State selective fragmentation of doubly ionized sulphur dioxide

Using multi-electron–ion coincidence measurements combined with high level calculations, we show that double ionisation of SO2 at 40.81 eV can be state selective. It leads to high energy products, in good yield, via a newly identified mechanism, which is likely to apply widely to multiple ionisation by almost all impact processes.

Double and Triple Ionisation of Isocyanic Acid

Double and triple ionisation spectra of the reactive molecule isocyanic acid (HNCO) have been measured using multi-electron and ion coincidence techniques combined with synchrotron radiation and compared with high-level theoretical calculations. Vertical double ionisation at an energy of 32.8 ± 0.3 eV forms the 3A” ground state in which the HNCO2+ ion is long lived. The vertical triple ionisation energy is determined as 65 ± 1 eV. The core-valence double ionisation spectra resemble the valence photoelectron spectrum in form, and their main features can be understood on the basis of a simple and rather widely applicable Coulomb model based on the characteristics of the molecular orbitals from which electrons are removed. Characteristics of the most important dissociation channels are examined and discussed.

Triatomic molecules

Double ionisation of the triatomic molecules presented in this chapter shows an added degree of complexity. Besides potentially having many more electrons, they have three vibrational degrees of freedom (three normal modes) instead of the single one in a diatomic molecule. For asymmetric and bent triatomic molecules multiple modes can be excited, so the spectral bands may be congested in all forms of electronic spectra, including double ionisation. Double photoionisation spectra of H2O, H2S, HCN, CO2, N2O, OCS, CS2, BrCN, ICN, HgCl2, NO2, and SO2 are presented with analysis to identify the electronic states of the doubly charged ions. The order of the molecules in this chapter is set first by the number of valence electrons, then by the molecular weight.

Core–core and core–valence double ionisation

Basic concepts of inner shell double ionisation phenomena are discussed and examples are presented. An empirical model to calculate single-site K-shell double ionisation energies is proposed and the enhanced chemical shifts in double L-shell ionisation are illustrated. Core–valence double ionisation spectra are shown to closely resemble photoelectron spectra from single ionisation in many cases. Core–valence double ionisation spectra of NH3, CO, CO2, OCS, CS2, CF4, Si(CH3)4, benzene, and C60 are presented with analysis. In the context of core ionisation it is customary to use the very economical ‘KLM’ notation, which is universally used in Auger spectroscopy, as well as the chemists’ familiar 1s, 2s, 2p, and so on. This chapter uses the KLM notation sparingly where its brevity is an advantage, so a short table list of equivalents is also included here.

Molecules with four, five or seven atoms

Double photoionisation spectra of NH3, C2H2, HCHO, C2N2, PCl3, CH4, the methyl halides CH3F, CH3Cl, CH3I, the methylene halides CH2Cl2, CH2Br2, CH2I2, the carbon tetrahalides CF4, CCl4, CBr4, germanium tetrahalides GeCl4, GeBr4, and SF6 are presented with analysis to identify the electronic states of the doubly charged ions. The effects of indirect double ionisation pathways are discussed. There are relatively few important molecules with just four atoms, but most of the ones included here are present and sometimes abundant in planetary and astrophysical environments. The range of five-atom molecules includes methane and all its simple derivatives. Where possible closely related molecules are grouped together in this chapter, as much of the discussion of their electronic structure is the same for all members of a group. This chapter also includes SF6 as a closely related molecule, even though its atom count goes beyond those of some molecules in later chapters.

Mainly aliphatic molecules

Double photoionisation spectra of homologous iodides and alcohols, acetonitrile, methyl mercaptan, acetaldehyde, acetone, norbornane, cyclooctatetraene, and TMMD are presented. Effects on the spectra of these molecules from electronic state congestion and geometry changes on ionisation mean that only the lowest dication states can be identified. As little detailed analysis of the individual spectra is possible, this chapter presents the molecules in groups rather than individually. In this chapter, molecules are ordered more thematically than strictly by size. The chapter starts with four homologous iodides and three homologous alcohols. Then this chapter takes some individual molecules with different substituent groups and proceed to a few larger molecules. The chapter demonstrates the dominant effect of the distance to which charges can separate in the dication on the double ionisation threshold.

Conjugated and aromatic molecules

In the vast majority of conjugated and aromatic molecules, the outermost occupied orbitals are either of π‎ character or non-bonding lone pairs belonging to heteroatoms. These are the orbitals from which double ionisation gives rise to most of the distinct bands that can be discerned in their spectra. Double photoionisation spectra of ethylene, butadiene, pyrrole, furan, thiophene, benzene, hexafluorobenzene, toluene, pyridine, pyrazine, pyrimidine, pyridazine, naphthalene, azulene, quinoline, biphenyl, TDME, iron pentacarbonyl, ferrocene, and TMPPD are presented with analysis where possible. The effects of inner valence Auger effects are also emphasised, which can greatly increase the intensity of double photoionisation. In this chapter, the molecules are ordered mainly in the usual way by number of atoms, then by molecular weight, but the authors have put closely related molecules together where possible.

Double Photoionisation Spectra of Molecules

This book contains spectra of the doubly charged positive ions (dications) of some 75 molecules, including the major constituents of terrestrial and planetary atmospheres and prototypes of major chemical groups. It is intended to be a new resource for research in all areas of molecular spectroscopy involving high energy environments, both terrestrial and extra-terrestrial. All the spectra have been produced by photoionisation using laboratory lamps or synchrotron radiation and have been measured using the magnetic bottle time-of-flight technique by coincidence detection of correlated electron pairs. Full references to published work on the same species are given, though for several molecules these are the first published spectra. Double ionisation energies are listed and discussed in relation to the molecular electronic structure of the molecules. A full introduction to the field of molecular double ionisation is included and the mechanisms by which double photoionisation can occur are examined in detail. A preliminary chapter covers double photoionisation of an atom in order to explain the basic principles of the technique, then five chapters present spectra of molecules of increasing size. A seventh chapter on the new fields of core–core and core–valence double ionisations, with selected examples, completes the main body of the book. Appendices explain the detailed mechanisms of double photoionisation, the calibration of the electron spectrometers, and give a brief summary of the methods by which double ionisation energies are calculated theoretically.

Introduction

After very brief historical notes, the basis of the TOF-PEPECO technique is explained and other techniques for spectra of doubly charged positive ions are described and compared with this modern method. The meaning of ionisation energies in the context of molecular double ionisation is discussed, with their relationship to electron orbital configurations. With the advent of photoelectron spectroscopy in the 1960s, new techniques allowed complete spectra of valence electron ionisations for each molecule to be revealed in a single measurement. The effects on the spectra of the different major pathways from starting molecules to final doubly ionised states are explained. Details of the experiments are given, including pulsed lamps, synchrotron radiation as light sources, and the magnetic bottle time-of-flight electron spectrometer.