US3622781A

US3622781A - Mass spectrograph with double focusing

Info

Publication number: US3622781A
Application number: US32931A
Authority: US
Inventors: Helmut Liebl
Original assignee: Institut fuer Plasmaphysik GmbH
Current assignee: Institut fuer Plasmaphysik GmbH
Priority date: 1969-07-30
Filing date: 1970-04-29
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23
Also published as: DE1938770B2; NL7004207A; GB1320206A; DE1938770A1; LU61429A1; FR2056163A5; BE749340A

Abstract

To provide for improved energy output and image resolution of Mattauch-Herzog type mass spectrographs, a beam-imaging means, which may be a separate electronic lens, or part of the electric field sector deflection arrangement suitably energized, is located in advance of the beam energy aperture, by a distance such that the exit slit of the ion source is imaged in the plane of the beam energy aperture. A second electronic lens is arranged between the beam energy aperture and the magnetic field sector deflection arrangement, of similar focal length, and arranged to image the target plane of the mass spectrograph at the same plane of the beam energy limiting means, so that the image focused thereon by the first imaging arrangement is received and focused on the target plane by the second electronic lens; the magnetic field sector deflection arrangement is rotated with respect to the electric field sector deflection arrangement by 180* to compensate for the effects of the lenses. The presence of the lenses permits independent adjustment of dispersion angles and energy band width of the ion beams.

Description

United States Patent 72] Inventor Helmut Llebl Echlng, Germany [2]] Appl. No. 32,931

[22] Filed Apr. 29, 1970 [45] Patented Nov. 23, 1971 73] Assignee Institut fur Plasmaphyslk G.m.b.H.

Garchlng, Germany [32] Priority July 30, 1969 [3 3 Germany [54] MASS SPECTROGRAPH WITH DOUBLE 3,233,099 2/l966 Berry et al. 250/4l.9 ME 3,524,056 8/1970 Enge etal. 250/419 ME Primary Examiner-Anthony L. Birch Attorney-Flynn & Frishauf ABSTRACT: To provide for improved energy output and image resolution of Mattauch-Herzog type mass spectrographs, a beam-imaging means, which may be a separate electronic lens, or part of the electric field sector deflection arrangement suitably energized, is located in advance of the beam energy aperture, by a distance such that the exit slit of the ion source is imaged in the plane of the beam energy aperture. A second electronic lens is arranged between the beam energy aperture and the magnetic field sector deflection arrangement, of similar focal length, and arranged to image the target plane of the mass spectrograph at the same plane of the beam energy limiting means, so that the image focused thereon by the first imaging arrangement is received and focused on the target plane by the second electronic lens; the magnetic field sector deflection arrangement is rotated with respect to the electric field sector deflection arrangement by 180 to compensate for the effects of the lenses. The presence of the lenses permits independent adjustment of dispersion an- 8 2 90555! b i qf iqi fi ion eam MASS SPECTROGRAPH WITH DOUBLE FOCUSING The present invention relates to a mass spectrograph in which the path of ions is so arranged that the ions are focused twice, in order to permit independent adjustment of the energy level ranges as well as of the angle of the beam.

Mass spectrographs of the Mattauch-l-lerzog type are widely used in industry and research. This type of mass spectrograph is distinguished from other types of mass spectrographs in that all particles can be focused simultaneously along a straight line-see, for example, H. Ewald and H. Hintenberger Methoden und Anwendungen der Massenspektroskopie," Chemieverlag Weinheim 1953. The ions are focused twice, that is focusing is obtained by a double-focusing arrangement. lons which are introduced into the entrance slit in a predetermined, small angular range and which fall within a predetermined energy level, are focused to a mass spectrum as a first approximation. Mass spectrographs of this type do, however, have the disadvantage that it is not possible to independently adjust the ranges of the angle and of the energy level. The reason is seen in that the entrance slit is placed in the focal length of the electrical sector field, so that ions of varying energy which leave the electrical sector field or the analyzer form differently oriented bundles of ions, the ions within the bundles themselves being parallel.

