bleaney, b. -- the fifty-third kelvin lecture. radiospectroscopy

8/13/2019 Bleaney, B. -- The Fifty-third Kelvin Lecture. Radiospectroscopy

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621.317.4:535.33:539.1 The Institution of Electrical EngineersPaper No. 4019Nov. 1962

THE FIFTY THIRD KELVIN LECTURE'RADIOSPECTROSCOPY'By Professor B. BLEANEY, F.R.S.

{ ecture delivered beforetheINSTITUTION,26th April,1962.)Introduction

The history of radio-spectroscopy begins in the 1920swith the experiments of Debye on solutions containingpolar moleculesmolecules with permanent electric dipolemom ents. When an electric field is applied, a liquid containingsuch molecules acquires an electric polarization, since the equi-librium situation is one where a small fractional excess of thedipoles have their moments parallel to the field. This fractionalexcess is of order y*E\kT,where (JLE is the potential energy ofthe dipole (which favours alignment of a dipole/xparallel to thefield E)andk T isthe thermal energy of random motion (kbeingBoltzmann's constant and T the absolute temperature) whichfavours random orientation of the dipoles and thus opposes thealignment. When the electric fieldE is switched on or off, thepolarization does not appear or disappear instantaneously sincethe dipoles are fixed in the molecules, which must thereforerotate to allow realignment of the dipoles. This rotatio n takesa finite time, its rate being regulated by th e viscosity of the solu-tion. The equilibrium situation is approached exponentiallywith a characteristic time,r, which is a time-constant like ther = RC which appears in the charging of a capacitance Cthrough a resistance R.

This analogy with a capacitance and resistance is ratherclose, since the solution behaves as an imperfect dielectric. Thepower dissipation is due to the work done in turning the mole-cules against the viscous forces in the liquid, and results in aheating of the solution. That part of the dielectric constantwhich is associated with rotation of the dipoles varies withfrequency as (1+jayf)~i, and is complex; the real part variesas (1 + (D2T2)~ X, which has its full value only at low frequencieswhen the dipoles can reverse quickly enough to keep in step withthe field, and falls to zero at very high frequencies when theycannot. The imaginary part varies as CUT/(1+ CO2T2), and hasa maximum when CUT= 1. This maximum occurs because thelosses are proportional both to the angle through which thedipoles turn in attemp ting to follow th efieldand to their angularvelocity; the former decreases, but the latter increases, as thefrequency of the alternating field goes up.As would be expected from the discussion, detection of therelaxation effect is accomplished by measurem ent ofthe electricalproperties of the lossy capacitor formed byfillinga good capaci-tor with the liquid. To test the theory, measurements must bemade over a range of frequencies such that cur varies from valuessmall to large compared with unity. With the limited frequencyrange of radio equipment then available, Debye could achievethis only for substances such as the higher alcohols(e.g. glycerol),where the viscosity is high and the value of r comparativelylong. For water at room temperature, r is about 10~nsecand accurate measurements were made only after the war-timedevelopment of microwave equipment. Some results are shownin Fig. 1.A similar effect occurs in paramagnetic substances, whichcontain permanent magnetic dipoles of atomic origin; this waswidely investigated by Gorter and other Dutch physicists in

8 0 r

Prof. Bleaney is Dr. Lee's Professor of Experimental Philosophy, University ofOxford.

