EPR spectrometer resonant cavity
United States Patent Hyde EPR SPECTROMETER RESONANT CAVITY James S. Hyde, Menlo Park, Calif.  
 [73] Assignee: Varian Associates, Palo Alto, Calif.  
 [22] Filed: July 19, 1973 [2|] Appl. No.: 380,647  
 [75] Inventor:  
 Primary Examiner-Michael J. Lynch Attorney, Agent, or Firm-Stanley Z. Cole; Gerald M. Fisher [Ill 3,878,454  
 [451 Apr. 15,1975  
 [57] ABSTRACT An EPR spectrometer resonant cavity having an aperture for sample introduction that is typically a rectangular or cylindrical pipe. The pipe is dimensioned so that it can be considered a waveguide which is not be&#39; yond cutoff. The aperture and pipe are precisely placed on the cavity wall so that, due to symmetry, the lowest frequency propagation mode is not excited. At the end of the pipe removed from the cavity a conveniently removable second rectangular or cylindrical pipe is provided which is coaxial with the first pipe. The second pipe is smaller than the first pipe and is dimensioned to be beyond cutoff for all propagation modes so as to enable convenient introduction of large and small samples without any disturbance of a permanent wall of the cavity and with virtually no radiation losses during operation.  
 6 Claims, 6 Drawing Figures EPR SPECTROMETER RESONANT CAVITY FlELD OF INVENTION The present invention relates generally to electron paramagnetic resonant (EPR) spectrometer and more particularly to an improved EPR spectrometer resonant cavity.  
 BACKGROUND OF THE INVENTION To examine a sample with an EPR spectrometer the sample is placed in a microwave resonant cavity. in the earliest cavities a sample was inserted into the cavity by removing the entire wall of the cavity. It has been found that repeated removal and replacement of a cavity wall was both tedious and deleterious to the cavity 0 and mechanical stability by reason of changes in the geometric and dimensional properties of the cavity. The Remple et al. U.S. Pat. No. 3 l22.7t)3. assigned to the present assignee. disclosed an improved cavity which permitted insertion of a sample into a cavity without requiring the removal of an end wall. The Remple apparatus includes a fixed support structure for positioning a sample in the cavity wherein the support is a hollow waveguide that is dimensioned to have a cutoff frequency beyond the resonant frequency of the cavity. thereby precluding loss of microwave energy through the support. This structure has been widely used even though the volume of sample which can be introduced through the properly dimensioned support is smaller than would be most desirable.  
  To permit introduction of larger samples. it is also known to provide a precisely located larger aperture in the cavity walls such that symmetry of the microwave fields differ from the symmetry of the first propagation mode through the aperture. This configuration permitted introduction of larger sample structures or dewars but such apparatus has the disadvantage of leaking microwaves and becoming degraded in performance due to changes in microwave symmetry caused by the introduction of the sample or mechanical irregularities.  
  For certain applications it is known to be desirable to irradiate a sample in an EPR resonant cavity with optical energy. One prior art structure for enabling a sample to be irradiated with optical energy involves providing the cavity with a wall formed of an RF shield of a thin metal grid. for example. see Remple et a]. US. Pat. No. 3.122.703. The spacing between wires forming the grid generally has an area on the same order of magnitude as the area ofthe wires forming the mesh. whereby transmitted light through the grid is on the order of fifty percent. Optical sources illuminating the sample through the grid have generally employed optical means to focus the source light on the sample. The spacing of the wires forming the mesh is sufficiently close to shield the microwave energy in the cavity and prevent the escape thereof from the interior of the cavity. It is also known to irradiate a sample in a chamber through an opening having dimensions such that the opening is not capable of supporting the resonant microwave energy in the cavity. While the opening is one hundred percent light transmissive. it is limited in size by the microwave resonant frequency of the cavity. whereby the area ofthe sample that can be illuminated by the optical energy is relatively restricted. It is. therefore. desirable to provide the EPR resonant cavity with a means for enabling a substantial area of a sample in the cavity to be illuminated while providing substantially more than fifty percent transmission ofthe optical energy.  
