He++N2 Charge Transfer Reaction: An Ab initio Analysis

An ab initio analysis on the involved potential energy surfaces is presented for the investigation of the charge transfer mechanism for the He⁺+N₂ system. At high collision energy, as many as seven low-lying electronic states are observed to be involved in the charge transfer mechanism. Potential energy surfaces for these low-lying electronic states have been computed in the Jacobi scattering coordinates, applying multireference configuration interaction level of theory and aug-cc-pVQZ basis sets. Asymptotes for the ground and various excited states are assigned to mark the entrance (He⁺+N₂) and charge transfer channels (He+N₂⁺). Nonadiabatic coupling matrix elements and quasi-diabatic potential energy surfaces have been computed for all seven states to rationalize the available experimental data on the charge transfer processes and to facilitate dynamics studies.     

Vibrationally inelastic collisions in H+ +CO system: comparing quantum calculations with experiments

State-resolved cross beam experiments [H. Udseth et al., J. Chem. Phys. 60, 3051 (1974); J. Krutein and F. Linder, J. Chem. Phys. 71, 599 (1979); G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)], coupled with proton energy loss spectroscopy for the inelastic scattering of H(+) from CO in the collision range of 10-30 eV show very low vibrational excitation of the target molecule. Stimulated by the experimentally observed low vibrational inelasticity in the system the ground and the first two low-lying excited electronic potential-energy surfaces have been computed using the ab initio multireference configuration interaction method. Quantum dynamics has been performed on the ground potential energy surface in the framework of vibrational close-coupling rotational infinite-order sudden approximation. The various computed dynamical attributes such as differential and integral cross sections, and average vibrational energy transfer are analyzed in detail, and compared successfully with the available experimental results.     

Ab initio potential energy surfaces and nonadiabatic interactions in the H+ +NO collision system

Ab initio calculations on the H(+)+NO system have been carried out in Jacobi coordinates at the multireference configuration interaction level employing Dunning's correlation-consistent polarized valence triple zeta basis set to analyze the role of low-lying electronic excited states in influencing the collision dynamics relevant to the experimental collision energy range of 9.5-30 eV. The lowest two adiabatic potential energy surfaces, asymptotically correlating to H(+)+NO(X (2)Pi) and H((2)S)+NO(+)(X (1)Sigma(+)), have been obtained. Using ab initio procedures, the (radial) nonadiabatic couplings and the mixing angle between the lowest two electronic states (1 (2)A' and 2 (2)A') have been obtained to yield the corresponding quasidiabatic potential energy matrix. The strengths of the computed vibrational coupling matrix elements reflect a similar trend, as has been observed experimentally in the magnitudes of the state-to-state transition probability for the inelastic vibrational excitations [J. Krutein and F. Linder, J. Chem. Phys. 71, 559 (1979); F. A. Gianturco et al., J. Phys. B 14, 667 (1981)].     

Ab initio adiabatic and quasidiabatic potential energy surfaces of lowest four electronic states of the H+ + O2 system

Ab initio global adiabatic and quasidiabatic potential energy surfaces of lowest four electronic (1-4 (3)A(")) states of the H(+)+O(2) system have been computed in the Jacobi coordinates (R,r,γ) using Dunning's cc-pVTZ basis set at the internally contracted multireference (single and double) configuration interaction level of accuracy, which are relevant to the dynamics studies of inelastic vibrational and charge transfer processes observed in the scattering experiments. The computed equilibrium geometry parameters of the bound [HO(2)](+) ion in the ground electronic state and other parameters for the transition state for the isomerization process, HOO(+)?OOH(+) are in good quantitative agreement with those available from the high level ab initio calculations, thus lending credence to the accuracy of the potential energy surfaces. The nonadiabatic couplings between the electronic states have been analyzed in both the adiabatic and quasidiabatic frameworks by computing the nonadiabatic coupling matrix elements and the coupling potentials, respectively. It is inferred that the dynamics of energy transfer processes in the scattering experiments carried out in the range of 9.5-23 eV would involve all the four electronic states.     

Quantum dynamics of inelastic excitations and charge transfer processes in the H+ +NO collisions

State-resolved differential cross section, integral cross section, average vibrational energy transfer, and the relative transition probability are computed for the H(+)+NO system using our newly obtained ab initio potential energy surfaces (PES) at the multireference configuration interaction level of accuracy employing the correlation consistent polarized valence triple zeta basis set. The quantum dynamics is treated within the vibrational close-coupling rotational infinite-order sudden approximation using the coupled ground state and first excited state ab initio quasidiabatic PES. The computed collision attributes for the inelastic vibrational excitation are compared with the state-to-state scattering data available at E(c.m.)=9.5 eV and E(c.m.)=29.03 eV and are found to be in overall good agreement with those of the experiments. The results for the vibrational charge transfer processes at these collision energies are also presented.     

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.     

