US3740552A - Method and apparatus for mass separation employing photo enhanced surface ionization - Google Patents

Abstract

Photo-assisted surface ionization techniques are employed for the separation of isotopes and the purification of materials, the formation of relatively pure and intense ion beams and plasmas and the detection of minor constituents, pollutants and leaks. A species intermingled with other constituents in an atomic or molecular beam or in a gas, for example, is selectively excited by irradiation at a resonance frequency in close proximity to an ionizing surface. The selected species, while still in the excited level, interacts with the surface and is ionized. The specimen is excited by the radiation to a level such that the energy required to ionize the atom is less than the work function of the solid surface.

Description

limited States Patent 1 Pressman METHOD AND APPARATUS FOR MASS SEPARATION EMPLOYING PHOTO ENHANCED SURFACE IONIZATION Inventor: Jerome Pressman, 4 Fessenden Way, Lexington, Mass. 02173 Filed: Apr. 29, 1971 Appl. No.: 138,640

POWER SUPPLY Primary Examiner-William F. Lindquist Attorney-Morse, Altman & Oates 1 1 ABSTRACT Photo-assisted surface ionization techniques are employed for the separation of isotopes and the purification of materials, the formation of relatively pure and intense ion beams and plasmas and the detection of minor constituents, pollutants and leaks. A species intermingled with other constituents in an atomic or molecular beam or in a gas, for example, is selectively excited by irradiation at a resonance frequency in close proximity to an ionizing surface. The selected species, while still in the excited level, interacts with the surface and is ionized. The specimen is excited by the radiation to a level such that the energy required to ionize the atom is less than the work function of the solid surface.

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POWER SUPPLY 8i CONTROLS i| lll 1H! |i |l l ||1| M 2? 'zs {{{I suPPLYe ll CONTROLS lu ||ll \24 LASER Patented June 19, 1973 3 Sheets-Sheet 1 EMISSION PROFILE OF LIGHT SOURCE N D LH T PCS mwm SREK BUHP ACCS ABSORPTION CURVE OF CHEMICAL SPECIES I X WAVELENGTH 0 A 1| 8 L E RT U 1 8-.

l/I/VE/VTOR JEROME PRESSMAN 5y mv-OMT ATTORNEYS Patented June 19, 1973 3 Sheets-Sheet I:

LASER WER SUPPLY 8 CONTROLS x) m am YO R EW N f/Y/ VOUO 2 OPSC 0 v 2 M P WM w w m 2 H M 2 2 6 2 v Q II I A I II I uA ll II I I II I I I "I I I l I I I I II I I I l IIIIIW I I I I I II w {I I I I I II I \l III II RY A: I I I I II m A WP Pw //VVE/V7'O/? JEROME PRESSMAN 5y Wm ,CZZZIW ATTORNEYS Patented June 19, 1973 3,740,552

3 Sheets-Sheet 3 IONIZING SURFACES MULTIPLE VACUUM CHAMBERS 34 LASER LASER LIGHT FLOW MOLECULAR BEAMS OVEN lNVE/VTOR JEROME PRESSMAN 5) 714 I (lax w. ATTORNEYS METHOD AND APPARATUS FOR MASS SEPARATION EMPLOYING PHOTO ENHANCED SURFACE IONIZATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a novel method and associated apparatus for separating isotopes and purifying materials, forming ion beams and plasmas and for detecting chemical species.

2. Summary of the Prior Art The separation of isotopes heretofore has been carried out by using techniques and equipment which have been relatively inefficient and expensive. One such technique involves a mass spectrometer in which a plasma moves through a magnetic field. Another technique makes use of photophysical acceleration, while another employs thermal diffusion principles. These techniques and their related equipment generally required the use of large quantities of power and have been relatively limited with respect to range of operation and productivity.

It is an object of the present invention to provide improvements in mass separation techniques and equipment. A further object of this invention is to provide a new and improved method and apparatus for forming ion beams and plasmas as well as to improve the detec tion of selected chemical species.

