Stanka Jerosimić

Ab initio study of the A 2Π–X 2Π electronic transition in HCCS

Potential energy surfaces for the electronic states of the HCCS radical correlating at linear nuclear arrangement with the A 2Π state are calculated by means of an extensive ab initio approach. They are used to compute the vibronic and spin-orbit structure of the A 2Π–X 2Π electronic transition. These calculations are carried out by means of a new variational approach based on the use of normal bending coordinates. The results of calculations question various interpretations of the available experimental data; on the other hand they do not offer reliable explanation of all features observed, pointing in this way at the shortages of the present, as well as of previous theoretical handling of the problem in question.

Mass spectrometric study of the structures and ionization potential of LinI (n = 2, 4, 6) clusters

We report a combined experimental and theoretical investigation of small heterogeneous clusters of lithium iodide. Apparatus based on magnetic sector instrument, where Knudsen cell is placed into the ionization chamber, provides suitable conditions for the detection of both the ionic and neutral components of LinI (n = 2, 4, 6) clusters. The clusters Li4I and Li6I were detected experimentally for the first time. Ionization potentials determined by the electron impact ionization mass spectrometry were (4.69 ± 0.25) eV for Li2I, (4.86 ± 0.25) eV for Li4I and (4.96 ± 0.25) eV for Li6I. The ionization potential of Li2I corresponds to previous experimental data obtained by thermal ionization mass spectrometry. The first theoretical data of the ionization potential and structure of the above mentioned clusters are presented in this work. A comparison with the experimental ionization potentials provides evidence for the theoretically calculated geometrical structures of the small heterogeneous clusters of lithium iodide.

Ab initio study of the hyperfine structure of the X2Π electronic state of HCCS

The results of an ab initio study of the magnetic hyperfine structure of the X2Π electronic state of HCCS and DCCS are reported. The potential surfaces for two components of the X2Π electronic state and the electronically averaged hyperfine coupling constants for H, D, 13C and 33S are obtained as functions of two bending vibrational modes by the density functional theory method. Since the bending potential curves calculated in the present study do not differ significantly from their counterparts computed in our previous work by means of an extensive configuration interaction approach, we employ the latter here to avoid the appearance of small differences between the vibronic energy levels presented in this work and those already published. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner–Teller effect and spin–orbit coupling. It is found that, due to the generally significant geometry dependence of the hyperfine coupling constants, it is necessary to carry out vibronic averaging of the corresponding functions in order to obtain values which can be compared with the results of measurements. The results of the present study are an aid to the reliable interpretation of recently published experimental data.

Underlying theory of a model for the Renner–Teller effect in tetra-atomic molecules: X2Πu electronic state of C2H2+

In the present study, we prove the plausibility of a simple model for the Renner-Teller effect in tetra-atomic molecules with linear equilibrium geometry by ab initio calculations of the electronic energy surfaces and non-adiabatic matrix elements for the X(2)Πu state of C2H2 (+). This phenomenon is considered as a combination of the usual Renner-Teller effect, appearing in triatomic species, and a kind of the Jahn-Teller effect, similar to the original one arising in highly symmetric molecules. Only four parameters (plus the spin-orbit constant, if the spin effects are taken into account), which can be extracted from ab initio calculations carried out at five appropriate (planar) molecular geometries, are sufficient for building up the Hamiltonian matrix whose diagonalization results in the complete low-energy (bending) vibronic spectrum. The main result of the present study is the proof that the diabatization scheme, hidden beneath the apparent simplicity of the model, can safely be carried out, at small-amplitude bending vibrations, without cumbersome computation of non-adiabatic matrix elements at large number of molecular geometries.

Theoretical investigation of vibronic and spin-orbit effects in the ground X 2Πu electronic state of the dicyanoacetylene cation.

