Double-charge-transfer (DCT) collisions of H+, OH+ and F+ 3keV beam ions with a series of alkyl isocyanate molecules were studied using mass spectrometric techniques. Measurement of the kinetic energies of H− ions so produced enabled the determination of double-ionization energies (DIE) for transitions to singlet doubly ionized states of the target molecules; those for triplet doubly ionized states were obtained similarly from measurements of the kinetic energies of OH− and F− ions. Values up to approximately 40eV were obtained in most cases and were found to be in close agreement with the predictions of ab initio calculations using propagator theory, also presented here. For n-butyl isocyanate (and by implication heavier molecules in the series) the density of doubly ionized states above 30eV was both observed and predicted to be too large and featureless for meaningful analysis, so establishing an effective upper limit on molecular size for the current application of these techniques. Significant configuration interaction was predicted for the final doubly ionized states, which justified theoretical analysis with a relatively complex method that accounts well for correlation effects.

Triple-electron-transfer (TET) collisions of 6 keV O2+ beam ions with gas-phase ethyne molecules were studied with mass spectrometric techniques. Measurement of the kinetic energies of the O− ions produced yielded triple-ionization energies (TIEs) of ethyne to states of its triply charged positive ion, once those transitions corresponding to the transfer of three electrons in a single collision were identified: eight such peaks were found between 65.1 and 80.1 eV. Possible O2+ states in the beam include 3P, 1D, 1S; according to spin conservation the singlet ions should populate only doublet states of C2H23+, while projectiles in triplet states would populate both its doublet and quadruplet states. Using ab initio propagator calculations for the triple-ionization transitions, the TIEs and quantum numbers for the states of C2H23+ associated with each peak observed in the TET spectrum could be identified. The overall match between the theoretical and experimental energies was good, so that it could be established that the population of quadruplet states was comparable in intensity with that for doublets. This is consistent with previous evidence for O2+(3P) being the dominant state in the projectile beam used.

The application of the second-order Algebraic Diagrammatic Construction (ADC(2)) propagator method for calculation of molecular double-ionization energies is extended to larger molecules than hitherto, through investigation of a diagonal approximation to the interactions between satellite configurations. The sparse 'arrowhead' structure thereby introduced to each Hermitian interaction matrix is then exploited by development of a 'subspace bisection' method for determination of the matrix eigenvalues, the predicted double-ionization energies, which is carried out within the small subspace of main configurations, so enabling the application of the ADC(2) method to molecules for which its computational demands would previously have been unreasonable. The diagonal approximation is tested against previous results for benzene and then applied successfully to the analysis of data from the double-charge-transfer spectroscopy of monofluorobenzene and hexafluorobenzene.

Experimental and computational methods were applied to study the double ionization of the molecules CH 3 Cl, CH 2 Cl 2 , and CHCl 3 . Double-charge-transfer spectroscopy was applied to measure the double-ionization energies of the molecules. The translational energies of the negative ions generated when fast-moving singly charged positive projectile ions acquire two electrons in collisions with the molecules provides information on the states of the dications populated. The use of H + , OH + , and F + projectile ions allowed transitions to singlet and triplet electronic states of the dications to be studied. Double-ionization energies to those states were calculated using the second order algebraic diagrammatic construction Green’s function method. The calculated and measured values for CH 3 Cl were found to be in good agreement. The computed data predicted that the density of states for CH 2 Cl 2 2+ and CHCl 3 2+ were much higher than for CH 3 Cl 2+ . On grouping the calculated double-ionization energies to close-lying states together, however, good agreement with the measured values was evident. The combined experimental and computational study reported here thus provides a detailed understanding of the double ionization of the three molecules to singlet and triplet electronic states of their dications.

The results of a combined experimental and theoretical study of the double ionization of sulphur hexafluoride are reported. Double-charge-transfer (DCT) spectroscopy was used in the experimental investigation. Double-electron-capture (DEC) reactions, on which DCT spectroscopy is based, are known to be subject to spin conservation. Consequently, double-ionization energies to singlet and triplet electronic states of SF62+ were measured by using H+ and F+ projectile ions, respectively. In the theoretical investigation, ADC(2) Green’s function calculations indicated a high density of singlet and triplet electronic states for SF62+. It was found, however, that groups of states could be identified that were well separated from adjacent groups. The average double-ionization energies (DIEs) for these groups are in good agreement with those measured. The combined experimental and theoretical approach has thus provided a considerably improved understanding of the double ionization of the SF6 molecule.