Fig.1.Realand imaginary parts of the dielectric constant of wateras a function of frequency (reciprocal of the wavelength).the 1930s. Since atomic magnetic dipoles are of magnitudeabout 10~20e.m.u., while molecular electric dipoles are about10 ~18e.s.u., the effects are rather mo re difficult to de tect. In asolid the characteristic time,T,is determined by the rate at whichthe magnetic dipoles achieve thermal equilibrium with the latticevibrations which form the reservoir of thermal energy. Thecoupling between the dipoles and the lattice varies widely, andso correspondingly does the relaxation time r (generally knownas the 'spin-lattice relaxation time', since in many cases themagnetic dipoles are associated with the electron spin). How-ever, at a low enough temperature the lattice vibrations willalways die away sufficiently to bring the relaxation time intothe millisecond region, long enough to make measurementspossible using standard audio-frequency bridge techniques, theparamagnetic sample being placed in a coil to whose inductanceand loss it makes a small contribu tion. (At radio frequenciesdifficulties arise from effects like dielectric loss, which, althoughcoupled only to the stray capacitance of the coil, can be com-parable in size.)The investigation of these two effects was carried out bymethods very different from those of conventional spectroscopy,a point which has been characteristic of radiospectroscopy ingeneral. We can endeavour to summarize the importantfeatures of radiospectroscopy as follows:(a ) As in all spectroscopy, some mechanism must exist toprovide an interaction between the substance and an electro-magnetic field. In nearly all the cases of interest to us, thismeans the substance must contain either permanent electric orpermanent magnetic dipoles of atomic magnitude.(b)The rate of spontaneous emission of radiation from atomsat radio frequencies is quite negligible. The spectra are thereforeobserved through interaction with a strong applied electromag-neticfieldgenerated by an electronic oscillator. Such oscillatorsprovide tunable sources which are virtually single frequency.For this reason no spectrometer of the conventional type isrequired to separate out radiatio ns of different frequencies.(c) The degree of resolution which can be attained is usuallydetermined, not by the resolving power of the spectrometer, butby the inherent width of the spectral lines themselves. In o rder

[457]


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458 BLEANEY: THE FIFTH-THIRD KELVIN LECTUREthat thermal equilibrium may be attained, some process mustexist through which exchange of energy between the system ofdipoles and the ordinary thermal molecular motion can takeplace. This process acts as a cause of damping in any forcedoscillatory motion of the dipoles and results in broadening ofthe spectral lines. In the phenomena considered at the start ofthis lecture, the dipoles have no natural oscillatory frequencyand their motion is determined by the relaxation processes;these are extreme cases where the line width is large comparedwith the frequency on which the line is centred. High resolutionrequires that the width be small compared with the naturalresonance frequency.(d) Molecules with electric dipoles (atoms and nuclei do nothave electric dipole moments) are firmly bound in solids and, asmentioned earlier, are subject to strong viscous forces in liquids.High-resolution spectroscopy is possible only with substances inan attenu ated state, e.g. gases at low pressure.(e) Magnetic dipoles, which can interact only with magneticfields, are relatively free, and high-resolution spectroscopy ispossible in both solids and liquids, as well as gases.(/ ) All spectral lines in the radio-frequency region areextremely weak compared with those in the optical region. Theproblem, therefore, is to detect very small changes in the powerreaching a receiver due to the presence of spectral lines, andspectrometers must be pu shed to the limits of sensitivity.(g ) With few exceptions, spectra observed at frequenciesbelow 1000 Mc/s are due to nuclear magn etic dipoles; spectradue to electronic dipoles (electric or magnetic) are observed atcentimetre wavelengths.

Nuclear Magnetic Resonance using Molecular BeamsSeveral important branches of radiospectroscopy involvemagnetic dipole moments, and we shall consider first the natureof the motion of the dipole in a steady magnetic field and thenthe effect of an additional oscillatory field.In both the atom and the nucleus, permanent magnetic dipolesoccur only in association with angular momentum, since theyarise from the spinning motion (using spin in a rather looseway to include orbital motion) of an electric charge. A mag-netic dipole therefore has the properties of a gyroscope, wherethe magnetic moment lies along the axis of the gyroscope. Themotion in a steady magnetic field, which exerts a couple onthe magnetic moment, is similar to that of an ordinary top inthe earth's gravitational field (that is, under the action of themechanical couple formed by the weight of the top acting throug hthe centre of gravity and the equal and opposite reaction at thepoint of support). When the top is spinning rapidly about itsown axis, this couple causes the axis to precess slowly abou t th evertical (the direction of the eart h's gravitational field), a pheno -menon w ith which we are all familiar as children. If dampingis neglected, the precession continues indefinitely with the axisof the top at a fixed angle to the vertical. In the magnetic casethe motion is similar. The magnetic mom ent, JX, bears a fixedratio,y, to the intrinsic angular momentum,G :

u = yG (1)where y has the dimensions of (electric charge)/mass if thequantities are in electromagnetic units and is called th e magneto-gyric ratio. In a steadyfield B,the angular velocity of precession(the so-called Larmor precession) is simply

= yB 2)

B-B,* - B ,

a result which is given by classical theory but is also correct inquantu m mechanics. The negative sign indicates that, if y isnegative (as for an electron with negative charge), the precession

6)Fig. 2.Motion of atomic or nuclear magnetic dipole in the presenceof steady and rotating magnetic fields.

