Multireference configuration interaction study of the electronic states of ZrC

The potential energy curves and spectroscopic constants of the ground and 32 low-lying electronic states of ZrC have been studied by employing multireference configuration interaction methods, in conjunction with relativistic effective core potentials and 5s3p3d1f, 3s3p1d basis sets con Zr and C, respectively. We have determined that the ground state is (3)Sigma(+). However there are two low-lying (1)Sigma(+) states (below 5000 cm(-1)) which strongly interact resulting in avoided crossings. The lowest (1)Sigma(+) state corresponds to a combination of 1sigma(2) Xsigma(2) 1pi(4) configurations whereas the second is an open shell singlet 1sigma(2) 2sigma(1) 3sigma(1) 1pi(4). Several avoided crossings were observed, for (1)Pi, (3)Pi, (1)Delta, (3)Sigma(+), and (3)Delta states. We have identified (3)Pi and (1)Pi lying at 4367 and 5797 cm(-1), respectively. The results are in good agreement with the recent experimental findings of Rixon et al. [J. Mol. Spectrosc. 228, 554 (2004)], and indicate that the (3)Pi-(3)Sigma(+), and (1)Pi-(1)Sigma(+), bands located between 16 000-19 000 cm(-1) are extremely complex due to near degeneracy of several (1)Pi and (3)Pi states. We also have identified a (1)Sigma(+) state in the same region that may interfere with the (1)Pi emission bands. The present results not only shed further light into the spectra of ZrC but also predict yet to be observed systems.     

A theoretical study of the excited states of CrH: potential energies, transition moments, and lifetimes

Ab initio calculations of low-lying electronic states of CrH are presented, including potential energies, dipole and transition dipole moment (TDM) functions, and radiative lifetimes for X (6)Sigma(+), A (6)Sigma(+), 3 (6)Sigma(+), 1 (6)Pi, 2 (6)Pi, 3 (6)Pi, and (6)Delta. Calculation of dynamic correlation effects was performed using the multistate complete active space second-order perturbation method, based on state-averaged complete active space self-consistent-field reference wave functions obtained with seven active electrons in an active space of 16 molecular orbitals. A relativistic atomic natural orbital-type basis set from the MOLCAS library was used for Cr. Good agreement is found between the current calculations and experiment for the lowest two (6)Sigma(+) states, the only states for which spectroscopic data are available. Potential curves for the 3 (6)Sigma(+) and 2 (6)Pi states are complicated by avoided crossings with higher states of the same symmetry, thus resulting in double-well structures for these two states. The measured bandhead T(0)=27 181 cm(-1), previously assigned to a (6)Pi<--X (6)Sigma(+) transition, is close to our value of T(0)=28 434 cm(-1) for the 2 (6)Pi state. We tentatively assign the ultraviolet band found experimentally at 30 386 cm(-1) to the 3 (6)Pi<--X (6)Sigma(+) transition for which the computed value is 29 660 cm(-1). The A (6)Sigma(+)<--X (6)Sigma(+) TDM and A (6)Sigma(+) lifetimes are found to be in reasonable agreement with previous calculations.     

Theoretical investigation of RbCs via two-component spin-orbit pseudopotentials: spectroscopic constants and permanent dipole moment functions

Potential energy curves for the 28 lowest LambdaSigma states and 49 Omega states of RbCs are obtained from large-scale multireference configuration interaction calculations using both spin-averaged and two-component spin-orbit energy-consistent effective core potentials. Spectroscopic properties of all states are compared across available data in literature to date. Variations of the permanent dipole moments on the internuclear separation (R) for the (1)Sigma(+), (3)Sigma(+), (1)Pi, and (3)Pi states are evaluated over a wide range of R. The most important effects of the spin-orbit interaction on the dipole moment distribution are discussed.     

