Single photon double ionization of nitrogen (NI), carbon monoxide (CO), nitric oxide (NO) and oxygen (0,) has been studied in an electron-electron coincidence experiment. Threshold photoelectrons coincidence (TPWO) spectra have k e n obtained using a dual penetrating field technique, in which only electrons with nearly zero kinetic energy (c20 meV) are detected. The high sensitivity and energy resolution attainable have allowed observation of several states of NF, CO", NO2+ and O?, excited essentially at their respective thresholds, resolving vibrational energy levels for nearly all states where they exist. Accurate energies of the features observed in the TPUCO spectra, and the molecular parameters derived from them, are presented, and the possible mechanisms of formation are discussed.

A versatile electron-ion coincidence spectrometer for photoionization studies is described. The spectrometer incorporates a 127 degrees cylindrical electrostatic energy analyser and a time-of-flight energy analyser. The two analysers are mounted on a turntable that can be rotated with respect to the polarization axis of the photon beam. Both analysers have a penetrating field stage which provides very high detection efficiency over a large solid angle and excellent threshold energy resolution ( approximately 3 meV) for the detection of charged particles. The analysers can also detect energetic particles. The time-of-flight analyser is used specifically for the separation and detection of different charge states of molecular photodissociation products. The use of the spectrometer to observe photoelectron-photo-ion coincidences and photoelectron-photoelectron coincidences in a variety of experiments (TPEPICO, PEPICO, TPEPECO) is illustrated by a study of the photo-double ionization of argon and of the dissociation products from the v=0 level of the c state of oxygen at 24.56 eV. Additionally, a spectrum of coincidences between threshold photoelectrons (TPESCO) is presented in the region of the 3P double ionization potential of argon.

TDCS for photo-double ionization of helium have been measured in a PhotoElectron-PhotoElectron COincidence (PEPECO) experiment. The TDCS have been obtained for the first time at very low excess energies E, 0.6 eV

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Double ionization of O2 induced by photons from a synchrotron radiation source has been studied in an electron-electron coincidence experiment. A new technique has been developed wherein only electrons with near zero energy are detected by use of the penetrating field method. In this way high sensitivity and energy resolution have been attained allowing nineteen vibrational levels of the (>2 2+ ground state to be observed and molecular parameters of this state to be obtained.