 BRIEF DESCRIPTION OF THE INVENTION 5 In accordance with the present invention. insertion of a sample into an EPR resonant cavity is accomplished without disturbance of a permanent wall of the cavity by providing a wall segment of the cavity with a larger aperture through which the sample may be inserted. The aperture has dimensions capable ofsupporting microwave resonant energy in the cavity. but the microwave energy in the cavity is not coupled through the aperture to any substantial extent because the aperture is symmetrically positioned. Large samples may be inserted into the cavity directly through the aperture. A pair of such apertures on diametrically opposed wall segments ofthe cavity may be provided. Leakage of the microwave energy is further considerably reduced by providing waveguide segments that connect to. are coaxial with and aligned with the apertures. The waveguide segments are dimensioned so that they have a beyond frequency beeyond the frequency of resonant energy in the cavity. The waveguide segments are fixedly positioned on demountable fixtures or cavity stacks that are selectively secured to exterior portions of a block forming the resonant cavity. Small samples can be inserted into the cavity through the hollow waveguide and aperture. while large samples may be inserted by removing the hollow waveguide. and inserting the large sample through the aperture and then securing the waveguide in situ. Removal and insertion of the waveguide segments have no effect on the dimensional and geometrical properties of the high 0 interior of the resonant cavity since the fixtures for the waveguide segments do not engage the inside of the cavity. Positioning the apertures and waveguide segments so they are symmetrical with respect to the microwave fields prevents virtually all the resonant energy in the cavity from escaping and the cavity Q is maintained stable at a relative high value for repeated use for both large or small samples.  
  To enable optical energy from a source outside of the cavity to irradiate a relatively large area of the sample without substantially affecting the Q of the cavity while providing optical energy transmittivity considerably in excess of 50 percent, on the order of 80 percent. a multiplicity of selectively spaced slots in a portion of the resonant cavity wall that is preferably at right angles to the wall segments through which the apertures extend are provided for cooperation with collimated optical energy. The slots are relatively closely spaced and dimensioned to be waveguides having a cutoff frequency greater than the cavity resonant frequency to preclude the escape of microwave energy from within the cavity. The spacing between adjacent slots is substantially less than the width of the slots so that the transmittivity of the cavity wall to the optical energy is on the order of 80 percent.  
  It is. accordingly. an object of the present invention to provide a new and improved EPR resonant cavity.  
  A further object is to provide a new and improved means for enabling a sample to be inserted into an EPR resonant cavity.  
  Another object of the invention is to enable a sample to be inserted into an EPR resonant cavity without disturbing a permanent wall of the cavity.  
  An additional object of the present invention is to provide an impro\ ed EPR resonant cavity having apertured walls for insertion of a large sample into the cavity. with the apertures being positioned to have no sub stantial effect on the O of the cavity and wherein the waveguide coa\ial with the apertures precludes leak age.  
  Another object ofthe invention is to provide an EPR resonant cavity wherein small samples can be inserted into the cavity through a waveguide dimensioned beyond cutoff and apertures dimensioned and positioned so that virtually no leakage of electromagnetic energy from within the cavity occurs.  
  A further object of the invention is to provide a new and improved EPR resonant cavity wherein a satnple in the cavity can be irradiated by collimated optical energy originating exteriorly of the cavity.  
  A further object ofthe invention is to provide an EPR resonant cavity wherein a sample within the cavity can be illuminated over a relatively large area by optical energy and considerably more than fifty percent of the optical energy is transmitted to the sample without substantially degrading the Q of the cavity.  
  The above and still further objects. features and ad vantages ofthe present invention will become apparent upon consideration of the following detailed descrip tion of one specific embodiment thereof. especially when taken in conjunction with the accompanying drawing BRIEF DESCRIPTION OF THE DRAWING FIG. I is an exploded perspective view of an EPR spectrometer in accordance with a preferred TM mode embodiment of the invention;  
 FIG. 2 is a side view ofthe resonant cavity illustrated in FIG. I.  
  FIG. 3 is a top view of the resonant cavity of FIG. I; FIG. 4 is a front view of the resonant cavity of FIG. I:  
  FIG. 5 is a side view of a hollow cavity stack for enabling insertion of a sample into the cavity of FIGS. l-4&#39;. and  
  FIG. 6 is a schematic of a T nlt mode embodiment of the invention.  
