D.E. Parry

Triplet-state energy levels of CO2+2, COS2+ and CS2+2

Abstract Double-charge-transfer spectroscopy has been used to measure the energies of triplet states of CO2+2, COS2+ and CS2+2 up to approximately 15, 13 and 9 eV, respectively, above the ground-state energies. In addition, the double-ionization energies have been calculated using Moller-Plesset perturbation theory with split-valence basis sets and are in excellent agreement with the measured values. The excited triplet state energies correlate well with values calculated in a previous investigation, and an assignment of these states is reported. Other experimental data on these dications have been reviewed and an assignment of these data is also described.

A combined experimental and theoretical investigation of C2H2+2 electronic-state energies

Abstract Double-charge-transfer spectroscopy has been used to investigate low-lying electronic states of C 2 H 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 3 Σ − g state, the 3 Π u state and the 3 Π 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 2 H 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.

Triplet-state energies of H2O2+: a combined experimental and theoretical study

Abstract OH + and F + projectile ions were used in a double-charge-transfer spectroscopy study of the water molecule. Double-ionization energies to three triplet states of H 2 O 2+ were determined, the values being 40.0±0.6, 43.2±0.8 and 45.5±0.6 eV. A theoretical study, based on a modified MSXα method, suggests that the states populated are 3 B 1 , 3 A 1 and 3 B 2 , respectively. An ab initio MP2/D95** calculation predicts 39.6 eV for the energy of the ground 3 B 1 state of H 2 O 2+ , in good agreement with the lowest double-ionization energy measured.

Energies of triplet electronic states of the N2O2+ ion

The energies of triplet electronic states of N2O2+ have been measured by double-charge-transfer spectroscopy using OH+ and F+ as projectile ions. Peaks in the spectra correspond to double-ionization energies of 35.7 ± 0.4, 38.8 ± 0.4, 41.8 ± 0.4 and 43.1 ± 0.4 eV. The lowest energy is identified with the 3Σ– ground state. The energy of this state was calculated using second-order Moller–Plesset perturbation theory with the 3–21G basis set, and found to be 35.84 eV. The other three measured double-ionization energies can be identified with previously computed triplet-state energies.

Double ionization of the ethyne molecule

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) Greens 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 1Δg and 1Σ+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 C2H2+2 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 1Δg and 1Σ+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 1Πu state and 1±g state, the double-ionization energies to which are predicted to be 39.0 eV and 41.0 eV, respectively.

Double ionization to singlet and triplet electronic states of CCl42

Abstract The semiempirical MSXα computational method has been applied to calculate the double-ionization energies to singlet and triplet electronic states of CCl42+. In addition, a high resolution double-charge-transfer spectroscopy study of CCl4 was undertaken in which the double-ionization energies to singlet and triplet states of the dication were measured. These are in good agreement with the calculated values, thus giving confidence in the overall predictions of the computational study.

Electronically excited states of the CH3I2+ ion

A double-charge-transfer spectroscopy study has provided evidence for the existence of four low-lying electronic states of the CH3I2+ ion, the double-ionization energies to which are 27.0 ± 0.3, 29.6 ± 0.3, 31.3 ± 0.5 and 36.5 ± 0.5 eV. Three of these energies agree, within experimental error, with those determined previously in a dissociative double photoionization study of CH3I. The present investigation reveals for the first time the state at 29.6 eV. The value of the double-ionization energy to the ground triplet state, calculated in the present investigation using the single-determinant Hartree–Fock approximation to the many-electron wavefunction with corrections of second-order Moller–Plesset perturbation theory for correlation effects, is 25.80 eV, somewhat lower than the measured value of 27.0 eV.

A combined computational and experimental study of the triple ionization of carbon disulphide

Abstract 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.

Electronic-state energies of the CH2I2+2, CHI2+3 and CI2+4 dications

The double-ionization energies of CH2I2, CHI3 and CI4 have been measured and calculated. Double-charge-transfer spectrometry was used in the experimental part of the investigation, and a modified multiple scattering Xα method was used to calculate the energies. In order to test the accuracy of the calculations, the computational method was initially applied to CH3I; the values obtained were found to be in good agreement with those measured previously. Calculated data for CH2I2 indicate that a large number of electronic states of the dication exist. By forming appropriate groups of states, mean calculated double-ionization energies were obtained which agree with the measured data. For CHI3 and CI4, the number of states calculated to exist is considerably less. When groups need to be formed, the numbers in them are much less than those for CH2I2. Good agreement between grouped calculated data and those measured was obtained for CHI3 and CI4. The exception was the four lowest states of CI42+ which appear not to be populated experimentally. A probable explanation is that the endoergicities of the relevant double-electron-capture reactions are too low, lying outside the operative reaction window of endoergicities.

Double-ionization energies to ground and electronically excited singlet and triplet states of CCl2+4, CCl3F2+, CCl2F2+2 and CClF2+3

Abstract Double-charge-transfer spectroscopy has been used to measure singlet and triplet electronic-state energies of CCl 2+ 4 , CCl 3 F 2+ , CCl 2 F 2+ 2 and CClF 2+ 3 ions. Because of spin conservation, singlet states were populated when using H + as the projectile ion and triplet states when using F + . Also, double-ionization energies to ground singlet and triplet states were calculated with second-order Moller—Plesset perturbation theory. The calculated energies are in reasonable to good agreement with those measured. Some of the energies have been measured previously, and the corresponding data from the present investigation are in good agreement with them. The singlet-state energies for the last three title ions have been measured for the first time in the present investigation, and the higher-lying triplet-state energies of all the ions have been measured more accurately using the F + projectile ion.