High resolution vacuum ultraviolet emission spectrum of D(2) from 78 to 103 nm: The D (1)Pi(u)-->X (1)Sigma(g) (+) and D(') (1)Pi(u) (-)-->X (1)Sigma(g) (+) band systems

The emission spectrum of the D(2) molecule has been studied at high resolution in the vacuum ultraviolet region 78.5-102.7 nm. A detailed analysis of the two D (1)Pi(u)-->X (1)Sigma(g) (+) and D(') (1)Pi(u) (-)-->X (1)Sigma(g) (+) electronic band systems is reported. New and improved values of the level energies of the two upper states have been derived with the help of the program IDEN [V. I. Azarov, Phys. Scr. 44, 528 (1991); 48, 656 (1993)], originally developed for atomic spectral analysis. A detailed comparison is made between the observed energy levels and solutions of coupled equations using the newest ab initio potentials by Wolniewicz and co-workers [J. Chem. Phys. 103, 1792 (1995); 99, 1851 (1993); J. Mol. Spectros. 212, 208 (2002); 220, 45 (2003)] taking into account the nonadiabatic coupling terms for the D (1)Pi(u) state with the lowest electronic states B (1)Sigma(u) (+), C (1)Pi(u), and B(') (1)Sigma(u) (+). A satisfactory agreement has been found for most of the level energies belonging to the D and D(') states. The remaining differences between observation and theory are probably due to nonadiabatic couplings with other higher electronic states which were neglected in the calculations.     

Dissociation of the OCS+ ion in low-lying electronic states studied using multiconfiguration second-order perturbation theory

Complete active space self-consistent-field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) calculations with atomic natural orbital basis sets were performed to investigate the S-loss direct dissociation of the 1 2Pi(X 2Pi), 2 2Pi(A 2Pi), 1 2Sigma+(B 2Sigma+), 1 4Sigma-, 1 2Sigma-, and 1 2Delta states of the OCS+ ion and the predissociations of the 1 2Pi, 2 2Pi, and 1 2Sigma+ states. Our calculations indicate that the S-loss dissociation products of the OCS(+) ion in the six states are the ground-state CO molecule plus the S+ ion in different electronic states. The CASPT2//CASSCF potential energy curves were calculated for the S-loss dissociation from the six states. The calculations indicate that the dissociation of the 1 4Sigma- state leads to the CO + S+ (4Su) products representing the first dissociation limit; the dissociations of the 1 2Pi, 1 2Sigma-, and 1 2Delta states lead to the CO + S+(2Du) products representing the second dissociation limit; and the dissociations of the 2 2Pi and 1 2Sigma+ states lead to the CO + S+(2Pu) products representing the third dissociation limit. Seams of the 1 2Pi-1 4Sigma-, 2 2Pi-1 4Sigma-, 2 2Pi-1 2Sigma-, 2 2Pi-1 2Delta, and 1 2Sigma(+)-1 4Sigma- potential energy surface intersections were calculated at the CASPT2 level, and the minima along the seams were located. The calculations indicate that within the experimental energy range (15.07-16.0 eV) the 2 2Pi(A 2Pi) state can be predissociated by 1 4Sigma- forming the S+(4Su) ion and can undergo internal conversion to 1 2Pi followed by the direct dissociation of 1 2Pi forming S+(2Du) and that within the experimental energy range (16.04-16.54 eV) the 1 2Sigma+(B 2Sigma+) state can be predissociated by 1 4Sigma- forming the S+(4Su) ion and can undergo internal conversion to 2 2Pi followed by the predissociation of 2 2Pi by 1 2Sigma- and 1 2Delta forming the S+(2Du) ion. These indications are in line with the experimental fact that both the 4Su and 2Du states of the S+ ion can be formed from the 2 2Pi and 1 2Sigma+ states of the OCS+ ion.     

Ab initio study of KN

The potential energy curves for the lowest (3)Sigma(-), (3)Pi, and (5)Sigma(-) states of the KN molecule have been calculated by the multireference singles and doubles configuration interaction method, including Davidson's corrections for quadruple excitations [MRCI(+Q)]. It is shown that the former two are bound, while the last one is repulsive. The electronic ground state of KN is predicted as (3)Sigma(-) state, although the term energy of the (3)Pi state is very small, 177.3 cm(-1). The binding energy for the (3)Sigma(-) state is evaluated as 0.838 eV, the rotational constant B(0) as 0.250 63 cm(-1), and harmonic frequency as 324.4 cm(-1). The spin-orbit coupling effects between the (3)Sigma(-) and (3)Pi states of KN are evaluated and discussed. The same MRCI(+Q) computational procedures are applied to the isovalent LiN, KC, KO, and KCl to confirm the accuracy of present calculations. Theoretical spectroscopic constants presented here will inspire experimental studies of KN.     