 DETAILED DESCRIPTION OF THE DRAWING Reference is now made to the figures wherein there is illustrated a cylindrical microwave resonant Cavity II formed from a metal right parallelepiped I2. Cavity II is excited by microwave energy coupled into the cavity from rectangular waveguide I3 through slotted iris 100 so that the cavity is excited to the TMM mode. Thereby. electric field vectors extend the length of cav ity II between parallel walls 14 and 1S and closed microwave magnetic field lines I7 are symmetrically excited in the cavity about a null electric field plane 18 that runs the length of cavity II and is vertically oriented so that it is at right angles to horizontal. parallel end faces I9 and 20.  
  A steady magnetic field H,, that may be swept on which is imposed an AC component of approximately 100 KHz is provided by an electromagnet including oppositely polarized pole faces 22 and 23 and cylindrical coils which are excited by a 100 KHz source. Pole faces 22 and 23 are positioned in proximity to walls I4 and IS so that the low frequency magnetic lines and micro wave electric field vectors extend through the cavity in the same axial direction between the walls. In response to a change in the resonate properties of cavity II, as reflected in changes in the electron paramagnetic resonant properties of a sample in the cavity. a detectable output signal is provided by a crystal detector circuit (not shown) connected to waveguide 13.  
  In accordance with one aspect of the present invention. insertion of a sample is facilitated without removing any permanent. interior walls or faces of cavity II and without disconnecting waveguide I3 from the cavity. To these ends. upper and lower faces I9 and 20 are provided with vertically extending apertures 31 (only the aperture in upper wall 20 is illustrated in FIG. I). Apertures 3I are relatively large and symmetrical rela tive to electric field null plane I8. The area and geometry of apertures 31 are such that microwave energy resonant in cavity II can propagate in the waveguides formed by the apertures. However, because of an arcuate intersection 32 between apertures 31 with the upper and lower wall portions of cavity 1 I and the symmetrical relationship ofthe intersection with null plane I8 there is no substantial amount of microwave energy coupled from cavity 11 into apertures 3]. In a typical embodiment wherein cavity II is resonant to a frequency of approximately 9.5 GHZ. as is achieved by providing the cylindrical cavity with a radius ofapproximately 0.760 inches, apertures 31 have a length of approximately 0.70 inches between end faces 14 and I5 and a width in the direction between side walls 33 and 34 of approximately 0.46 inches. The length of one of apertures 3I from top face 20 to intersection 32 is ap proximately 0.487 inches. Thereby. the aperture is capable of supporting and propagating the 9.5 GHz resonant energy in resonant cavity 1] but this tendency is substantially minimized as indicated supra.  
  To virtually eliminate the possibility of microwave energy propagating through apertures 31, cavity stacks 133 (FIG. 5] are removably mounted on end walls I9 and 20. Each of cavity stacks 133 comprises a hollow,  
 cylindrical waveguide element 34 having a cutoff fre-.  
 quency greater than the microwave frequency which is exciting cavity II. Waveguides 34 have longitudinal aligned axes coaxial with the axes of apertures 3I and are formed by providing a central circular aperture on each of plates 35. Coaxially aligned with and having substantially the same diameter as the circular aperture on each of plates 35 is a bore of cylindrical sleeve 30 that is fixedly secured to the plates. Each of cylindrical waveguides 34 has a sufficient length. as defined by the distance from the inside edge of plate 35 to the end of sleeve 30, to present a high impedance to the microwave resonant energy in cavity II. a length on the order of (QM/3. where equals the wavelength of the resonant frequency of resonant cavity II. The bore of sleeve 30 and its aligned aperture in plate 35 have the same diameter that forms the waveguide 34, having a cutoff frequency greater than the resonant frequency of cavity II. To these ends. for the 9.5 GHz microwave excitation and resonant frequency of cavity 11, the length and diameter of waveguide 34 are approxi mately 084 inches and 0.46 inches. respectively.  
  To enable large samples (samples which do not fit through the bores of sleeves 30) to be inserted and removed from cavity II, cylindrical waveguides 34 are mounted on metal plates 35. Plates 35 are selectively secured to end faces I9 and 20 by screws 36 which are inserted into aligned bores 37 and 38 in end plates 35 and end faces I9 and 20. la normal operation. a sample 39 is located in cavity II so that it intersects electric null field plane 18. If the sample is in a dielectric capillary tube. the capillary tube is usually positioned so that the longitudinal axis thereof lies wholly in the plane 18 and extends vertically. at right angles between end faces 19 and 20. and equidistance between end walls 14 and I5. If the sample is located in a dielectric cell having substantially planar dimensions. the cell is located so that its planar area is substantially coincident with electric field null plane 18. However. it is to be understood that a planar sample holder can be oriented at right angles to the electric field null plane. provided symmetry is maintained with respect to the null plane. as described by l. S. Hyde. Review qfScivnrifir Instr-w mums. Volume 43. page 629 (1972).  