(o) Precession of a magn etic mome nt |JI (associated with spin a bou t its own axis)around a magnetic field B.(6) In a system of co-ordinates rotating in synchronism with a circularly polarizedoscillatory field, B\, this field B\ is a constant vector. The appar ent steady field isthenBB i, whereBi is the steady field at which the magn etic moment would precessin synchronism withB\ . In this rotating system, the appar ent motion of the magneticmoment is a simple precession about the resultant of the two fields By an d BB L-

is in the sense of a right-handed screw advancing along B, asin Fig.2(a).Suppose we start with the magnetic dipole parallel to thesteady field B and then apply an oscillatory magnetic field ofsmall amplitude in the plane normal toB. This will have littleeffect unless its frequency coincides with the natural precessionfrequency of the dipole. A forced precession takes place inwhich the angle d between the dipole and the steady field Bincreases gradually from zero to a finite value, returns to zeroand then increases again, repeating the cycle. The rate at whichthe angle 6changes depends on the size of the oscillatory field(or, more precisely, on the size of its circularly polarized com-ponent B{ which rotates in synchronism with the natural pre-cession); the maximum angle dm = 2a attained is given bytan a = Bxj(JBBf), whereBis the actual value of the steadyfield andBLis the value required to mak e the natural precessionfrequency coincide exactly with the applied frequency [seeFig. 2(6)]. Thus the maximum angle reached depends on howclose the applied frequency is to the natural frequency; wherethey coincide exactly, the maximum angle is 180, i.e. the dipolecan be completely reversed. This is a resonance phenomenonwhich occurs in such a striking fashion only when damping canbe neglected.

The magnetic moments of atoms were studied in the 1920sby the experiments of Stern and others, where the sidewaysdeflection of a beam of atoms moving throu gh an inhomogeneousmagnetic field was observed. This field exerts a translationalforce which is proportional to the field gradient and the parallelcomponent of the magnetic moment; the spatial quantizationof angular momentum was demonstrated in this manner, butthe accuracy with which magnetic moments could be determinedwas limited by the spread of molecular velocity in the beam,which means that different atoms spend varying times intraversing the inhomogeneous field and so suffer varyingdeflections.A natural extension of this method is to measure nuclearmoments, but these are so much smaller (about 10~23e.m.u.)than electronic moments (about 10~ 20e.m.u.) that extremedifficulties are encountered . In the late 1930s, by a stroke ofgenius, Rabi conceived the idea of using the resonance pheno-menon to reverse the orientation of a nuclear dipole in a knownuniform magnetic field,B,and then to detect its reversal by thesubsequent change in its path through an inhomogeneous field.In a molecular beam the atoms are quite free from collisionsand the reversal of the dipole can occur as outlined above,provided that the ratio of the applied frequency to the steady


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BLEANEY: THE FIFTY-THIRD KELVIN LECTURE 459

DETECTOR

Fig. 3.Principle of atomic beam magnetic resonance experiment.Atoms (or molecules) from a source O are deviated downwards in the magnet Athrough the translational force exerted by the inhomogeneous field of Aon the nuclearmagnetic mom ent. The atoms which then penetrate a slit S suffer a similar upwardsdeflection in the magnet B (whose gradient of field is in the reverse sense from thatof A) and reach a detector. Magn et C produces a homogen eous field and in theregion F a radio-frequency field (if at the right frequency) flips the nuclear momentover; the atom then follows one of the paths indicated by broken lines and fails toreach the detector.

field is correct for resonance . This ratio gives the value of y,and hence the nuclear moment is found provided that the nuclearspin (in units of h\2TT where h is Planck's constant) is knownfrom spectroscopic evidence. The apparatus devised for thispurpose is shown in Fig. 3 and a typical resonance line is shownin Fig. 4. The line width, Ato, is determined by the time, A/,

1 0 0

>

i- 90UJHI

8 0

MAGNETIC FIELD1-5x105I I

I I

IN

AMP/METRE1-6x10 5I I

i i

1800 1900 20 0 0MAGNETIC FIELD IN E.M.U.Fig. 4.Drop in beam intensity reaching detector when magnetic fieldof C is swept through the value required to make the nuclearprecession frequency equal to the frequency of the radio-frequencyfield.