Relativistic configuration interaction study of the electronic spectrum of SnTe and SnTe+

Ab initio based relativistic configuration interaction calculations have been performed to study the electronic spectrum of the heaviest tin chalcogenide and its monopositive ion. Potential energy curves and spectroscopic constants of low-lying states of both species within 7 eV are reported. The ground-state dissociation energies of SnTe and SnTe+ are computed to be 3.48 and 2.50 eV, respectively. The spin-orbit splitting between the two components of the X 2Pi state of SnTe+ is about 3030 cm(-1). Effects of the strong spin-orbit coupling on the potential curves and spectroscopic properties of both the species are investigated in detail. The electric dipole moments of some of the low-lying states of SnTe and SnTe+ are reported. Transition moments of some important spin-allowed and spin-forbidden transitions are calculated from the configuration interaction wave functions. The radiative lifetime of the excited E 1sigma0+(+) state of SnTe is about 39 ns. The X2-X1 transition in SnTe+ is found to be more probable than the similar transition in the lighter ions. The vertical ionization energy of SnTe in the ground state is estimated to be 8.22 eV.     

Electronic structures and bonding of CeF: a frozen-core four-component relativistic configuration interaction study

We study the electronic structure of the ground and several low-lying states of the CeF molecule using Dirac-Fock-Roothaan (DFR) and four-component relativistic single and double excitation configuration interaction (SDCI) calculations in the reduced frozen-core approximation (RFCA). The ground state and two low-lying excited states are calculated to have (4f)1(5d)1(6s)1 configurations with Omega = 3.5, 4.5, and 3.5, and the resulting excitation energies, T0, are, respectively, 0.319 and 0.518 eV. The experimental configurations for these states are the same, although the experimental T0 values are approximately 0.3 eV smaller than those calculated. Experimentally, the red-degraded band was observed to be 2.181 eV above the ground state, having the configuration (4f)1(5d)1(6p)1 with Omega = 4.5. The calculation for this state gives 2.197 eV and configuration (4f)1.0(5d)1.7(6p)0.3 with Omega = 4.5. We found that Omega, Re, and nu(1-0) obtained by CI agree well with experiment. Bonding between the Ce and the F is highly ionic. The 4f, 5d, and 6s valence electrons are localized at the Ce+ ion, because they are attracted by the Ce4+ ion core, and are excluded from the bonding region because of the electronic cloud around the negatively charged fluoride anion. The bonding in the ground and excited states of the CeF molecule is significantly influenced by the 6s and 5d electron distributions between the Ce and the F.     

Ab initio potential energy surfaces and nonadiabatic collision dynamics in H(+)+O(2) system

The adiabatic potential energy surfaces for the lowest five electronic states of (3)A" symmetry for the H(+)+O(2) collision system have been obtained at the multireference configuration interaction level of accuracy using Dunning's correlation consistent polarized valence triple zeta basis set. The radial nonadiabatic coupling terms and the mixing angle between the lowest two electronic states (1 (3)A" and 2 (3)A"), which adiabatically correlate in the asymptotic limit to H((2)S)+O(2) (+)(X (2)Pi(g)) and H(+)+O(2)(X (3)Sigma(g)(-)), respectively, have been computed using ab initio procedures at the same level of accuracy to yield the corresponding quasidiabatic potential energy matrix. The computed strengths of the vibrational coupling matrix elements reflect the trend observed for inelastic vibrational excitations of O(2) in the experiments at collision energy of 9.5 eV. The quantum dynamics has been preformed on the newly obtained coupled quasidiabatic potential energy surfaces under the vibrational close-coupling rotational infinite-order sudden framework at the experimental collision energy of 9.5 eV. The present theoretical results for vibrational elastic/inelastic excitations of O(2) are in overall good agreement with the available experimental data obtained from the proton energy-loss spectra in molecular beam experiments [F. A. Gianturco et al., J. Phys. B 14, 667 (1981)]. The results for the complementary charge transfer processes are also presented at this collision energy.     

Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.     

9,10-二氰基蒽和杜烯间光诱导电子转移的电荷分离态和电子耦合

采用从头算方法,讨论了9,10-二氰基蒽(DCA)和杜烯(DUR)间光诱导电子转移反应的态-态跃迁.考虑基组重叠误差(BSSE)对相互作用能的校正,用MP₂方法优化得到重叠式[DCA…DUR]配合物的稳定构型.用单激发组态相互作用(CIS)方法讨论了[DCA…DUR]配合物的光诱导电荷分离和电荷复合过程.根据广义Mulliken-Hush(GMH)模型,计算了电荷复合过程的电子耦合矩阵元.结果表明,[DCA…DUR]配合物的S₀→S₁和S₀→S₂跃迁产生了两个强的局域激发态,S₀→S₃跃迁直接导致电荷分离态,小的振子强度预测该电荷转移(CT)跃迁是一弱跃迁,电荷分离态S₃衰变到低局域激发态或基态的电荷复合是可能的.     