SUMMARY OF THE INVENTION This invention features the method of separating masses, forming beams and detecting chemical species comprising the steps of directing a diffuse mixture of particles towards an ionizing surface having a work function related to the ionization potential of the selected species, and irradiating the diffuse mixture with a beam of an electro-magnetic energy in close parallel proximity to the surface, the frequency of the beam being resonant only with the selected species to excite the species to a level where it may be surface ionized with a high degree of probability. The technique may be used to advantage in the separation of isotopes, purification of materials as well as the formation of relatively pure and intense ion beams, plasmas as well as the detection of minor constituents, pollutants and leaks.

This invention also features apparatus for separating a selected atomic or molecular species from a mass including an ionizing surface, means for directing the mass against the surface and a source of electromagnetic energy adapted to radiate the mass in an area adjacent the surface and at a frequency resonant with the selected species for raising the species to an excited level to exclusively enhance surface ionization of the species.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating resonance ionization of an excited atom at a surface,

FIG. 2 is a sectional view in side elevation, somewhat schematic, of a photo-assisted surface ionization apparatus made according to the invention,

FIG. 3 is a diagram of selective chemical species excitation by special light source,

FIG. 4 is a schematic representation showing a modification of the invention,

FIG. 5 is a schematic representation showing a further modification of the invention,

FIG. 6 is a perspective view, partially broken away and somewhat schematic, showing an apparatus for material purification or isotope separation made according to the invention, and,

FIG. 7 is a diagram showing the natural resonant lines of Lithium 6 and 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention relates to a novel method and associated apparatus for achieving photo-assisted surface ionization useful in the separation of isotopes and the purification of materials, the formation of relatively pure and intense ion beams and plasmas and the detec' tion of minor constituents, pollutants and leaks. The terms photo assisted surface ionization" as employed herein are used to refer to a surface ionization which is made selective and enhanced by radiating the desired atomic or molecular species with electro-magnetic energy, typically light, at a selected frequency which is resonant with the species to raise the species to an excited level. While still in the excited state, the species impinges a selected ionizing surface and interacts with it to be ionized.

By way of background, surface ionization is the phenomenon in which atoms or molecules are adsorbed on a heated surface and then quickly evaporated in an ionic state. Both positive and negative ions may be formed in this fashion. The atom or molecule striking the surface gives up to or acquires an electron from the surface and evaporates as a positive or negative ion. Theoretically, if the ionization potential of an electropositive atom incident on a hot surface is less than the surface work function, then it is probable that the atom will be adsorbed on the surface and lose an electron to the surface material. Depending upon the temperature of the surface, the ion may remain adsorbed or evaporate as a positive ion or as a neutral atom.

Referring now to FIG. 1 there is graphically illustrated the energy levels of an atom or molecule that has been raised by radiation of a frequency hv W,,, to an excited level W such that the energy required to ionize the atom (W, W is less than the work function (8) of an impinged solid surface where W, is the atoms ionization potential. This is represented by the follow ing equation:

a w,-W,

The ability to raise an atom to an excited level then permits that the atom to be surface ionized with a high degree of probability, assuming equation (-1) is satisfied, whereas in the ground state the probability of surface ionization may be almost zero. The theoretical fraction of a species which will be contact ionized is given by f,, where Where T equals temperature, V kt/e, where k equals Boltzmann's constant and e equals electronic charge.

For an excited atom (at level W,.,) W, in equation (2) must be replaced by W, W, and equation (2) becomes 'fx= P rH(Wr e l1" The theoretical separation factor for excited isotope raised to level W over a non-excited isotope is then I fX/fA Similarly from equations (2) and (3) it may be seen that for any given chemical species which can be excited substantially, this species can then be given in a suitable arrangement with an appropriate a preferential ionization. This preferential ionization can then be used for species separation, beam or plasma formation or detection purposes.

