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~9
MI-CRO~VE PHtN~lENA I~ GASES
By
Dakin
Research Laboratories
Westinghouse Electric Corporation
East Pittsburgh, Pa.
Tnt,roduction
The investigation of microwave phenomena in gases has in the year 1947
grown into a flourishing field of research with more than fifty papers and letters
published by more than a dozen laboratories. The field can be divided into two
main divisions. (a) the investigation of the absorption of microwave energy by
gases, and (b) the investigation of electric discharge phenomena at high frequencies.
The field of measurement of absorption of microwave by gases has become
known as "microwave spectroscopy". Microwave absorption studies have for the most
part been concerned with the observation and frequency measurement of the line
absorption spectra of various gases. Absorption of energy occurs when the energy
quanta (huff 9 Where h is Plancl<'s constant and ~ is the frequency) of the incident
radiation coincide with the difference in energy between energy levels of the gas
molecules From a measurement of the frequency and intensity of the absorption
lines of particular molecules, one can calculate, using the theory of quantum
mechanics, a great deal of information about the molecules. The moments of inertia
of the molecules are directly determined in most cases. The interatomic bond
distances are also calculable from the moments of inertia when molecules ~ith-
isotopic atoms or diatonic molecules are observed. The electric dipole moment of
the molecule can also be calculated in two ways, one from the intensity of the
line and another from frequency shift or splitting of the line by a superimposed
d-c electric field (Stark effect).
In most cases where one, according to the simpler quantum theory, ex-
pected to find only a single absorption line, a group of lines has been observed.
This "hyperfine structure" has been explained as the effect of interaction of the
electric quadr~pole moment of the nucleus, Q. in the internal electric field
gradient, d2 t',in the molecule With the overall molecular rotation. The frequency
shift dZZ produced by this interagt~on~is proportional to the product of the
above quantities, so that~if either is determined independently, the other can be
calculated. From the intensity and spacing of the fine structure lines, one can
also determine the nuclear spin quantum number. From the microwave spectroscopy,
therefore, one is able to determine considerable information not only of the
physical and electrical structure of ~noIecule, but of the nuclei as sell. Bleaneg
wrote a general review of the application of microwave spectroscopy to the study
of nuclei and molecular structure. Kyhl 4 also reviewed the subject before~the
1947 Conference on Electrical nsulation. !
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JO
Apparatus and Methods
Considerable advancement has been.rnade during 1947 in met,hods cuff detecting
absorption lines at microwave frequencies. The lines must be observed at Con
pressures to be seen sharply and in many cases they are very Weak absorbers.
Therefore, much has been done to try. to .i~ilprovp the serisiti~rity. The most,2inter-
esting anti sensitive method of de;tect~on.~as.re~ported by;~Hugnes and. Wilson Who
utilized the Stark effects sp:Litting of the ob$=ption line. The molecular energy
is modulated by a variation of the high voltage electric field of about 80 Kc
frequency. A sensitive amplifier tuned to this frequency was used then as ~
detectors Most other investigators in the field have since tried this scheme, too,
including use of both sine wave and square wave modulating field. Watts and
Williams report.= modification using a fine wire down the cen~er.of the Wave guide
for the high ~nod,~lating Frontage instead of the usual. plates This wire produces a
highly non-uniform electric field, and has some merits for detecting lines, but is
not useful for.meas~xements. SUCGeSS has also.~cen.obta~ned in.using a low radio
frequency, don ampl~tude.~odulation of the Preq'~enc~r of .the oscillator tube. supers
imposed on the low frequency Creep mod.ldation which i' used for oscilloscope ob-
servation: Gordy and Kessler4 and Watts and Williams' have reported on this scheme.
C. K. Jen claims a great deal ot.success with.a resonant cavity scheme of measure-
ment instead.of a Wave guide as an absorption cell. Be used a 3 cm. Have meter
cavity &1 about 1 cm..~avelenath. Andy lOC Kc frequency modulation in addition to
the low'[requency sweep. .