A theoretical analysis is presented of the conservation of electronic reflection symmetry for all nuclear configurations of a system consisting of a proton, or other atomic ion having zero electronic orbital angular momentum, interacting with a diatomic molecule. Transitions between Sigma(+) and Sigma(-) states of the molecule are associated with a change in the parity of the system under reflection of the electronic coordinates in the plane of the three nuclei, which is shown to be forbidden in collisions to which the Born-Oppenheimer approximation may be applied. This selection rule is consistent with the absence of Sigma(+) <-> Sigma(-) excitations noted in recent high-resolution studies of the collisions of protons with diatomic molecules. Its extension to the double- and triple-ionization of diatomic molecules in electron-transfer collisions with suitable atomic projectile ions is examined and the case of a target linear molecule with more than two nuclei also explored. (C) 1999 John Wiley & Sons, Inc.

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 SO22+ 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 SO22+ are also found to be consistent with the results of earlier studies of SO2 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 SO2 up to 50 eV in energy, allowing an analysis of some earlier PIPICO data for SO2.

Double-ionization energies of CH3Br, CH2Br2 and CHBr3 to singlet and triplet electronic states of their dications were calculated and measured. Double-charge-transfer spectroscopy was employed in the experimental part of the study; the computational work was carried out using a semi-empirical form of the multiple-scattering Xα method. For CH3Br, each spectral peak could be identified with a calculated double-ionization energy thus giving a clear insight into the electronic transitions which give rise to the spectra. The predicted densities of states for CH2Br22+ and CHBr32+ were, however, much higher than that for CH3Br2+, so that groupings of states had to be established to correlate with the information obtained from some peaks of the spectra. © 1998 John Wiley & Sons, Ltd.

Triple-charge-transfer (TCT) spectrometry was used to study the triple ionization of allene (C3H4). Ten clearly defined peaks were observed in the spectra, from which triple-ionization energies to electronic states of C3H43+ were determined. Triple-ionization energies to quadruplet states were calculated using the ADC(2) Green's function method. The calculated values agree well with those measured, thus giving an insight into the electronic transitions which gave rise to the spectral peaks. Quadruplet states were considered since these will be populated if spin is conserved in the triple-electron-capture reactions on which TCT spectrometry is based.

The double-ionization energies of hexafluorobenzene to singlet and triplet electronic states of its dication have been measured by double-charge-transfer spectroscopy. Because the energy-loss spectrometer used has a high resolving power, more information about the electronic-state manifolds of C6F62+ was obtained than existed from the results of previous studies. The values of double-ionization energies determined are compared with those calculated in a previous investigation using the Green's function method. Because the predicted density of singlet and triplet states is high, only the lower energies can be identified with specific electronic transitions. For the higher energies, many states could be populated to generate each of the spectral peaks observed.

The newly developed technique of triple-charge-transfer spectrometry was used to measure eight distinct triple-ionization energies of CS2 in the range 54–66 eV. A semi-empirical method was used to calculate the energies, and excellent agreement was obtained between measured values and those calculated for triple ionization to quadruplet states of CS23+, consistent with spin conservation in the triple-electron-capture reactions of Cl2+ projectile ions with CS2 molecules. The lowest triple-ionization energy measured is 54.3 ± 0.5 eV, which is close to the value 53.6 ± 0.5 eV measured previously using the appearance-energy method.

Double-ionization energies of acetonitrile (methyl cyanide) to singlet and triplet electronic states of the dication were measured and calculated. In the experimental part of the investigation, double-electron-capture reactions of fast-moving singly charged positive ions with acetonitrile molecules were investigated. Double-ionization energies were determined from the translational energies of the negative ions generated. Because of spin conservation in the reactions, triplet states of CH3CN2+ were populated when using OH+ projectile ions, and singlet states when using H+ projectile ions. The double-ionization energies measured were compared with values calculated using two different computational methods. Ab initio MP2J3-21G predictions of the double-ionization energies to the lowest singlet and triplet states agreed well with the measured values of 34.3 ± 0.2 and 33.3 ± 0.3 eV, respectively. Double-ionization energies to higher-lying states were calculated using an ab initio Green's function method. In general, the theoretically predicted results agreed well with those measured, thus identifying the electronic transitions which take place in the double-electron-capture reactions.

Ab initio propagator calculations for triple ionizations of the carbon disulphide molecule to quadruplet states of CS23+ have been performed for transition energies up to 80 eV, using the second-order algebraic diagrammatic construction (ADC(2)) method. Energies predicted for main transitions, arbitrarily defined as those to CS23+ states having at least 20% weight in 3-hole configurations, are found to correlate well with those of a range of peaks exhibited in triple-charge-transfer spectra for the collisions of 6 keV Cl2+ ions with gas-phase CS2. The ADC(2) results also enable an appraisal of the predictions of a semi-empirical method previously used to analyse the spectra.

