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 calculation of (2+1) resonance enhanced multiphoton ionization spectra and lifetimes of the (D,3)2Sigma- states of OH and OD

High-level ab initio potential-energy curves and transition dipole moments for the OH X 2Pi, 2 2Pi, 1 2Sigma-, D 2Sigma-, 3 2Sigma-, A 2Sigma+, B 2Sigma+, 1 2Delta, 1 4Sigma-, and 1 4Pi states are computed. The results are used to estimate the (2+1) resonance enhanced multiphoton ionization spectrum for the (D,3)2Sigma-(upsilon')<--2hnuX 2Piupsilon") transitions, which are compared with experiments by Greenslade et al. [see M. E. Greenslade, M. I. Lester, D. C. Radenovic, J. A. van Roij, and D. H. Parker, J. Chem. Phys. 123, 074309 (2005), preceeding paper]. We use the discrete variable representation-absorbing boundary condition method to incorporate the effect of the dissociative intermediate 1 2Sigma- state. We obtain qualitative agreement with experiment for the line strengths. Radiative and predissociative decay rates of the Rydberg (D,3)2Sigma- states of OH and OD were computed, including spin-orbit coupling effects and the effect of spin-electronic and gyroscopic coupling. We show that the lifetime of the Rydberg 2Sigma- states for rotationally cold molecules is limited mainly by predissociation caused by spin-orbit coupling.     

Fluorescence and metastability of N2O2+: theory and experiment

State-selective mass spectrometry has revealed one conclusive and another probable metastable state of the N2O2+ dication, assigned respectively as 1 3Pi at 38.5 eV and 2 3Pi at 42.5 eV. Photon coincidence experiments confirm that dissociation of 1 3Pi is preceded by a fluorescent transition to X 3Sigma- and also indicate that an identical mechanism occurs for 2 3Pi. Highly correlated MRCI calculations are performed at a range of N2O2+ geometries, from which both N-N and N-O bond stretching curves are generated. Substantial barriers along both coordinates are observed for 1 3Pi and 2 3Pi, although the increasing density of states at higher energy may allow spin-orbit or vibronic predissociation for 2 3Pi. Fragment emissions derived from N2O+ and N2O2+ are analyzed with the aid of glass filters, from which NO (X 2Pi<--A 2Sigma+) and vibrationally excited N2+ (X 2Sigmag+<--B 2Sigmau+) transitions are deduced.     

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.     

Spectroscopic constants and potential energy curves of yttrium carbide (YC)

The potential energy curves of the low-lying electronic states of yttrium carbide (YC) and its cation are calculated at the complete active space self-consistent field and the multireference single and double excitation configuration interaction (MRSDCI) levels of theory. Fifteen low-lying electronic states of YC with different spin and spatial symmetries were identified. The X (4)Sigma- state prevails as the ground state of YC, and a low-lying excited A (4)Pi state is found to be 1661 cm(-1) higher at the MRSDCI level. The computations of the authors support the assignment of the observed spectra to a B (4)Delta(Omega=72)<--A (4)Pi(Omega=52) transition with a reinterpretation that the A (4)Pi state is appreciably populated under the experimental conditions as it is less than 2000 cm(-1) of the X (4)Sigma- ground state, and the previously suggested (4)Pi ground state is reassigned to the first low-lying excited state of YC. The potential energy curves of YC+ confirm a previous prediction by Seivers et al. [J. Chem. Phys. 105, 6322 (1996)] that the ground state of YC+ is formed through a second pathway at higher energies. The calculated ionization energy of YC is 6.00 eV, while the adiabatic electron affinity is 0.95 eV at the MRSDCI level. The computed ionization energy of YC and dissociation energy of YC+ confirm the revised experimental estimates provided by Seivers et al. although direct experimental measurements yielded results with greater errors due to uncertainty in collisional cross sections for YC+ formation.     

Electronic structure and bonding of the 3d transition metal borides, MB, M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu through all electron ab initio calculations

The electronic structure and bonding of the ground and some low-lying states of all first row transition metal borides (MB), ScB, TiB, VB, CrB, MnB, FeB, CoB, NiB, and CuB have been studied by multireference configuration interaction (MRCI) methods employing a correlation consistent basis set of quintuple cardinality (5Z). It should be stressed that for all the above nine molecules, experimental results are essentially absent, whereas with the exception of ScB and CuB the remaining seven species are studied theoretically for the first time. We have constructed full potential energy curves at the MRCI/5Z level for a total of 27 low-lying states, subsequently used to extract binding energies, spectroscopic parameters, and bonding schemes. In addition, some 20 or more states for every MB species have been examined at the MRCI/4Z level of theory. The ground state symmetries and corresponding binding energies (in kcal/mol) are 5Sigma-(ScB), 76; 6Delta(TiB), 65; 7Sigma+(VB), 55; 6Sigma+(CrB), 31; 5Pi(MnB), 20; 4Sigma-(FeB), 54; 3Delta(CoB), 66; 2Sigma+(NiB), 79; and 1Sigma+(CuB), 49.     

