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The single- and double-electron capture (SEC, DEC) processes of He2+ ions colliding with Ne atoms are studied by utilizing the full quantum-mechanical molecular-orbital close-coupling method. Total and state-selective SEC and DEC cross sections are presented in the energy region of 2 eV/u to 20 keV/u. Results show that the dominant reaction channel is Ne+(2s2p 6 2 S) + He+(1s) in the considered energy region due to strong couplings with the initial state Ne(2s 22p 6 1 S) + He2+ around the internuclear distance of 4.6 a.u. In our calculations, the SEC cross sections decrease initially and then increase whereby, the minimum point is around 0.38 keV/u with the increase of collision energies. After considering the effects of the electron translation factor (ETF), the SEC cross sections are increased by 15%–25% nearby the energy region of keV/u and agree better with the available results. The DEC cross sections are smaller than those of SEC because of the larger energy gaps and no strong couplings with the initial state. Due to the Demkov-type couplings between DEC channel Ne2+(2s22p 4 1 S) + He(1s 2) and the dominating SEC channel Ne+(2s2p 6 2 S) + He+(1s), the DEC cross sections increase with increasing impact energies. Good consistency can also be found between the present DEC and the experimental measurements in the overlapping energy region.

Gas phase emission spectra of the hitherto unknown free radical LiSb were measured in the NIR range with a Fourier-transform spectrometer. The emissions were observed from a fast-flow system in which antimony vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. A number of blue-degraded bands observed in the range 6200–7800 cm−1 are assigned to transitions from four excited states A21, A30+, A40− and a2 to the X10+ and/or X21 components of the X3Σ− ground state. The 0–0 bands of six transitions were measured at high spectral resolution and rotationally analysed. The rotational and vibrational analyses have yielded the spectroscopic parameters of all six states. 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 energy curves and spectroscopic constants for the low-lying states of LiSb as well as electric-dipole transition moments and radiative lifetimes.

The nonradiative charge-transfer processes of Be3+(1s)/B4+(1s) colliding with He(1s2) are investigated by the quantum-mechanical molecular orbital close-coupling (QMOCC) method from 10 eV/u to 1800 eV/u. Total and state-selective cross sections are obtained and compared with other results available. Although the incident ions have the same number of electrons and collide with the same target, their cross sections are different due to the differences in molecular structure. For Be3+(1s) + He(1s2), only single-electron-capture (SEC) states are important and the total cross sections have a broad maximum around E = 150 eV/u. While for B4+(1s) + He(1s2), both the SEC and double-electron-capture (DEC) processes are important, and the total SEC and DEC cross sections decrease rapidly with the energy decreasing.

Gas phase emission spectra of the hitherto unknown radicals NaTe and KTe 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 sodium or potassium vapor in an observation tube. Excitation of the radicals mostly occurred by near-resonant E-E energy exchange from added metastable oxygen O2(a1Δg). Like for the previously studied LiTe molecule, for both radicals two systems of blue-degraded bands were measured in the range 4500–8500 cm−1. They were assigned to the transitions A2Σ+(A1/2) → X12Π3/2(X13/2) and A2Σ+(A1/2) → X22Π 1/2(X21/2). For all four subsystems vibrational analyses were performed which yielded ground state splittings of 2265.39(13) cm−1 (NaTe) and 1544.19(11) cm−1 (KTe) as well as A2Σ+ state energies of 7935.98(9) cm−1 (NaTe) and 6404.71(11) cm−1 (KTe) and vibrational constants of the X13/2, X21/2 and A1/2 states of both molecules. The 0-0 bands of both sub-transitions of NaTe were measured at high spectral resolution and rotationally analyzed. 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 NaTe and KTe and also electric dipole transition moments connecting them.

The nonradiative single-electron-capture (SEC) processes for N5+(1s2) colliding with He atoms are investigated by using the quantum-mechanical molecular-orbital close-coupling (QMOCC) method. Total and state-selective SEC cross sections are obtained in the energy range of 3 – 1700 eV/amu, for which the ab initio potential curves and nonadiabatic coupling matrix elements (involving physical and pseudo states) are computed by using the multireference single- and double-excitation configuration-interaction (MRD-CI) method. It is found that the pseudo states are also important to obtain the convergent results, especially in the low energy region. Moreover, our results indicate that the n = 3 states are the dominate states in the considered energy region, and that the electron translation factor effects (ETF effects) become important when the energy is above 1 keV/amu. In addition, it is observed that the total and state-selective cross sections for SEC are in general agreement with all available experimental data.

