G Dawber

Threshold photoelectrons coincidence spectroscopy of doubly-charged ions of nitrogen, carbon monoxide, nitric oxide and oxygen

Single photon double ionization of nitrogen (N2), carbon monoxide (CO), nitric oxide (NO) and oxygen (O2) has been studied in an electron-electron coincidence experiment. Threshold photoelectrons coincidence (TPEsCO) spectra have been obtained using a dual penetrating field technique, in which only electrons with nearly zero kinetic energy (<20 meV) are detected. The high sensitivity and energy resolution attainable have allowed observation of several states of N22+, CO2+, NO2+ and O22+, 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 TPEsCO spectra, and the molecular parameters derived from them, are presented, and the possible mechanisms of formation are discussed.

A penetrating field electron-ion coincidence spectrometer for use in photoionization studies

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.

Double photoionization of CO2, OCS, C2H2, CF4, and C6H6 studied by threshold photoelectrons coincidence (TPEsCO) spectroscopy

Abstract Double photoionization of CO 2 , OCS, C 2 H 2 , CF 4 and C 6 H 6 molecules has been studied in an electron-electron coincidence experiments, where nearly zero kinetic energy (⩽20 meV) electrons are detected. This first application of TPEsCO spectroscopy to polyatomic molecules confirms the ability of this experimental technique to determine accurate double ionization potentials and locate excited electronic states. The spectrum of benzene reveals what appears to be the first observation of vibrational structure in a polyatomic dication.

Threshold photoelectrons coincidence spectroscopy of doubly charged ions of hydrogen chloride and chlorine

Single photon double ionization of hydrogen chloride (HCl) and chlorine (Cl2) has been studied in an electron-electron coincidence experiment. Threshold PhotoElectrons COincidence (TPEsCO) spectra have been obtained by using a penetrating field technique, in which only electrons with nearly zero kinetic energy (<20 meV) are detected. The high sensitivity and energy resolution attainable has allowed observation of several states of HCl2+ and Cl22+, excited essentially at their respective thresholds, including vibrational energy levels for some states. Accurate energies of the dicationic (doubly charged ion) features observed in the TPEsCO spectra are presented, and the possible mechanisms of formation are discussed.

High resolution threshold photoelectron and photoion spectroscopy of oxygen in the 12-50 eV photon range

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 2 Pi g, a 4 Pi u, A 2 Pi u, b 4 Sigma g-, B 2 Sigma g-, c 4 Sigma u- O2+ states, several other series of vibrational structure are observed in the 21-24 eV range. One series is interpreted to result, not from an ion state, but from an autoionization process involving a neutral state and the nuclear continuum of an ion state. Above 26 eV, 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 O2+ states have been studied using a threshold photoelectron-photoion coincidence technique (TPEPICO). These observations identify the weak, broad feature at 24.96 eV as the nu =2 level of the c 4 Sigma u- state.

Observation of the argon photoelectron satellites near threshold

The authors have studied near-threshold photoionisation of argon in the region of the valence satellites (32 to 49 eV) in two ways. They have measured photoelectron spectra for various constant values of photoelectron energy, from zero to 0.48 eV. Secondly, they have measured relative partial ionisation cross sections for many of the ionic states in this region, over the energy range from zero to 1.0 eV above their respective thresholds. The dominant contribution to these cross sections is observed to come from the decay of doubly excited neutral states or resonances.

Near-threshold photoionization of the Ar 2p subshell

Near-threshold photoionization of argon in the region of the 2p subshell has been studied. Firstly we have measured the threshold photoelectron spectrum in the energy regions of the 2p12 ,3/2/-1 main lines and of the 2p53p5 nl satellite states and have discerned a large number of new features in the satellite region. Secondly the angular distributions of the photoelectrons ejected from the 2p subshell have been measured from 0.25 to 30 eV above threshold.

THRESHOLD PHOTOELECTRON SPECTROSCOPY OF HCL UP TO 40 EV

The threshold photoelectron spectrum of HCl has been recorded using synchrotron radiation over the energy range encompassing both outer and inner valence ionization. A large enhancement in the intensity of the v+ = 0–1 bands of the HCl+ 2∏32 subsystem was observed in comparison with the same vibrational bands of the 2∏12 subsystem which are attributed to autoionization processes. Resonance autoionization was also found to be responsible for the populating of upper vibrational levels (v+ = 2–14) of the X 2∏i system in the Franck-Condon gap region preceding the onset of the A 2∑+ band system. The observation of vibrational-band broadening for v+ = 7–10 of the A 2∑+ system is attributed to combined autoionization processes between repulsive neutral Rydberg states and bound vibrational levels of the A 2∑+ state and the continuum of repulsive ion states, the latter of which are responsible for predissociation in the A 2∑+ state of HCl+. In the inner valence ionization region (20.0–40.0 eV), pronounced resonance structure was observed which correlates well with the calculated energy positions of 4σ−1 satellite ions states of HCl. In addition, the direct populating of repulsive ion states in the 20.0–25.2 eV range is suggested by the observation of several broad, structureless band features, and the detection of three vibrational bands in this same energy range suggests the presence of a bound neutral Rydberg state that undergoes autoionization into the continuum of a repulsive ion state.

Near threshold TDCS for photo-double ionization of helium

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<E<2 eV, for both equal and unequal energy sharing between the two outgoing electrons. The measured data are compared with the predictions of the Wannier model, and also with recent, non-Wannier, ab initio calculations. In addition, also for the first time, the relative magnitudes of the various TDCS measured have been determined in this excess energy region, suggesting a departure from the predictions of the Wannier model at the largest excess energy studied, E=2 eV.

A study of and satellite states observed by `threshold photoelectrons coincidence' spectroscopy

The and spectra have been studied from their respective thresholds up to their respective triple ionization potentials by threshold photoelectrons coincidence (TPEsCO) spectroscopy. The and (n = 2, 3 in Ne and Ar, respectively) states of the dications as well as the satellite states, where two electrons are ejected and one is promoted into an unfilled orbital, have been observed. This study is the first extensive observation of the satellite states of dications by electron emission spectroscopy and extends the previous observations by optical emission spectroscopy to the region of the triple ionization threshold. New features have been discerned in the spectra and an assignment is proposed based on previous results from optical spectroscopy and on calculations with an atomic multiconfigurational Dirac - Fock code.