Ab initio investigation of the electronic structure and bonding of BH, BH(-), and HBBH molecules

By correlating all electrons and employing core-tuned correlation consistent basis sets of quintuple-zeta quality, we applied multireference and coupled-cluster methods to study 32 electronic states of the diatomic BH molecule, two bound states of BH(-), and three states of the linear HBBH molecule. We have constructed full potential energy curves and profiles, reporting binding energies, geometries, spectroscopic parameters, dipole moments, and energy separations, whereas our numerical results are in excellent agreement with available experimental numbers. We are trying as well to interpret the binding modes of a large number of the examined states. 18 states of BH are of Rydberg character, with the BH(-) anion revealing similar structural characteristics to the isoelectronic CH species. The first three states of HBBH X 3Sigma g (-), a 1Delta g, and b 1Sigma g + diabatically correlate to two a 3Pi BH fragments, they are similar to the states b 3Sigma g (-), B 1Delta g, and B' 1Sigma g + of the isoelectronic molecule C2, however, their ordering follows that of the first three states of the O2 molecule.     

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

An ab initio study of the ground and valence excited states of GaF

Ab initio calculations on the ground and valence excited states of the GaF molecule have been performed by using the internally contracted multireference electronic correlation methods (MR-CISD, MR-CISD + Q, and MR-AQCC) with entirely uncontracted all-electronic basis sets and Douglas-Kroll scalar relativistic correction. The potential energy curves of all valence states and the spectroscopic constants of bound states are fitted. It is the first time that the 12 valence Lambda-S states of GaF molecule and all of the 23 Omega states generated from the former are studied in a theoretical way. Calculation results well reproduce most of the experimental data. The effects of the size-extensivity correction and the avoided crossing rule between Omega states of the same symmetry are analyzed. The transition properties of the A 3Pi0+, B 3Pi1, C 1Pi1, and 3Sigma1+ states are predicted, including the transition dipole moments, the Franck-Condon factors and the radiative lifetimes. The radiative lifetime of the C 1Pi1 state of GaF molecule is of the order of nanosecond, implying that it is a rather short-live state. The lifetimes of the B 3Pi1 and 3Sigma1+ states are of the order of microsecond, while the lifetime of the A 3Pi0+ state are the order of millisecond.     

Vacuum ultraviolet photoionization of C3

Photoionization efficiency (PIE) curves for C(3) molecules produced by laser ablation are measured from 11.0 to 13.5 eV with tunable vacuum ultraviolet undulator radiation. A step in the PIE curve versus photon energy, obtained with N(2) as the carrier gas, supports the conclusion of very effective cooling of C(3) to its linear (1)Sigma(g)(+) ground state. The second step observed in the PIE curve versus photon energy could be the first experimental evidence of the C(3)(+)((2)Sigma(g)(+)) excited state. The experimental results, complemented by ab initio calculations, suggest a state-to-state vertical ionization energy of 11.70 +/- 0.05 eV between the C(3)(X(1)Sigma(g)(+)) and the C(3)(+)(X(2)Sigma(u)(+)) states. An ionization energy of 11.61 +/- 0.07 eV between the neutral and ionic ground states of C(3) is deduced using the data together with our calculations. Accurate ab initio calculations are performed for both linear and bent geometries on the lowest doublet electronic states of C(3)(+) using Configuration Interaction (CI) approaches and large basis sets. These calculations confirm that C(3)(+) is bent in its electronic ground state, which is separated by a small potential barrier from the (2)Sigma(u)(+) minimum. The gradual increase at the onset of the PIE curve suggests a geometry change between the ground neutral and cationic states. The energies between several doublet states of the ion are theoretically determined to be 0.81, 1.49, and 1.98 eV between the (2)Sigma(u)(+) and the (2)Sigma(g)(+),( 2)Pi(u), (2)Pi(g) excited states of C(3)(+), respectively.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.     