Spectrographs as referred to may limit the permitted range of the angle at the inlet side of the electrical sector field by a diaphragm opening. An aperture or opening between the electrical and the magnetic sector field can limit the energy ranges of the ions. This aperture may be referred to as an energy diaphragm, and it functions as a beam energy limiting means. The energy aperture however decreases the angular range of the ion beam and even if the energy aperture is infinitesimally small, ions which should be limited by the ion aperture can still pass therethrough.

To obtain reasonable resolution of masses, both the angle and the energy range of the ions in the beam must be made small to avoid line spread at the image plane. Usually three image aberrations of second order are present-see, for example, L. A. Kir'nig and H. Hintenberger, Nucl. Instr. 3, 133 (I958 One is the aperture aberration (a: aberration), the other the energy aberration (B aberration) and the third mixed aberration (a/Baberration). Of the three, the mixed (01B) aberration is the greatest. To reduce this aberration, both the aperture diaphragm as well as the energy aperture, or diaphragm must be made small, substantially decreasing the transmission of ions, that is decreasing their intensity, that is the overall ion energy, or ion current being applied to the target.

It is an object of the present invention to provide a mass spectrograph of the above referred to Mattach-Herzog type in which the energy range and the angular range can be independently adjusted while having an output at the image plane which is highly linear and nondistorted in spite of a high transmitted ion current or energy.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, double focusing is provided. Between the entrance slit and the energy aperture, an electrical beam-imaging means is provided, such as an electrical lens. An electronic lens is arranged behind the energy aperture by a distance equal to its focal length. The magnetic sector field follows this electronic lens and is so poled that the ions are deflected therein in the same sense as in the electrical sector field.

The beam-imaging means may wholly consist of the electrical sector field, which is then so formed and dimensioned that the ions are focused in the plane of the energy aperture. Alternatively, the beam-imaging means may include an electronic lens located either between the entrance slit and the electrical sector field, or behind the electrical sector field, and designed and dimensioned to provide ion beams in which the ions of similar energy level are emitted in parallel, the further electronic lens being located the entrance side to the energy aperture by a distance equal to its focal length.

'The focal lengths of one, or both of the electrical lenses are preferably adjustable, independently or conjointly.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic presentation illustrating one example of the structure of the present invention;

FIG. 2 is a presentation of the theoretical path of the beams in a portion of the mass spectrograph of FIG. 1;

FIG. 3 is a schematic presentation of a different embodiment of the present invention; and

FIG. 4 is a schematic presentation of an arrangement in which the sector field images the entrance slit in the plane of the energy aperture.

The mass spectrograph of FIG. 1 has an entrance slit 10 to which ions are applied from an ion source 12, schematically indicated only, and known in the art. In the direction of the path taken by the ions, the structure then includes: an aperture 14, located immediately in front of an electrical field sector deflection arrangement 16. Behind the electrical field sector deflection means 16 is a first electronic lens assembly 18 which focuses ions into the plane of an energy aperture 20. The ions emitted from the electrical sector field are parallel, but randomly oriented bundles of varying energy, which are now all focused on aperture 20. The distance between energy aperture 20 and lens 18 is equal to the focal length f of the lens 18. A second, electronic lens 22, located by a distance equal to its focal length from aperture 20, which preferably is the same as the focal length f of the first lens, is located behind the energy aperture 20 and which again restores the parallel relationships of the ion bundles. The parallel ion bundles are then introduced into a magnetic field sector deflection means 24, which is limited at one of its end planes by an imaging plane 26, for example a photographic plate.

The mass spectrograph of the present invention thus differs from the known instruments of this type by the presence of the

lenses

18, 22; additionally, the magnetic sector field 24 is rotated with respect to the mean entrance direction of the ions by l, so that the ions are deflected in opposite sense by the electrical and magnetic sector fields.

Energy focusing without

lenses

18 and 22 is obtained from the following relationship L =N l in which L is the angular dispersion coefficient of the electrical sector field and N is the angle-dispersion coefiicient of the magnetic sector fieldsee, for example, the above referred to literature reference by Kiinig and Hintenberger.

