Excitation of bands of the first negative system of the N 2 + ion in collisions of N+ and O+ ions with N2 molecules

Absolute values of the excitation cross sections of the (0,0) bands [for O⁺(⁴ S), O⁺(² P)-N₂ pairs] and the (0,0), (0,1), (1,2), and (2,3) bands [for N⁺(³ P)-N₂ pairs] of the first negative system of the N ₂ ⁺ ion have been measured in collisions with nitrogen molecules of nitrogen and oxygen ions in the ground state and in a metastable state in the interval of ion energies 1–10 keV. The process of excitation of the (0,0) band of the first negative system of the N ₂ ⁺ ion by oxygen ions in the metastable ² P state is of a quasi-resonant character. The presence in the beam of ions in metastable states was monitored by measuring the excitation efficiency of the (0,0) band λ3914 Å of the N ₂ ⁺ ion in different operating regimes of the highfrequency ion source. For N⁺ ions in the ³ P ground state, as the collision frequency is decreased the relative vibrational population of the v′=1 and v′=2 levels of the B ²Σ _u ⁺ state of the N ₂ ⁺ ion is observed to deviate strongly from the value calculated in the Franck-Condon model.     

The optical spectrum of the doubly charged molecular nitrogen ion: Rotational analysis of the (0-0) and (1-1) bands of the D1Σu+-X1Σg+ transition of N22+: Comparison of observations with Ab-initio calculation results

Emission spectra obtained in the 1550–1650 Å region with a 10-m vuv spectrograph are conclusively assigned to the N₂²⁺ ion. The 1589-Å band, previously observed by Carroll, and a new band of the same system, have been rotationally analyzed. Ab-initio calculations have been performed which support the assignment of these two bands to the D¹Σ_u⁺-X¹Σ_g⁺ system. The calculations also explain the observed breaking-off points in the branch structure as well as weakening and broadening of the other expected bands. These phenomena arise from electron configuration changes and perturbation effects in the ground state.     

Exploration of large amplitude motions in the Ca+Ar2 complex

ABSTRACT

We present a theoretical study of the ground electronic state potential of the Ca⁺Ar₂ complex and of its photoabsorption spectra, simulated at temperatures ranging between 20 and 220?K. These calculations exploit a Monte-Carlo (MC) method, based on a one-electron pseudo-potential approach. A pairwise additive potential fitted to coupled cluster ab initio points, is used to model the Ca⁺Ar₂ complex. Our study shows that the most stable form of Ca⁺Ar₂ is a bent C_2v structure, whereas the linear isomer is located at around 90?±?10?cm^?1 above in energy. The analysis of the photoabsorption spectra establishes that a structural transition from bent Ca⁺Ar₂ to linear ArCa⁺Ar occurs at T～100?K. Trends in binding energies of both isomers, bond lengths and bond angles are also discussed. Molecular orbital overlaps provide an explanation for the order of stability between the bent and linear structures.     

Charge exchange in collisions of C60+ ions with laser-excited aligned Na atoms

We have performed the first measurement of charge exchange in keV C₆₀⁺ ion collisions with Na atoms in their 3s ground state as well as in the laser-excited and aligned 3p state. To interpret the results, we have calculated the correlation diagram of the C₆₀⁺–Na collision system using the Discrete Variational Density Functional Method. Possible channels for electron capture are studied by choosing the collision path along an axis perpendiculax to a pentagon and to a hexagon, respectively.     

Polarized electron capture by3He2+ at 20–28 keV

Nuclear polarization was measured by means of beam foil spectroscopy for a³He⁺ ion produced by an electron capture process of a³He²⁺ from a polarized sodium atom in an incident energy range from 20 to 28 keV. Assuming that a polarized electron of a sodium atom is predominantly captured to the 3d orbital of a³He⁺ ion andcascades down to the 1s ground state via the 2p orbital, an alignment factorA ₀ ^col (L=2) for the 3d orbital of a³He⁺ ion was extracted by comparing the observed initial sodium polarization andfinal nuclear polarization. The observedA ₀ ^col (L=2) showed a less pronounced energy dependence andwere qualitatively reproduced by the theoretical calculation.     