. .
. . . .
Precision frequency measurements of the micro~.~ave absorption 'lines have
been jisc.ussed. bar Good and Coles ~ and a frequency marker scheme by.Ca:rter and
Smith .. Strandberg9 Kyhl' t..entink and Fillgerl° also discussed precision frequency
measurements. These ~.eci~ion~measurements are made by calibrating With a high
ham onic of the, Bureau Qf Staniards 5 megacycle precision frequency broadcast.
Inversion_Spectrum of Ammonia ,.'
The microwave absorption spectrum. of ammonia is somewhat of a freak
inasmuch as ammonia is the of y molecule so far investigated. in this f=,equency'
range where the absorption lines are not due to tr~nsitions,betv,~-en q,uant:.zed.
rotational levels of the mol'ecules. .The ammonia absorption lines are due to .
. . . . .
transitions between a-symmetric and anti-symmetri.c state of the molecule e9 or the
inversion of the pyramidal molecule. Many pap~ys,and letters have been published ,
concerning this spectrum. Bleaney and Penrose ~ report 29.1ines and discuss the
pressure broadeningl2.at higher pressures. Pond and Cannonl,3,,and Smith and Carters
and others discuss.saturation effects due to too strong ' microwave fields9 there '
the molecule es are lifted~to2.the,h~gher energy level faster than they can return'by
kinetic processes. .
. The Superfine siruc.ture of the ammonia lines have been thel~ge~4 ~8'
numerous letters'by Jauch 5,,Gordy and Kessler .9 ~atts'and ~il:.iams ' 9
This fine structure'.due to.'the electric quadrupole moment of the nitrogen 14
nucleus, had been ~rc~vious~y'exclained by Coles,and'Good and Van Vleck. An
anomalous frequency value for the ammonia absorption lines Nvi.th K = 3 (K is the
quanta "number for rotation fib out the synch try axis! Noms noticed by Good and
Coles ' and by Strandberg' Xyhl, ~enti~k and Hi~lgerl0~21 following precision
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51
measurement of the frequency of the lines. Coles suggested it was due to a k type
s~littin~2of the line with one of the components missing and later Nielsen and
Denni.son '23 gave a detailed explanation of the effect, confirming the suggestion
o~ Coles.
Dictation Spectra of linear Molecules
The linear molecules, of course, present the simplest spectra, but even
this becomes complex when nuclear fine structure effects are present and When higher
modes of vibration are also excited. Prevision measurements of the J - ~ to 29
= 3 to .4 and2: = 4 to 5 transitions.of carbon ox~su3.fide hove been made by Dakin,
:~o].e~ an] Good 9 and Hider, Strandberg, ~entink and K~h] . In two cases the
li.rles with the St34) isotope in the molec-~.e were Observed and the C-O and CAMS bond
distances calculated. Tonnes' Holden and I.herriot2 discuss the linear molecule
spectra and point out the effect of the zero crder vibration which is always present,
on the calculated bond distances. The magnitude of this correction to the bond
distance is still uncertain, but it ma,., be as m~.~.ch as several hundredths of an
angstrom unit... The OOS molecule ro+~.~ona.~..spectra does not appear to have~any
detectable fine str,..tct~.~re.
Coles, Elyash and Gorrnan27 report the absorption lines for the J = 0 to
J = ~ transition of nitrous oxide' N20, for both the M(14) and N(15) Isotopic
molecules. Sond distances are calculated. higher cress,..tre measurements were made
by VIeic~ner28 on the Iodine moncc~-loride absorpt on Dices at 69.~0 and 6634 t.1c for
the J = 0 to J = 1 transition. Cyanogen bromide (BrCN? and chloride (C1CN) spectra
Here studied by Townes, Holden and Merritt2' and by Gordy, Smith and Simnons'°. Tn
this case the molecu]..e has both the f ne structure due to the nuclear quadr~pole
moment of the t.axogen atom and. a]..so a higher vibrational mode excited, Which makes
the sr~ectr~.~m quite complex*
The mol.ec~.~le~.' do not need to possess an electric dipole moment to show
rotational absorption 'ones, if the moue has a magnet'..c moment and that is the
case with. atmvepiler-`c oxygen Which shows an absorption line at about 5 ~m. ~ave-
lengtY.. Van VI.eck authors a Caper discussing the absorption of oxygen
. . .