Extensive theoretical study on the low-lying electronic states of silicon monofluoride cation including spin-orbit coupling

Ab initio calculations on the low-lying electronic states of SiF+ are performed using the internally contracted multireference configuration interaction method with the Davidson correction and entirely uncontracted aug-cc-pV5Z basis set. The effects of spin-orbit coupling are accounted for by the state interaction approach with the full Breit-Pauli Hamiltonian. The entire 23 Omega states generated from the 12 valence Lambda-S states, which correlate with the first dissociation channel are studied for the first time. Good agreement is found between the calculated results and the available experimental data. The spin-orbit coupling effects on the potential energy curves and spectroscopic properties are studied. Various curve crossings are revealed, which could lead to the predissociation of the a3Pi, A1Pi, and (2)3Sigma+ states and the predissociation pathways are analyzed based upon the calculated spin-orbit matrix elements. The calculated ionization potentials of the ground-state SiF to a few states of SiF+ are in good agreement with the available experimental measurements. Moreover, the transition dipole moments of the dipole-allowed transitions and the transition properties for the A3Pi0+ -X1Sigma+ 0+ and B3Pi1-X1Sigma+ 0+ transitions are predicted, including the Franck-Condon factors and the radiative lifetimes.     

The NaK 1(b) 3Pi(Omega=0) state hyperfine structure and the 1(b) 3Pi(Omega=0) approximately 2(A) 1Sigma+ spin-orbit interaction

We have measured the hyperfine structure of mutually perturbing rovibrational levels of the 1(b) 3Pi0 and 2(A) 1Sigma+ states of the NaK molecule, using the perturbation-facilitated optical-optical double resonance method with copropagating lasers. The unperturbed 1(b) 3Pi0 levels are split into four hyperfine components by the Fermi contact interaction bFIS. Mixing between the 1(b) 3Pi0 and 2(A) 1Sigma+ levels imparts hyperfine structure to the nominally singlet component of the perturbed levels and reduces the hyperfine splitting of the nominally triplet component. Theoretical analysis relates these observations to the hyperfine splitting that each 1(b) 3Pi0 level would have if it were not perturbed by a 2(A) 1Sigma+ level. Using this analysis, we demonstrate that significant hyperfine splitting arises because the 1(b) 3Pi0 state cannot be described as pure Hund's case (a). We determine bF for the 1(b) 3Pi0 levels and also a more accurate value for the magnitude of the singlet-triplet spin-orbit coupling HSO=[1(b) 3Pi0(vb,J)(H(SO))2(A) 1Sigma+(vA,J). Using the known spectroscopic constants of the 1(b) 3Pi state, we obtain bF=0.009 89+/-0.000 27 cm(-1). The values of (H(SO)) are found to be between 2 and 3 cm(-1), depending on vb, vA, and J. Dividing (H(SO)) by calculated vibrational overlap integrals, and taking account of the 1(b) 3Pi(Omega) rotational mixing, we can determine the magnitude of the electronic part H(el) of H(SO). Our results yield (H(el))=(16.33+/-0.15) cm(-1), consistent with our previous determinations using different techniques.     

Electronic excited-state mixing in NeCl2

Ab initio calculations that explicitly include spin-orbit interactions are reported for the NeCl2 system of electronic states. A surprising curve crossing is observed for the C2v, T-shaped geometry. Away from the C2v geometry, the states mix, as expected. On the basis of these new results we propose a new mechanism for electronic energy transfer from highly vibrationally excited levels of the B electronic state of the chlorine molecule. It is proposed that as long as vibrational predissociation of NeCl2 proceeds by direct coupling of the initial state to the continuum states the Ne atom does not sample geometries that efficiently quench the Cl2 B electronic state. However, when the vibrational dynamics changes to the intramolecular vibrational relaxation regime the Ne atom becomes quite effective at coupling the Cl2 B3Pi0u+ state with a 3Pi2g state.     

