Vertical Double Ionization Energy Research Articles

Core-valence double photoionization of the CS2 molecule

Double photoionization spectra of the CS(2) molecule have been recorded using the TOF-PEPECO technique in combination with synchrotron radiation at the photon energies hν=220, 230, 240, 243, and 362.7 eV. The spectra were recorded in the S 2p and C 1s inner-shell ionization regions and reflect dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. MCSCF calculations were performed to model the energies of the dicationic states. The spectra associated with a S 2p vacancy are well structured and have been interpreted in some detail by comparison to conventional S 2p and valence photoelectron spectra. The lowest inner-shell-valence dicationic state is observed at the vertical double ionization energy 188.45 eV and is associated with a (2p(3/2))(-1)(2π(g))(-1) double vacancy. The spectrum connected to the C 1s vacancy shows a distinct line at 310.8 eV, accompanied by additional broad features at higher double ionization energies. This line is associated with a (C 1s)(-1)(2π(g))(-1) double vacancy.

Synchrotron radiation VUV double photoionization of CHF 2Cl

VUV double photoionization of CHF 2Cl in an energy region 32–40 eV was investigated with photoionization mass spectroscopy by using synchrotron radiation. The double ionization energy of CHF 2Cl and appearance energies for its main fragment dications (CHCl 2+, CF 2 2+ and CHFCl 2+), were determined with photoionization efficiency spectroscopy for the first time. The single point energies of CHF 2Cl and its parent dication (CHF 2Cl 2+) were calculated using Gaussian 03 program and density functional theory (DFT and B3LYP functional). The vertical double ionization energy of CHF 2Cl was predicted by using B3LYP method and empirical equation. According to our research results, the experimental double ionization energy of CHF 2Cl is in good agreement with the theoretically calculated vertical double ionization energy. The mechanism of double photoionization of CHF 2Cl was discussed based on the comparison of our experimental results with those predicted theoretically.

Vertical double ionization of the sulphur dioxide molecule

Vertical double-ionization energies, for transitions from the ground state of the neutral sulphur dioxide molecule to both singlet and triplet energetically low-lying electronic states of the SO 2 2+ dication, have been studied both experimentally with double-charge-transfer spectroscopy and theoretically with ab initio propagator calculations; consistently good agreement is found between the theoretical and experimental results. The experimental data for transitions to singlet states of SO 2 2+ are also found to be consistent with the results of earlier studies of SO 2 employing Auger spectroscopy, which also match closely the theoretical predictions in the higher-energy range not currently accessible with double-charge-transfer spectroscopy. The results of this paper, therefore, provide a comprehensive survey of the vertical double ionizations of SO 2 up to 50 eV in energy, allowing an analysis of some earlier PIPICO data for SO 2.

Ab initio study of the chloromethane dication fragmentation

The chloromethane dication dissociation pathways have been studied using the complete active space self-consistent field (CASSCF) method followed by a multi-reference perturbative configuration interaction (CI). The vertical double ionization energy is calculated to be 31.64 eV for the first singlet state and 30.69 eV for the first triplet state, the latter value being close to the experimental determination of 31.5 ± 0.5 eV. For most two-body dissociation processes, the appearance thresholds of the ion pairs observed in coincidence experiments are in good agreement with the results of the calculations. In the case of the CCI +/H 3 + pair, the dissociation is indirect and comes from the ionization of a Rydberg excited state of the CH 3CI + monocation. For three-body processes, the uncertainties of the available experimental results and the large number of possible dissociation channels do not allow identification uniquely of the involved mechanisms in most cases.

The allene dication: calculated and measured singlet-electronic-state energies

Vertical double-ionization energies of the allene molecule to singlet electronic states of the dication have been measured by double-charge-transfer spectroscopy, and are in excellent agreement with values predicated using a semi-empirical MSXα-based method. The present data, when combined with those of a previous investigation in which energies to triplet states were determined, provide comprehensive electronic-state information for the allene dication in a range up to ca. 12 eV above that for the triplet ground state.

Mechanisms of the charge separation in the phosphorus tetramer dication P42+

The charge separation reactions of P42+ have been investigated using the PEPIPICO (photoelectron-photoion-photoion coincidence) technique at photon energies between 25 and 48 eV. All four possible ion pairs are formed. The relatively low appearance energy of the ion pair P+ + PP3+ provides an estimate of ⩽ 25.4 eV for the vertical double ionization energy of P4. Dissociations producing P++P2+ or 2P+ are mainly sequential, proceeding via the intermediate ions PP32+ or PP22+ respectively.