Essentially, the technique of photo-assisted surface ionization disclosed herein may be considered as the" selectivepseudo-lowering'of the ionization potential of a species or. isotope by means of photon absorption. This condition thus enhances the surface ionization for that species or isotope. Alternatively, the technique may be viewed as the pseudo-raising of the work function of a surface forone particular species or isotope.

Referring now to FIG. 2 of the drawings, there is illustrated somewhat schematically a system useful in the purification of materials or for separating isotopes, there being no fundamental differences between the different applications. In FIG. 2 the apparatus is generally organized about a housing to which is connected avacuum pump 12 adapted to produce an evacuated chamber within the housing for the formation of a molecular beam 14 generated by means of an oven 16 at one end of the housing and directed against an ionizing surface 18 at the opposite end of the housing. The oven 16 is separatedfrom the evacuated chamber by means of an aperture'd wall 20 through which the beam is focussed. At mess of the housing and adjacent the ionizing surface 18 a window 22 is provided to pass a beam 26 from a laser 24 mounted exterior to the chamber and positioned to direct its beam along a path parallel to the ionizing surface 18 and perpendicular to the 4 molecular beam 14. The laser beam 26 intersects the molecular or atomic beam 14 in close proximity to the surface l8 whereby a selected species within the beam 14 will be excited as close as possible to the surface 18 toiinsure thatfthe optimum number of selectively excited species contact the surface 18 still in the excited state for surface ionization.

I Optical considerations The system to be used is that of an atomic or molecular beam being illuminatednear and hitting a suitable ionizing surface such as the surface 18. A source of electro-magnetic energy, preferably the laser 24, is tuned to the resonance level of the desired species in where T the life time of the excited state and k ab sorption at line center, approximately one-third of the species in the beam 14 will be excited. For example, for T, 10 seconds and k i0 cm P5 is 10 photons/second, or on the order of a watt. For P P, the ratios of the excited species to the ground state approaches equality. The time required for absorption or a deexcitation by stimulated emission T,, from the excited state is given by:

For example, for k, 10 7" cm and P 10 photons/sec cm an atom in the field of the laser beam 26 is oscillating between the excited state and the ground state every 10 seconds..lt its velocity is 1.4. cm/sec'then the distance it moves on the average inthe excited or ground state, X, is given by:

If p, 10 cm/sec then for the values above X 10? x 10- 10 em 110 A a For T, =1osec x,',= 10 cm i ,t In terms of the surface ionization the behavior of the atom away from the surface is irrelevant. Consequently, this fact dictates that the interaction zone of the resonance radiation be adjusted so that it'paralle'ls the surface and is'X wide, subject tothe practicalities of achieving this kind'of flat-thin beam. Under the circumstances, half of the desired species would be ionized and separated out at the surface and, ideally, only one photon would be required for each atom. Consequently, the efficiency of the process would be high. A major advantage of the process is that because of [the above-described efficiency the throughput of the system can be greatly improved by slicing the-light bean 26, which typically might be 1 cm by 1 cm in area, into separate sections and using each section to operate in conjunctionwithdifferent ionizing surface areas. If for example, X 1p. 10 cm then the throughput can be increased by a factor'of 10. as suggested in FIG. 4.

Referring now to FIG. 4 of the drawings, there is il 3213, etc. Each group is connected to a housing 34 formed with a plurality-of vacuum chambers 36A, 36B,

etc., with each chamber connected to an oven 38 adapted to'produce separatemolecular beams 40A, 40B,-etc., directed against individual ionizing surfaces 42A, 42B, etc. The number of different beams corresponds with the number of fiber groups 32A, etc., and each fiber group is positioned to direct a laser beam perpendicularly with respect to the molecular flow and in close proximity to the ionizing surfaces 42A, etc. Thus a single laser may service a number of molecular beams and a corresponding number of ionizing surfaces. While five such beams and fiber groups are illus trated, obviously the number may be increased or decreased as desired.