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.

Wave functions obtained employing a standard complex Hamiltonian matrix diagonalization procedure are square-integrable and therefore cannot accurately describe the asymptotic character of resonance solutions of the Schrodinger equation. The nature of this approximation is investigated by means of explicit calculations which are based on diabatic RKR potentials for the B 1Σ+ — D′ 1Σ+ vibronic resonance states of the CO molecule. It is shown that expanding the basis of complex harmonic oscillator fuctions gradually improves the description of the exact resonance wave functions out to ever larger internuclear distances on the real axis before they take on artificial bound-state characteristics due to the square-integrable character of the basis functions. In order to solve the diagonalization problems for as many as 500 such basis functions, it proves necessary to employ specialized numerical techniques such as Gauss-Hermite quadrature to evaluate the required Hamiltonian matrix elements.

Based on recently improved quantum-chemical data, the lifetimes of the vibrational states of the He(21 S)-Ne weakly bound molecule are calculated. The resulting estimates take into account both bound-bound and bound-free transitions. For the first time, the temperature dependence of the effective lifetime is determined.

To illustrate the role of quantum mechanical electronic structure methods in providing accurate predictions of the cross sections for ionization and electron capture processes that occur as a result of proton collisions in the high-eV to low-keV energy region.Calculations are carried out to compute potential curves and non-adiabatic coupling elements for a variety of collision partners employing the ab initio multireference single- and double-excitation configuration interaction (MRD-CI) method. These results are used as input for the electronic-state close-coupling (ESCC) method in order to compute cross sections for various transitions that occur because of collisions.Emphasis is placed on proton-molecule collisions that play a key role in various medical applications as well as in astrophysics and nano-scale materials manufacturing. Examples discussed include the Na-He pair of atoms of interest in recent astrophysical observations, proton collisions with various hydrocarbons and also with water and oxygen, and key ionic products such as H(2)O(+) and H(3)O(+) .The computed cross sections provide a detailed picture of radiation damage dynamics in Monte Carlo simulations. This information can potentially enhance the efficiency of proton beam focusing and also reduce undesirable side effects on body tissues.

Ab initio multireference configuration interaction calculations including spin–orbit coupling effects have been carried out for four LiX molecules (X = C, Si, Ge and Sn). Potential energy curves of the ground and low-lying excited states have been obtained in each case as well as the corresponding spectroscopic constants. Transition moments have also been computed in order to give estimates of the radiative lifetimes of the excited states for each system. Trends in a variety of quantities such as T e values, spin–orbit splittings, equilibrium bond lengths and vibrational frequencies for this series of molecules are discussed in detail and comparison with the corresponding data reported earlier for the PbLi system is also made.

The calculation of the radiative lifetime for weakly bound He(21 S)–Ne molecules in the lowest vibrational state based on recent improved quantum chemical data is performed for the first time. The obtained value of 13 ns is counterintuitively of the same order as lifetimes for allowed optical transitions. The radiative decay is determined for both the bound-bound and bound-free transitions.

Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) wave functions have been employed to compute the annihilation rates (AR) of positronic molecular complexes of four alkali hydrides. The first step in these calculations is the evaluation of integrals of the two-particle Dirac delta function δ+− over pairs of electronic and positronic basis functions. MRD-CI wave functions calculated with the same basis are then employed to obtain expectation values of the δ+− operator (Zeff), which in turn are proportional to the corresponding annihilation rates (AR) of the associated many-particle states. The importance of removing near-linear dependencies in the basis sets employed is stressed as well as the advisability of placing diffuse (small-exponent) functions in the basis only at the most electronegative center of the molecule. A tendency to underestimate the Zeff values is noted because of the impracticality of including sufficiently high-l basis functions in the basis for general molecular systems. However, comparison with the relatively accurate values for the four-electron e+LiH complex obtained by Quantum Monte-Carlo (QMC) and other methods indicates that the fractional error is nearly constant over a large range of internuclear distance, consistent with the expectation that missing correlation effects in the MRD-CI treatment are predominantly atomic in nature. A scaling procedure based on the asymptotic δ+− value, which is the same for all four alkali hydrides, is then shown to produce good agreement with the QMC AR data for e+LiH. The same procedure has been applied to the δ+− values for the positronic complexes of the heavier alkali hydrides for which no other theoretical results are available. Trends in the variation of the AR results with bond distance are discussed.

Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) calculations have been carried out for magnesium oxide (MgO) and lithium oxide (LiO) and their positronic complexes. These results are compared with previous theoretical data obtained earlier for beryllium oxide (BeO) and the series of alkali hydrides with and without an additional positron. Potential curves have been constructed for each of the systems, MgO, e + MgO, LiO and e + LiO. Positron affinities (PAs) of 0.472 eV and 0.304 eV, respectively, have been computed for the ground states of MgO and LiO. Because of the relatively low ionization potential of the Li atom, it is found that the dissociation limit in the latter case is Li + + PsO (Oe + e − ), whereas it is Mg + e + O in the former case, in close analogy to what has been found for e + BeO. Significant changes in bond lengths, vibrational frequencies and dissociation energies are indicated as a result of binding a positron to each of these oxides as well as to the alkali hydrides studied earlier. The general trend observed is toward increased bond length and decreased frequency as a result of attaching a positron to these systems in their various low-lying electronic states.

K-vacancy Auger states of Nq+ (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.

The ab initio multireference single- and double-excitation configuration interaction (MRD-CI) method has been used to calculate the potential surfaces for the six lowest-lying electronic states of the TeOH molecule. The 2A″ ground state is predicted to have a bent equilibrium geometry. The first excited state, 2A′, is calculated to lie 2695 cm−1 above the ground state. The MORBID program package has been used for the rotation–vibration analysis of the electronic ground state, for which the term values of the fundamental levels are calculated as 582 cm−1 for the Te–O stretching mode, 959 cm−1 for the bending mode, and 3655 cm−1 for the O–H stretching mode.

A theoretical study of the photoinduced C-H bond cleavage in H 2 CO adsorbed on Ag(111) surface is carried out by employing an ab initio embedding approach and multireference configuration interaction method. Pathways for one- and two-step fragmentation into CO + 2H are investigated as well as adsorption properties of the H and HCO products. The π* electron-attachment state, H 2 CO -(a) , formed due to photoinduced electron transfer from the surface to an adsorbate, serves as intermediate for the dissociation process. An effective reaction pathway includes initial nuclear relaxation of the H 2 CO -(a) anion followed by electron detachment and subsequent dissociation of the neutral H 2 CO (a) molecule. The present calculations also show that, in contrast to gaseous H 2 CO, cleavage of the C-H bond in adsorbed H 2 CO needs higher excitation energy than the C-O bond. The corresponding energy difference is obtained to be about 1.6 eV.

The optical potential and semiclassical methods have been applied to the studies of radiative charge transfer in collisions of ground-state and metastable Ne2+ ions with neutral helium. Cross sections are presented with relative collision energies between 0.1 meV and 10 keV. For all transitions concerned, the cross sections display rich resonance structures in the low-energy region. From the calculated cross sections, rate coefficients are obtained with temperatures between 10 and 20 × 106 K and are found to be in agreement with available experimental data. The multireference single- and double-excitation configuration-interaction (MRD-CI) method was used to obtain the relevant adiabatic molecular potentials and dipole transition matrix elements.

Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) calculations have been carried out to study the manner in which the electronic structure of the series of neutral alkali hydride molecules is affected by the interaction of a lone positron. Four systems have been treated: LiH, NaH, KH and RbH. A new computer program has been constructed for this purpose that makes use of the Table-Direct-CI method for molecular calculations within the Born–Oppenheimer approximation. The main emphasis in the present work is on internuclear geometries with relatively large bond distances near the dissociation limit. Molecular charge density contour diagrams are presented to illustrate the bonding mechanism in each case. Trends in a variety of quantities such as positron affinities at both equilibrium molecular and separated atomic geometries, dissociation energies and positronium formation energies are computed and analyzed as the atomic number of the alkali atom is increased.