Theoretical investigation of the ground and low-lying excited states of nickel carbide, NiC

The electronic structure and bonding of 19 states of the diatomic nickel carbide (NiC) has been studied by multireference methods. Potential energy curves have been constructed for all states, whereas for the three lowest states of symmetries X (1)Sigma(+), a (3)Pi, and A (1)Pi well separated from the rest of the states, special attention was paid through the use of very large basis sets and the calculation of core-valence correlation and scalar relativistic effects. The recommended binding energies for these states are 91, 67, and 54 kcal/mol with respect to the ground state atoms. Our results in general can be considered in fair agreement with the limited experimental findings.     

Ab initio study of the low lying electronic states of ZnF and ZnF-

Highly correlated ab initio calculations have been performed for an accurate determination of the electronic structure and of the spectroscopy of the low lying electronic states of the ZnF system. Using effective core pseudopotentials and aug-cc-pVQZ basis sets for both atoms, the potential curves, the dipole moment functions, and the transition dipole moments between relevant electronic states have been calculated at the multireference-configuration-interaction level. The spectroscopic constants calculated for the X(2)Sigma(+) ground state are in good agreement with the most recent theoretical and experimental values. It is shown that, besides the X(2)Sigma(+) ground state, the B(2)Sigma(+), the C(2)Pi, and the D(2)Sigma(+) states are bound. The A(2)Pi state, which has been mentioned in previous works, is not bound but its potential presents a shoulder in the Franck-Condon region of the X(2)Sigma(+) ground state. All of the low lying quartet states are found to be repulsive. The absorption transitions from the v=0 level of the X(2)Sigma(+) ground state toward the three bound states have been evaluated and the spectra are presented. The potential energy of the ZnF(-) molecular anion has been determined in the vicinity of its equilibrium geometry and the electronic affinity of ZnF (EA=1.843 eV with the zero energy point correction) has been calculated in agreement with the photoelectron spectroscopy experiments.     

Electronic states and spectroscopic properties of SiTe and SiTe+

Ab initio based configuration interaction calculations have been carried out to study the low-lying electronic states and spectroscopic properties of the heaviest nonradioactive silicon chalcogenide molecule and its monopositive ion. Spectroscopic constants and potential energy curves of states of both SiTe and SiTe+ within 5 eV are reported. The calculated dissociation energies of SiTe and SiTe+ are 4.41 and 3.52 eV, respectively. Effects of the spin-orbit coupling on the electronic spectrum of both the species are studied in detail. The spin-orbit splitting between the two components of the ground state of SiTe+ is estimated to be 1880 cm(-1). Transitions such as 0+ (II)-X1Sigma(+)0+, 0+ (III)-X1Sigma(+)0+, E1Sigma(+)0+ -X1Sigma(+)0+, and A1Pi1-X1Sigma(+)0+ are predicted to be strong in SiTe. The radiative lifetime of the A1Pi state is less than a microsecond. The X(2)2Pi(1/2)-X(1)2Pi(3/2) transition in SiTe+ is allowed due to spin-orbit mixing. However, it is weak in intensity with a partial lifetime for the X2 state of about 108 ms. The electric dipole moments of both SiTe and SiTe+ in their low-lying states are calculated. The vertical ionization energies for the ionization of the ground-state SiTe to different ionic states are also reported.     

Observation and calculation of the Cs2 2 3Delta(1g) and b 3Pi(0u) states

The Cs(2) 2 (3)Delta(1g) and b (3)Pi(0u) states have been observed by infrared-infrared double resonance spectroscopy for the first time. 221 2 (3)Delta(1g)<--A (1)Sigma(u) (+)<--X (1)Sigma(g) (+) double resonance lines have been assigned to transitions into the 2 (3)Delta(1g) v=6-13 vibrational levels. Resolved fluorescence into the b (3)Pi(0u) v(')=0-48 levels has been recorded. Molecular constants and potential energy curves are determined by the global fit of the entire set of the experimental data. Theoretical potential energy curves of the 2 (3)Delta(g) and b (3)Pi(u) states have been determined in the framework of the pseudopotential method and are compared with the experimental results.     