Tunable KrF laser-induced fluorescence of C2 in a sooting flame

2 in a flame, excited by a tunable KrF laser near 248 nm. The first comprises several P and R lines of the (1,0) band of the e ³Π_g-a ³Π_u Fox–Herzberg system, with fluorescence bands extending past 350 nm. The second is the band head region of the (7,1) band of the D ¹Σ_u ⁺←B^′ 1Σ_g ⁺ system, with fluorescence at 232 nm from D to the X ¹Σ_g ⁺ ground state. Neither band has been previously observed in any environment. The flame in these experiments is highly sooting, and the C₂ seen here is likely produced by laser vaporization of the soot with subsequent laser photolysis of a C₂ precursor. In a rich flame, this fluorescence could cause interferences in other studies such as KrF laser Raman scattering. Moreover, signal level calculations suggest native C₂ near 10 ppm could be readily observed using the Fox–Herzberg excitation. Raman measurements of major species (X≥0.01) in the same flame, using the KrF laser, are in good agreement with a model prediction. Received: 2 April 1998/Revised version: 8 June 1998     

Dunham coefficients of 14N2 from CARS measurements of high vibrational states in a low-pressure discharge

Spectroscopic constants of the X ¹Σ_g⁺ ground state of ¹⁴N₂ are deduced from CARS spectra recorded in a 4 Torr d.c. N₂ glow discharge. Vibrational states up to ν = 14 have been observed but only the 11 lower levels which have a good signal-to-noise ratio have been processed. The Dunham constants that were deduced yield vibrational band centre positions in good agreement with those of Lofthus and Krupenie.     

The reactions of Cl+ (3P) and Cl+ (1D) with hydrogen sulphide. A G2 molecular orbital study

G2 ab initio molecular orbital calculations have been performed to study the potential energy surfaces (PESs) associated with the reactions of Cl⁺ in its ³P ground state and in its ¹D first excited state with hydrogen sulphide. [H₂, Cl, S]⁺ singlet and triplet state cations present very different bonding characteristics. The latter are systematically ion-dipole or hydrogen-bonded weakly bound species, while the former are covalent molecular ions. As a consequence, although the Cl⁺(³P) is 34.5 kcal mol^?1 more stable than Cl⁺(¹D), the global minimum of the singlet PES lies 37.3 kcal mol^?1 below the global minimum of the triplet PES. Both singlet and triplet potential energy surfaces show significant differences with respect to those associated with Cl⁺ + H₂O reactions as well as with SH₂ reactions with F⁺. In both cases, the major product should be SH⁺ ₂; SH⁺ and HCl⁺ being the minor products, in agreement with the experimental evidence. The estimated heat of formation for the most stable H₂SCl⁺ singlet state species is 198 ± 1 kcal mol^?1.     

Theoretical study of ground and excited state potential energy surfaces for the Ca+-H2 complex

The potential energy surfaces of the Ca⁺-H₂ complex are calculated using the internally contracted multireference CI method (ICMR CI) and complete active space SCF (CAS SCF) reference wave functions. The calculations involve both the ground and the excited states correlating to (3d)²D and (4p)²P Ca⁺ terms and are carried out for C_∞v and C_2v configurations. Anisotropy of the potential surfaces has also been analysed by computing the interaction energy for some representative points as a function of the angle between the H₂ molecular axis and the Ca⁺—centre of mass of H₂ bond axis. The calculations have revealed the existence of a conical intersection of the lowest excited (3d)²B₂ potential surface with the ground state one. The obtained global energy minimum of the (3d)²B₂ potential surface lying 0.683 eV below the asymptote indicates a possible stabilization of the Ca⁺-H₂ complex towards formation of an exciplex in the (3d)Ca⁺-H₂(v = 0) collision process. The dependence of the vibrational energy levels of H₂ on the distance from Ca⁺ in the C_2v configuration has also been studied.     

