T J Reddish

Dual toroidal photoelectron spectrometer for investigating photodouble ionization in atoms and molecules

A photoelectron-photoelectron coincidence spectrometer, incorporating two independent toroidal analyzers, has been developed to study photodouble ionization of gas targets using synchrotron radiation. The energy-resolved and angle-dispersed electron images are recorded using two-dimensional position-sensitive detectors. The design and performance of the spectrometer is described, with particular emphasis on the electron optical properties of toroidal deflectors and their associated electrostatic lenses. The operation and calibration of the spectrometer are discussed and sample (γ,2e) results of helium are presented.

2e) cross section measurements of and He

Angular distributions of the two ejected electrons following photodouble ionization have been measured for molecular and He with . The results were obtained at the Daresbury Synchrotron Radiation Source using a multicoincidence spectrometer consisting of two toroidal analysers. The measured distributions show strong similarities, as well as differences which suggest that molecular effects are manifest at these energy conditions.

Theoretical study of space focusing in linear time-of-flight mass spectrometers

In response to continued improvements in the production of “cold” atoms, molecular beams, and in electronic timing resolution, the issue of space focusing in linear time-of-flight (TOF) mass spectrometers is reevaluated. Starting with the Wiley–McLaren [W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum. 26, 1150 (1955)] condition for first-order space focusing in the conventional two-field system, we extend the approach to higher orders in more complicated situations. A general, solvable, set of equations for satisfying n-order space focusing in an m-field regime is derived. We demonstrate quantitatively that if higher orders of space focus are employed, then provided the initial velocity distribution of the ions is sufficiently narrow, a significant improvement in the mass resolution can be achieved. The conclusions drawn have important implications for the design of the next generation of TOF instruments.

Photodouble Ionization of Helium at an Excess Energy of 40 eV

Angular correlation in the two-electron continuum of the He double photoionization has been studied, both experimentally and theoretically, for equal and unequal energy sharing conditions at the photon energy of 40 eV above the threshold. The triple differential cross sections have been measured in the plane perpendicular to the photon direction using the multi-coincidence detection technique of Reddish et al at the Daresbury Synchrotron Radiation Source. Recent modifications to the bending-magnet beam line allowed an effective cancellation of the circular polarization at the target, leaving a relatively high degree of linear polarization (Stokes parameters: S 3 = S 2 0, S 1 = 0.8). The measured cross sections are compared with the calculations using the 3C method of Maulbetsch and Briggs and the convergent close coupling method of Kheifets and Bray. Good agreement between theory and experiment has been found in most cases, except for the unequal energy sharing when one of the escaping electrons is detected in a direction close to the polarization axis.

Optimization of momentum imaging systems using electric and magnetic fields

Time-of-flight (TOF) momentum imaging systems utilize the x, y, t information from charged particles striking a position-sensitive detector to infer the x, y, and z components of the particles’ initial momenta. This measurement capability can lead to the complete experimental determination of multi-ionization/fragmentation dynamics. In the case of electron detection, the addition of a magnetic field leads to a significantly increased operational energy range. This study shows that the TOF system has to be carefully designed in order to optimize the magnetic confinement effect. Expressions for the optimal dimensions of a single electric field TOF system are derived and factors contributing to the resolution are discussed, along with their application to an existing imaging system.

Helium (γ, 2e) triple differential cross sections at an excess energy of 60 eV

γ, 2e) triple differential cross sections (TDCS) are presented for helium at 60 eV above the photodouble ionisation threshold with energy sharing ratios ( R = E2/E1) for the two ejected electrons of R = 1, 5 and 11. The measurements were taken using a toroidal spectrometer and linearly polarised light from an undulator beamline (SU6) at the Super-ACO synchrotron. Good agreement is found with TDCSs obtained by the CCC method and by the length gauge of 3C theory.

Photo double ionization of molecular deuterium

A helium-like description of photo double ionization of molecular deuterium is extended to derive a dependence of molecular excitation amplitudes on electron energy sharing and dynamical quantum numbers labelling internal modes of excitation of the escaping electron pair. Both linear and circular polarizations are considered, and predictions regarding circular dichroisms in the angular distributions of the electron and ion pairs are given. A detailed comparison of the model with recent (,2e) measurements is also presented.

Photodouble ionization differential cross sections for D2 with various electron energy sharing conditions

The mutual angular distributions of the two ejected electrons following direct photodouble ionization have been measured for D2 at an excess energy (E) of 25 eV using linearly polarized light. These (γ ,2 e) ‘t riple’ differential cross sections (TDCSs) were obtained for asymmetric electron energy conditions with energy sharing ratios (R = E2/E1) of R = 24, 11.5, 4a nd2.57. In all cases the ‘reference’ electron (energy = E1 )w as orie nted along the direction of the electric field vector (e )a nddetected in coincidence with a second electron (energy = E2) coplanar with e and the photon beam direction (kγ ). For comparison, helium TDCSs were obtained for the same E and R values under nearly identical spectrometer conditions. These show very good agreement with the results of hyperspherical-� -matrix with semi-classical outgoing waves calculations, thus providing even more confidence in the D2 TDCSs where there is as yet no accurate ab initio theory. The similarities and differences between the experimental results associated with the two targets are qualitatively discussed in terms of Feagin’s model (Feagin J M 1998 J. Phys. B: At. Mol. Opt. Phys. 31 L729).

The design and performance of an effusive gas source of conical geometry for photoionization studies

The design, construction, and performance of an effusive gas source of conical geometry is reported. This gas jet enables experiments that require the gas and photon beams to be essentially collinear and has a focusing multicapillary array with the central portion left free for the photon beam. In total, the source comprises 90 “tubes” in three layers and has been designed by modeling free molecular gas flow and optimizing for highest gas density ∼2–3 mm from the jet’s exit plane. The nature of the gas flow through the source and its focusing properties are investigated theoretically and experimentally.

An angle-resolved dissociative photoionization study of the c4Σ−u state in O+2 using the TPEPICO technique

We have investigated the angular distributions of O + ( 4 S) ions produced from dissociative photoionization of O + c 4 � − u (ν = 0, 1) using the TPEPICO technique, i.e. by measuring the coincidence yield between threshold photoelectrons and photoions. The vibrational levels have distinctly different lifetimes, τ ν, which diminish their inherent anisotropic photoion angular distribution characterized by a β parameter. We obtain τ1 = 6.0 ± 0.3 × 10 −14 s and a lower limit on τ0 of ≈1 × 10 −12 s, in broad agreement with other experimental studies using different methods, and find that β = 0.40 ± 0.05, which is significantly at variance with the predicted value of 1.6.