The Fully Computer Controlled


Computer Optimised

(e,2e) Coincidence Spectrometerat Manchester

Page constructed by Andrew Murray

This Page has been updated on the 12th January, 2006.

Papers from the group about (e,2e) processes in Manchester since 1992:

Murray A J, Turton B C H & Read F H, Rev. Sci. Inst. 63 3346-3351  (1992)
"Real-time computer-optimized electron coincidence spectrometer"

Murray A J,  Woolf M B J and  Read F H, J. Phys. B  25 3021-3036  (1992)
"Results from symmetric and non-symmetric energy sharing (e,2e) experiments in the perpendicular plane"

Murray A J and Read F H, Phys. Rev. Lett.  69 2912-2915  (1992)
"Novel exploration of the helium (e,2e) ionization process"

Murray A J and Read F H, J. Phys. B  25 L579-L583  (1992)
"Coplanar doubly-symmetric He (e,2e) measurements with excitation of the residual ion"

Murray A J and Read F H, Phys. Rev. A 47 3724-3732  (1993)
"Evolution from the coplanar to the perpendicular plane geometry of helium (e,2e) differential cross section symmetric in scattering angle and energy"

Murray A J  and Read F H, J. Phys. B 26 L359-L365  (1993)
"Exploring the helium (e,2e) differential cross section at 64.6eV with symmetric scattering angles & non-symmetric energies"

Murray A J, Read F H and Bowring N J, J. de Phys. IV 3 51-58 (1993)
"(e,2e) Collisions at intermediate energies"

Murray A J, Read F H and Bowring N J, Phys. Rev. A  49 R3162-R3165  (1994)
"Decomposition of Experimentally Determined Atomic (e,2e) Ionization Measurements"

Murray A J, Read F H and Bowring N J  J. Phys. B 30 387-402  (1997)
" Parameterisation of low energy symmetric (e,2e) differential cross section measurements"

Bowring N J, Murray A J and Read F H  J. Phys. B 30 L671-L676  (1997)
" Near-threshold doubly symmetric (e,2e) measurements on helium"

Bowring N J, Read F H & Murray A J,  J.  de Physique 9 Pr6-45-49  (1999)

" Dips and backward peaks in the helium (e,2e) differential cross section at low energies"

Bowring N J, Read F H, Murray A J   J. Phys. B 32 L57-L63 (1999)
"Two-electron interference in the helium (e, 2e) differential cross section at 64.6eV"

Murray A J and Read F H    J. Phys. B  33 L297-L302  (2000)
"Low energy (e,2e) differential cross section measurements on neon from coplanar to perpendicular plane geometry"

Murray A J, Bowring N J and Read F H   J. Phys. B  33 2859-2867  (2000)
" Comparison of argon and helium (e,2e) differential cross sections at 64.6eV using symmetric detection energies and angles"

Murray A J and Read F H    Phys. Rev. A  63 0127141-0127144  (2001)
    "Deep Interference Minima in Experimental Ionisation Differential Cross Sections"

Murray A J  Meas. Sci. Tech.  13 pN12 (2002)
"Construction of a gravity fed circulating liquid nitrogen dewar for experiments in high vacuum”

Hussey M and Murray A J   J. Phys. B 35 3399-3409 (2002)

       "Low energy (e, 2e) differential cross-sections on the 3sigmag and 1piu molecular orbitals of N2"

Cvejanovic D and Murray A J    Meas. Sci. Tech. 13 1482-1487 (2002)
"Design and characterization of an atomic beam oven for combined laser and electron impact experiments"

Murray A J    Meas. Sci. Tech. 14 N1-4 (2003)
“Design of a non-magnetic translator for use in vacuum systems”

Cvejanovic D & Murray A J    J. Phys. B 36 3591-3605 (2003)
“Single ionization of calcium by electron impact”

Murray A J & Cvejanovic D    J. Phys. B 36 4889-4910 (2003)
“Low Energy Superelastic Scattering from the 41P1 state of Calcium in an (e,2e) spectrometer.”

