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.

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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.

Near-threshold inner-shell photoionization of neon has been studied by measuring threshold photoelectron spectra in the energy regions of the 1s ' main line and the 1s2p'nl satellite states. The measured linewidth (0.22+0.03 eV) of the 1s -+3@ transition is more consistent with the observation of the 1s hole state linewidth than previous determinations.Features, assigned as doubly excited states and conjugate shakeup transitions, have been discerned in the 1s2p'nl satellite spectrum. PACS number(s): 32.80.HdThe high sensitivity and energy resolution of threshold photoelectron spectroscopy (TPES) have made this technique a unique tool to study photoionization of valence and inner valence shells of a,toms [1 -3] and molecules [4 -6]. Recently, the application of TPES has been extended to the photoionization of inner shells of the rare gases [7,8] and small molecules [9,10]. This paper de-

High resolution threshold photoelectron and photoion spectra of the oxygen molecule have been measured in the photon energy range 12-50 eV using a penetrating field electrostatic analyser and a synchrotron radiation source. Besides the well known X 'nS, a 'K, A 'nu, b 4 q , B 'r; , c 'Xi 0 : states, several other series of vibrational structure are observed in the 21-24eV range. One series is interpreted to result, not from ad ion state, but from an autoionization prh-ess involving a neutral state and the nuclear continuum of an ion state. Above 26eV, three relatively strong features are observed at approximately 27.3 eV. 32.5 eV and 39.5 eV. interspersed with several weaker ones. Dissociation processes from some 0 : states have been studied using a threshold photoelectron-photoion coincidence technique (TPBPICO). These observations identify the weak, broad feature at 24.96 eV as the u=2 level of the c %state.

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A new type of photoelectron/photoion coincidence spectrometer using a penetrating field technique has been used to investigate photo-double ionization processes in helium. Threshold photoelectron/photoion coincidence measurements have been used to determine the value of the exponent in the Wannier theory and its range of validity. The value obtained is consistent with the Wannier prediction. Photoelectron/photoion coincidence measurements have also been used to investigate the behaviour of the asymmetry parameter, beta , for near threshold photo-double ionization and a near constant value of beta , close to -0.4, has been observed. This result is in disagreement with the predictions of the Wannier theory which appears to underestimate the angular correlation between the two electrons.