Ab initio MO calculations on the electronic and geometric structure of fullerene dications, C 2+60, and the double-ionization energy of C 60

Ab initio SCF calculations have been performed to determine the equilibrium geometry of the doubly charged fullerene, C 2+ 60. A significant Jahn-Teller distortion is found to be operative. The adiabatic double-ionization energy for the process C 60(I h → C 2+ 60 (D 5d) is calculated to be IE a = 18.9 eV; this number is in pleasing agreement with the recently determined energy of 19.0 eV. Our calculated vertical double-ionization energy of IE v = 19.6 compares well with the result of a photon ionization experiment, 19.5 eV.

Measured and calculated double-ionization energies of the chlorofluoroethane molecules CF 3CF 2Cl, CF 3CFCl 2, CF 3CCl 3, CF 2ClCF 2Cl, CF 2ClCFCl 2, CF 2ClCCl 3, CFCl 2CFCl 2 and CFCl 2CCl 3

The double-ionization energies of the eight chlorofluoroethane molecules have been measured by double-charge-transfer spectrometry. Several peaks were observed in most of the spectra giving information about the energies required to populate electronically excited states (or groups of states) of the dications as well as those to the ground states. In addition to the experimental study, the vertical double-ionization energies to the lowest triplet levels of the dications were calculated. In general, the calculated values are lower than those determined experimentally, but the addition of a uniform shift of 0.4 eV to them brings all calculated data to within ±0.7 eV of those measured. This agreement is quite good since the uncertainty associated with the measured double-ionization energies to the ground states is ±0.5 eV.

A combined experimental and theoretical investigation of C 2H 2+2 electronic-state energies

Double-charge-transfer spectroscopy has been used to investigate low-lying electronic states of C 2H 2+ 2. The projectile ion used was OH + and, because of spin conservation in the double-electron-capture reaction with is central to this type of spectroscopy, it was expected that triplet states of the dication would be populated. Three peaks in the spectrum were observed corresponding to vertical double-ionization energies of 32.7 ± 0.3, 37.9 ± 0.4 and 39.6 ± 0.5 eV. By comparing these with calculated data, the energies are identified with the groun 3Σ − g state, the 3Π u state and the 3Π g state, respectively. Incorporating the present data with previously determined singlet-state data, and with computed values, provides a comprehensive electronic-state distribution for C 2H 2+ 2 up to 13 eV above the ground state. Vertical double-ionization energies calculated in the present investigation using a modified MSXα method are accurate agreement with this distribution.

Prediction of the electronic energy levels of doubly charged positive molecular ions with the aid of the multiple scattering Xα method

Approximate electronic structure calculations with the multiple scattering Xα method have been employed to predict the energy differences, for NH 3 and N 2O, between the neutral molecule ground state and energetically low-lying states of the doubly charged positive molecular ion with the neutral molecule's geometric structure. Comparison with experimental values of such vertical double ionization energies, observed with double charge transfer spectroscopy, indicates that direct application of the method gives unsatisfactory results. Instead a combination of an interaction energy calculated with the method and experimentally determined single ionization energies can predict double ionization energies to an accuracy sufficient to aid the analysis of double charge transfer spectra.

Collisions of doubly charged nitrogen molecules with rare gas atoms

The authors report on single electron capture in slow collisions of doubly charged nitrogen molecules with the rare gases He, Ne and Ar. The vertical double ionization energy of N2 as well as vertical excitation energies of N22+ have been calculated by means of the multiconfiguration coupled electron pair approximation (MC-CEPA) method. Furthermore, experiments have been carried out applying the technique of translational energy spectroscopy. The energy gain spectra of the resulting N2+ molecules have been analysed, showing the single electron capture to be dominated by N22+ (X 1 Sigma +g) projectile ions, as well as by N22+ ions in the low-lying metastable c 3 Sigma +u state. The position of the experimentally observed reaction window is discussed in terms of Franck-Condon transitions and is compared with predictions of classical descriptions.