The double-ionization energies (DIEs) of 2-butyne, 2-pentyne, 2-hexyne and 3-hexyne molecules to singlet and triplet electronic states of their dications have been measured using double-charge transfer spectroscopy. Values of DIEs to the lowest singlet and triplet states of the four dications were calculated using an ab initio method and the split-valence 3-21G and 6-31G basis sets. The pattern of those DIEs for the four molecules (and the previously studied propyne molecule) agree with that observed in the experimental values, thus giving an insight into the double-ionization processes of part of the alkyne family of molecules. The spectra showed many other peaks which correspond to double ionization to electronically excited states, For 2-butyne, the DIE values to those states calculated using a semi-empirical form of the multiple-scattering X alpha computational method allowed the relevant electronic transition to be identified. (C) 1997 John Wiley & Sons, Ltd.

The double-ionization energies of methylamine, dimethylamine, trimethylamine, ethylamine, propylamine and butylamine were measured and calculated. Transitions to singlet states of the ions were considered in order to complement the previously obtained information on transitions to triplet states. The double-ionization energies were determined experimentally from the positions of peaks observed in double-charge-transfer spectra. Information on the electronic transitions giving rise to the peaks was obtained by comparing the measured energies with those calculated using a semi-empirical form of the multiple-scattering Xα computational method. It was shown that for some of the dications the density of states is high. An ab-initio computational method, used with the 3-21G and 6-31G∗ basis sets, predicted quite accurately the trend in the lowest double-ionization energies measured.

It has been proposed that the endoergicities of triple-electron-capture (TEC) reactions involving doubly charged ions must fall within a reaction window from 12 to 33eV in order that the reactions have a finite probability. The upper limit has been confirmed previously in experiments involving Cl2+ and O2+ ions interacting with krypton. The lower limit was studied by reacting O2+ with CS2 molecules. It was shown that TEC reactions did not occur significantly below an endoergicity of about 10eV, thus confirming the approximate lower limit for the window.

Double ionization of C2H5I, n-C3H7I and n-C4H9I has been investigated experimentally using double-charge-transfer spectroscopy. On the assumption that spin is conserved in the double-electron-capture reactions, and because of the use of OH+ as the projectile ion, it is probable that triplet states of the dications were populated. Peaks in the spectra were interpreted in terms of the electronic transitions giving rise to double-ionization energies calculated using a semi-empirical version of the multiple scattering X alpha (MSX alpha) method of molecular orbital calculations. The density of states was found to increase as the alkyl chain-length increased, and most of the peaks observed were interpreted as being due to transitions to groups of close-lying states. In addition to the use of the MSX alpha method, ab initio double-ionization energies to the lowest triplet states of the dications were calculated.

When previously measured double-ionization energies of the ethyne (C2H2) molecule to singlet and triplet electronic states of its dication are compared with previously calculated values, it becomes apparent that some of the predicted values are not matched with equivalent experimental data. In the present investigation, the results of ADC(2) Green's function calculations indicate that some of the transitions are to satellite states and so too weak to allow double-ionization energies to those states to be measured. The (1) Delta(g) and (1) Sigma(g)(+) states are, however, main states and transitions to them should be sufficiently strong to give two separate double-ionization energies. Only one peak was seen in the appropriate region using both Auger-electron spectroscopy and double-charge-transfer spectroscopy. This may be due to limitations in resolving power since the states are predicted to lie within 0.8 eV of one another To investigate this possibility, the double-ionization energies of C2H2 to singlet states of C2H22+ have been measured using a double-charge-transfer spectrometer which is capable of resolving spectral peaks 0.7 eV apart. Four peaks were observed, the first two corresponding to double-ionization energies of 33.6+/-0.3 eV and 34.3+/-0.2 eV, These are identifiable with transitions to the (1) Delta(g) and (1) Sigma(g)(+) states, corresponding to calculated energies of 33.8 eV and 34.6 eV, respectively. The two other peaks correspond to double-ionization energies of 38.7+/-0.4 eV and 40.7+0.5 eV, They identify with transitions to the (1) Pi(a), state and (1) Pi(g), state, the double-ionization energies to, which are predicted to be 39.0 eV and 41.0 eV, respectively.

Double-ionization energies of the fluorobenzene molecule to singlet and triplet electronic states of its dication have been measured using double-charge-transfer spectroscopy. The values obtained are compared with those calculated using a semi-empirical form of the multiple-scattering Xα computational method. The calculated data show that the density of electronic states of C6H5F2+ increases markedly as the energy increases. However, the first six measured energies to triplet states, and the first five to singlet states, closely match the energies of predicted electronic transitions, thereby verifying the association of these measured energies with double ionization to specific states of C6H5F2+.