The resonance shown is for the 19F nucleus in the molecule NaF, and the radiofrequency is about 8 Mc/s at resonance.spent by the molecule in traversing the region occupied by theoscillator field, since this limits the length of the wave traininvolved, and the product AcoAr is approximately u nity. Inpractice it is simpler to work at fixed frequency and to passthrough resonance by varying the magnetic field, as in Fig. 4,where the line width appears in terms of magnetic field. Inthis way the nuclear moments of the proton, deuteron and anumber of other simple nuclei were determined to about onepart in a thousand.Since 1945, the determination of nuclear moments of stableisotopes has been carried out more simply by nuclear magneticresonance in liquids or solids (as we shall see later), but beammetho ds have been fruitfully employed in o ther fields of rad io-spectroscopy. These have used the electronic magnetic momentsof atoms which, being so much larger than nuclear moments,give correspondingly larger deflections in inhomogeneous mag-netic fields and make the whole technique much simpler. Aningenious trick due to Ramsey using two separated oscillatory

fields has mad e it simple to increase A/, the time required totraverse the space between them, and to decrease line widthsso that measurements are readily made with an accuracy ofabout 1kc/s in frequency; in special cases much higher accuracyhas been attained. High-precision measurements on the hydro -gen atom have greatly increased our understanding of atomictheory and have yielded much more precise values of severalfundamental constants.Atoms which possess both electronic and nuclear magneticmoments have been a special field of interest. Each of thesemoments is acted on by the magnetic field of the other moment,and precesses about the direction of the total resultant angularmomentum, which is, of course, constant for a free atom notsubject to any external couple. The precession frequency ischaracteristic of the atom; for hydrogen it is approximately1420Mc/s, a spectral line which is the basis of an importantbranch of radio-astronom y. The electron-nuclear interactionhas provided detailed information abo ut bo th atomic and nuclearstructure for many isotopes; for highly radioactive isotopes(whose activity makes them easily detectable), atomic-beammethods provide almost the only way of obtaining reliablevalues of spins and moments.A characteristic atomic frequency which can be measuredwith high precision forms the natural basis of an atomic fre-quency standard which is free from the uncertainties of quartzclocks or the mean period of rotatio n of the earth. Stablecaesium atoms (isotope 137) are convenient for this purpose,since an atomic beam of low velocity can readily be obtained.Their characteristic frequency is approximately 91 63 Mc/s andthe precision of atomic clocks based on this spectral line hasimproved steadily, notably thro ugh the work of Essen. Aprecision of about 1 part in 1011 can be obtained, which isroughly a factor of 100 better than the reliability with whichthe mean period of the earth's rotation round the sun (theastronomical unit of time) can be determined. At special radiostations the transm issions are now maintained within 1 par t in1010 of their nominal frequencies by checking against atomicclocks, and time signals are co-ordinated to 1m s.

Microwave Gas SpectroscopyDuring the Second World War requirements for the accuratelocation of targets by radio led to the development of centimetre-wave rada r. Initially 10 cm, then 3cm, and finally l cmwavelengths were developed . As the wavelength was decreased,scattering by raindrops (the magnitude of which depends onthe behaviour of the complex dielectric constant of water inthe anomalous Debye region) and absorption by the gases ofthe atmosphere became increasingly importan t. Much of thelatter was due to the tails of absorption lines at still shorter

wavelengths; the work of Van Vleck and W eisskopf on th e shapeof lines where the length of the wave-train is determined by theinterval between molecular collisions was stimulated by theneed for more accurate estimates of the atmospheric absorption.In this way the attention of physicists was drawn to the presenceof molecular spectra at these wavelengths, while at the sametime, equipment (oscillators, detectors, waveguide components)became available to open up a new region of the spectrum forinvestigation.A strong absorption band at about 1cm wavelength inammonia gas was known from the pioneer work of Cleeton andWilliams in 1934. The NH 3 molecule has the form of a flatpyramid which vibrates between two equilibrium positions withthe plane of the hydrogen triangle respectively above and belowthe nitrogen atom. The vibrational frequency, about 24G c/s,is comparable with the collision frequency at atmosphericpressure and the absorption line is very broad . In 1945,


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460 BLEANEY: THE FIFTY-THIRD KELVIN LECTURE

- 3 0 0

- 2 0 0 -

- 100

hv

0-65 0-70 0-75 0-80WAVENUMBER. CM ' 0-85Fig. 5.Absorption spectrum of ammonia gas at a pressure ofl - 2 m m H g .The numbers above the absorption lines are the values of /, K; whereJ(hl2n) isthe total angular momentum oft hemolecule and K determines the angle at which theaxis of the molecule moves round the vector /.