, . .
So. m~netr~..c Tom Mo]..ec~;..t ~ es
These molecules can be considered as t4&Vi3:g the next more complex spectra
after tile ii.near molecules. Methyl code and bromide have been investigated by
lordly' Simmons and Smithy. Their found lines for ~,c~th chlorine (J _ O to ~ transi-
-'-::on) and t~romix~e fJ = 1 to 2 tra-ns~tion) isotonic ~nclec,~es. These invest) gators
Awe also studied the J - ~ to 2 transition for methyl iocli~de32 where only one
isotope c)f the halogen exists. The f~ ne structure due to quadr'tpole moments and
nucI.~ar spins of thee h=~.ogt?n atoms are discussed.
. .
. ,
~leth:1 cyanide (3 = 1 to 2) and isoc~a~lide (J = ~ Lo 1) rotational lines
here found by Ping, Edwards, it.essler and Cordy 3. The nuclear coupling coefficient
alas evaluated tiere. Ire this case the N(14) nucleus is responsible for the fine
struc ture .
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Representative terms from entire chapter:
absorption spectra
52
Metric T~Io'ect,:es
Ire the case of molecules ~;~liich dc ;nct ha-fro a thre~fcld or greater is of
symmetry, the spectrum is vet complex id iffy to -interpret. N-~h~?le;~s
a majority of mo'ec~.~es are Ash netric raid their ro=,~tional spectra clan be c;x-
planned, even if done pith more l='cor. The water molecule falls in this cites cuds
because of its univorsa1 ar~peara.nce, was 1n~r~st.~ga+ed firsts TviC th~oreti-~1
papers, a result of ~:iartitnc research, ct?p~arcd In '94? Or`e of ties Gas a?~t'~'r~d
by NTan iTleck;4 and the other by King;, lIai~ler and Cross35. The ·.~-ter Cater also
discusses rele.ted molecules. H2S, H2Se,
53
Radio Frequency Srect~'oscoPy of Gases
__..._... A...__ . _~., .__, _. . .. .._~,.. _~.
~C any transitions between Stark and freeman levels of single rotational
states c:~ mo].ec.~des can be expected to occur in the presence of an el.ectrlc or
magnetic fie]-d at radio frequencies of thejorder of several megacycles to several
tens of megacycles in froquency-. In most cases, the absorption line intensity
could be very weak. This field, although it does not apply to microwave frequencies
can apples to gases aced is a natural. oll1.tgro~Jth of the microwave frequency spectros
co,-oy. Such a transiticr~ bet,~;sen Stark levels of a molecule in a d-c electric field
has 'omen observed with a molecular beam teci~n~-que bar H. K. Hughes49. Transitions
of this sort would show absorption lines in the radio frequency range in a d-c
electric field. It remains for further investigation to show whether or not they
can be detected. Analogous to this are the nuclear magnetic moment transitions
whichg for examine, have been recent!:, detected in gaseous hydrogen by Purcell, Poland
and Rloembergen With a resonant cavity in a magnetic field. Similar nuclear
tran.sltions have previously been observed With solid paraffin and other materials.
If. Frenuengy Dicshar~e Phenomena In Gases
X].though considerable Work has been ~n progress in this field, only a
few pul:ilshed papers have anpes.red. Several are due to appear earl,,r in 1948.