Referring now to FIG. 5 of the drawings, there is illustrated a modification of the optical beam-slicing system and, in this embodiment, a plurality of individual mirrors 44 or the like arranged in a bank are employed in the path of a laser light beam 46 with each individual mirror being tilted at a different angle causing the beam 46 to be broken up into a plurality of beams 46A, 468, etc., depending upon the number of mirrors. Each separate beam 46A, 468, etc., is directed into a different molecular beam system and ionizing surface such as suggested in the FIG. 4 embodiment.

In the preferred form of the invention, the optical light source is a laser tuned to the resonance frequency of the selected atom which is to be separated or detected. Suitable laser devices which may be employed in this invention include continuous wave as well as pulse type dye lasers, parametrically tuned lasers and semiconductor lasers, depending upon the wave length range of the laser and the chemical species involved. Where the process is to be employed to separate isotopes or detect a particular substance in addition to the many laser fine tuning techniques available such as diffractive gratings, etc., an absorption cell may be used external to the molecular beam in which the undesired isotope absorb out their components of the radiation before the light beam enters the molecular beam apparatus.

Purification of Materials & Isotopes Separation In the system illustrated in FIG. 2, materials may be purified or isotopes separated insofar as both processes are essentially the same. For purifying materials, the light source 24 must be tuned to the resonance frequency of the desired species in the beam 14 and, in this regard, reference is made to the absorption characteristic curves shown in FIG. 3 with respect to two different species. Also, the ionizing surface 18 must be selected from a material that will cause only the selectively excited species to ionize to the exclusion of other species in the ground state. While it is known that preferential contact ionization has been proposed for material purification, the method disclosed herein extends preferential contact ionization of chemical species by the pseudo-reduction of the ionization potential, thus creating new possibilities for surface ionization, viz., for elements or compounds of higher ionization potential than would otherwise be considered for this technique.

Once the photo-assisted surface ionization technique has preferentially created the unique desired ionic species, any one of numerous techniques available may be employed to remove the ionized species. For example, there is illustrated in FIG. 6 a magnetoplasma device in which the separation is achieved through a plasma 50 moving parallel to a magnetic field generated by coils 52 about an evacuated housing 54. The plasma moves through an orifice and deposits on perpendicular or transverse plates. Alternatively, ion beam pullout can be achieved and the desired atoms or molecules can be removed from the ionizing surface in the form of ions. The specific nature of removable process is not critical in the present invention insofar as a variety of possible arrangements of electrical electrodes and/or magnetic fields used in conventional surface ionization devices may be employed for this purpose.

The method and apparatus utilized for photo-assisted surface ionization involved in the separation of isotopes is similar to that employed in the purification of materials. The only exception is that the light source must be tuned to the resonance frequency of one of the isotopes and not to that of the others. The isotope in the excited state is then preferentially ionized in accordance with equations (1 through (4). This technique is dependent upon the fact that the optical spectra of isotopes are different. By way of illustration, in FIG. 7 there are shown the resonance lines of Lithium 6 and 7. While these atomic isotopic lines have a small shift, presently available light sources, such as tuned lasers, monoisotopic lasers, use of depleted isotope absorption cells, etc., are able to resolve this difference. Molecular isotopic shifts are considerably larger, being on the order of one part in one hundred in comparison with atomic shifts on the order of one part in one thousand.

By way of example, there is described herein the parameters relating to the separation of Uranium 235 (U To separate this isotope an apparatus such as shown in FIG. 2 is employed and, in this instance, the ionizing surface 18 is fabricated from Tungsten whose work function, 8, is 4.54 electron volts. This surface is heated and serves to do the ionizing. The surface is operated at a typical temperature, T, such that V 0.2 volts. The laser 24 is tuned to the Uranium 235 line at 4153A where the isotopic shift is 0.7A, this being much larger than the uranium line width. An alternative possibility is another Uranium 235 resonance line at 5027A. It has been shown that these lines have small hyperfine structures (M. Deringer, Compt. Rendu. 250 (l960) 828. As a result, a minimum amount of interfer ence should develop between Uranium 235 and its re lated isotopes and neighboring lines of uranium and other elements. Moreover it has been shown experi mentally in uranium atomic absorption work (A. Goleb, Anal. Chem Acta (1966) I35 165) that the Uranium-234, 236 and 238 components of the 4 I 55A line do not interfere with the Uranium-235 component.

In the event that the precise laser tuning to the U-235 wavelength 4153A, or 5027A prove difficult, it may be advantageous to use an absorption cell with U-234, 236, and 238 to absorb out these components before the light beam performs the separation process.

For uranium atoms impinging on the tungsten surface, the degree of ionization (f,) for V 0.2ev, 5 4.54ev is from equation (2)f 2 X 10. For uranium excited by )t 4153A, the excited level W 2.98ev and with W; 6.22ev there is obtained for equation (3) W, W 8 6.22 2.98 4.54 orf =1 and therefore, percent ionization should occur.

Consequently, of the ionized species coming off the tungsten surface, almost all of the U235 is ionized and only 2 X 10" of the other isotopes is ionized.

Detection of Minor Constituents, Pollutants & Isotopes The present method depends on the existence of differential optical spectra of the minor constituent pollutants which is the only species excited to a resonance level and subsequently preferentially ionized at the surface. Whereas in the purification or isotopic separation application a continuous light source is desired to obtain a large throughput, in the detection application, a pulse or modulated light signal is preferred in order to utilize the wellknown advantages in signal-to-noise ratios of a narrow band signal for detection purposes.

Such detection devices are available and include locked amplifiers, etc., and are well known in the art.

The technique disclosed herein thus represents photon-assisted surface ionization techniques in which the minor constituent or pollutant ion current can be uniquely marked with an alternating or modulated current or, if desired, a direct current. In the present technique, the photon is used to place the atom in the excited state for subsequent surface ionization in contract to other techniques which detect by photon absorption or scattering with an associated photo-detector.

Formation of Specific Ion Beams and Plasmas The foregoing techniques may also be employed to create unique ion sources and beams and plasmas for various purposes. The photo-assisted surface ionization techniques in effect create a new type of ion source since the effective work potential of the ionizing surface becomes:

Where S effective work functions, 8 work function of the surface, and h v photon energy.

For example, if a surface has a 8 (work function) of 8 electron volts, for species absorbing 2ev photons and having an ionization potential of lev, abundant ionization can be created when the ground state would have little ionization. Consequently new types of ion beams, or if a plasma mode of extraction is preferred, new types of plasmas may be created. This new type of vion source and beams have applicability in ion implantation systems such as tailoring the surface properties of semi-conductor devices, mass spectroscopy, etc. The new type-plasma source may be used for scientific purposes such as the construction of new types of Q machines or industrially in electronics for such applications as forming thin films, chemical reactions, etc.

Having thus described the invention, what I claim and desire to obtain by Letters Patents of the United States is:

l. The method of separating certain particles which absorb radiation of one frequency from other particles that absorb radiation of different frequencies, comprising the steps of directing all types of said particles against an ionizing surface and irradiating in close proximity to the ionizing surface all of said particles with electromagnetic energy of said one frequency whereby said certain particles are selectively excited to a level less than the work function of said surface to be surface ionized upon contact with said surface, and then separating the surface ionized particles from all other particles.

2. The method of claim 1 wherein said energy is light.

3. The method of claim 2 wherein said light is a laser beam.

4. The method of claim 1 wherein said particles are in the form of a beam.

5. The method of claim 1 wherein said particles are in the form of a gas.

6. The method of claim ll wherein said particles and said surface are in a vacuum.

7. The method of claim 1 including the step of collecting the surface ionized particles.

8. The method of claim 7 wherein said surface ionized particles are formed into a plasma beam and focussed into a receptacle.

9. The method of claim ll wherein said energy is pulsed.

10. The method of claim 1 wherein said energy is continuous.

11. The method of claim 1 including the step of measuring electrical current flow at said surface.

12. Apparatus for separating certain particles which absorb electromagnetic radiation of one frequency from other particles that absorb radiation of different frequencies, comprising a. a housing,

b. a member of selected material in said housing and providing an ionizing surface,

0. means for directing particles against said surface,

d. means for heating said surface,

e. radiation means for irradiating said particles in close proximity to said surface with radiation of said one frequency whereby said certain particles will be selectively excited to a level less than the work function of said surface to be surface ionized upon contact with said surface, and,

f. means for separating the surface ionized particles from all other particles.

13. Apparatus according to claim 12 wherein said radiation means is tuneable to different discrete frequencles.

14. Apparatus according to claim 12 wherein said radiation means is a laser.

15. Apparatus according to claim 14 including a plurality of spaced ionizing surfaces in said housing and optical means operatively associated with said laser for dividing said laser beam into a plurality of separate beams and forming each said separate beam into proximity to each of said spaced surface.

16. Apparatus according to claim 15 wherein said optical means includes optical fibers.

17. Apparatus according to claim 15 wherein said optical means include mirrors.

18. Apparatus according to claim 12 including evacuating means connected to said housing for evacuation thereof.

19. Apparatus according to claim 12 wherein said directing means includes an oven adapted to generate a beam of said particles.

20. Apparatus according to claim 12 including collection means in said housing for collecting the selectively ionized particles.

Claims (20)

1. The method of separating certain particles which absorb radiation of one frequency from other particles that absorb radiation of different frequencies, comprising the steps of directing all Types of said particles against an ionizing surface and irradiating in close proximity to the ionizing surface all of said particles with electromagnetic energy of said one frequency whereby said certain particles are selectively excited to a level less than the work function of said surface to be surface ionized upon contact with said surface, and then separating the surface ionized particles from all other particles.

2. The method of claim 1 wherein said energy is light.

3. The method of claim 2 wherein said light is a laser beam.

4. The method of claim 1 wherein said particles are in the form of a beam.

5. The method of claim 1 wherein said particles are in the form of a gas.

6. The method of claim 1 wherein said particles and said surface are in a vacuum.

7. The method of claim 1 including the step of collecting the surface ionized particles.

8. The method of claim 7 wherein said surface ionized particles are formed into a plasma beam and focussed into a receptacle.

9. The method of claim 1 wherein said energy is pulsed.

10. The method of claim 1 wherein said energy is continuous.

11. The method of claim 1 including the step of measuring electrical current flow at said surface.

12. Apparatus for separating certain particles which absorb electromagnetic radiation of one frequency from other particles that absorb radiation of different frequencies, comprising a. a housing, b. a member of selected material in said housing and providing an ionizing surface, c. means for directing particles against said surface, d. means for heating said surface, e. radiation means for irradiating said particles in close proximity to said surface with radiation of said one frequency whereby said certain particles will be selectively excited to a level less than the work function of said surface to be surface ionized upon contact with said surface, and, f. means for separating the surface ionized particles from all other particles.

13. Apparatus according to claim 12 wherein said radiation means is tuneable to different discrete frequencies.

14. Apparatus according to claim 12 wherein said radiation means is a laser.

15. Apparatus according to claim 14 including a plurality of spaced ionizing surfaces in said housing and optical means operatively associated with said laser for dividing said laser beam into a plurality of separate beams and forming each said separate beam into proximity to each of said spaced surface.

16. Apparatus according to claim 15 wherein said optical means includes optical fibers.

17. Apparatus according to claim 15 wherein said optical means include mirrors.

18. Apparatus according to claim 12 including evacuating means connected to said housing for evacuation thereof.

19. Apparatus according to claim 12 wherein said directing means includes an oven adapted to generate a beam of said particles.

20. Apparatus according to claim 12 including collection means in said housing for collecting the selectively ionized particles.

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