Autoionization and neutral dissociation of superexcited HI studied by two-dimensional photoelectron spectroscopy

Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10-14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI*(RA) converging to HI+(A 2Sigma1/2(+)). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(X 2Pi) over a photon energy region from approximately 12 to 13.7 eV, which is attributable to the autoionizing feature of the 5 dpi HI*(RA) state. A noticeable set of stripes in the photon energy region of 13.5-14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I* converging to I+ (3P0 or 3P1). The formation of I* is understood in terms of predissociation of multiple HI*(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Pi1/2).     

Photodissociation of bromobenzene, dibromobenzene, and 1,3,5-tribromobenzene

Quantum chemical calculations have been performed on the ground state and several low-lying excited states of bromobenzene, ortho-, meta-, and para-dibromobenzene, and 1,3,5-tribromobenzene using high-level ab initio and hybrid density-functional methods. Experimental observations of ultrafast predissociation in these molecules are clarified from extensive theoretical information about all low-energy potential-energy curves together with symmetry arguments. The intriguing observation that o- and m-dibromobenzene have two ultrafast predissociation channels while bromobenzene, p-dibromobenzene, and 1,3,5-tribromobenzene only have one such channel is explained from the calculated potential-energy curves. These show that the lowering of point-group symmetry from C2v to Cs along the main photodissociation reaction coordinate, which only occurs in o- and m-dibromobenzene, opens up a new predissociation channel. Dynamical quantum simulations based on the calculated potential-energy curves are used to estimate the coupling strength at the intersystem crossing point in bromobenzene.     

Rotational effects in the band oscillator strengths and predissociation linewidths for the lowest 1Pi u-X 1Sigmag+ transitions of N2

A coupled-channel Schrodinger equation (CSE) model of N2 photodissociation, which includes the effects of all interactions between the b, c, and o 1Pi u and the C and C' 3Pi u states, is employed to study the effects of rotation on the lowest-upsilon 1Pi u-X 1Sigmag+(upsilon,0) band oscillator strengths and 1Pi u predissociation linewidths. Significant rotational dependences are found which are in excellent agreement with recent experimental results, where comparisons are possible. New extreme-ultraviolet (EUV) photoabsorption spectra of the key b 1Pi u<--X 1Sigmag +(3,0) transition of N2 are also presented and analyzed, revealing a b(upsilon=3) predissociation linewidth peaking near J=11. This behavior can be reproduced only if the triplet structure of the C state is included explicitly in the CSE-model calculations, with a spin-orbit constant A approximately 15 cm(-1) for the diffuse C(upsilon=9) level which accidentally predissociates b(upsilon=3). The complex rotational behavior of the b-X(3,0) and other bands may be an important component in the modeling of EUV transmission through nitrogen-rich planetary atmospheres.     

First principles investigation of chromium carbide, CrC

We have investigated the electronic structure of 14 states of the experimentally unknown diatomic molecule chromium carbide, CrC, using standard multireference configuration interaction methods and high quality basis sets. We report potential curves, binding energies, and a number of spectroscopic parameters. The ground state of CrC, X 3Sigma-, displays triple-bond character with a binding energy of D(e)=89 kcal/mol and an internuclear separation of r(e)=1.63 A. The first excited state (1 5Sigma-) lies 9.2 kcal/mol higher. All the states studied are fairly ionic, featuring an electron transfer of 0.3-0.5e- from the metal atom to the carbon atom.     

Ab initio study of the electronic structure and bonding of aluminum nitride

For the diatomic aluminum nitride (AlN), we have constructed potential energy curves for 45 states employing multi-reference variational methods and quantitative basis sets. Thirty-six states are relatively strongly bound, five present local minima, and four are of repulsive nature. Almost all states are of intense multi-reference character rendering their calculation and interpretation quite problematic. Our tentative assignment of the ground state is 3Pi, while a 3Sigma- state is above by less than 1 kcal/mol. Our best estimate for the binding energy of the X3Pi state is D0 = 56.0 +/- 0.5 kcal/mol at re = 1.783 A, in good agreement with the experimental values of D = 66 +/- 9 kcal/mol and re = 1.7864 A. The binding energy of the A3Sigma- state is very similar to the X state because they both correlate to the ground-state atoms, but the bond distance of the former is 0.13 A longer. The first seven states can be tagged as follows: X3Pi, A3Sigma-, a1Sigma+, b1Pi, c1Delta, B3Sigma+, and d1Sigma+, a rather definitive order with the exception of X and A states.     