V=rela5 and Kirclmer) discuss, the elect of & superimposed d-c potential with a
high frequency Voltage. The d-c field decreases the rate of initiation of discharge
and reduces the de-io.~izatlon time. Babat53 leas experimentally lovestlgated elec-
trodeless discharge and. allied prvtlems in the frequency range of 1 to 100 mega-
c~rc].es and discusses theoretically the situation Where the electrode separation is
greater than a wavelength where it is Possible to obtain a breakdowns in an elec-
tromagnetic ray. This is the sort or s tuation present With microwave apparatus.
˘,.
54
BIBI,TrjC-.RAPH,7
1. B Blear~ey, Physica,-12' 595-605, (I)ec. 19~)
· . ~.
2. R. U0 Hughes and E. B. 8.;lson, Jr., P~.:Rd~r. 719 5629 (197IJ7) .
P. J.
CWO
W.
.. l1e
71
llo B.
12. B.
13. T.
14. E.
15.
36~ r R. J. ~atts and Dudley TVi]liams, Phys. Rev., 72, 9809 (1947>
A . . .
fialter Gordy and.Myer Fessler, Phys. Resr., 729 644, f~947)
Watts and I;udley W~ iliams' Phys. Re~rO g rI.?, 7]229 (1947),
. - · . .;
:~. Jen, PIlys. Re~r. j '729 986, (1947)
· ·
E. Gcod and D. K. OGlee, PllysO Rev. ~ 729 1579 (19.~7)
E. C-ood and D. X. Colee, Ph~rs. Rev. g 719 383, (3947)
1~. Car~ er arld- C-. Vq Sm~thg Phys O Rev., 72, :1265 , ( 1947)
. . . . . . . . .
. . · . .
T0 P. Str~dberg5-R. Kyh:L, T0 T;;ent~nx9 Jr. and R. E. lIiliger, PbysO Rev,
- 3269 (~:9~7)
. .
. . . . . . . . .
Pileane~- and R. P. Penrose9. Proc. Roy O SocO 'S9A, 358~71, (Dda~y' 19k~7;
.. -. . ~. . . . . -
. . . . . .
Bleane;y and R. r. Penrose, Proc~ Phys. SOC*9 London, ~, 4.~-289 (1947?
A. Pond and ~. F. Cannon, Ph~rs, P`ev. 729 11219 (1947)
V. Smith and R. T, Carter, Phys. Rev, 72, 6389 (19b7?
J. M. Jauch, Phys. Rev. 729 535 9 ~ 3 947) 0 J. M. uauch, Phys. Re-;. -29 715 723
( 19~7
16. Falter Gordy ancl M~rer Kessler9 r-h,,rs. Rev. 719 64~?, (1947)
17. R. u. 11;atts und D~ley Tiillic~ms, Phy=. Rev. ~ 7l, 635, (1947)
I8. R. J. i~atts and Dud' ey V.~illiams, PLys. Re~r. 723 263-265, (1947)
19. Dud~ey Ti~liams, Phys. Rev. 72, 9749 (~.947)
2(~. R. J. ~atts and Dudley I'i]li~^ms, PL,,~. Rev. 72? 157, (1947)
210 ~o ~. P. Strs~i~ndberg, R. ,~;~-h], T. ~ent~.nk and Ro E. Hi.llger, Phys. Bev. =`
639, (]94~7>
h0 F. 7~ielsen and D. Mo Dennison9 Phys. Rev. 72' ll()l~ll()g, (1947!
23. F. F. Mielsen and D. ~q Denn~ SOn9 Ph:'rs. Re~rO 72, 86' (~947)
t
.
