M A MacDonald

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.

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.

Direct observation of vibrational states of CO2+ and N22+ by threshold photoelectrons coincidence spectroscopy

Double ionization of CO and N2 induced by photons from a synchrotron radiation source has been studied in an electron-electron coincidence experiment. A new technique for this type of study has been developed wherein only electrons with near-zero energy are detected by the use of the penetrating field method. In this way high sensitivity and energy resolution have been attained and this has allowed vibrational structure to be observed for the doubly-charged ions of both these molecules and accurate energies for the states to be obtained.

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.

A photoelectron study of resonance structure in the argon correlation satellites

Doubly excited neutral states of argon have been investigated by observing their decay into the 3p and 3s main ionisation lines and into the satellite ionic states. These were measured using photoelectron spectroscopy and monochromatic synchrotron radiation in the range 31.0 to 42.3 eV. The 3p and 3s main line constant ionic state spectra show structure due to the decay of doubly excited states but in general these are much less prominent than in the spectra for the satellites which are dominated by resonances, particularly close to threshold. There is also a strong selectivity with marked differences in the resonance structure observed in the decay to different satellite states. Four Rydberg progressions have been observed each of which is split into two series.

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.

Near-threshold single photon double ionization of neon, argon, krypton and xenon observed by electron-electron coincidence spectroscopy

A photoelectron spectrometer composed of two analysers of different types has been used in an electron-electron coincidence mode to study photo-double ionization phenomena in neon, argon, krypton and xenon. Relative partial cross sections of the lowest three double ion states measured close to threshold would indicate that the predicted state selectivity is roughly adhered to except in the case of neon. The near-threshold angular behaviour of the ejected photoelectrons did not indicate a trend dependent on the symmetry of the state of the double ion as would be expected from theory.

Threshold photo-double ionization processes in helium studied by a photoelectron/photoion coincidence technique

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.

Double and single ionization of neon, argon and krypton in the threshold region studied by photoelectron-ion coincidence spectroscopy

Photoionization and photo-double ionization of neon, argon and krypton have been studied in the threshold region for double ionization by measuring coincidences between zero energy photoelectrons and the singly or doubly charged product ions (TPEPICO technique). The efficiency of this technique has been much improved by the use of the penetrating field method for the detection of both the photoelectrons and the ions. The observations show that autoionization of satellite states makes a dominant contribution to the double ionization process and that these states decay solely into this channel when it is energetically allowed. beta parameter measurements up to 2 eV above the double ionization potential or argon for electrons correlated to Ar2+ ions do not reveal the behaviour expected from theoretical predictions.