Energies of the lowest singlet and triplet states of molecular dications determined using O + and C + ion double-charge-transfer spectroscopy

The potential for using O + and C + ions as projectiles in double-charge-transfer (DCT) spectroscopy has been investigated. Target-gas molecules of CO 2, OCS and CS 2 are shown to be double ionized into triplet states in reactions with ground-state O + and C + ions whereas the metastable ions in these beams populate singlet dication states. The superpositions of spectral peaks arising from these processes have allowed precise determinations to be made of the energy separation between lowest singlet and triplet states which are consistent with previous experimental and theoretical values. CH 4 target gas was also studied, the vertical double ionization energy to the 3T 1 state are being found to be 38.3 ± 0.4 eV and the separation between this state and the lowest singlet state 1E being 0.3 ± 0.2 eV. Additional peaks observed in the DCT spectra of C +( 2P) ions have been tentatively attributed to spin-disallowed processes or to reactions involving radiative loss.

Vibronic and electronic states of doubly charged H2S studied by Auger and charge transfer spectroscopy and by a b i n i t i o calculations

Doubly ionic states of H2S are investigated by means of Auger and double charge transfer spectroscopy. From the kinetic energy distribution of H− ions arising from double charge-transfer of protons impinging on gaseous H2S several singlet state energies of H2S2+ have been resolved in the 30 to 50 eV energy region. The most intense experimental peak is narrow proving that the doubly ionized ground state is stable or quasi-stable. The LII,IIIVV Auger electron spectrum exhibits a number of well-defined structures which exhibit vibrational fine structure in the outermost bands. The assignments of the charge transfer states and of the Auger bands are given by ab initio MCSCF electronic structure calculations. We also present vertical double ionization energies, optimized geometries and normal coordinate analysis for the neutral, single and double ionized states. A vibrational analysis of the resolved Auger bands is carried out by employing a recently derived theory for vibrational decay of short-lived core hole states in polyatomic molecules.

Experimental and theoretical studies of the doubly charged NO2+2 ion

Six features observed in the energy distribution of H− ions arising from the single collision double charge transfer of 3–5 keV protons on nitrogen dioxide are interpreted as vertical double ionization energies of NO2 of 35.1, 38.1, 39.5, 41.0, 43.2, and 46.8 eV to form doublet states. Theoretical energies of the first 13 doublet and quartet electronic states of NO2+2 have been calculated by the ab initio CIPSI method at a fixed N–O distance and for several O–N–O bending angles. Thresholds and kinetic energy releases observed in dissociative double photoionization of NO2 are interpreted by comparison with the calculations and charge transfer results. The N- and O-Auger spectra of NO2 have been recorded and some of the 40 features observed have been assigned, confirming and improving the spectrum of NO2+2.

Doubly charged benzene and isomeric dications. Fragmentation energetics and charge distributions calculated by MINDO/3 molecular orbital theory

AbstractMINDO/3 calculations for singlet and triplet doubly charged benzene [C6H6]2+ are in satisfactory agreement with the experimentally determined values of the vertical double ionization energy of benzene; calculations for straight chain isomeric structures are consistent with the observed kinetic energy release on fragmentation to [C5H3]+ and [CH3]+. Symmetrical doubly charged benzene ions relax to a less symmetrical cyclic structure having sufficient internal energy to fragment by ring opening and hydrogen transfer towards the ends of the carbon chain. Fragmentation of [CH3C4CH3]2+ to [CH3C4]+ and [CH3]+ is a relatively high energy process (A), whereas both (B): [CH3CHC3CH2]2+ to [CHC3CH2]+ and [CH3]+ and (C): [CH3CHCCHCCH]2+ to [CHCCHCCH]+ and [CH3]+ may be exothermic processes from doubly charged benzene. Furthermore, the calculated energy for the reverse of process (A) is less than the experimentally observed kinetic energy released, whereas larger energies for the reverse of processes B and C are predicted. Heats of formation of homologous series [HCn]+, [CH3Cn]+, [CH2Cn−2CH]+, [CH3Cn−2CH2]+ and [CH2CHCn−3CH2]+ with 1 < n < 6 are calculated to aid prediction of the most stable products of fragmentation of doubly charged cations. The homologous series [CH2Cn−2CH]+ is relatively stable and may account for ready fragmentation of doubly charged ions to [CnH3]+; alternatively the symmetrical [C5H3]+ ion [CHCCHCCH]+ may be formed. Dicoordinate carbon chains appear to be important stabilizing features for both cations and dications.