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By clicking Confirm, you are committing to buy this item from the seller if you are the winning bidder and have read and agree to the Global Shipping Program terms and conditions - opens in a new window or tab. As noted on previous pages, (e,2e) experiments which measure the angular correlations between scattered and ionised electrons following a collision provide a rigorous test of theories of electron impact ionisation of atoms.
These experiments have almost always been concerned with ionisation in which the residual ion is left in its ground state, and have principally been carried out in either coplanar geometry or in perpendicular plane geometry. These studies have almost all been carried out at incident electron energies in excess of 650eV. These experiments have been of the glancing type, with one of the outgoing electrons carrying away most of the energy and being deflected in the forward direction. In this page results are presented for a coplanar non-glancing experiment at a much lower incident energy. The excitation energy of the excited (n=2) electron in the He+ ion core is comparable to the kinetic energy of the detected electrons in this experiment, and so strong correlations between all three electrons in the final state might be expected. Measurements are also presented for direct ionisation to the ionic ground state, where the energy of the detected electrons was selected to be the same as for ionisation with excitation.
The doubly-symmetric differential cross-section (DCS) was obtained by varying the angle q and measuring the (e,2e) coincidence signal as a function of this angle. The ionic 2P substate populations and coherences could additionally be determined by measuring the angular distribution or polarisation of the emitted photons. The spectrometer used to collect these results is a fully computer controlled and real-time computer optimised (e,2e) coincidence spectrometer.


The spectrometer is maintained at its optimum during data accumulation, with frequent adjustment for any long term drifts.
The spectrometer is mounted in a vacuum chamber at a background pressure of typically 10E-8 torr. Two hemispherical deflection analysers with acceptance half angles of 3A° rotate in the detection plane (figure 2). Following energy selection the electrons are detected by channel electron multipliers, whose pulses are pre-amplified and noise discriminated. The discriminators feed a time to amplitude converter via appropriate delay lines, resulting in an 8ns FWHM coincidence signal which accumulates in a multi-channel analyser located on the IBM 80286 PC computer bus. The computer software optimises the spectrometer at regular intervals using a modified simplex technique.
The direct ionisation experiments carried out at 104.6eV incident energy were interleaved with the experiments where the ion was left in an excited state. 18 counts per second to approximately 1 count every 30 seconds, for a total runtime of 4.8E5 seconds.
These extremely low coincidence rates mean that the spectrometer must be stable for the very long times that data has to be accumulated to become statistically meaningful.
The ratio of differential cross-sections for the two processes is therefore given by the ratio of observed coincidence count rates. Figures 3 and 4 show the normalised differential cross-sections plotted on linear and logarithmic scales in order to highlight the differences in the forward and backward scattering directions.
The ratio of cross-sections is seen to be approximately 100 : 1 and so separate scales have been used for the two sets of measurements. It has been suggested that this results from elastic scattering of the incident electron through 180A° from the atomic core followed by a quasi-free binary collision. By contrast, the result for ionisation with excitation does not appear to exhibit this deep minimum or structure, although the very low measured coincidence counts at these angles make any definite conclusion difficult.
There is a slight structure observed at theta ~ 60A° for ionisation with excitation which does not appear for direct ionisation, and since these results were taken at the same time, this is not considered to be an instrumental effect. Direct ionisation from a p-orbital exhibits structure in the cross-section with a splitting of the binary and backscatter peaks, a secondary peak appearing roughly at 50 - 60A° in the forward lobe. The cross-section for excitation to the 2S ionic state would be expected to forward peak at approximately theta = 45A°, as in the direct ionisation process.
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The incident electron energy ranges widely from near threshold to many keV above threshold, allowing different theoretical approximations to be investigated. The measurements show a marked contrast to those for direct ionisation, with the backscatter peak being larger than the forward scatter peak.
This allows a direct comparison to be made between the two processes, since the analyser efficiencies remained unchanged.
The (e,2e) Experimental Geometry chosen to be in Coplanar Geometry with equal selection of energy for the scattered and ejected electrons.
Such an experiment would, however, be very difficult in practice due to the extremely low triple coincidence yields that would be obtained. This is essential in experiments which require long data accumulation times such as the one described here.
If the spectrometer drifts over these times in energy, electron beam current , tuning conditions or gas density then the results are far more difficult to interpret.
The DCS for ionisation and excitation of the ion plotted on a linear scale, normalised to unity at theta = 90 degrees. The DCS for ionisation and excitation of the ion plotted on a logarithmic scale, normalised to unity at theta = 90 degrees.
The forward scattering cross-sections are qualitatively similar in form, in contrast to the results for the high energy asymmetric geometry. It is perhaps not unreasonable to assume that analogously, excitation to the excited 2P state might exhibit similar structure. A similar structure might be expected in a backscatter peak at approximately 120A°, where the incident electron is initially scattered elastically by the atom prior to ionisation and excitation, and a suggestion of such a peak can be seen in figure 4.
If you reside in an EU member state besides UK, import VAT on this purchase is not recoverable. The use of the computer control to maintain the operating conditions at a stable point over the 2 months continuous accumulation time was essential for success of these experiments.
Even with this sophisticated feedback control, there is evidence of variations in the cross sections obtained as seen in figures 3 & 4. 23 L641(1990) at 100eV as expected, although the magnitude differs slightly because the incident energy is different.



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