Penrose and the author found that, on reducing the pressure toobtain smaller line width, a complex spectrum is observed (seeFig. 5). The vibrational frequency depends quite strongly onthe flatness of the pyramid, and varies with the rotational stateof the molecule because of centrifugal distor tion . Discrete linesare obtained because the angular momentum of rotation isquantized; the two quantum numbers required to specify thestate of rotation are shown above each line in Fig. 5. Moreresolution is obtained by lowering the pressure still further, andat a bout 0-01 mm Hg a hyperfine stru cture due to the electricquadrupole moment of the nitrogen nucleus was discovered byGo od. The limiting line width is attained at a pressure wherethe molecular mean free path is of the order of the wavelength;the width due to collisions is then comparable with the widthdue to the Doppler effect, and is about one-millionth of thefrequency of the sp ectral line.The discovery of hyperfine structure provided a furtherstimulus to the investigation of other spectra. These are mostlyassociated with molecular rotation, and can be observed if the

molecular has a permanent electric dipole moment (or, in rarecases, a permanent magnetic moment, as in O 2, which has anabsorption band at wavelengths around 6mm). As the angularmomentumisquantized, observation of the frequency of rotatio ngives the moment of inertia of a molecule; observations with thesame molecule containing isotopes of different mass then yieldinformation about the inter-nuclear distances which is moreaccurate than can be obtained by other means. For example,in the linear molecule OCS, the oxygen-carbon and carbon-sulphur distances are thu s determined as 1-161 and 1-561 A,respectively. Fro m hyperfine structure of such spectra muchnew nuclear information has been obtained. Splitting of thelines in magnetic and electric fields has also been studied; thelatter gives a direct measurement of the size of the electric dipolemoment of the molecule.As in all radiospectroscopy, the sensitivity of spectrometershas been pushed to the limit in the search for weak lines. Inmicrowave spectroscopy the limit corresponds to an absorptionof about 10 of the power in a path length of 1000 km.From a quantum mechanical viewpoint, any spectral line isthe result of a transition between a pair of energy levels whoseseparation is equal to the quantum of radiation, hv . In theradio-frequency region this energy separation is always verysmall compared with the energy of molecular fluctuation, kT ,where k is Boltzman n's constant. Since the popu lation of alevel with energy W is proportional to exp{WjkT), the dif-ference in the numbers of molecules in two levels differing inenergy by AW= hv < kT is approximately hv\kT under con-ditions of thermal equilibrium; for a wavelength of lcm androom temperature, the difference in population is about \ .

w,

12Q0.K8m

2

5hv

NORMALPOPULATION INVERTED POPULATIONREQUIRED FOR MASERACTIONFig. 6.Transition between two energy levels W\, Wi caused byquantumhvsuch thathv W W\.