Radiative Transitions and Temperature Time Dependences in Pulse Excited Microwave Discharges – (N2, Ar + N2)

Quantitative spectral and microwave measurements of vibrational temperatures and electron densities were performed for 2400 MHz non-isothermic pulse excited discharges in flowing nitrogen and argon at pressures (60–2700) Pa. A detailed analysis of the N₂ vibrational states population for the N₂ C³Π_u, X¹Σ_g⁺ electronic states has been carried out. The basic difficulties encountered when comparing the spectroscopically determined values of vibrational temperatures with corresponding quantities of the ground electronics state are mentioned and the time resolved dependences of the translational gas temperature in N₂ during the microwave pulse is evaluated. The steady state in the nitrogen pulse excited microwave plasma is reached within 3 · 10^?4 s, but generally, this time depends on the gas pressure in the discharge tube. In the Ar + N₂ mixtures the excitation conditions are complicated by the metastable argon atoms (3P_2,0) creating the nonequilibrium populations of electronic, vibrational and rotational N₂ states.     

Coulomb Distortion of Carbododecahedron C20 2+

The electronic structure, geometry, and potential energy surface of the dication of carbododecahedron C₂₀ ²⁺ have been investigated by the PM3 semiempirical method of molecular orbitals. An analog of the Jahn–Teller dynamic effect for this system is revealed, caused by the repulsion of uncompensated positive charges on carbon atoms (Coulomb distortion). The D ₃ symmetry of the ground state of C₂₀ ²⁺ is predicted. The IR vibrational spectrum of the dication of carbododecahedron is calculated, the observation of which could confirm the predicted effect. Evaluational calculations of C₂₀ ⁺ and C₂₀ are also carried out.     

Wavepacket dynamics and quantum mechanical energy densities in the quartet N+ 2 + O2 system

Surface crossing of the potential energy surfaces (PESs) and the formulation of differentiable PES functions are discussed in the quartet N⁺ ₂ + O₂ system containing the T shape intermediate complex. Consideration of the ion-dipole interaction between N₂ and O₂ fragments due to the induced dipole moment of the neutral molecule gives a good image of the surface crossing between the quartet reactant state (N⁺ ₂ ···O₂) and the quartet product state (N₂ ···O⁺ ₂) for the charge transfer (CT) reaction in terms of the conventional harpooning mechanism. Differentiable functions of PESs are constructed for the reactant and product states that may be applied for 3-dimensional dynamics calculations by means of wavepackets, and examples of 2-dimensional wavepacket dynamics calculations are introduced. Further, electron transfer processes and local electronic nature in the CT reaction are discussed in terms of the quantum mechanical energy densities based on regional density functional theory. Regional partitioning by using the kinetic energy density provides a new concept of electron transfer, and the tension density provides new images of microscopic electronic stresses.     

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne₂ ⁺, Ar₂ ⁺ and Kr₂ ⁺. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar₂ ⁺, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne₂ ⁺ suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne₂ ⁺ and Ar₂ ⁺ to possess a double-well potential and that of Kr₂ ⁺ to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr₂ ⁺ and Xe₂ ⁺ dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar₂ ⁺ does not.     

Zeeman relaxation of N2 + (2Σ+) in collisions with 3He and 4He

We compare the cross sections for the transitions changing the projection of the total angular momentum of N₂ ⁺(²Σ) in collisions with ³He and ⁴He at very low collision energy. The fundamental states of the two nuclear spin isomers of N₂ ⁺ are considered as well as the two fine structure levels of the first excited para level N=2. It is shown that the two fundamental states of the two nuclear spin isomers behave differently. For the fundamental para level N=0 of N₂ ⁺, the projection changing cross section is always negligible compared to the elastic one for both He isotopes. For the fundamental ortho level N=1 of N₂ ⁺, the spin-rotation interaction couples the different spin levels directly so the spin relaxation becomes a first order process. The associated resonances increase the projection changing cross section which remains smaller but becomes comparable with the elastic one. This is in contrast with the excited rotational levels of N₂ ⁺, which for the rotational deactivation and elastic channels are found to be equal around the resonances for the collisions involving ³He. These two channels are always larger than the projection changing one. We also find that, for transitions involving the fundamental rotational state, the domain of validity of the threshold laws discussed by Krems and Dalgarno [Phys. Rev. A 67, 050704 (2003)] for a potential decreasing faster than 1/r2 is shortened, due to the long range charge induced dipole potential. This effect is illustrated for the collisions of ³He with the fundamental para state of N₂ ⁺.     

Theoretical study of the HeN2+ 2 dication

The structure and stability of the helio nitrogen molecular dication, HeN²⁺ ₂, is investigated by means of standard quantum chemical methods based on both single-reference and multi-reference formalisms. Sequences of correlation consistent basis sets are employed to establish convergence with respect to the size and the quality of the single-particle basis set. MRCI and CASPT2 calculations are reported for the HeN²⁺ ₂ ground-state ion which is found to be metastable. The barrier for the transition state of the reaction HeN²⁺ ₂→ N⁺ …NHe⁺ → N⁺ + NHe⁺ is calculated to be 59.5 kcals mol^?1. The structure of the transition state NHeN²⁺ was also determined from calculations using the MRCI and CASPT2 methods.     

The vacuum uv emission spectrum of the 15N22+ molecular ion

The vacuum uv emission of the ¹⁵N₂²⁺ ion has been recorded for the first time. Rotational analysis of two bands, analogous to those already observed in the case of the natural isotope, confirm their assignment to the D¹Σ_u⁺-X¹Σ_g⁺ (0, 0) and (1, 1) bands. More precise data are also obtained for the ³Σ_g^? state which perturbs ground state vibrational levels.     

Einstein A-coefficients for rotational transitions in the HN2O+

EinsteinA-values for the electric dipole transitions between the rotational levels up to 540 cm⁻¹ andJ=11 in the ground vibrational state of the protonated N₂O (i.e., HN₂O⁺) are calculated. The coefficients are used to compute the mean radiative lifetimes of the levels. TheseA-values can be used for analysing the spectra from astronomical objects, if observed.     

Newly calculated absolute cross-section for the electron-impact ionization of C2H2 +

New measurements of the cross-section for electron impact ionization of the molecular ion C₂H₂⁺ have been carried out recently. These data differ significantly from earlier data, because cross-sections corresponding to all the possible dissociative ionization processes were determined. The new data in conjunction with the significant discrepancies between the earlier data and the results of various calculations, which disagreed among themselves by a factor of 3, motivated a renewed attempt to apply the semi-classical Deutsch-M?rk (DM) formalism to the calculation of the absolute electron-impact ionization cross-section of this molecular ion. A quantum chemical molecular orbital population analysis for both the neutral molecule and the ion revealed that in the case of C₂H₂⁺ the singly occupied molecular orbital (i.e. the “missing” electron) is highly localized near the site of a C atom in the molecule. This information is explicitly incorporated in our formalism. The results obtained by taking the ionic character directly into account are in excellent agreement with the recent experimental data.     

Positron annihilation from excited states of [X-e+] systems

The relative probabilities for the radiative de-excitation 2p₊ → ls₊ versus 2γ-annihilation for the hdot; ns²np⁶2p₊, ²P state of the [X^-e⁺] system with X = F, Cl, Br, and I are presented. It is shown that a positron captured into a 2p₊ orbital undergoes annihilation with electrons of the system instead of radiative transition to the ground state of the [X^-e⁺] system.