Coupled-channel study of the Rydberg-valence interaction in HBr.

We report an ab initio study of the low-lying valence and Rydberg states of HBr. The calculations are carried out employing the multireference single- and double-excitation configuration interaction method including the spin-orbit interaction. The first excited adiabatic potential of 1Σ+ symmetry presents two minima which correspond to the Rydberg E1Σ+ and valence V1Σ+ observed states. We calculate the vibrational levels of these two states using a coupled-channel treatment based on the two diabatic potentials deduced from the ab initio adiabatic potentials and the Rydberg-valence interaction. The chaotic energy separations between the observed levels are well reproduced in the calculations. We have also obtained for the first time theoretical data for numerous Rydberg states of HBr lying in the (66-79) × 103 cm-1 excitation energy interval. The calculated spectroscopic parameters are found to be in good agreement with experiment and provide a basis for future studies of radiative and non-radiative processes in the HBr molecule.

Rydberg, valence, and ion-pair quintet states of O2.

We report an ab initio study of the quintet states of molecular oxygen. The calculations are carried out employing the multireference single and double excitation configuration interaction package. Potential energy curves of the six quintet valence states dissociating into ground state atoms and of the four quintet states dissociating to ion-pair atoms were computed. A number of bound quintet Rydberg series converging to the a(4)Πu and b(4)Σg(-) states of the O2(+) cation have been identified.

Complex multireference configuration interaction calculations for the K-vacancy Auger states of N(q+) (q = 2-5) ions.

K-vacancy Auger states of N(q+) (q = 2-5) ions are studied by using the complex multireference single- and double-excitation configuration interaction (CMRD-CI) method. The calculated resonance parameters are in good agreement with the available experimental and theoretical data. It shows that the resonance positions and widths converge quickly with the increase of the atomic basis sets in the CMRD-CI calculations; the standard atomic basis set can be employed to describe the atomic K-vacancy Auger states well. The strong correlations between the valence and core electrons play important roles in accurately determining those resonance parameters, Rydberg electrons contribute negligibly in the calculations. Note that it is the first time that the complex scaling method has been successfully applied for the B-like nitrogen. CMRD-CI is readily extended to treat the resonance states of molecules in the near future.

Adiabatic versus diabatic descriptions of the lowest Rydberg and valence 1Σ+ states of HCl.

In this contribution we first report new ab initio self-consistent field configuration interaction calculations of the first excited adiabatic potential of (1)Σ(+) symmetry, the 2(1)Σ(+) or B(1)Σ(+) state, which presents two minima and can thus be seen as made up of the Rydberg E(1)Σ(+) and the valence V(1)Σ(+) states. Based on the computed 2(1)Σ(+) potential, we devised a theoretical procedure to compute the vibronic structure in order to try to explain the energy levels observed in the region above 76 254.4 cm(-1) which display an irregular vibrational structure, indicative of spectral perturbations. We try to find out which representation of the electronic states, the diabatic or the adiabatic one, is best suited to replicate the lowest observed vibronic levels of the E and V states. To this end, we deduce, from the 2(1)Σ(+) potential and its complementary adiabatic potential, two diabatic potentials. We then carry out a coupled equation treatment based on these diabatic potentials. The results of this treatment indicate that, in the present case, the adiabatic representation is better than the diabatic one to describe the observed vibronic levels. This is due, as expected, to the existence of a strong electrostatic interaction between the two diabatic potentials.

An interpretation of the anomalous (1)Pi vibronic structure in the far-UV spectrum of CO.

The far-UV spectrum of carbon monoxide presents numerous abnormal (1)Pi rovibronic levels in the region 92,000-105,000 cm(-1) which have been observed by several experimentalists. Yet, and in spite of various attribution attempts carried out over the past two decades, the nature of these levels is poorly understood and they still lack a definitive assignment. The absorption lines in this energy region are characterized by irregular energy level positions and spacings, and odd, smaller than expected, rotational constants. In the current contribution we address this puzzle by relying on recent ab initio calculations of several Rydberg and valence states of CO [G. J. Vazquez, J. M. Amero, H. P. Liebermann, and H. Lefebvre-Brion, J. Phys. Chem. A 113, 13395 (2009)], and on further new calculations in which we compute electronic transition moments between the ground state and several excited (1)Pi states. We focus on the perturbations between the adiabatic (1)Pi states, specifically on the interaction between the second and third potential energy curves, reported in our previous paper. The second adiabatic potential energy curve, which we refer to as E-E('), displays a distorted shape with two minima as a result of an avoided crossing with the third one. We report here the computation of the lowest vibronic levels of a system of two electronic states which undergo a strong Rydberg-valence interaction. Our vibronic calculations proceed as follows: from the second and third computed adiabatic curves we first obtain approximate diabatic curves for the (X (2)Sigma(+))3ppi E (1)Pi Rydberg state and for the valence E(') (1)Pi state. Then we solve a system of 2x2 coupled equations in order to obtain the perturbed vibronic energy levels and wave functions for the interacting E and E(') states. The computed vibronic levels obtained from the coupled equation treatment are compared to the first six observed (1)Pi levels. A good agreement is found with experiment for the four lowest vibronic levels and a reasonable accord for two higher levels.

Potential energy curves for the (1)Sigma(+) and (1,3)Pi states of CO.

