John H. D. Eland

Theoretical and experimental studies of the triatomic doubly charged ions CO2+2, OCS2+, and CS2+2

The term schemes of the doubly charged ions CO2+2, OCS2+, and CS2+2 have been calculated by the CIPSI method using a new and economical basis set of polarized atomic orbitals and difference orbitals. The calculated energies are compared with existing and new experimental data obtained from Auger spectra, double charge transfer, and photoionization including the PIPICO technique. A complete assignment of the manifolds of singlet states is achieved with good agreement between theory and experiment. Only the lowest doubly charged ion states are well described by simple two‐hole configurations, while three‐hole one‐particle configurations are important at higher energy. It is confirmed that the triplet ground states of these ions are well populated by photoionization, while the excited singlet states are revealed most clearly by the double charge transfer technique.

Velocity imaging photoionization coincidence apparatus for the study of angular correlations between electrons and fragment ions

A new apparatus has been developed to detect and measure angular correlations between energy-selected photoelectrons and coincident mass-analyzed fragment ions from photoionization at selected wavelengths. It achieves velocity imaging for electrons and ions simultaneously and has high collection efficiency for both particles, with moderate mass and energy resolution. Angular and energy correlations between the two particles are measured, as are the angular distributions of each particle independently relative to the light polarization direction. Fixed-molecule electron angular distributions are deduced in cases of pure axial recoil. Examples of angular distributions from photoionization of diatomic molecules are reported.

Single photon double ionization studies of CS2 with synchrotron radiation

Single photon double ionization of CS2 has been investigated, using mass spectrometric coincidence techniques: both metastable and dissociative CS++2 states have been observed; three different dissociation pathways of CS++2 have been demonstrated, including one bond (S++CS+) and two bonds (S++C+S+ and S++C++S) breakings; simulation of the observed dissociation provided some insight into the dissociation mechanisms. Monochromatized synchrotron radiation enabled us to measure the excitation spectra of these relatively intense processes, in the 25–75 eV photon energy range. Our results provide an approach to the spectroscopy of the doubly charged CS++2 ion; comparison with a SCF‐LCAO‐MO calculation leads to a tentative assignment of the observed states.

Reaction dynamics of three-body dissociations in triatomic molecules from single-photon double ionization studied by a time- and position-sensitive coincidence method

Application of the position-sensitive photoelectron - photoion - photoion (PSD-PEPIPICO) method to dissociative double-ionization processes in and at photon energies of 40.8 and 48.4 eV provides measurements of total kinetic energy releases and complete descriptions of the kinematics for three-body dissociations of the type . Comparisons of experimental observations with predictions from sequential and concerted bond breaking mechanisms suggest a unified model in which both bonds break on a time scale of , and electron ejection may be either faster (direct ionization) or competitive (indirect) on this time scale.

Lifetimes of metastable molecular doubly charged ions

Abstract Methods for the determination of molecular dication decay constants using the charge separation mass spectrometry technique have been developed. Mean lifetimes have been determined for CO2+2 (900+400−200ns), N2O2+ (450+250−200ns) and CO2+ (600+600−200ns). In other cases metastable dication decay must be described by more than one mean lifetime.

Femtosecond X-ray-induced explosion of C 60 at extreme intensity

Understanding molecular femtosecond dynamics under intense X-ray exposure is critical to progress in biomolecular imaging and matter under extreme conditions. Imaging viruses and proteins at an atomic spatial scale and on the time scale of atomic motion requires rigorous, quantitative understanding of dynamical effects of intense X-ray exposure. Here we present an experimental and theoretical study of C60 molecules interacting with intense X-ray pulses from a free-electron laser, revealing the influence of processes not previously reported. Our work illustrates the successful use of classical mechanics to describe all moving particles in C60, an approach that scales well to larger systems, for example, biomolecules. Comparisons of the model with experimental data on C60 ion fragmentation show excellent agreement under a variety of laser conditions. The results indicate that this modelling is applicable for X-ray interactions with any extended system, even at higher X-ray dose rates expected with future light sources.

Molecular photodissociation studied by VUV and soft x-ray radiation

A brief account of the developments in photodissociation studies of free molecules by VUV and soft x-ray sources is given as an extended introduction. Then two typical experimental setups are described for multiple-ion coincidence momentum imaging and high-resolution Auger electron–ion coincidence momentum imaging. Finally, some individual cases of molecular dissociation following core-hole creation are examined, to illustrate how the complex multidimensional data produced can be represented and interpreted. The new experimental techniques based on Coulomb explosion are shown to allow direct characterization of the initial nuclear motions which follow electronic excitation.

Electronic States and Decay Mechanisms of the N2o2+ Dication

Energies of the electronic states of the triatomic dication N2O2+ in the Franck–Condon zone of neutral N2O have been determined by a combination of (1) double charge transfer spectroscopy to locate singlet states, (2) photoionization measurements to locate the lowest triplet state, and (3) configuration‐interaction calculations to identify the states and to predict the energies of other triplets. It seems likely that two distinct charge separation reactions compete in the relatively slow decay of the N2O2+ ground state.

Spectroscopy and metastability of CO22+ 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.

Complete double photoionisation spectrum of NO

Abstract A new time-of-flight photoelectron–photoelectron coincidence technique, which gives complete two-dimensional e−–e− spectra in single-photon double photoionisation, is applied to nitric oxide. Direct double photoionisation is the major process at photon energies of 40.8, 48.4 and 50 eV, but dissociative ionisation with autoionisation of atomic fragments is also significant at these wavelengths and is dominant at 38.7 eV photon energy. The energy of the v=0 level of the A2Π state of NO2+ is corrected, leading to a new prediction of the wavelength of optical emission. The electron energy distributions in formation of the bound states of NO2+ are smooth, lacking sharp autoionisation features. Broad peaks found in the photoelectron distributions coincident with autoionisations of atomic oxygen are at different energies for each different O* state, indicating the involvement of several distinct dissociative superexcited states of NO+.