The positive water and hydronium ions are of interest in a variety of chemical and biological applications. Here we study the steric effect in charge transfer collisions, i.e. the spatial dependence of single electron capture, in collisions mediated by these ions. In particular, the steric effect is demonstrated in the O+(2D)/H2 and H+/H2O charge transfer collisions in the energy range of 100 eV/amu to 10 keV/amu.

Potential energy curves and properties of the low-lying electronic states of tellurium oxide have been computed using a configuration interaction treatment that includes the spin-orbit coupling interaction. Relativistic effective core potentials (RECPs) are used to describe the inner shells of both the Te and O atoms. Good agreement is obtained for the spectroscopic constants of the X1-X2(3)sigma-, a1delta, and b1sigma+ states for which experimental data are available. The ratio of the parallel and perpendicular b-X transition moments, as well as the radiative lifetime of the b state, was computed, and both results were also found to be in good agreement with measurement. The energetic order of the electronic states in TeO appears to be very similar to that observed for the isovalent O2 molecule, but the Rydberg valence-mixing effects that are so prominent in the latter's spectrum (e.g., for the Schumann-Runge bands) are totally absent in TeO.

The Bi2(2-) anions that have been characterized in (K-crypt)2Bi2 are isoelectronic with O2 but are diamagnetic and EPR-silent, unlike O2. The UV-vis spectrum measured for (K-crypt)2Bi2 shows two broad absorption peaks located at 2.05 and 2.85 eV, but no absorption at lower energies down to 0.62 eV. To account for these observations, the electronic structures of the isoelectronic diatomic dianions Q2(2-) (Q = N, P, As, Sb, Bi) were compared on the basis of relativistic density functional theory calculations, and the electronic excitations of Bi2(2-) were analyzed on the basis of relativistic configuration interaction calculations. The extent of spin-orbit coupling, brought about by the relativistic effect, increases steadily in the order NPAsSbBi such that the "closed-shell" state is more stable than the "open-shell" state for Bi2(2-), while the opposite is the case for N2(2-), P2(2-), As2(2-), and Sb2(2-). The nature of the electronic excitations of Bi2(2-) was assigned and discussed from the viewpoint of molecular orbitals in the absence of spin-orbit coupling.

The photodissociation of H(2)Te through excitation in the first absorption band is investigated by means of multireference spin-orbit configuration interaction (CI) calculations. Bending potentials for low-lying electronic states of H(2)Te are obtained in C(2v) symmetry for Te-H distances fixed at the ground state equilibrium value of 3.14a(0), as well as for the minimum energy path constrained to R(1)=R(2). Asymmetric cuts of potential energy surfaces for excited states (at R(1)=3.14a(0) and theta;=90.3 degrees ) are obtained for the first time. It is shown that vibrational structure in the 380-400 nm region of the long wavelength absorption tail is due to transitions to 3A('), which has a shallow minimum at large HTe-H separations. Transitions to this state are polarized in the molecular plane, and this state converges to the excited TeH((2)Pi(1/2))+H((2)S) limit. These theoretical data are in accord with the selectivity toward TeH((2)Pi(1/2)) relative to TeH((2)Pi(3/2)) that has been found experimentally for 355 nm H(2)Te photodissociation. The calculated 3A(')--XA(') transition dipole moment increases rapidly with HTe-H distance; this explains the observation of 3A(') vibrational structure for low vibrational levels, despite unfavorable Franck-Condon factors. According to the calculated vertical energies and transition moment data, the maximum in the first absorption band at approximately 245 nm is caused by excitation to 4A("), which has predominantly 2(1)A(") ((1)B(1) in C(2v) symmetry) character.