  Cavity stacks I33 are normally maintained in situ on end faces 19 and 20. To insert relatively small samples. such as are maintained in capillary tubes. into cavity ll. the sample is inserted through the bore of sleeve 30.  
 thence through the aligned aperture of plate 35 and aperture 31 into cavity I]. To facilitate insertion of such a sample. the sample holder may be provided with relatively long dielectric arms that extend beyond the ends of sleeves 30. Thus. the small samples can be inserted into cavity II without removal of any parts. For insertion of larger samples that are in holders which do not pass through the hollow bores of sleeves 30 and the aligned apertures of plates 35. but which do pass through apertures 3l. one of cavity stacks I33 is removed and the sample holder is placed in the cavity by insertion through apertures 31. Such a sample holder is also provided with elongated dielectric arms that extend through aperture 3I and the bore in sleeve 30. to facilitate insertion of the sample and enable the sample position to be controlled in the cavity without removal of cavity stack 133 which is secured in situ on end face 20 after the sample holder has been inserted into cavity 1]. Even though one of cavity stacks 133 must be removed to enable a relatively large sample to be inserted into cavity II. the removing and replacing operations have no substantial effect on the Q of cavity 11 because no walls actually forming the cavity interior are disturbed.  
  In accordance with another feature of the invention. a relatively large area of a sample can be irradiated by optical energy originating outside of block 12. and significantly more than 50 percent of the optical energy can be coupled to the sample without substantially degrading the Q of the cavity. To this end. side wall 33 is provided with a number of substantially identical. par allel slots -44. FIG. I and FIG. 4. that extend horizontally into cavity I1 and intersect a portion of the cavity wall that is generally at right angles to the inter- Sections 32 of the cavity wall segment with apertures 31. Slots 40-44 have substantially the same dimensions and geometries so that they effectively form wave guides having cutoff frequencies greater than the resonant and excitation frequency of cavity ll. Thereby. the resonant energy in cavity 11 cannot be coupled through the slots and the slots do not materially adversely affect the Q of the cavity. To enable substantially more than fifty percent of the optical energy to be coupled through slots 40-44 to sample 39. the space between adjacent horizontal edges of the slots is approximately one-fourth of the width of the slots. e.g.. the vertical distance separating adjacent edges 46 and 47 of slots 40 and 4] is approximately one-quarter of the distance between horizontally disposed edges 47 and 48 across slot 40. In one embodiment of the invention. the width of slots 40-44 between parallel edges 47 and 48 is on the order of M/lO. the length of the slots in the horizontal direction is on the order of at least ()t )/4. and the distance between side wall 33 and cavity wall portion 45 is on the order of at least (A )/4 to provide sufficient microwave attenuation in the slots. For the 9.5 GHz resonant cavity 11. the width of each of slots 40-44. as between edges 47 and 48. is approximately 0.125 inches. the distance between adjacent edges. as between edges 46 and 47 of slots 40 and 4] is approximately 0.03 [2 inches. and the length of slots 40-44. as between side edges 49. is approximately 0.45 inches.  
  The structure is especially suitable for irradiation of the sample with parallel light. such as provided by a laser.  
  While there has been described and illustrated one specific embodiment of the invention. it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example. the cavity can be formed as a rectangular right parallelepiped instead of a cylinder. In such a case the cavity may be excited to the TE mode so that the null electric field plane is coincident with the null electric field plane of the cylindrical cavity TM mode. The cavity might also resonate in the cylindrical TE mode. A TE mode geometry actually constructed by us operating at 9.5 GHz employs a symmetrically placed aperture not beyond cutoff (l inch inner diameter) and a detachable coaxial waveguide beyond cutoff aperture of 0.46 inches.  