When ions having an energy eU,, leave the electrical sector field 16 parallel to the exit axis then the paths of the ions having an energy level e( U,,+A U) are inclined with respect to the output axis by an angle F 4 AU/2 U (2) Analogous considerations arise with respect to N for the magnetic sector field, if one considers the ion beams coursing counter the actual direction of the beam from the focusing point in the imaging plane 26. The relationship for the angle 7, of these ion beams with respect to the entrance axis of the magnetic sector field (which is coincident with the exit axis of the electrical sector field) will be as follows:

Contrary to the known mass spectrograph of the above referred to type, the two

electrical lenses

18, 22 are located between the electrical sector field l6 and the magnetic sector field 24. The first lens 18 images an intermediate image of the entrance slit 18 in its focal plane which is coincident with the plane of the energy aperture 20. Ions of energy eU are focused at the axis of the energy aperture. Ions having an energy e( U,,+A U) are focused by a distance .-=f v.-=f 1 All/2 in whichfis the focal length of lens 18 which, in the example illustrated in FIG. 1, is equal to half of the distance d between the two

lenses

18 and 22.

Let it be assumed, for purposes of discussion, that ions of a predetermined mass are emitted from the imaging plane 26 of the magnetic sector field 24 and travel counter the actual direction of the ion path. Let it be further assumed, for purposes of discussion, that ions of energy eU are focused by lens 22 on the axis indicated in FIGS. 1 and 2 in the chain-dotted line, then ions having an energy e(U,,+AU) are focused at a distance ym f 'Yn| f 2 All2 In order to obtain energy focusing, y,.= and thus which, as will be seen is the same as the relationship (1) above. The mathematical requirements of the known mass spectrographs are thus complied with except for the sign. A positivesign indicates that the deflection in both cases would be in the same direction or sense, so that, by rotating one of the fields with respect to the other by 180, the structure of the system of FIG. 1 willmeet all mathematical requirements. All other parameters of the mass spectrograph, particularly those of the electrical and magnetic sector field may be dimensioned as is well known in apparatus of this type.

The energy aperture 20 will determine in the mass spectrograph in accordance with the present invention the energy level range independent of the beam aperture, due to the presence of the two

lenses

18, 22, located symmetrically with respect to the opening of the energy aperture 20, in accordance with FIG. 1. Introducing the two

lenses

18, 22, has the further advantage that fine focusing and fine adjustment can be done entirely electrically. Mass spectrographs of the above referred to type, as known, require additionally a mechanical displacement of the entrance slit in order to finefocus the beam. Fine adjustment of energy focusing, as is known, is done by moving the grounding point of the electrical sector field, which however, usually again changes the directional focusing. In mass spectrographs of the present invention, no mechanical motion of the entrance slit is necessary and energy focusing can beachieved as noted, independently of the directional focusing.

Use of two lenses provides for double focusing of ions of energy e( U,,+AU), when the intermediate image projected by the first lens is coincident with the theoretical image of ions which would be emitted from the imaging plane 26, if they would travel backwards, through the magnetic sector field. As

' seen in FIG. 2, in which the theoretical intermediate image is indicated by a vertical arrow, coincidence of this theoretical intermediate image can be readily obtained by adjustment of the focal length of the two lenses.

In order to obtain double focusing of all masses along a straight line in imaging plane 26, it is only necessary to so adjust and design the apparatus that ions having the same energy enter the magnetic field in parallel and that, considering the ion path backwards, the lens 22 in advance of the magnetic field 24 forms 'a theoretical intermediate image of the entrance slit which is coincident with the image formed by the other lens, with respect to those ions having the energy e( U,

The intermediate image of the entrance slit 10 can be obtained by various arrangements. For example, it is possible to form the intermediate theoretical image solely by the electric sector field, as is being done by a known mass spectrograph see E. G. Johnson and AD. Nier, Phys. Rev. 91, I0 I953).

High transmission and low image aberration is obtained when the combination of an. electrostatic lensand a sector field is used, and the lens is used in advance of the sector field-see H. Liebl, J. Appl. Phys. 38, 5277 (1967). An example is seen in FIG. 3 in which an imaging arrangement to obtain a theoretical intermediate image entrance slit 10 is shown. It includes an electric sector field l8, and an electronic lens 16' in advance thereof. The relationship to obtain double focusing'for the case, in which only a single lens, namely the one in advance of the magnetic field, is located between the f... focal length of the lensin front of the magnetic sector K, exit dispersion coefficient of the electrical sector field I. angular dispersion coefficient of the electrical sector N, angle dispersion coefficient of the magnetic sector Reference is here also made to the above referred to publication by Konig and Hintenberger.