A combined high-resolution (6 meV) HeI photoelectron (PE) and ab initio theoretical study of the A 2Σ+ ion system of HI and DI has been conducted to elucidate the origin of a peculiar “holelike” feature in the vibrational distribution found in the high-resolution (6 meV) threshold photoelectron (TPE) spectrum of HI. The PE and TPE spectra were found to yield essentially identical results. Ab initio potential energy curves for the low-lying cationic states of HI have been calculated for the first time with and without spin–orbit contributions included. It has been found that the diabatic A 2Σ+ state of HI+ is strongly predissociated due to spin–orbit coupling with the 4Σ−, 2Σ−, and 4Π repulsive states leading to a complex set of adiabatic curves. It is shown that the adiabatic A 2Σ1/2+ state is only slightly bound (by 260 cm−1 after suitable adjustments of the positions of the various repulsive potentials relative to that of the A state are made based on observed atomic spectral data) and should support at...

A procedure is developed to obtain pseudo-canonical localized molecular orbitals for use in ab initio multireference CI calculations of the electronic spectra of molecules adsorbed on the surfaces of large clusters of metal atoms. The Pt97CO system is employed as a test and potential curves are computed for a large number of its electronic states. The present method is able to accurately describe internally excited states of the adsorbed CO molecule without computing most of the charge-transfer states of lower energy which occur in the same MRD-CI secular equations. It is found that the potentials of the σ–π* states are repulsive along the Pt–C coordinate and hence that their transition energies from the ground state are much higher (11.4–11.6 eV) than for the isolated CO molecule. The corresponding π–π* states are all relatively strongly bound, however, similarly as for the ground state, with transitions to them occurring as much as 1 eV lower than when the Pt97 cluster is absent. This behavior can be traced to the fact that the Pt–C bonding orbital in the ground state has a large amount of CO sigma character, consistent with earlier calculations reported for the Pt7CO model system, whereas neither the π nor the π* MO is strongly affected by the approach of the metal cluster.

A new theoretical result of the total charge transfer cross section in collisions of O+ ion in the ground state with H2 molecule is reported. The collision energy of the incident O+(4S) ion ranges from 1 eV/u to 10 keV/u, covering the available experimental data. States of 4A2 and 4B1 symmetry are included in the calculation. The state-resolved cross sections are obtained by using both the semiclassical and full quantum molecular orbital close coupling method. Based on our results, we propose a possible explanation for the large discrepancies in recent measurements among various experimental groups.

The interaction of ethylene with a single proton has been studied theoretically by means of multireference configuration interaction calculations. Potential energy surfaces have been obtained as well as both radial and rotational nonadiabatic coupling matrix elements. Three different approaches of the proton toward the midpoint of the target molecule have been considered for eight collision channels. Along the π direction a number of deep potential minima have been found and the ground-state proton affinity is calculated to be 7.25 eV on this basis. There is a strongly avoided crossing in this case between the higher-energy C2H4/H+ incoming channel and the ground C2H/H singlet state, and the corresponding radial coupling matrix element has a broad shape as a result. For the approaches along the in-plane perpendicular axes of the molecule there is a potential crossing for these two states due to their different symmetries in these orientations, from which it is clear that the incoming channel is strongly bound whereas the charge-transfer state is repulsive. Comparison with available experimental results indicates that the computed energy differences between the electronic states are accurate to within 0.1–0.2 eV. For the approach along the CC bond there is a weakly avoided crossing between the lowest triplet state of C2H4 and the C2H state corresponding to ionization from the σ molecular orbital (MO). One of the potential minima occurring for the π approach is seen to result from a doubly excited state in which the two π electrons are transferred to a π* MO and the incoming proton, respectively, so that a relatively stable three-center bond is formed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

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.

Ab initio configuration interaction calculations have been carried out on the potential energy surfaces of the ground and the doublet excited electronic states of CaOH. The calculated transition energies are in good agreement with previous theoretical values and with the available experimental data. The calculated Ca–O stretching potentials at different values of the bond angle show typical Rydberg minima at short internuclear distances but are complicated for large RCa–O. The calculated bending potentials are rather shallow for deviations of 5°–10° from linearity and, depending on the values of the Ca–O and O–H bond lengths, off-linear geometries have low energies. There is no evidence for the existence of an electronic state with bent equilibrium geometry, which would correspond to the reported state correlating with F 2Π of CaOH. The 7 2A′ state (which does correlate with F 2Π) is found to be quasi-linear with a barrier to linearity of only 92 cm−1, and similarly the 4 2A″ state (correlating with C ...