In thermal equilibrium, the population of the lower level is greater than that of theupper level, and more quanta are absorbed by raising molecules from W\ to W2thanare emitted by molecules jumping from Wi to W\ . With an inverted population (moremolecules in upper level) the net effect is changed from absorption to emission.A beam of radiation (see Fig. 6) is equally effective in causingtransitions from the lower to the upper level (stimulated absorp-tion) as from the upper to the lower level (stimulated emission),and the net effect depends on the difference in the populationsof the two levels. In thermal equilibrium the excess of popula-tion is in the lower level, and energy is absorbed by the m oleculesfrom the beam of radiation . (The smallness of the excesspopulation in the lower state is one reason why radio-spectraare so weak). If by some trick an excess of molecules can becreated in the excited state, they can be stimulated to emitradiation and feed power into a tuned circuit. This counteractsthe damping through ordinary resistive losses, giving an effectivenegative resistance. If this is sufficient to outweigh the positiveresistance of the circuit, spontaneous oscillation will occur atthe natural frequency of the radiation.This exciting possibility was realized independently byTownes and Prokhoro v in 1954. A beam of ammo nia mole-cules passes through an inhomogeneous electric field, whichproduces a spatial separation of the molecules in the excitedstate from those in the ground state. The former then enter ahighly tuned resonant cavity where stimulated emission occurs,and oscillation is set up provided that the beam is sufficientlyintense. Estimated power outpu ts of 10~10W or more wereobtained, with a long-term frequency stability of 1part in 1010and short-term stabilities of a few parts in 10 12. The devicecan also be used as an amplifier, but it is untunable. As anoscillator it has been used to open up a new (but experimentallyrather difficult) field of molecular spectroscopy, but its mostfar-reaching achievement has probably been to stimulate ideas.This is shown by the wide range of use of the word 'maser'coined by Townes as an acronym (Microwave Amplification byStimulated Emission of Radiation) for the device, from whichhave followed words such as 'laser' for a parallel device(suggested by Townes and Schawlow) for light.At present th e most prom ising device for th e atomic frequencystandard of the future is a maser essentially similar to theammonia oscillator but based on the 1420 Mc/s line of atomichydrogen. Since the rate of radiation from atomic magneticdipoles is much less than from electric dipoles, the atoms mustbe stored in the cavity for correspondingly longer times.Fortunately this is possible because wall collisions have littleinfluence on the magnetic dipoles of hydrogen atom s. A longerstorage time gives the possibility of higher accuracy, and workat Harvard University under Ramsey envisages 1 part in 10 13in frequency precision.

Nuclear Magnetic ResonanceDetection of the precession of nuclear magnetic momentsabout an applied magnetic field in bulk matter was first accom-plished in 1945. It is convenient to discuss the motion in terms


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BLEANEY: THE FIFTY-THIRD KELVIN LECTURE 461of the nuclear magnetization, the vector resultant of the indi-vidual nuclear dipole moments, which obeys the same equationsof motion. In thermal equilibrium the magnetization is, ofcourse, parallel to the steady magnetic field. If an oscillatorymagnetic field is applied perpendicular to the steady field, themagnetization vector undergoes a forced precession about thesteady field, but owing to relaxation effects, the precession isat an angle which is appreciably different from zero only atresonance and is negligibly small otherwise. This rotatin gcomponent of magnetization can be detected in two ways:

(a)It induces an oscillatory voltage in a pick-up coil, which isnormally oriented at right-angles to the driving coil to avoiddirect pick-up; this is the method of 'nuclear induction' inventedby Bloch.(b) The transfer of power from the driving coil to the nucleito replace that dissipated through the relaxation processesresults in a damping of the coil, which can be observed bymaking it part of a tuned circuit; this is the method of 'nuclearmagnetic resonance' due to Purcell. The additional damping issmall compared with the n atural resistive damping of the circuit,but it can be picked o ut by sweeping the steady m agnetic fieldback and forward across the resonance value at a low frequency.The radio-frequency voltage across the coil is then modulatedby the absorption at this low frequency, and the modulationvoltage can be amplified after detection and displayed on anoscillograph (Fig. 7). A similar trick is used in nuclearinduction.

Fig. 7.Absorption line due to nuclear magnetic resonance ofprotons in liquid water, displayed on oscillograph.The vertical deflection is proportional to the strength of the absorption and thehorizontal deflection to the variation in the applied magnetic field.A limit to the degree of resolution attainable in magneticresonance is set by the line width which arises from the dam pingeffect of the spin-lattice relaxation, but in practice a much moreserious limitation is set by interaction between the dipoles them-selves ('spin-spin' interactio n). Each dipole is subject no t onlyto the external magnetic field but also to the fields of neigh-

bouring dipoles; the latter vary in orientation, giving a spread

A B

in the local field which amounts to a few gauss in a solid wheremost of the nuclei have dipole moments, such as proton-richorganic substances or simple inorganic fluorides. The effect issimilar to that of inhomogeneity in the applied field, but wherethe nuclei are identical and precess at the same frequency, theeffect is enhanced by a resonant coupling between them. Inliquids, however, effects of this kind are averaged out by therapid molecular motio n. The precession frequency of a dipolewill be displaced by the local field of a neighbour only if thisfield remains constant for a period greater than the duration ofthe wave train absorbed by the dipole from the applied oscillatoryfield . This cond ition is far from fulfilled in a liquid of lowviscosity like water, where the Debye relaxation time of about10~ nsec is a measure of the time for which a molecule remainsfixed in orientation. Except in very viscous liquids, such asglycerol at low temperatures, the random local fields areaveraged out very effectively, and resolution of the order of1 part in 108can be obtained. This requires very high homo-geneity in the magnet used to supply the steady field; residualvariation in field over the sample is averaged out by spinningthe sample rapidly. As Purcell has said, the nucleus rides outthese storms like a miniature gyroscope on perfect gymbals.The interest in such high resolution results from the fact thatit is possible to observe the very tiny variation in magnetic fieldat different nuc lear sites in a molecule. The local field differsfrom the external field because of the precession of the electronspins which gives rise to the normal diamagnetism; variation ofthe electron density from point to point within the moleculegives a corresponding variation in the local field. There arealso interactions between different nuclear dipoles within themolecule which are not averaged out by the molecular motion.Both these effects are studied in order to elucidate details ofchemical structure, and the fact that characteristic spectra areobtained from groups such as CH 3 provides a further tool for

the analytical chemist. Fig. 8 shows the nuclear magnetic-resonance spectrum of ethyl alcohol, where the three groups oflines belong to the protons in the OH, CH 2 and CH3 groups,respectively. The separation between the groups is due tovariation in the diamagnetic shielding at the different groups;thefinestructure within each group is due to spin-spin interactionbetween the proto ns. The spectrum was measured at 30Mc/s(a magnetic field of about 7000e.m.u.), while the smaller split-ings observed are less than 1c/s.A relatively strong nuclear magnetic-resonance signal such asthat of proton s in water can be detected fairly easily; for example,by the decrease in the output of an oscillator whose level ismarginal and therefore sensitive to the damping of its tunedcircuit. The strength of magnetic fields which are reasonablyhomogeneous can be most easily measured accurately by proton

magnetic resonance, since the only quantity to be determined is

Fig. 8.Nuclear magnetic resonance spectrum at 30 Mc/s ofethylalcohol.Left, proton signal from OH; centre, proton signal from CH2 group; right,proto n signal from CH3 group. The high resolution achieved isillustrated by the separation ofHnes A, B; this is about c/s.


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462 BLEANEY: THE FIFTY-THIRD KELVIN LECTUREthe resonance frequency and frequencies can readily be measuredwith high accuracy using simple equipment. Even fields assmall as the ea rth's field can be determined precisely by special,but not specially difficult, methods. In Oxford, the ResearchLaboratory for Archaeology and the History of Art has appliedsuch techniques for the detection of buried pottery kilns, whoseclay retains a permanent magnetic moment which causes ananomaly in the earth's magnetic field. Surprisingly, evenfilled-in pits and ditches can be located in this way, giving thearchaeologists an invaluable tool for locating sites of possibleinterest without digging over vast areas, a costly and oftenunrewarding exercise. Fig. 9 shows a proton magnetometer inoperation on a site.

Electron Spin Magnetic ResonanceIn substances which contain permanent magnetic dipolesassociated with electrons, magnetic resonance can readily beobserved. The magnetogyric ratio of electrons is about athousand times higher than that of nuclei, so that the precession

