John H.D. Eland

Multielectron coincidence spectroscopy has been used to study multiple ionization of atomic mercury. The binding energies of triply and quadruply ionized states of Hg have been determined from three- and fourfold electron coincidences. Relativistic ab initio theory has been used to calculate the state energies and predict the experimental findings.

Final-state trication spectra and electron distributions produced by soft x-ray single-photon triple ionization of rare gas atoms have been obtained by a multiple-coincidence technique using storage-ring synchrotron radiation. The technique uses electron time of flight with ion detection to overcome the problem of high repetition rates in single-bunch operation. A correction needed to the triple-ionization energy of Kr currently listed in standard tables is confirmed, and the method’s ability to examine the three-electron distributions, characterizing the ionization mechanisms and post-collision interactions, is illustrated.

The electronic structure and fragmentation of the cyanogen dication formed by double photoionisation of the neutral NCCN molecule, are investigated by means of time of flight photoelectron–photoelectron coincidence (TOF-PEPECO) and photoelectron–photoion–photoion coincidence (PEPIPICO) techniques. Large scale ab initio computations at the cc-pVDZ/RCCSD(T) and cc-pVQZ/CASSCF levels of theory are performed in order to provide detailed structures, harmonic wavenumbers, vertical excitation energies and unimolecular decay pathways of the NCCN⁺⁺ and CNCN⁺⁺ isomers. Both theoretical and experimental data show that the NCCN⁺⁺ dication is metastable; it converts rapidly into the CNCN⁺⁺ most stable isomer for internal energies above 2 eV. Multi-step dissociation pathways are proposed. The dominant production of ions from unsymmetrical fragmentations is rather unexpected and is explained by rapid rearrangement to the CNCN⁺⁺ isomer.

The double photoionization of CF3I and the electronic structure and the dissociation dynamics of the CF3I++ dication have been investigated using large ab initio calculations and coincidence techniques. The double photoionization spectrum of CF3I consists of a continuous background with a number of narrow bands superimposed. The spectrum is attributed here to the population of groups of close lying electronic states interacting mutually by spin-orbit, spin-spin, and rovibronic couplings. At energies near the vertical double ionization threshold, CF3++I+ ionic fragments are produced. At higher energies, a very specific dissociation with double charge retained on one fragment, CF3I++→CF2I+++F becomes dominant and is attributed to a specific group of dication electronic states.

State-selective mass spectrometry has revealed one conclusive and another probable metastable state of the N2O2+ dication, assigned respectively as 1Π3 at 38.5eV and 2Π3 at 42.5eV. Photon coincidence experiments confirm that dissociation of 1Π3 is preceded by a fluorescent transition to XΣ−3 and also indicate that an identical mechanism occurs for 2Π3. Highly correlated MRCI calculations are performed at a range of N2O2+ geometries, from which both N–N and N–O bond stretching curves are generated. Substantial barriers along both coordinates are observed for 1Π3 and 2Π3, although the increasing density of states at higher energy may allow spin-orbit or vibronic predissociation for 2Π3. Fragment emissions derived from N2O+ and N2O2+ are analyzed with the aid of glass filters, from which NO (XΠ2←AΣ+2) and vibrationally excited N2+ (XΣg+2←BΣu+2) transitions are deduced.