55
24. T. U. Dal;: ins ~ E. Good and D. K. Coleus Ph;iso Rev c 71, 64(719 (1947)
25. R E. HiTLlger, L ~*P. Str2.ndLerg, T. 11:entlnk and R.L X~yLl'PY,Ts Rev 72,l57g(1947
26. C.~. Tonnes, Ao N. Hol.den, F. R. l.~errltt,~ell. Te1~.Labs,Ph:rs.Rev.72,51.3-1.4 (1947)
D. K. Coles9 E. S. Eyelash and J. G. '~Torm.an, Pk~J~. Rev. 729 9739 (1947)
R. T" Te~ deer, P.l:ys* Rev. 72, 1.2~S, (1947)
C. H. Tonnes, A. i. Fo~d.en anc3. ~0 Ret Merritt,, Phys. Rev. 719 6k., (1947)
27.
28 ~
290
30. ;~`llter.Gord:;-, ~ G. Smith and James Simmons, Phases. Rev. 71, 917, (1947)
Walter Gordon, fares E. Simmons and A. G. Smith, Pays. Rev. 72, 344, (1947)
later GordNr, A. ITS Smiths and James ~- Simmons, Phrase Rev. 72' 249 g (1947)
Harold Ret ng g Howard Ed~ard.s 91.]yer .~.~es sler and Halter Cordy 9 I; S ~ Rev 87291263h947)
J. H. lT-.n Fleck, Ph.xTs. Rev. 71, 425-4.32, `1947)
G. A. King, R. kilo plainer and PalLl C. Cross, Pays. Rev. 7I, 433-43, (1947)
|3a Pe Dailey, Ph-fs. Beer. 72, 84' (1~4.7)
Ala D. Pershberger and Tohn Turkevi cdl, ~3JSo Rev. 71' 5549 (1947
B. P. Dai.le:~ and E 72. Vail SQn' JrO ~ Pb~rSo Rev. 7~9 522, ll947)
B. P. Da.iley9 SO Golden and E0 B. Wilson, ~r.9 Phys. Rev. 72' 3719 (194.7)
C.!'. Tonnes, Ac No ETo:Ldeng ~I' . .6crdeen, F.Ro 15erri+.19Phys.Rev.71,644, (1947)
F72]ter Gorky V7.V. Smith, koGe Smiths Harold Rings Pliers. Rev. 729 259, (~947!
B. T. Feicl, Ph:;-sO Rev. 729 ].~ 6, (1 947)
43. I,.Davi s, Jr., B.T.Feld, C.lTi. ZŁ~l~el9 IT, R. Z2char~.as9Ph.7s.Rev~7:39525-26, (~°48)
44°
45.
O
47 ~
D. X. Coles anc? i`. E. Good, Phy=. Rev. 729 1.579 (39471
5~ X. Tonnes and. Fix :~. I.lerrit~, Phys. Rev O /2, 1266? (1947)
T.` ~ V. Smith' Ph,rs. Rear. 71~ 1265 (~947?
C. I. heard and. .730 PO D-.`ile~, Abstracts of papers presented at ].71.2th rneeti.ng
of the American C,^:em~ca] Society, 1':e,;~.~ York, (September, 194.7) ~ pO i]~Oa
48 . J . ~i. . NT ~ n V:l e: k g Lays . Rev . 71 ~43 3 1+24 s ~ --947 ,
~6
49. H. K. rouges i I' .~.,TS o Rev. _2, 63 4-(!23 ~ ( ~ 947)
50. E.~.~. Purcell, R.V. Pound and 17. Bloembergen' Phise Refire
51 . A o A . V=rela , Ph. rs . Rev ~ '.7', 124~ 5 ~ ~ 1947)
52 . F. Arc inner g Phys . Rear ~72 5 IS, (1947)
709 0~-79 (194~)
53e Ge le PsabatJ., JO Inst. Elect Engrs., Pt. I.Lr., 9~, 27-379 \]94?)
R. T!. -Kyhig ALS+,rSCIS~ir An~UŁ: RePO.r! 19h7 COnIerer~e on E:'ect.-ic<-1
InSU1atiOO9 (SePTe~]e~ 1947~9 P. 39
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