Relativistic core-potential calculations have been carried out on Ω states resulting from the interaction of Xe* (5p56s, 3P, 1P) with ground-state Kr atoms as well as for the system Ar* (3p54s, 3P, 1P) with ground-state Ne, using different basis sets and configuration interaction procedures. The present calculations on ArNe, employing larger sets of Rydberg functions than those of the previous calculations, yield totally repulsive potentials for the excited states of ArNe. Similar calculations on XeKr obtain shallow minima (600–860 cm−1) in the potential energy curves of the excited states at large internuclear distances (6.9–7.8 bohr). Dipole transition moments have been calculated and strong radiative transitions are predicted from excited states to the ground state. The 1(I) state, correlating with the metastable 3P2 state of Xe is found to have a small but nonzero dipole transition moment at short and intermediate nuclear distances leading to a radiative lifetime for the v=0 level of this state of 21....

Multireference single- and double-excitation configuration interaction (MRD-CI) calculations of transition energies for the Hg atom and spectroscopic constants for the HgH molecule are carried out with the generalized relativistic effective core potential (GRECP) method. A new selection criterium for the reference configurations is discussed. The calculated spectroscopic constants are compared with experimental data and results of calculations of other groups. Improvement of accuracy is mainly observed for bond lengths from the GRECP/MRD-CI calculations (without applying the T = 0 correction) with respect to the results of other groups. Analysis of the quality of the approximations employed is carried out. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

The electronic structure of the lead monofluoride molecule is studied by means of ab initio configuration interaction (CI) calculations including the spin-orbit interaction. Potential-energy curves are generated for a large number of electronic states, of which only the X1 2Π1/2 ground and X2 2Π3/2 and A 2Σ+ excited states have been observed experimentally. Two different methods are compared for the inclusion of spin-orbit effects in the theoretical treatment, a contracted CI which employs a basis of large-scale Λ–S eigenfunctions to form a rather small matrix representation of the full relativistic Hamiltonian (two-step approach), and a more computationally laborious technique which involves solution of a secular equation of order 250 000 S2 eigenfunctions of different spin and spatial symmetry to achieve a potentially more evenly balanced description of both relativistic and electron correlation effects (one-step approach). In the present application, it is found that both methods achieve quite good agr...

The goal of a full configuration interaction (CI) calculation in a flexible atomic orbital basis can be approached rather closely for small molecular systems by designating a set of reference configurations from which only single- and double-orbital substitutions are allowed to generate the many-electron basis. The present review discusses how configuration-driven algorithms can be applied to form the corresponding Hamiltonian matrix representation from which approximate energy eigenvalues and eigenvectors can be computed. Tables are constructed which simplify the analysis of configuration relationships that determine how a given interaction is calculated. This Table-CI procedure eliminates the need for carrying out lineup permutations of spin orbitals, which was one of the major bottlenecks in early implementations of the multireference CI method. Although this approach is fundamentally variational in character, it can be easily combined with perturbative techniques that allow one to approximate the corresponding full CI energies to a high degree of accuracy. The speed with which Hamiltonian matrix elements can be calculated makes it feasible to use the Direct CI approach which requires that these values must be recalculated in each iteration of the diagonalization procedure, thereby achieving a considerable increase in the overall efficiency of the computations. The effects of spin–orbit coupling and other terms in the relativistic Hamiltonian have been incorporated in a straightforward manner through the use of effective core potentials.

Relativistic core-potential calculations have been carried out on the ground and excited Ω states of ArHe and HeNe correlating with the atomic limits Ar*(3p54s, 3P,1P) and Ne*(2p53s, 3P,1P), respectively, and with ground-state He atoms. The potential energy curves of the excited states of ArHe are repulsive. The potential energy curves of the 0−(I), 1(I) and 0+(II)2pσ→3s states of HeNe show a local Rydberg minimum at 2.8 bohr and a barrier at 4.0 bohr, while the other excited state potentials of HeNe are repulsive. For both ArHe and HeNe, large dipole transition moments are calculated connecting the ground state with states dissociating to atomic limits from which radiative transitions to the atomic ground state are allowed. Small but nonzero dipole transition moments are obtained for short internuclear distances for the transitions from the 1(I) state, which correlates with the metastable 3P2 state of Ar (in ArHe) or Ne (in HeNe). The radiative lifetime of the 1(I) state of HeNe (estimated from the verti...