Dissociation potential curves of low-lying states in transition metal hydrides. 3. Hydrides of groups 6 and 7

The dissociation curves of low-lying spin-mixed states in monohydrides of groups 6 and 7 were calculated by using an effective core potential (ECP) approach. This approach is based on the multiconfiguration self-consistent field (MCSCF) method, followed by first-order configuration interaction (FOCI) calculations, in which the method employs an ECP basis set proposed by Stevens and co-workers (SBKJC) augmented by a set of polarization functions. Spin-orbit coupling (SOC) effects are estimated within the one-electron approximation by using effective nuclear charges, since SOC splittings obtained with the full Breit-Pauli Hamitonian are underestimated when ECP basis sets are used. The ground states of group 6 hydrides have Omega = (1)/(2)(X(6)Sigma(+)(1/2)), where Omega is the z component of the total angular momentum quantum number. Although the ground states of group 7 hydrides have Omega = 0(+), their main adiabatic components are different; the ground state in MnH originates from the lowest (7)Sigma(+), while in TcH and ReH the main component of the ground state is the lowest (5)Sigma(+). The present paper reports a comprehensive set of theoretical results including the dissociation energies, equilibrium distances, electronic transition energies, harmonic frequencies, anharmonicities, and rotational constants for several low-lying spin-mixed states in these hydrides. Transition dipole moments were also computed among the spin-mixed states and large peak positions of electronic transitions are suggested theoretically for these hydrides. The periodic trends of physical properties of metal hydrides are discussed, based on the results reported in this and other recent studies.     

Interactions of transition metal atoms in high-spin states: Cr2, Sc-Cr, and Sc-Kr

The high-spin van der Waals states are examined for the following dimers: Cr(2) ((13)Sigma(g)(+)), Sc-Cr ((8)Sigma(+), (8)Pi, (8)Delta), and Sc-Kr ((2)Sigma(+), (2)Pi, (2)Delta). These three systems offer a wide range of van der Waals interactions: anomalously strong, intermediate, and typically weak. The single-reference [coupled cluster with single, double, and noniterative triple excitations, RCCSD(T)] method is used in the calculations for all three systems. In addition, a range of configuration-interaction based methods is applied in Cr(2) and Sc-Cr. The three dimers are shown to be bound by the dispersion interaction of varying strength. In a related effort, the dispersion energy and its exchange counterpart are calculated using the newly developed open-shell variant of the symmetry-adapted perturbation theory (SAPT). The restricted open-shell time-dependent Hartree-Fock linear response function is used in the calculations of the dispersion energy in Sc-Cr and Sc-Kr calculations, while the restricted open-shell time-dependent density functional linear response function is used for Cr(2). A hybrid method combining the repulsive restricted open-shell Hartree-Fock (or complete active space self-consistent field) interaction energy with the dispersion and exchange-dispersion terms is tested against the RCCSD(T) results for the three complexes. The Cr(2) ((13)Sigma(g)(+)) complex has the well depth of 807.8 cm(-1) at the equilibrium distance of 6.18a(0) and the dissociation energy of 776.8 cm(-1). The octet-state Sc-Cr is about four times more strongly bound with the order of well depths of (8)Delta>(8)Pi>(8)Sigma(+) and a considerable anisotropy. The enhanced bonding is attributed to the unusually strong dispersion interaction. Sc-Kr ((2)Sigma(+), (2)Pi, (2)Delta) is a typical van der Waals dimer with well depths in the range of 81 cm(-1) ((2)Delta), 84 cm(-1) ((2)Sigma(+)), and 86 cm(-1) ((2)Pi). The hybrid model based on SAPT leads to results which are in excellent qualitative agreement with RCCSD(T) for all three interactions.     

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.     

Theoretical study of the ArH+ photodissociation

The multireference Spin-Orbit (SO) Configuration Interaction (CI) method in its Lambda-S Contracted SO-CI (LSC-SO-CI) version is employed to calculate potential energy curves for the ground and low-lying excited states of the ArH(+) cation. For the first time, electric dipole moments are also computed in the approach, including SO coupling for transitions to the states responsible for the first absorption continuum (A-band) of ArH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that absorption in the A-band is dominated by the parallel A(1)Sigma(+)<--X(1)Sigma(+) transition. In the low-energy part of the band (<95 x 10(3) cm(-1)) the absorption is caused by the perpendicular B(1)Pi<--X(1)Sigma(+) excitation, but transitions to the b(3)Pi(0(+),1) states are also not negligible. The branching ratio Gamma for the final photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or= 10(5) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Ar(+)((2)P(1/2)) ions, and thus leads to the inverse population of the Ar(+)((2)P(1/2)-(2)P(3/2)) ion states.