The aim of the present study is to predict by means of ab initio calculations the vibronic and spin-orbit structure in the X (2)Π(u) electronic state of NC(4)N(+) and to elucidate some details in an observed laser-induced fluorescence spectrum of this species, particularly those interpreted in terms of the bending overtones and the spin-orbit splitting. The ground electronic state of NC(4)N(+) was investigated by density functional (B3LYP) and complete active space self-consistent field-multi-reference configuration interaction (CASSCF-MRCI) methods. The bending vibrational frequencies ω(T1) = 558 cm(-1), ω(T2) = 266 cm(-1), ω(C1) = 459 cm(-1), and ω(C2) = 113 cm(-1) were obtained. The spin-orbit coupling constant was calculated using state-average CASSCF wave functions in the framework of the MRCI method, and the value of A(SO) = -44 cm(-1) was determined. This quantity and the data for the bending frequencies and Renner parameters were employed for handling a combined effect of the vibronic and spin-orbit coupling, according to a model developed in our earlier studies.

An ab initio calculation of the vibronic energy levels of the X Π2 and 1 Δ2 electronic states of C2P

In the present study, the results of an ab initio calculation of the vibronic energy levels in the X (2)Pi and 1 (2)Delta electronic states of C(2)P are reported. This work is motivated by recent measurements carried out by [Sunahori et al. J. Chem. Phys. 128, 244311 (2008)]. The vertical electronic spectrum, excitation energies, bending potential curves, and spin-orbit constants for the title molecule are computed by means of the state-average complete active space self-consistent field and multireference configuration interaction approach. Vibronic energy levels of the X (2)Pi and 1 (2)Delta states are calculated with the help of a simple, effectively one-dimensional model. The results of the present study strongly support the analysis of experimental data by Sunahori et al. and offer reliable predictions for experimental searches for heretofore unobserved electronic states.

Ab initio study of the 1 Δ2-X̃ Π2 electronic transition of C2As

We report the results of ab initio calculations on the 1 (2)Delta-X (2)Pi spectral system of C(2)As. The present study is closely related to the recent comprehensive experimental and theoretical work by Wei et al. [J. Chem. Phys. 129, 134307 (2008)]. By means of the state-average complete active space self-consistent field and multireference configuration interaction approach, we computed the vertical excitation energies to the low-lying doublet electronic species, potential surfaces and spin-orbit constants for the X (2)Pi and 1 (2)Delta states, as well as the components of the electric dipole moment for the transition between these two species. Using these data, we calculated the vibronic energy levels, the spin-orbit structure of the spectrum, and the vibronic transition moments of the X (2)Pi-1 (2)Delta system. The results of the present study for the X (2)Pi state agree with those derived from experimental findings by Wei et al., they elucidate the vibronic and spin-orbit structure in the 1 (2)Delta species, and offer predictions for experimental searches of heretofore unobserved electronic states.

Theoretical investigation of the vibronic spectrum in the XΠu2 electronic state of C6

In this study we employ the recently developed model for handling the Renner-Teller effect in Pi electronic states of six-atomic molecules with linear equilibrium geometry to calculate the vibronic spectrum in the X(2)Pi(u) electronic state of the C(6)(+) ion. The applied model Hamiltonian excludes the stretching vibrations and end-over-end rotations. On the other hand, it considers the interplay between the vibronic and spin-orbit couplings. The parameters determining the shape of the bending potential energy surfaces are computed by means of a Density functional theory, and the spin-orbit coupling constant by the Multireference CI program using state-averaged complete active space self-consistent field (SA-CASSCF) wavefunctions. The results of the present study are expected to motivate and help future experimental investigations on C(6)(+).

Topological study of nonadiabatic effects in Π electronic states of tetra-atomic molecules

ABSTRACT In this study, we investigate the topological structure of the adiabatic potential energy surfaces and nonadiabatic matrix elements in the framework of a simple model for handling the Renner–Teller effect in tetra-atomic molecules. The reliability of the transformation from the adiabatic to diabatic electronic basis functions is discussed. As a concrete example, we consider the X 2Πu electronic state of C2H2 +. GRAPHICAL ABSTRACT

Variational calculation of the vibronic spectrum in the X2Пu electronic state of C6

A variational approach for ab initio handling of the Renner–Teller effect in six-atomic molecules with linear equilibrium geometry is elaborated. A very simple model Hamiltonian suitable for description of small-amplitude bending vibrations in Π electronic states of arbitrary spin multiplicity is employed. The computer program developed in the framework of the present study is tested on the example of the X 2 Π u state of C 6 – . The results are compared with those generated in the corresponding perturbative calculations.