The above referred to advantage of the exclusively electric fine adjustment of the double focusing is retained in the example of FIG. 3.

The arrangement described above, as well as the arrangement in accordance with FIG. 1 is well suited to incorporation of the Mattauch-Herzog type mass spectrograph, to obtain independent adjustment of aperture and energy width. To give an example, the invention can be applied to the known double focusin Stigmatically imaging mass spectrograph illustrated in Zeitschrift fiir Naturforschung, Vol. 140, Number 2, 1959, pp. 1-29 to 14 I. This known mass spectrograph has been modified by locating, in accordance with the arrangement illustrated in FIG. 1, two electronic lenses between the electrical and the magnetic sector field, and by rotating the magnetic sector field by The lenses used consist of three cylindrical electrodes of equal inner diameter D, the length (thickness) of the central electrode being D/2,' and the distance between the center electrode and the outer electrode, each, being D/4. A potential is applied to the center electrode of U A; 0.5 U (wherein U is the energy of the ion/electron discharge). Focal length f g 10 D. If D=l cm., then f=l0 cm., and the distance d of the two lenses between themselves will be equal to 2f=20 cm. These two lenses are introduced between the electric and magnetic sector fields, with the energy aperture therebetween. The distances between the electric and magnetic sector fields, at their final limits, are 24.6 cm., and the magnetic field is rotated by 180 about its exit direction.

Other types of lenses may be used, such as extended apertures which, analogous to optical cylindrical lenses focus in only one plane. They will then be so arranged that the focusing plane is coincident with the deflection plane of the mass spectrograph.

Focusing the

electronic lenses

18, 22, can readily be accomplished by connecting the elements of the electronic lenses to a source of voltage, as schematically indicated in FIG.- 2.

Potentiometers

32, 38 are provided, connected to a source,

from which tap points are connected to the center elements of J the electrostatic lens. As schematically indicated by the f focusing or for separate balancing of manufacturing tolerances.

FIG. 4 illustrates an arrangement in which the sector field alone images the entrance slit in the plane of the energy aperture. The mathematical relationships for double focusing are' identical to those discussed in connection with the embodiments of FIGS. 1 to 3, namely formula (7) is equally applicable.

Various angles and parameters are indicated in FIG. 4. These angles, and parameters, in accordance with a calculated example, may be as follows:

The sector field is a toroidal condenser, having an mean axial radius of curvature R,.=3.22r,.. The toroidal condenser, and the axial effect of the lenses in combination with the slanting limits of the magnetic field results in focusing the ions also perpendicularly to the plane of the drawing (see for example: R. Herzog ActaPhys. Austriacae 4 (I950) p. 413). The example of FIG. 4 illustrates that the image plane does not coincide with the trailing limit of the magnetic field, but rather forms an angle in therewith, which intersects at its origin with the intersection of the beam axis with the leading limit of the magnetic field.

The significance of the remaining dimensions and parameters will be obvious from a consideration of FIG. 4. Focusing of the lens 22 can, again, be readily accomplished by connecting the center electrode to a source of varying potential, for example to the tap point of a potentiometer.

1 claim:

1. Mass spectrograph comprising an ion source (12) generating a beam of ions;

an entrance slit through which said beam of ions passes;

a target plane (26) to receive the ion beam;

means directing said ion beam from said slit to said target plane (26) in a predetermined path; electric field sector deflection means (16) located in the path of said beam electrically deflecting said beam;

magnetic field sector deflection means (24) located in the path of said beam and magnetically deflecting said beam;

beam energy limiting means (20) located in the path of said beam and disposed between said electric and magnetic field sector deflection means; beams imaging means (16-18; 16'-l8') located between the entrances slit and the beam energy limiting means and focusing said beam, in its path, to image said entrance slit in the plane of the beam energy limiting means (20) and in advance of said magnetic field sector means (24);

an electric lens (22) located in the path of said beam in advance of said magnetic field sector deflection means spaced from said beam energy limiting means (20) by a distance equal to its focal length and energized to transform the beam entering said electronic lens (22) to a parallel beam so that the beam entering the magnetic field sector deflection means 24) is parallel;

and wherein said magnetic field sector deflection means (24) is poled and oriented to deflect said ions in the same sense and direction as said electric field sector deflection means l6).

