Radiant energy – Electron energy analysis
Patent
1991-03-11
1992-11-24
Berman, Jack I.
Radiant energy
Electron energy analysis
250306, 250281, 250294, H01J 3705
Patent
active
051665190
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an image band pass filter operating in a photoelectron spectromicroscope and particularly to the low pass filter elements of that image band pass filter.
BACKGROUND OF THE INVENTION
An image band pass filter operating in a photoelectron spectromicroscope is known and comprises two stages, the first comprising a region in which a beam of imaging electrons is deflected in the crossed electrostatic and magnetic field between a succession of electron mirrors which are designed to remove electrons of a certain energy and reflect those of a particular energy which are to form the electron image.
The beam from the first stage is then transferred to the second stage through a high pass filter into a region of the second stage and then deflected in an electrostatic and magnetic cross field along a path corresponding to a restoration of the displacement of the original beam in the first stage from its original path of travel.
In the first and second stage of the analyser as above described, the electrostatic field of the cross field is designed to be non-uniform whereby to control motion of the electrons in the cross field and correct for distortion of the image in the output beam.
In such spectroscopy the signal to background ratio is of high significance particularly in the detection of weak signals. The first low pass filter encountered by the image stream in the band pass analyser as described above, plays a vital role in determining the signal/scattered electron background ratio. This arises from the unwanted higher energy electrons obtaining axial momentum in the band pass range after elastic and inelastic collision with the absorber plate of the low pass filter and hence being transported through the analyser.
Such scattered electrons may have both large energy and low axial momentum thus contributing a diffuse background within the pass band of the subsequent low pass and high pass elements of the analyser.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome this defect and to reduce the proportion of elastically and inelastically scattered high energy electrons escaping the energy filters.
According to the invention there is provided a device for energy filtering a beam of imaging charged particles such as positions or electrons travelling under the control of a crossed electrostatic and magnetic field comprising a filter housing provided with an aperture for accepting the charged particles in the imaging beam, means for providing a virtual electrical potential surface within the housing for reflecting oncoming charged particles in the imaging beam having a potential energy below that of the virtual surface, and charged particle trap means for preventing oncoming higher energy charged particles passing through said virtual surface returning through said aperture via said virtual surface thereby to be lost to the system.
Advantageously the electron or other charged particle trap is provided by electrostatic field means acting transverse to the direction of travel of the oncoming higher energy electrons to transfer the electrons to magnetic flux lines of the prevailing magnetic field outside the aperture.
The electrons whose path of travel has been so diverted by the electrostatic field means, may then be absorbed by an electron absorption element arranged in their path of travel.
The electrostatic field means may incorporate sooted mesh structures held at varying potentials with respect to the wall defining the electron trap so creating non-axial electrostatic fields driving the electrons into complex motions and trajectories and eventually out of the system.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings wherein:
FIG. 1 is a schematic view of an imaging band pass electron energy analyser;
FIGS. 2(a), 2(b) and 2(c) are front and rear perspective views and a schematic view respectively of a low pass filter inco
REFERENCES:
patent: 3579270 (1971-05-01), Daly et al.
patent: 3774028 (1973-11-01), Daly
patent: 3786359 (1974-01-01), King
patent: 4292519 (1981-09-01), Feverbaum
patent: 4417175 (1983-11-01), Curren et al.
patent: 4810883 (1989-03-01), Turner
Berman Jack I.
Beyer James
Price, Jr,. Stanley J.
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