with those derived by Tang and Saito [J. Chem. Phys. 105, 8020 (1996)] and thus strongly support the interpretation of their experimental findings.

Ab initio study of the vibronic and spin–orbit coupling in the X 2Πu state of C2H+2

A variational approach for treating the Renner–Teller effect in tetra‐atomic molecules [Peric et al., Mol. Phys. 55, 1159 (1985)] is extended to account for the effect of spin–orbit coupling. The approach is applied to compute the spin–orbit splittings of the vibronic levels in the X 2Πu state of C2H+2. The bending potential curves employed in a previous study [Peric and Peyerimhoff, J. Chem. Phys. (in press)] are improved by carrying out ab initio calculations at a higher level of sophistication. The results of the computations enable a reliable interpretation of recent experimental findings [Pratt et al., J. Chem. Phys., 99, 66 (1993)].