Recombination of simple molecular ions studied in storage ring: dissociative recombination of H2O+

Dissociative recombination of vibrationally relaxed H2O+ ions with electrons has been studied in the heavy-ion storage ring CRYRING. Absolute cross-sections have been measured for collision energies between 0 eV and 30 eV. The energy dependence of the cross-section below 0.1 eV is found to be much steeper than the E-1 behaviour associated with the dominance of the direct recombination mechanism. Resonant structures found at 4 eV and 11 eV have been attributed to the electron capture to Rydberg states converging to electronically excited ionic states. Complete branching fractions for all dissociation channels have been measured at a collision energy of 0 eV. The dissociation process is dominated by three-body H + H + O breakup that occurs with a branching ratio of 0.71.     

Theoretical study of the CH2 + O photodissociation of formaldehyde adsorbed on the Ag(111) surface

Configuration interaction calculations of the ground and excited states of the H2CO molecule adsorbed on the Ag(111) surface have been carried out to study the photoinduced dissociation process leading to polymerization of formaldehyde. The metal-adsorbate system has been described by the embedded cluster and multireference configuration interaction methods. The pi electron-attachment H2CO- and n-pi* internally excited H2CO* states have been considered as possible intermediates. The calculations have shown that H2CO* is only very weakly bound on Ag(111), and thus that the dissociation of adsorbed formaldehyde due to internal excitation is unlikely. By contrast, the H2CO- anion is strongly bound to Ag(111) and gains additional vibrational energy along the C-O stretch coordinate via Franck-Condon excitation from the neutral molecule. Computed energy variations of adsorbed H2CO and H2CO- at different key geometries along the pathway for C-O bond cleavage make evident, however, that complete dissociation is very difficult to attain on the potential energy surface of either of these states. Instead, reneutralization of the vibrationally excited anion by electron transfer back to the substrate is the most promising means of breaking the C-O bond, with subsequent formation of the coadsorbed O and CH2 fragments. Furthermore, it has been demonstrated that the most stable state for both dissociation fragments on Ag(111) is a closed-shell singlet, with binding energies relative to the gas-phase products of approximately 3.2 and approximately 1.3 eV for O and CH2, respectively. Further details of the reaction mechanism for the photoinduced C-O bond cleavage of H2CO on the Ag(111) surface are also given.     

Electronic states and nature of bonding of the molecule PdSi by all electron ab initio HF-CI calculations and mass spectrometric equilibrium experiments

Six low-lying electronic states of the PdSi molecule have been investigated by performing all electron ab initio Hartree-Fock (HF) and configuration interaction (CI) calculations. The molecule is predicted to have a³∏ ground state and two low-lying excited states,³Σ^? and¹Σ⁺. The electronic structure of the PdSi molecule has been rationalized in a simple molecular orbital diagram. As part of the PdSi molecule the Pd atom essentially retains its (4d)¹⁰ ground term configuration. The chemical bond in the PdSi molecule has been interpreted in terms of donation and back-donation of charge. The bond is polar with charge transfer from the Pd to the Si atom. The dissociation energy of the PdSi molecule has been determined from the mass spectrometric equilibrium data in combination with the theoretical results asD ₀ ^o =257±12 kJ mol^?1.     

The electric dipole polarity of the ground and low-lying metastable excited states of NF.

NF (nitrogen monofluoride, fluoroimidogen) is isoelectronic with O2, and, like O2, it has a triplet configuration in the ground state, with two low-lying metastable singlet excited states. The dipole moment of the a 1Delta excited state was measured in 1973 to be 0.37 +/- 0.06 D; at the time its polarity was assumed to be normal (i.e., with the negative charge on the fluorine). However, high-level electronic structure calculations, which reproduce with high accuracy the known spectroscopic constants of the ground and excited states of NF, predict a dipole moment of -0.388 D for a 1Delta NF, indicating that, despite the electronegativities, this molecule carries a positive charge on fluorine. The other singlet state is predicted to have an even larger negative dipole moment; the ground-state triplet should have a very small positive moment. Singlet NF resembles in this respect CO and BF, from the N2 isoelectronic series, both of which also have negative dipole moments.     

Diabatic potential energy surfaces of H+ + CO

Ab initio adiabatic and diabatic surfaces of the ground and the first excited electronic states have been computed for the H⁺ + CO system for the collinear (γ = 0°) and the perpendicular (γ = 90°) geometries employing the multi-reference configuration interaction method and Dunning’s cc-pVTZ basis set. Other properties such as mixing angle before coupling potential and before coupling matrix elements have also been obtained in order to provide an understanding of the coupling dynamics of inelastic and charge transfer process.     

Spectroscopy and metastability of CO2 2+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.     

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+-rare gas collisions: an experimental and trajectory study

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

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.