  For example. referring to FIG. 6. if the rectangular cavity 101 is excited in the cylindrical TE mode through iris I04 and a circular aperture and connecting cylinder [02 are coaxial. then the microwave fields in the cavity will not excite any TM (transverse magnetic) transmission modes in the cylinders. The lowest (first) frequency TE (transmission electric) transmission mode is TE... The cutoff wavelength is given by the formula 3.412 a. where a is the radius in cm.  
  In a structure actually built. a /2 inch. or A 4.3 cm. Thus. at 9.5 GHz (A 3.2 cm) the cylinder I02 is not beyond cutoff for the TE mode. However. the TE transmission mode has a plane of symmetry. while the TE resonance mode in the cavity is cylindrically symmetrical. As long as this cylindrical symmetry is preserved and the cylinder 102 is precisely coaxial with the cavity. the microwave fields and currents in the cavity resonator will not excite the TE propagation mode. The fields and currents in the cavity can. of course. excite the TE. propagation mode. The cutoff wavelength for this TE... mode is given by the expression A 1.640 a. where a is the radius in cm. At 9.5 GHz. u L93 cm. Thus the diameter of cylinder I02 must be less than 1.5 inches. Similarly. to suppress the TE propagation mode in the attached cylinder the inner diameter must be less than 0.73 inches. In practice both the inner diameter of cylinder I02 and the inner diameter of cylinder 103 are selected to be less than these theoretical limits; for example. 1 inch and 0.46 inches. respectively. This selection compensates for the dielectric properties of quartz sample holders which lower the cutoff frequency from that calculated when the cylinders are empty.  
  A similar explanation may be provided for the invention of FIG. 1. Thus the aperture 31 may be regarded as a rectangular wmeguide. The lowest (first) fre quency transmission mode has the designation TE,., and A 2a in cm. where a is the inside broad dimension and is 0.700 inches in this embodiment. Thus 3.55 cm which is not beyond cutoff at 9.5 GHZ. However. an inspection of the symmetries of the fields in this propagation mode and in the cylindrical TM mode shows that ifthe aperture 31 is located symmetrically with respect to the plane of zero RF electric field. this propagation mode will not be excited.  
  Mode designations and cutoff frequencies for waveguides are given in standard microwave manuals as. for example. The l/Iit&#39;rmvuvt&#39; Engineers Handbook. Horizon House. Microwave. lnc. 197i. page El (1).  
 What is claimed is:  
  I. An EPR spectrometer resonant cavity container for carrying and supporting a resonant cavity and a sample in said resonant cavity. said cavity being resonant to a microwave frequency and being excited by a low frequency magnetic field. said container including a resonant cavity.  
  first waveguide having a cutoff frequency greater than the resonant frequency of said cavity. second waveguide having a cutoff frequency less than the resonant frequency of said cavity. said first waveguide being displaced from said resonant cavity and microwave coupled to said resonant cavity through said second waveguide. said second waveguide being fixedly connected about an aperture in an interior wall of said cavity. said aperture being symmetrically positioned and con 8 figured in said interior wall such that microwave currents in said cavity do not excite the first propagation mode of said second waveguide. and  
 said first waveguide being removably connectable to said second waveguide.  
  2. The apparatus of claim 1 wherein said resonant cavity is a right parallelepiped for supporting a TM,,., mode.  
 3. The apparatus of claim 1 wherein said resonant n cavity is rectangular for supporting a TE mode.  
  4. The apparatus of claim 2 wherein the chamber of said resonant cavity and said second waveguide are both formed out of a single metallic member having no welded or soldered joints connecting said cavity and s said second waveguide.  
  5. The apparatus of claim 4 wherein the said chamber is formed as a bore through said single metallic member and the second waveguide is cut into said second metallic member so as to intersect said chamber bore in an In arcuate intersection.  
  6. The EPR spectrometer according to claim I wherein a wall of the cavity substantially at right angles to the aperture includes slots for enabling collimated optical energy outside of the cavity to be coupled into the cavity to irradiate the sample while substantially preventing the escape of micro ave energy from within the cavity. said slots being closely spaced individual slots dimensioned to be waveguides having a cutoff frequency greater than the cavity resonant frequency. the  
  spacing between adjacent ones of said slots being substantially greater than the width of the slots so that substantially more than fifty percent of the optical energy impinging on the area bounded by the slots passes through the slots.