Spin-orbit coupling in O2(upsilon)+O2 collisions: I. Electronic structure calculations on dimer states involving the X 3Sigmag-, a 1Deltag, and b 1Sigmag+ states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.     

Role of the electric dipole moment in positron binding to the ground and excited states of the BeO molecule

Self-consistent-field and multireference single- and double-excitation configuration interaction (CI) calculations have been carried out for various electronic states of the beryllium oxide molecule and their positron-attached counterparts. Particular emphasis is placed on the correlation between the polarity of a given BeO state and the magnitude of the positron binding energy as the internuclear distance is varied. Potential curves are computed for all BeO states that correlate with the first three atomic limits for this system and good agreement is found between the experimental and calculated spectroscopic constants in all cases. The present level of CI treatment is known to underestimate the positron affinities of atoms by at least several tenths of an eV, and this fact needs to be taken into account in evaluating the results for positron binding to molecules. The lowest BeO excited states (3,1Pi) are not found to bind with a positron in the Franck-Condon region due to their comparatively small dipole moments caused by O to Be charge transfer relative to the X 1Sigma+ ground state, which in turn does have a fairly sizeable positron affinity. The situation changes significantly as dissociation proceeds, however, with both 4,2Pi and 2Sigma+ positronic states lying several tenths of an eV lower than their neutral counterparts over a broad range of internuclear distance.     

Theoretical investigation on the electronic and geometric structure of GaN2+ and GaN4+

The electronic and geometric structures of gallium dinitride cation, GaN2+ and gallium tetranitride cation, GaN4+ were systematically studied by employing density functional theory (DFT-B3LYP) and perturbation theory (MP2, MP4) in conjunction with large basis sets, (aug-)cc-pVxZ, x = T, Q. A total of 7 structures for GaN2+ and 24 for GaN4+ were identified, corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces, and bonding mechanisms for some low-lying electronic states. The calculated dissociation energy (De) of the ground state of GaN2+, X1Sigma+, is 5.6 kcal/mol with respect to Ga+(1S) + N2(X1Sigmag+) and that of the excited state, ?3Pi, is 24.8 kcal/mol with respect to Ga+(3P) + N2(X1Sigmag+). The ground state and the first excited minimum of GaN4+ are of 1A1(C2v) and 3B1(C2v) symmetry with corresponding De of 11.0 and 43.7 kcal/mol with respect to Ga+(1S) + 2N2(X1Sigmag+) for X1A1 and Ga+(3P) + 2N2(X1Sigmag+) for 3B1.     

OOCO+ cation. II. Its role during the atmospheric ion-molecule reactions

For the charge transfer and vibrational and electronic deexcitations between O2/O2+ + CO+/CO, O/O+ + CO2+/CO2, and C/C+ + O3+/O3, multistep reaction pathways are discussed in light of the theoretical data of this and previous paper together with close comparison with the experimental observations. Our calculations show that these pathways involve both the long range and molecular region ranges of the potential energy surfaces of the electronic states of the stable isomers of OOCO+ and mostly those of the weakly bound charge transfer complex OOCO+. The couplings between these electronic states such as vibronic, Renner-Teller, Jahn-Teller, and spin orbit are viewed to play crucial roles here. Moreover, the initial orientation of the reactants, in the entrance channels, strongly influences the reaction mechanisms undertaken. We propose for the first time a mechanism for the widely experimentally studied spin-forbidden exothermic O+((4)S(u))+CO2(X (1)Sigmag+)-->O2+(X (2)Pi(g))+CO(X (1)Sigma+) reaction where the O turns around the OCO molecule.     

Heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) and their cations

The heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) have been studied by high-level ab initio calculations. The RCCSD(T) method has been employed, combined with large flexible valence basis sets. All-electron basis sets are used for potassium and sulfur, with effective core potentials being used for the other metals, describing the core electrons. Potential-energy curves are calculated for the lowest two neutral and cationic states: all neutral monosulfide species have a (2)Pi ground state, in contrast with the alkali-metal monoxide species, which undergo a change in the electronic ground state from (2)Pi to (2)Sigma(+) as the group is descended. In the cases of KS, RbS, and CsS, spin-orbit curves are also calculated. We also calculate potential-energy curves for the lowest (3)Sigma(-) and (3)Pi states of the cations. From the potential-energy curves, spectroscopic constants are derived, and for KS the spectroscopic results are compared to experimental spectroscopic values. Ionization energies, dissociation energies, and heats of formation are also calculated; for KS, we explore the effects of relativity and basis set extrapolation on these values.