Ab initio multireference single- and double-excitation configuration interaction calculations have been carried out for potential curves of the B 3Σu−, 1Πu, 3Πu, 5Πu, and 3Σu+ electronic states of the O2 molecule. Spin-orbit interaction matrix elements among the above states and rotational interaction matrix elements between the B 3Σu− and the 3Πu states have been evaluated in order to investigate the observed line broadening in the Schumann–Runge bands. Predissociation linewidths of rovibrational levels of the B 3Σu− state are determined by using the complex rotation method in conjunction with the Gauss–Hermite quadrature procedure. It has been found that the 5Πu and 3Πu states play dominant roles in the predissociation of the B 3Σu− state. The calculated linewidths for rovibrational levels of v=0–13, N=0–36 of the F2 fine-structure components of the B 3Σu− state are in very good agreement with experimental results. Isotopic effects in the predissociation of the B 3Σu− state are studied by means of calcu...

Ab initio multireference configuration interaction calculations including spin–orbit coupling are carried out for valence electronic states of the TeF molecule and compared to the results for the isovalent IO system obtained earlier at a similar level of theoretical treatment [S. Roszak et al., J. Phys. Chem. A 104, 2999 (2000)]. The calculated spectroscopic constants are in good agreement with available experimental data. It is shown that the X 2Π(σ2π4π*3) ground state is much more strongly bound in TeF (calc. De=25 480 cm−1) due to the greater ionic character of bonding in this system as compared to IO. The lowest excited states of TeF are found to be A 4Σ1/2,3/2− and B 2Σ1/2− which result from the π→σ* electronic excitation. In contrast to IO, the 2Π(σ2π3π*4) excited state has a repulsive potential curve and is not expected to be a factor in the low-energy spectroscopy of TeF. Particular emphasis is placed on computation of the transition moments and radiative lifetimes of the TeF electronic states. Mo...

Excited electronic states for CO adsorbed on Pt(1 1 1) are investigated using a seven-atom monolayer model of the surface in which CO is adsorbed atop a Pt atom. Configuration interaction theory is used to resolve the electronic states and to distinguish states involving the adsorbate from those corresponding to metal–metal excitations. A large number of excited electronic states occur in the range 3–9 eV above the ground state. Potential curves for singlet and triplet states are reported. Many of the states are charge-transfer states arising from Pt 6s and 5d transitions to CO π* levels. Although the CO σ to π* transitions for an isolated CO molecule fall in the 6–9 eV range, the CO σ orbital is stabilized to such an extent on adsorption that excitations from it lie very high in energy. It is concluded that the broad band observed in HREELS experiments is due to the charge-transfer excitations.

Nonadiabatic calculations of vibrational energies and wave functions are carried out for the E(0+, 3P2) and f(0+, 3P0) ion-pair states of the ClF molecule. It is shown that strong radial coupling between these 0+ states is caused by a significant variation of their 3Σ− and 3Π Λ-S contributions with internuclear distance and results in vibrational energy shifts as well as changes in the corresponding adiabatic vibrational wave functions. Both resonance and nonresonance interactions between vibronic levels of these two adiabatic states are found to be important, but significant mixing of the adiabatic wave functions can occur only for the nearly resonant levels located around f,v=3; E,v=7 and f,v=8; E,v=13. Nonadiabatic interactions are found to be responsible for the appearance of long-wavelength maxima in the f,v=3,4 emission spectra that was the subject of the discrepancy between theoretical and experimental data discussed in the previous paper [A. B. Alekseyev, H.-P. Liebermann, R. J. Buenker, and D. B....