Ab initio potential energy curves of CO are calculated to address a number of problems remaining in the interpretation of the experimental VUV absorption spectra. The calculations are of the type SCF MRSD-CI. We employed the aug-cc-pVQZ basis set for both carbon and oxygen, augmented with twelve diffuse functions, of s-, p-, and d-type, located on both atomic centers. We focus on the energy region 85,000 cm(-1) < E < 110,000 cm(-1) characterized by strong interactions between Rydberg and valence states. In this work we deal specifically with the (1)Sigma(+), (1)Pi, and (3)Pi states lying in this region. Some of the relevant findings are as follows: The minimum of the C'(1)Sigma(+) valence state is found 1920 cm(-1) above the value inferred from an extrapolation of experimental data. A new (1)Pi valence state, labeled E', is found to perturb strongly the (X(2)Sigma(+))3ppi E(1)Pi Rydberg state. The electrostatic perturbation of the (X(2)Sigma(+))3ppi c(3)Pi Rydberg state by the k(3)Pi valence state is confirmed. The energy position of the (A(2)Pi)3ssigma (3)Pi state, the triplet companion of the W(1)Pi Rydberg state, is predicted.

Insight into the Rydberg states of CH.

Ab initio electronic structure calculations of a relatively large number of Rydberg states of the CH radical were carried out employing the multireference single and double excitation configuration interaction (MRD-CI) method. A Gaussian basis set of cc-pV5Z quality augmented with 12 diffuse functions was used together with an extensive treatment of electron correlation. The main focus of this contribution is to investigate the 3d Rydberg complex assigned by Watson [Astrophys. J. 555, 472 (2001)] to three unidentified interstellar bands. The authors' calculations reproduce quite well the absolute excitation energies of the three components of the 3d complex, i.e., 2Sigma+(3dsigma), 2Pi(3dpi), and 2Delta(3ddelta), but not the energy ordering inferred from a rotational assignment of the 3d<--X 2Pi laboratory spectrum. The computation of the 4d complex is reported for the first time along with a number of other higher lying Rydberg species with an X 1Sigma+ core. The lowest Rydberg states belonging to series converging to the a 3Pi and A 1Pi excited states of CH+ are also calculated.

Use of exchange maximization to generate starting vectors for self-consistent field calculations on metal cluster/adsorbate systems.

Localized molecular orbitals (LMOs) derived from exchange maximization with respect to all atom-centered basis functions in the basis set are shown to generate a good starting electronic field for self-consistent field calculations on extended systems such as metal clusters, for which well-defined chemical bonds are not present. Examples studied are a cluster of 20 Ni atoms and the Pt(97)CO, Ag(43)/H(3)CNON, Ag(91)/H(2)CO, and vinylidene/Ni metal cluster plus adsorbate systems. It is also shown that improved starting vectors can be obtained by remixing a subset of the LMOs with the largest exchange eigenvalues through diagonalization of the Fock matrix computed with a null electronic field. Employing only a subset of the exchange-maximized LMOs in the first iterations, and then gradually expanding the space in which the diagonalizations are carried out in succeeding cycles, is shown to be an effective means of guiding the SCF procedure to the converged full-basis solution.

Experimental and Theoretical Study of the Electronic States and Spectra of TeH and TeLi.

Gas-phase emission spectra of the hitherto unknown free radical TeLi have been measured in the NIR range with a Fourier-transform spectrometer. The emissions were observed from a fast flow system in which tellurium vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. Two systems of blue-degraded bands were measured at high spectral resolution in the ranges 8000-9000 and 5700-6700 cm(-1) and vibrational and rotational analyses were performed. In order to aid in the analysis of the experimental data, a series of relativistic configuration interaction calculations has been carried out to obtain potential curves for the low-lying states of TeLi and the isovalent TeH and also electric dipole transition moments connecting them. As in the TeH system, the ground state of TeLi is found to be X(2)Pi(i), but with a remarkably smaller spin-orbit splitting. The TeLi calculations indicate a strongly bound A(2)Sigma(+) state, while in TeH the analogous state is computed to lie significantly higher at approximately 32 000 cm(-1), and it is strongly predissociated. Based on the theoretical analysis, the observed TeLi band systems are assigned to the transitions A(2)Sigma(+)(A1/2)-->X(1)(2)Pi(3/2)(X(1)3/2) and A(2)Sigma(+)(A1/2)-->X(2)(2)Pi(1/2)(X(2)1/2). Analysis of the spectra has yielded the molecular constants (in cm(-1)) X(1)(2)Pi(3/2):omega(e)=457.49(3), omega(e)x(e)=2.482(9), B(0)=0.408908(8); X(2)(2)Pi(1/2): T(e)=2353.44(3), omega(e)=456.28(4), omega(e)x(e)=2.635(8), B(0)=0.414954(8), p(0)=1.00637(4); A(2)Sigma(+): T(e)=8574.64(2), omega(e)=437.81(3), omega(e)x(e)=2.581(8), B(0)=0.423903(8), p(0)=-0.19915(2), where the numbers in parentheses are the standard deviations of the parameters. Comparison of the isovalent TeLi and TeH systems emphasizes that the difference in bonding character (ionic in TeLi vs covalent in TeH) is responsible for qualitative differences in the electronic spectra of these two molecules. Copyright 2001 Academic Press.