2. Mass spectrograph according to claim 1, wherein the energy band path width of said beam energy limiting means (20) is adjustable.

3. Mass spectrograph according to claim 1, including a beam width limiting means (14) located between the entrance slit 10) and the beam-imaging means (l6, l8).

4. Mass spectrograph according to claim 3, wherein the beam pass width of said beam width limiting means 14) is adjustable.

5. Mass spectrograph according to claim 1, wherein said beam-imaging means comprises said electric field sector deflection means (16) and means energizing said deflection means to simultaneously deflect said ion beam and to focus the image of said slit by said beam in the plane of the beam energy limiting means (20).

6. Mass spectrograph according to claim 1, wherein said beam-imaging means comprises an electronic lens assembly (18) located behind said electric field sector deflection means (16) in the path of said beam, said electronic lens assembly being spaced from said beam energy limiting means (20) by a distance equal to the focal length (I) of the electronic lens assembly 18).

7. Mass spectrograph according to claim 1, wherein said beam imaging means comprises an electronic lens assembly (16) located in advance of said electric field sector deflection means (18) in the path of said beam.

8. Mass spectrograph according to claim 7, wherein the focal length of the beam imaging means (16'18) is adjustable.

9. Mass spectrograph according to claim 1, wherein the focal length of the electronic lens (22) located in advance of the magnetic field sector deflection means (24) is adjustable.

10. Mass spectrograph according to claim 1, wherein the focal length of said beam imaging means (16, 18) and said electronic lens (22) is adjustable;

and means (30) are provided for conjointly varying the focal length of said beam energy means (16) and said electronic lens (22).

Claims

1. Mass spectrograph comprising an ion source (12) geNerating a beam of ions; an entrance slit (10) through which said beam of ions passes; a target plane (26) to receive the ion beam; means directing said ion beam from said slit to said target plane (26) in a predetermined path; electric field sector deflection means (16) located in the path of said beam electrically deflecting said beam; magnetic field sector deflection means (24) located in the path of said beam and magnetically deflecting said beam; beam energy limiting means (20) located in the path of said beam and disposed between said electric and magnetic field sector deflection means; beams imaging means (16-18; 16''-18'') located between the entrances slit and the beam energy limiting means and focusing said beam, in its path, to image said entrance slit in the plane of the beam energy limiting means (20) and in advance of said magnetic field sector means (24); an electric lens (22) located in the path of said beam in advance of said magnetic field sector deflection means spaced from said beam energy limiting means (20) by a distance equal to its focal length and energized to transform the beam entering said electronic lens (22) to a parallel beam so that the beam entering the magnetic field sector deflection means (24) is parallel; and wherein said magnetic field sector deflection means (24) is poled and oriented to deflect said ions in the same sense and direction as said electric field sector deflection means (16).

3. Mass spectrograph according to claim 1, including a beam width limiting means (14) located between the entrance slit (10) and the beam-imaging means (16, 18).

4. Mass spectrograph according to claim 3, wherein the beam pass width of said beam width limiting means (14) is adjustable.

6. Mass spectrograph according to claim 1, wherein said beam-imaging means comprises an electronic lens assembly (18) located behind said electric field sector deflection means (16) in the path of said beam, said electronic lens assembly being spaced from said beam energy limiting means (20) by a distance equal to the focal length (f) of the electronic lens assembly (18).

7. Mass spectrograph according to claim 1, wherein said beam imaging means comprises an electronic lens assembly (16'') located in advance of said electric field sector deflection means (18'') in the path of said beam.

8. Mass spectrograph according to claim 7, wherein the focal length of the beam imaging means (1618'') is adjustable.

10. Mass spectrograph according to claim 1, wherein the focal length of said beam imaging means (16, 18) and said electronic lens (22) is adjustable; and means (30) are provided for conjointly varying the focal length of said beam energy means (16) and said electronic lens (22).