The Fully Computer Controlled
(e,2e) Coincidence Spectrometerat Manchester
Page constructed by Andrew Murray
This Page has been updated on the 12th January, 2006.
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
Atomic, Molecular & Laser Manipulation Group, Department of Physics & Astronomy, SchusterLaboratory, Manchester University, Manchester, M13 9PL, United Kingdom.
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
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).
Figure 1. The electron-Photon coincidence experiment.An incident electron excites the target to an intermediate excited state,losing energy in the process. This electron scatters through an angle thetawith respect to its incident direction, and is subsequently detected usingan energy and angle selecting analyser. The excited target relaxes eitherback to the ground state or to a lower intermediate state, releasing energyas a photon during this process. Detection of this photon either as a functionof angle with respect to the incoming electron, or by measuring the polarisationof the photon in coincidence with the scattered electron yields detailedinformation about the excitation process.
Examples of electron-photon experiments can be found in:
Figure 2. The stepwise Laser electron-Photon coincidenceexperiment. An incident electron excites the target to an intermediateexcited state, losing energy in the process. This electron scatters throughan angle theta with respect to its incident direction, and is subsequentlydetected using an energy and angle selecting analyser. The excited targetis further excited using single mode CW laser radiation whose power, polarisationand radiation direction is accurately controlled. The upper laser excitedstate relaxes either back to a lower intermediate state, releasing energyas a photon during this process. Detection of the polarisation of thisphoton in coincidence with the scattered electron yields detailed informationabout the electron excitation process. Details of the Laser pumping processcan also be determined from these measurements.
Examples, including theoretical and experimental papers which detailthis type of electron-photon coincidence experiment can be found in:
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:
Figure 3. The (e,2e) interaction region and scatteringgeometry. An incident electron of sufficient energy collides with a targetat the interaction region. The target is subsequently ionised, the incidentelectron losing energy in the process and scattering into an angle theta.The target is split into an ion and an ejected electron. This ejected electronleaves the interaction region at some angle, as does the scattered incidentelectron. Both electrons are detected in coincidence using energy and anglesensitive analysers. The angular correlation between these electrons measuredas the detectors roam over various angles then reveals detailed informationabout the ionisation process.
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.
Figure 4. The (e,2e) scattering geometry accessible inthe Manchester experiment. The spectrometer allows full access to the coplanargeometry, where the incident, scattered and ejected electrons are all inthe same plane, through to the perpendicular plane geometry, wherwe thescattered and ejected electrons emerge perpendicular to the incident electrondirection. In these experiments it is convenient to define the detectionplane as the plane of spanned by the ejected and scattered electrons, incontrast to the scattering plane defined in figure 1. Clearly the ionisationis independant of the chosen geometry, and so results obtained in one geometrycan be defined in the alternate geometry via a simple transformation.
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.
For a more complete description of the (e,2e)Spectrometer Hardware and results, follow the links given below: