US2994836A - Molecular beam apparatus - Google Patents

Description

1961 J. H. HOLLOWAY 2,994,836

MOLECULAR BEAM APPARATUS Filed May 29. 1959 2 Sheets-Sheet l SYSTEM OUTPUT 23 OSC L MASTER FREQUENCY FREQUENCY OSCILLATOR CONTROJL SYNTHESIZER F 3-| F l G. 2

INVENTOR. Joseph HouCm dy kma/cz gw, M5 5 4% ATTORNEYS Aug. 1961 .1. H. HOLLOWAY 2,994,836

MOLECULAR BEAM APPARATUS Filed iviay 29. 1959 2 Sheets-Sheet 2 K/ "lf/ I A INVENTOR. 62 U 29 k/OSG D/z Holloway F e. 4 BY 21 WJ/m 7 fi'TORNEYS United States Patent O tional Company, Inc., Malden, Mass., a corporation of Massachusetts Filed May 29, 1959, Ser. No. 816,938

24 Claims. (Cl. 331-3) This invention relates to an improved atomic or molecularbeam resonance unit of the type incorporated in molecular beam frequency standards and to an improved frequency standard incorporating the resonance unit. More particularly, it relates to a molecular beam resonance unit which may be used in a frequency standard having increased resolution, yet occupying a smaller space when compared with prior frequency standards of this type.

A frequency standard incorporating a molecular beam resonance unit is disclosed in the copending application of J. R. Zacharias et al., Serial No. 693,104, filed October 29, 1957. The standard described therein utilizes as a reference the substantially invariant frequency corresponding to the transition of a molecule or atom from one energy state to another. A beam of molecules, for example, may be passed through a magnetic or electric separator which screens out the molecules in the lower of the two states. The beam then enters a resonant cavity in which it encounters radiation from an oscillator whose frequency nominally equals the molecular or atomic resonance frequency corresponding to the difference in the energy levels of the two states as given by,

in-W1 where v is the frequency, (W W is the difference in energy between the two states, and h is Plancks constant.

. The molecules absorb energy from the radiation and enter a superposition state between the first two states.

Upon leaving the resonant cavity, the beam passes through an intermediate region where the molecules are essentially undisturbed by outside effects, and it then enters another resonant cavity to which energy from the oscillator is fed. A number of the molecules are lifted to the higher state and the others are returned to the lower state. The closer the frequency of the radiation corresponds to the resonant frequency, the greater is the number of molecules lifted to the higher state. The beam then passes through another separator which discards the molecules in the lower energy state and directs those in the higher state to a detector. The detector provides an electrical signal proportional to the number of molecules impinging thereon. This signal is fed back to the oscillator to control the frequency thereof in such manner as to maximize the number of molecules reaching the detector, thus maintaining the oscillator frequency at the value determined by the difference in energy between the two energy states utilized.

The words molecule and molecular are used in their generic sense herein, as referring to the smallest particle in a gas capable of independent movement. Since such particles, particularly in the case of cesium and other preferred metals, may consist of single atoms, these words are used interchangeably with atom and atomic.

The operation of the resonance unit may be explained in terms of classical physical concepts. Assume, for example,the use of the f=4, m and f=.3, m =0 states of the cesium, Cs atom. These states correspond to orientation of the spin axis of the outermost electron with and opposite to the nuclear magnetic axis. The spinning electron is a magnetic dipole, and in the higher energy 4,0 state, it is aligned with the nuclear field. In

2,994,836 Patented Aug. 1, 1961 the 3,0 state its direction is opposite to that of the nuclear field.

The electron does not, however, line up exactly with the nuclear field, and therefore a torque is exerted on the electron by this field. Gyroscopic action results and the spin axis of the electron precesses about the field. The rate of precession is essentially invariant as it depends on the angular momentum and magnetic field generated by the spinning electron, both fixed quantities, and the nuclear field which is constant for all molecules of the same substance.

Energy can be fed to an electron in the 3,0 state by applying electromagnetic radiation from a local oscillator, the frequency of the radiation depending upon the natural precession rate of the electron. In the case of Cs this energy is in the microwave region. The direction of the alternating magnetic field of such radiation is oriented perpendicular to the nuclear magnetic fields which are aligned in the same direction by means of a weak, nonvarying magnetic field. If the frequency of the alternating magnetic field is close to the natural precession rate of the electron, it will cause the amplitude of precession to become larger and larger until the electron spin axis is perpendicular to the nuclear field. This is the superposition state and, as pointed out above, it is reached in the first resonant cavity.

Exposure of the cesium atoms to radiation in the first cavity serves also to correlate the electron spin prece s sions in the various. atoms comprising the molecular beam. That is, it forces the electrons to precess in step with the alternating magnetic field. Thus, as the atoms leave the first resonant cavity, the electrons in question are precessing in phase with each other. Also, in the region between the two cavities, the electrons are undisturbed by outside forces, and thus they precess at their invariant natural rate.

In the second resonant cavity, the electromagnetic energy, in phase with the energy then reaching the first cavity, is again applied with its magnetic field perpendicular to the nuclear field. If the frequency of the radiation is the same as the atomic resonant frequency, i.e., the precession rate 'of the electrons, and has not varied since the atoms left the first cavity, the precessing electron and the radiation will be in phase. The electron will therefore absorb additional energy and move from the superposition state to the 4,0 state, with its field aligned with the nuclear field.

On the other hand, if the frequency of the oscillator supplying the electromagnetic energy has varied, the electron precessions will no longer be in phase with the oscillator. The exact phase difference in each case will depend on the amount of the frequency variation and the velocity of the individual atom, i.e., the time it takes the atom to traverse the distance between the two resonant cavities. If the phase difference between the supplied energy and the electron precession is sufiicient, the radiation will not increase the energy of the electron, but rather will extract energy from it, and it will revert to the 3,0 state.

' The number of the atoms reaching the 4,0 state depends on the proximity of the oscillator frequency to the atomic resonant frequency, fewer atoms reaching the upper state as the oscillator frequency departs from this value. It

will also be apparent that by increasing the time the atoms. spend between the resonant cavities, the phase differencein the second cavity will also be increased. This magnifies the eifect of small departures of oscillator frequency from the resonant value and thus decreases the number of atoms reaching the 4,0 state for oscillator frequencies only slightly different from the resonant frequency.

Thus, the resolution of the frequency standard is dependent on the length of time the molecules remain in the intermediate region between the cavities, free from perturbing effects. The longer this time interval, the greater is the resolution. Various schemes have been proposed to lengthen this interval. For example, since the length of the interval is linearly related to the distance between the two resonant cavities, increased physical separation of the cavities provides greater resolution. However, only a small increase in resolution can be obtained in this manner without an inordinate increase in the size of the apparatus. This is of particular importance when it is desired to use atomic frequency standards in airborne or other mobile applications where space is at a premium. Lengthening the path between the chambers also increases the difficulty of providing the exact in-phase relationship required between the microwave energy injected into two cavities.

Accordingly, it is a principal object of my invention to provide an improved molecular beam resonance unit adapted for use in a high resolution frequency standard utilizing an atomic or molecular resonance as a stable reference. The apparatus is of the type in which a beam is subjected to radiation at a resonance frequency during two spaced time periods, and it is a further object of my invention to increase the interval between these periods in order' to improve the resolution. Another object of my invention is to minimize the phase matching problems heretofore involved in the utilization of two separate resonant cavities for particle-radiation interaction. A further object of the invention is to provide apparatus of the above character whose size is compatible with requirements for mobile and particularly airborne use. Another object of the invention is to provide an improved frequency standard incorporating a resonance unit of the above character. Other objects of my invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, the combination of elements and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIGURE 1 is a simplified schematic diagram of a frequency standard incorporating a molecular beam resonance unit made according to my invention,

FIGURE 2 is a simplified plan view, partly in section, of a molecular beam resonance unit incorporating the features of my invention,

FIGURE 3 is a view partly in section taken along line 33 of FIGURE 2, and

FIGURE 4 is a simplified elevation of the separating magnet taken along line 4-4 of FIGURE 2.

My invention makes use of a chamber in which the particles in the beam are stored after first being exposed to radiation from the oscillator; the particles later issue from the chamber to undergo exposure to the radiation a second time. Separation according to energy state and detection of the particles which have changed energy state follows the second such exposure. Molecules entering the chamber strike its inner surface and rebound across the chamber to encounter the surface again. The exit aperture from the chamber is small compared to the total inner surface area, and therefore the particles, on the average, traverse the interior of the chamber a great many times before leaving it. The chamber thus serves, in effect, as a long tube whose length is equal to the distance traversed by the particles in their multiple reflections within the chamber interior.

In my preferred embodiment, the entrance and exit apertures to the chamber are the same, although this is not necessary for the practice of my invention, and therefore the particles enter and leave it in substantially opposite directions. Accordingly, the same resonant cavity may be utilized for both particle-radiation interactions, and after the second of these interactions, the returning beam may undergo separation by the same separator as the entering beam. This reduces the physical length of the resonance unit by another factor of 2 and also greatly simplifies the structure thereof. Another highly advantageous result is the automatic mechanical phasing of the radiation as between the first and second exposures of the particles thereto.

The permitted number of bounces or reflections and thus the effective length of the chamber depends on the perturbing effect of the bounces on the particles in the beam. The greater this effect, the fewer number of bounces which may be made on the average without suffering a loss of coherence in the electron spin precessions. Again referring to the classical model described above, each bounce alters for an instant the precession rate of the electron spin axis. If the amount of such change is significant, only a few bounces will occur before the electrons undergo substantial phase changes and thereby impair the resolution of the system. Where the beam comprises cesium atoms, the interior wall of the chamber may be coated with certain hydrocarbons with which the particles undergo substantially elastic collisions, i.e., without appreciable energy transfer. This minimizes undesirable changes in phase, and thus increases the permitted number of bounces within the chamber, thereby providing a longer time within the chamber.

As seen in FIGURE 1, a molecular beam resonance unit generally indicated at 10 and incorporating the features of my invention includes a molecular beam source 12 adapted to project a stream of particles through the aperture 14 of a separator generally indicated at 16. Assuming the utilization of atoms of Cs in the molecular beam, the separator includes a magnet, whose structure will bedescribed below, which produces a strong inhomogeneous field in the aperture 14, separating the atoms into two groups. One of these groups includes the particles in the f=3, m =0 energy state and the other includes those in the f=4, m =0 state. The latter particles are directed off the axis of the unit in the direction of the dotted line 17 and strike an envelope schematically indicated at 18 adapted to maintain vacuum conditions within the unit. The envelope may be lined with suitable getter material (not shown) to absorb these atoms. The 3,0 atoms are deflected along the axis 19 of the uni-t to pass into a resonant cavity 20 powered by a frequency synthesizer 22 whose input is from an oscillator 23. The synthesizer frequency corresponds to the difference in energy between the 3,0 and 4,0 levels, and accordingly, a number of the atoms passing through the cavity 20 absorb radiation therein and reach the superposition state intermediate the 3,0 and 4,0 states.

Next, the beam enters a bounce chamber 24, preferably of spherical shape, where the particles undergo multiple reflection prior to emerging once again from the chamber and passing through the cavity 20 and the aperture 14 of the separator 16. In the cavity 20, some of the atoms are now elevated to the 4,0 state and others forced back to the 3,0 state. The separator again splits the particles into two groups, and the group including the particles in the 4,0 state is directed toward a detector 26, the other group being discarded in the manner described above. During the time they are in the cavity 20 and chamber 24-, the atoms are subjected to a weak magnetic field extending between a pair of pole pieces 25.

The output of the detector 26 is fed to a frequency control unit 28 which controls the frequency of the oscillator 23 in such manner as to maximize the electrical output signal from detector 26, i.e., obtain transition of a maximum number of particles from the 3,0v to the 4,0 state. The synthesizer 22, oscillator 23, detector 26 and frequency control unit 28 may take the form disclosed in the application of W. A. Mainberger, for Frequency In FIGURES 2, 3 and 4, I have illustrated in greater detail the molecular beam resonance unit described in connection with FIGURE 1. As shown in FIGURE 2, the unit is mounted on a base 29. The beam source 12 and detector 26 are mounted in a housing 30 connected to a tube 34 extending through the aperture 14 of the separator 16 and along the axis 19 of the unit to the resonant ,cavity 20. A baflle 33 between the source 12 and detector 26 prevents diffusion of molecules directly from the source to the detector. A tube 36 connects the cavity with the bounce chamber 24.

As best seen in FIGURE 3, the pole pieces are the arms of a deep channel-shaped member generally indicated at 38. The member 38 has flanges 40 welded to the base 29. The channel member 38 is preferably of low reluctance magnetic material such as iron, and thus it serves to shield the cavity 20 and chamber 24 from external magnetic fields, including that of the earth. This prevents such external fields from affecting the alignment of the nuclear magnetic fields. A coil 42 is Wound longitudinally of the member 38 on the web 44 thereof. During operation of the unit, a small current is passed through the coil 42 sufficient to produce a magnetic flux of approximately 0.05 gauss extending between the pole pieces 25. This field serves to align the atomic nuclei for the purpose described above.

In FIGURE 4, I have illustrated a preferred form for the separator 16. As shown therein, a magnet assembly generally indicated at 46 comprises soft iron pole pieces 50 and 52 extending inwardly from permanent magnets 54 and 56. The pole pieces and permanent magnets are fastened to iron side members 57 and 58 by clamps 59. An iron base 60, bolted to the members 57 and 58 by bolts 61, completes the iron return for the magnetic flux. The base 60 is secured to the base 29 by bolts 62. A cover 63 is fastened to the side members 57 and 58 by bolts 64.

As illustrated, the pole piece 50 is convex and the pole piece 52 concave, in order to provide an inhomogeneous field in the aperture 14. Within the aperture the tube 34 is shaped to interfit with the pole pieces, and to facilitate fabrication of the resonance unit, the tube may be formed in two parts, 34a and 34b, joined by a suitable coupling 65, as best seen in FIGURE 2. It is desirable that the molecular beam passing through the magnet assembly have a substantially rectangular cross section, and therefore the beam source 12 is preferably provided with a collimator adapted to form a beam of this shape, such as the collimator described in the copending application, Serial No. 693,104, heretofore identified.

The resonant cavity 20 is preferably a section of circular wave guide excited in the TE mode, with the radio-frequency magnetic field therein parallel to the D.-C. field generated between the pole pieces 25 and thus perpendicular to the nuclear magnetic fields of the atoms passing therethrough. The tubes 34 and 36 have crosssectional dimensions smaller than the cut olf values for the frequency of the radiation in the cavity 20 to prevent the electromagnetic energy in the cavity from being propagated to other portions of the resonance unit, particularly the chamber 24. If diesired, the tube 36 may be removed altogether to conserve space, with the cavity 20 connected directly to the chamber 24. When this is done, the cavity aperture 65 leading to the chamber 24 should also be limited in size for this reason.

The chamber 24 preferably comprises a glass sphere 66 on the interior surface of which is a lining 68. It is the lining 68 which the atoms engage in their multiple rebounds within the chamber 24. The material of the 6 lining should therefore be such as to provide for essentiaL ly elastic collisions between the atoms in the beam and the molecules in the lining to minimize the undesirable phase shifting effects described above. A material having suitable characteristics for this application is a hydrocarbon of the form C H Preferably, it is a substance in which n is at least 20, so as to provide a lower vapor pressure and, therefore, less likelihood of evaporation under the high vacuum conditions within the resonance unit. A substantial number of gaseous molecules of the lining material within the chamber 24 and the tube 36 to the left thereof (FIGURE 2) will result in a significant probability of collision with the atoms in the molecular beam, thereby causing undesirable diffusion of the beam.

I have also provided means for controlling the temperature of the chamber 24, cavity 20 and tube 36 to optimize the operating characteristics of the resonance unit. As seen in FIGURES 2 and 3, a system of tubing 70 enveloping these parts and in contact therewith ac-' commodates the flow of a suitable heat exchange fluid which maintains the temperature at the walls of the cavity 20, chamber 24 and tube 36 at a desired level. For example, the fluid may be a refrigerant which chills the walls and thereby reduces the tendency of the lining to evaporate; further, any molecules which would evaporate under such conditions and later strike the chilled walls would tend to condense and adhere thereto.

'The size of the chamber 24 and the diameter of the aperture 72 formed therein determine the average number of rebounds the atoms in the beam will undergo before re-emerging from the chamber. The size of the chamber also determines the distance and time between rebounds. to some extent the definition of the emerging beam, since, as the diameter of the aperture is decreased, atoms travelling within a smaller angle with respect to the axis of the resonance unit will pass therethrough. A suitable design is one in which the sphere 66 has a diameter of' 5-10 cm. with the area of the aperture 72 ranging from 0.1 to 1 square centimeter. The number of beam atoms in the chamber is small enough to keep the probability advantages pointed out above, the same separator 16 would be used twice, and also, the distance traversed by the atomic beam between interactions would be longer than in a prior resonance unit of the same length.

The molecular beam frequency standard of which the resonance unit 10 is a part is internally evacuated to pro vide as complete a vacuum as possible, preferably 10- millimeters mercury or better to minimize the number of residual molecules or atoms of gas in the path of the beam, and thereby minimize scattering of the beam by collision with such particles. In view of this consideration and also to provide a high degree of mechanical strength as well as relative ease of fabrication, the housing 30 and tubes 34 and 36 and cavity 20 are preferably,

of oxygen-free copper with the connections therebetween including the coupling 65, made by brazing. Further, the glass sphere 66 of the chamber 24 may be encased in a copper housing (not shown) brazed to the tube 36 to eliminate the glass-to-metal seal 74 (FIGURE 2) otherwise required at the aperture 72.

Thus, I have described an improved molecular beam frequency standard incorporating a resonance unit adapted to provide much greater resolution than prior devices of this type. The resonance unit includes a chamber which the particles in the molecular beam may undergo rebounds from the interior surface between their first and second interactions with radiation from the local oscillator 4 The size of the aperture 72 also determines Thus, the same resonant cavity 20 would be utilized for both particle-radiation interactions with the and associated circuits whose frequency is to be controlled. The use of the chamber thus eifectively lengthens the time between these interactions and thereby inn proves the resolution of the frequency standard. The chamber also permits the beam to be returned substantially along its initial path through the same resonant cavity and magnetic separator from which it originally entered the chamber. This further shortens the resonance unit as well as eliminating a second separator and cavity heretofore required. Also, the problem in prior devices of obtaining an exact in-phase relationship between two resonant cavities has been eliminated.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description are efficiently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of my invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. Molecular beam apparatus which utilizes the change in state of molecules between two energy levels, said apparatus comprising a separator for separating said molecules into two groups, one of which contains the predominant portion of molecules in one of said states and the other of which contains the predominant portion of molecules in the other of said states, a reaction chamber in which said molecules may be subjected to radiation corresponding to the difference in the energy levels of said states, a second chamber having an aperture formed therein and means for projecting a beam of said molecules through said separator and thence through said reaction chamber and through said aperture into said second chamber, whereby said molecules may rebound from the interior surface of said chamber and emerge therefrom through said aperture to pass again through said reaction chamber and separator.

2. The combination defined in claim 1 in which said second chamber is spherical in shape and its size and the diameter of said aperture are such as to provide for multiple rebounding of said molecules from said interior surface.

3. The combination defined in claim 1 including means for detecting the molecules in one of said energy states emerging from said second chamber and passing through said separator.

4. The combination defined in claim 1 including means for subjecting said molecules to a weak, non-v=arying, uniform magnetic field from the time they first enter said reaction chamber until they leave said reaction chamber after emerging from said second chamber.

5. The combination defined in claim 4 including means for propagating said radiation in said reaction chamber with the magnetic field thereof parallel to said uniform field.

6. Molecular beam apparatus which utilizes the transitions of molecules between first and second energy states, said apparatus comprising a source arranged to project a first beam of said molecules therefrom, a separator arranged to split said beam emerging from said source into second and third beams, said second beam containing the predominant portion of said molecules in said first state and said third beam containing the predominant portion of molecules in said second state, a reaction chamber disposed in the path of said second beam to the subject the molecules passing therethrough the radiation whose frequency corresponds to the energy difference between said first and second states, a second chamber having an aperture disposed in the path of said second beam of molecules emerging from said reaction chamber, an exit aperture formed in said second chamber, whereby molecules entering said entrance aperture will rebound from the interior surface of said second chamber and then exit therefrom through said exit aperture, a reaction chamber disposed in the path of the molecules emerging from said exit aperture to subject said molecules to radiation of said frequency a second time and a separator disposed in the path of the second beam of molecules which have undergone reaction with said radiation a second time and arranged to split said second beam into fourth and fifth beams, said fourth beam containing the predominant portion of said molecules in said second beam which have undergone the transition from said first to said second state and remained in said second state.

7. The combination defined in claim 6 including a detector disposed in the path of said fourth beam to detect the molecules therein.

8. The combination defined in claim 6 in which said second chamber has a spherical interior surface.

9. The combination defined in claim 6 in which the interior surface of said second chamber is lined with a hydrocarbon having the empirical formula C I-1 10. The combination defined in claim 9 in which n is at least 20.

11. The combination defined in claim 9 including means for controlling the temperature of said second chamber.

12. The combination defined in claim 6 including means for maintaining high vacuum conditions in the paths traversed by said beams.

13. Molecular beam apparatus which utilizes the transitions of molecules between first and second energy states, said apparatus comprising a separator having an aperture and arranged to split a beam of said molecules projected into said aperture into first and second resultant beams energing therefrom, said first resultant beam containing the predominant portion of said molecules in said first state and said second beam containing the predominant portion of said molecules in said second state, a cavity which resonates at a frequency corresponding to the difference in energy between said first and second states, a chamber having an aperture, said reaction chamber and said chamber aperture being disposed on the axis of said first resultant beam whereby the molecules in said first beam emerging from said separator pass through said resonant cavity to be subjected a first time to radiation of said frequency, then pass into said bounce chamber to rebound from the interior surface thereof and return from said chamber through said resonant cavity to be subjected again to said radiation and then pass through said separator a second time to be split into third and fourth beams according to the energy states thereof, and means for maintaining said beams under high vacuum conditions.

14. The combination defined in claim 13 in which said separator includes magnetic means generating an inhomogeneous magnetic field in said aperture thereof.

15. The combination defined in claim 13 including means for subjecting said molecules to a weak, non-varying, uniform magnetic field from the time they first enter said cavity until they last exist therefrom.

16. Atomic beam apparatus utilizing the transitions of Cs atoms between the 3,0 and 4,0 energy states, said apparatus comprising an atomic beam source which projects therefrom a first beam of said atoms, a magnet having an aperture disposed in the path of said first beam, said magnet generating an inhomogeneous magnetic field in said aperture to thereby split said first beam into second and third beams according to the 3,0 and 4,0 energy states, said second beam containing the predominant portion of said 3,0 molecules, a resonant cavity disposed on the axis of said second beam, said cavity being resonant at the frequency corresponding to the difference in energy between said states, whereby molecules in said second beam passing therethrough may absorb energy from radiation in said cavity to undergo a transition to the 4,0

state, a chamber having an aperture disposed in the path of said second beam emerging from said resonant cavity, whereby the molecules of said second beam enter said chamber to rebound from the interior surface thereof and exit said chamber through said second aperture and pass once again through said resonant cavity and then said aperture of said magnet, said magnet separating the returning molecules into fourth and fifth beams according to the 3,0 and 4,0 energy states, and means for applying a weak, non-varying, uniform magnetic field to said molecules from the time they first enter said cavity until they last exit therefrom.

17. The combination defined in claim 16 in vwhich the interior surface of said chamber is substantially spherical in shape.

18. The combination defined in claim 16 in which said interior surface of said chamber is lined with a material having the empirical formula C H n being at least 20.

19. The combination defined in claim 16 including means for imposing a weak, non-varying, substantially uniform magnetic field on said molecules from the time they first enter said cavity until their last exit therefrom and means for exciting said cavity with radiation of said frequency with the magnetic field of said radiation substantially parallel to said uniform magnetic field.

20. An atomic beam resonance unit comprising, in combination, means for forming an atomic beam, a first means for separating from said beam a first beam of atoms having a like energy state, means for exciting said separated beam of atoms to a superposition energy state intermediate said first energy state and a second energy state, a chamber having an entrance and an exit aperture, means connecting said exciting means to the entrance aperture of said chamber, means connected to said exit aperture of said chamber for exciting said atoms in said superposition state to said second energy state, separating means for separating from said beam, after excitation to said second state, a further beam of atoms in said second energy state, a detector, said detector being located to intercept said further beam of atoms in said second energy state- 21. The combination defined in claim 20 in which said chamber has a single aperture and including a single exciting means and a single separating means.

22. The combination defined in claim 20 in which said chamber is spherical in shape.

23. A molecular beam frequency standard which utilizes the change in state of molecules between two energy levels, said standard comprising, in combination, a molecular beam source which projects a first beam of molecules therefrom, a resonance unit, said resonance unit including a separator for separating said molecules from said source into two groups, one of which contains the predominant portion of molecules in a first one of said levels and the other of which contains the predominant portion of molecules in the second of said levels, a reaction chamber in which said molecules may be subjected to radiation whose frequency corresponds to'the diflerence in said energy levels, a second chamber having an entrance aperture formed therein, said entrance aperture being disposed in the path of said molecules in said first level coming from said separator, means forming an exit aperture in said second chamber, whereby molecules may enter said second chamber and rebound from the interior surface thereof and emerge from said exit aperture, a reaction chamber in which said molecules emerging from said exit aperture may be exposed to radiation at said frequency a second time, and separating means adapted to separate the molecules which have been exposed to said radiation a second time into two further groups according to said energy levels, a detector disposed in the path of said further group containing said molecules in said second level, a radiation source for supplying radiation at said frequency and means responsive to the number of molecules reaching said detector for controlling the frequency of said source.

24. The combination defined in claim 23 in which said second chamber has a single aperture and said resonance unit includes a single separator and a single reaction chamber.

References Cited in the file of this patent Article by N. F. Ramsey, vol. 28, pages 57 to 58, January 1957, in The Review of Scientific Instruments.

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