Gas phase emission spectra of the hitherto unknown free radical PbLi have been measured in the NIR range with a Fouriertransform spectrometer. The emissions were observed from a fast flow system in which lead vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. Five electronic transitions have been found in the wavenumber range 3800–10 000 cm � 1 . Bands from two excited states to the ground state were measured at high spectral resolution such that rotational analyses could be performed and accurate molecular parameters derived. 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 PbLi and also electric dipole transition moments connecting them. As in the lighter molecules of this group, CLi and SiLi, the ground state of PbLi is found to be X 4 R � ðX1 1=2; X2 3/2) with a spin splitting of about 2000 cm � 1 . The first excited state is A 2 P1=2 (A 1/2), and two observed band systems are assigned to the transitions A ! X1 and A ! X2. Two more excited states, B 2 P3=2 (B 3/2) and C 1/2, are identified from the observed spectra with the help of the computed data, and their spectroscopic constants are determined. In contrast to PbH and PbF, the ab initio results indicate a very complicated low-energy electronic structure for the PbLi radical, with 19 bound electronic states calculated to lie below 3 eV. 2002 Elsevier Science (USA). All rights reserved.

An ab initio configuration interaction (CI) study including spin-orbit coupling is carried out for the ground and low-lying excited states of the HI molecule by employing a relativistic effective core potential for the iodine atom. The computed spectroscopic constants for the X 1Σ+ ground and b 3ΠΩ Rydberg states are in good agreement with available experimental data, as are the vertical excitation energies for the repulsive a 3Π1, a 3Π0+, and A 1Π1 states of the A band. The a 3Π0+ state is found to possess a shallow minimum of 600 cm−1 depth outside the Franck–Condon region, at ≈5.1 a0. The electric-dipole moments have also been calculated for transitions from the ground to the A band states. Contrary to what is usually assumed, the a 3Π1, A 1Π1←X0+ transition moments are found to depend strongly on internuclear distance. Employing the computed potential energy and transition moment data, partial and total absorption spectra for the A band are calculated and the I* quantum yields, ΦI*(ν), are determined ...

Ab initio configuration interaction calculations of the electronic binding energies, spin–orbit coupling matrix elements and transition dipole moments of NaI are presented. The results are used to construct adiabatic and diabatic representations of the 0+ molecular states relevant to predissociation. The dynamics of photopredissociation is elucidated by multichannel time-dependent wave packet propagation in the diabatic representation. Specific features associated with the spatial and temporal evolution of the wave packet are ascribed to those observed in femtosecond pump–probe experiments. In particular, the rate of decay of the electronically excited NaI* complex is found to be in close agreement with time-resolved experimental studies. Partial photoabsorption cross sections for the production of iodine atoms in the ground (2P3/2) and excited (2P1/2) spin–orbit states are calculated and found to peak at excitation wavelengths of 322 and 263 nm, respectively, in accord with experimental data.

The electronic spectrum of carbon dioxide is predicted by means of multireference single- and double-excitation configuration interaction (MRD–CI) calculations. Bending potential curves for a large number of electronic states are calculated and the generalized oscillator strengths (GOS) obtained for the corresponding transitions are compared with recent electron energy loss spectra (EELS) obtained by Tanaka and co-workers as well as with optical intensity data. The CO2 absorption spectrum is dominated by vertical Rydberg transitions from the linear ground state but valence excited states with bent equilibrium geometries are also found to be important. Emphasis is placed on the intense spectral features in the 11.0–11.4 eV region of the spectrum. The two most intense peaks are both found to originate from π–3pπ transitions, one of which only becomes allowed upon molecular bending. Good agreement with the EELS results is obtained for the variation of the GOS with K2 and for the associated transition energies.

An ab initio configuration interaction (CI) study including spin−orbit coupling is carried out for the ground and excited states of the IO radical by employing relativistic effective core potentials. The computed spectroscopic constants are in good agreement with available experimental data, with some tendency to underestimate the strength of bonding. The first excited state, a4Σ-, which has not yet been observed experimentally, is predicted to be bound by 30.1 kJ/mol and to have a significantly larger equilibrium distance than the ground state. It is split by spin−orbit interaction into 1/2 and 3/2 components, with the 1/2 component being the lower one with a calculated spin−orbit splitting of 210 cm-1. The most interesting state in the low-energy IO spectrum, A12Π3/2, is shown to be predissociated due to interaction with a number of repulsive electronic states. Predissociation of the A1, v‘ = 0, 1 vibrational levels is attributed to a fairly weak spin−orbit coupling with the 2Δ3/2 state, while rotationa...