Murray A J & Cvejanovic D   J. Phys. B 36 4875-4888 (2003)
“Coplanar symmetric (e,2e) measurements from calcium at low energy.”

Murray A J & Atkinson S   Meas. Sci. Tech. 15 N31- N34 (2004)
“An Automatic Controller for Filling and Maintaining Liquid Nitrogen Levels in Dewars”

Murray A J, Hussey M J and Venables A    Meas. Sci. Tech. 16 N19-N23 (2005)
“Design of a non-magnetic high accuracy linear translator for use in vacuum systems”

Murray A J    J. Phys. B 38 1999–2013 (2005)

“(e,2e) studies of H2 in the intermediate energy regime”

Hussey M J  & Murray A J    J. Phys. B 38 2965–2977 (2005)

“Low energy (e,2e) differential cross-sections on the 1pig and 4sigma
g molecular orbitals of CO2.”

Murray A J    Phys. Rev. A 72 062711 – 062725 (2005)
“(e,2e) ionization studies of alkali and alkali earth targets: Na, Mg, K and Ca, from near threshold to beyond intermediate energies.”

Gao J, Madison D H, Peacher J L, Murray A J & Hussey M J, J Chem. Phys. 124 194306.1 – 194306.8 (2006)
“Experimental and theoretical (e, 2e) ionization cross sections for a hydrogen target at 75.3 eV incident energy in a coplanar asymmetric geometry”

Murray A J, Hussey M J, Gao J & Madison D H, J. Phys. B 39 3945-3956 (2006)

“(e,2e) Ionization measurements from the  3 sigma
g and 2sigmau*  states of H20  – comparison between experiment and theoretical predictions of the effects of two-centre interferences”

Lookat the (e,2e) Computer Controlled Spectrometer Hardware

Lookat the Symmetric (e,2e) Data collected by this spectrometer

Lookat the Symmetric data parameterisation

Lookat the Data where the Ion is left in an Excited State

Lookat the 64.6eV Data where the detected electrons have unequal energies

Lookat the results that were collected in the Perpendicular plane ionisingHelium

Link to the Experiments conductedin the Laser Collisions Laboratory at Manchester

Link to theManchester Electron Scattering group Home Page

Link to the Atomic, Molecular & Laser Manipulation Group Home Page

Linkto the Manchester Physics & Astronomy Department Home Page


The purpose of this page is to detail for the interested reader thefully computer controlled and computer optimised(e,2e) coincidence experiment at

Although there is an implicit assumption that the reader has a familiaritywith coincidence experiments, this is not completely necessary to enjoythis page and the associated pages that accompany this home page.

For those who are interested, many excellent reviews on this subjectcan be found, including articles by:

Overview of Atomic Coincidence Experiments

Coincidence experiments are used in atomic physics principally in twoways.

The first way that they are used is as a 'noise' filter, whereunwanted signals from events occurring at the interaction region are rejectedby carefully timing the detected events so as to exclude those that cannotarise from the reaction. As an example, such experiments may be used toexclude the cascade contributions to lifetime measurements of atomic fluorescencewhen states lying higher than the state under investigation are excited.

The principle use of coincidence experiments however, is to investigatesingle event processes that occur when either electrons, ions, atomsor photons interact with the target atoms or molecules. This is usuallyachieved by detecting the momentum of the scattered particle (in futurehere described as an electron) and observing the correlated reaction ofthe atomic system.

Three types of coincidence experiment are briefly described :

Further types of coincidence measurements are possible. These include

1.Electron-Photon Coincidence experiments

In the case of electron photon coincidence experiments, the atomicsystem excited by electron impact reacts by emitting a photon, which issubsequently detected either in angular correlation with the scatteredelectron, or the polarisation of the correlated emitted photon is measured(Figure 1).

electron-photon scheme

Examples of electron-photon experiments can be found in:

1.Stepwise Electron-Photon Coincidence experiments

stepwise laser electron-photon scheme

Examples, including theoretical and experimental papers which detailthis type of electron-photon coincidence experiment can be found in:

3. The (e,2e) Ionisation coincidence experiment

An alternate type of experiment to those investigating EXCITATIONof the target are those where the incident electron has sufficientenergy to IONISE the target. For theseso-called (e,2e) experiments the energy lost by the incident electron uponscattering from the target atom is sufficient to promote ionisation ofthe target, which ejects either a valence electron or an inner shell electron(should the incident energy be sufficiently large). The angular correlationwhich exists between the electron emitted from the target and the scatteredelectron is then measured, as shown in figure 3:

Examples of (e,2e) experiments can be found in the reviews by

It is these ionisation coincidence experiments that are performed usingthe spectrometer in Manchester.

The Manchester (e,2e) Experiment (1982-1990)

The Manchester (e,2e) experiment has been designed primarilyto study angular correlations arising between the scattered electron andan electron ejected from a valence state of the target. As such, the incidentelectron energy is adjustable from 20eV to 300eV.

The spectrometer has the advantage that all possible geometriesare accessible from coplanar geometry to the perpendicular planegeometry (see figure 4). The original experiments by Hawley-Jones etal, J Phys B 25 2398 (1992) measured ionisation close to thresholdto study the so-called Wannier effect. These experiments were carried outin the perpendicular plane using a hemispherical energy selected electrongun.

coplanar-perp plane geometry

Further work using an unselected electron gun by M.B.J. Woolf (Ph.DThesis, Man. Univ. (1989)) measured the angular correlated differentialcross section for electron scattering from a helium target for incidentenergies from 10 to 80eV above the ionisation threshold in the perpendicularplane, with the scattered and ejected electrons emerging from the scatteringregion with equal energy.

At the completion of these experiments hardware was installed for testingthe computer controlled optimisation routines, allowing a feasibility studyto be conducted. The computer controlled hardware and associated softwarewas tested by optimising signal from resonance states in helium. Full detailsof these experiments may be found in the thesis of B.C.H. Turton (Ph.DThesis University of Manchester (1990)).

The Manchester (e,2e) Experiment (1990-present)

Following these initial optimisation experiments, the apparatus wasmodified to allow coincidence experiments to be conducted using the computercontrol and optimisation hardware. This required considerable modificationto the apparatus and the software controlling the experiment. In addition,the efficiency of the experiment was reviewed and significant improvementswere made to increase both the analyser efficiency and the timing resolution.Implementationof these improvements occupied most of 1990, and coincidence data collectionre-commenced in November 1990, once more in the perpendicular plane.

For full details of the computercontrol hardware, see

The purpose of the perpendicular plane experiments was twofold.

Results verified that the computercontrol and optimisation significantly improved the experimentaldata when compared with data obtained with manual operation, both in statisticalaccuracy and angular symmetry. This data accumulated in less time thancould be obtained manually, since the experiment ran 24 hours a day.

Since these initial measurements data has been obtained in the perpendicularplane for symmetric and non-symmetric energies over the incident energyrange from 34.6eV to 104.6eV. For full details, see

Following from these perpendicular plane measurements, results couplingthe coplanar symmetric differential cross section to the perpendicularplane differential cross sectionhave been obtained from 44.6eV to 74.6eV incident energy, confirmingthat rapid changes occur to the differential cross section in this energyregime.See

These results, together with additional results closer to the ionisationthreshold, have been parameterisedin terms of a set of orthogonal angular functions defining thecorrelation between three vectors in space, in this case chosen to be theincident, scattered and ejected momenta of the electrons taking part onthe reaction process. For details of this parameterisation,see

The parameterisationallows the angular and energetic parts of the differential crosssection to be separated, and allows a common basis to be defined for allionisation processes.

Measurements of the cross section for ionisationwith excitation of the ion have also been carried out with thisspectrometer, these experiments confirming the advantages afforded by computercontrol which allows the stability of the experiment to be maintained overa period of months.For details, refer to:

Experiments to obtain the differential cross section for ionisationof Argon ranging from the perpendicular plane to the coplanar geometryhave also been conducted, although the results of these experiments havenot as yet been published.

Additional measurements at 27.6eV, 29.6eV and 34.6eV incident energyon Helium ranging from the coplanar to the perpendicular plane have recentlybeen completed. These results also have not yet been published.