frequencies in fields of a few thousand e.m.u. correspond towavelengths of a few centimetres, and the subject forms anotherbranch of microwave spectroscopy. The increased size of thedipoles means t hat local fields are correspondingly bigger, beingseveral hundred e.m.u. for a typical paramagnetic salt. Linewidths are therefore of this order, an effect which can be over-come by 'diluting' the magnetic ions; that is, a substance suchas zinc-potassium sulphate is used, where rather less than 1of the diamagnetic zinc ions are replaced by paramagneticcopper ion s. The residual line widths are then only a fewe.m.u., owing to the nuclear moments of the protons in thewater of crystallization. Coupling between the magnetic electronsand the thermal vibrations of the crystal lattice may also bestrong, giving a very short spin-lattice relaxation time; this canbe overcome by working at low temperatures, where the thermalvibrations have died out. This coupling is very variable, well-resolved spectra being observable at room temperatures in somesalts, while in others cooling to liquid-helium temperatures4K) may be necessary. Fortun ately, the large dipole momentsalso give high intensity, so that spectra can be observed from asfew as 1012ions. This makes it possible to observe spectra ofmagnetic centres formed by damage of the crystal latticefollowing exposure to ionizing radiation.Electron-spin resonance spectra are complex for a numberof reasons. First, the motio n of themagnetic electron is distorted by theelectric fields of neighbouring ligands,and its magnetic behaviour is aniso-tropic; that is, the spectrum observeddepends on the orientation of theapplied magnetic field relative to thecrystal axes. This makes it almostalways essential to use single crystals.Second, hyperfine structure may bepresent owing to the nuclear magneticdipole moment of the ion itself (seeFig. 10), or of the nuclei of neighbour-ing ions. The latter is enhanced whenthe binding is partially covalent, so thatthe magnetic electron is not localizedon a single ion but is to some degree'shared ' between several ions. Conver-sely, the complexity of the spectra hasresulted in a wealth of informationconcerning the paramagnetic propertiesof ions of the transition groups and thenature of their chemical binding in a

solid. A large number of the main advances in thisfieldhave beendue to my colleagues at the Clarendon Laborato ry. The truenature of a number of point defects due to radiation damage hasalso been revealed, confirming or replacing earlier ideas whichwere little more than informed speculation.An important practical development from electron spin reso-nance is the solid-state maser, suggested by Bloembergen andrealized experimentally (and independently) by Scovil. Thismakes use of paramagnetic salts which have at least threeunequally spaced levels between which transitions can takeplace throu gh the magnetic-resonance phenomenon. At lowtemperatures, where the energy separations are of order kT ,the popu lations of the three levels are appreciably different, asshown in Fig. 11. If a strong microwave magnetic field isapplied at a resonant frequency, such as that corresponding tothe transition between levels A and C, transitions between thispair of levels can be induced at a rapid rate; transitions in eitherdirection are equally probable, and if this were the only process,the populations of the two levels would rapidly become equal.This process is offset by relaxation effects which tend to restorethe equilibrium ratios of the popu lations. At low temperaturessuch relaxation processes are rather slow, and only a moderatepower is needed from the oscillator to make the populations oflevels A and C nearly equal. As a result of this pumpingaction, the population of level C may exceed that of level B,thus giving the possibility of maser action at the frequencycorresponding to the separation of these two levels. In other

Fig. 9.Proton magnetometer.This device gives an audible indication of thedifference in proton magnetic resonance frequen-cies in two bottles of water, thus detecting local

anomalies in the earth's field and eliminatingeffects due to diurnal variations or distant dis-turbances such as d.c. electric traction. Fielddifferences of 5 x 10~ 5e.m.u. can be detected.


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BLEANEY: THE FIFTY-THIRD KELVIN LECTURE 463

/ IFig. 10.Electron paramagnetic resonance spectrum of silver atomsin KC1 at a frequency of 9354Mc/s.

The two widely separated bands show the splitting due to the nuclear magneticmoment of the silver nucleus, which can take up two orientations such that its fieldcither adds to or subtracts from the external field. The structure within each band isdue to the magnetic fields of the magnetic moments of the six nearest chlorine nuclei.The spectrum shown is the derivative of the absorption signal.words, if a small signal is applied at this frequency, it may bemagnified by stimulated emission by the ions falling from levelC to level B, which supplies power at the signal frequency inthe right phase to make the signal grow. The importance of

ABSORPTION ATPUMP FREQUENCY

POPULATION OFTHREE LEVELSIN UNDISTURBEDTHERMAL EQUILIBRIUM

STIMULATEDEMISSIONAT SIGNALFREQUENCY

POPULATIONS UNDERPUMPING ACTION,SHOWING INVERSION OFRELATIVE POPULATIONIN LEVELS B AND C

Fig. 11.Principle of three-level maser.In thermal equilibrium the populations of the three levels decrease in ascendingorder of the energy. A strong microwave signal (the 'pum p' signal) is applied betweenlevels A and C, resulting in a greater population in level C than in level B. Maseraction (amplification or oscillation) can then be obtained at the frequency correspond-ing to transitions between levels C and B.

this as an amplifier is that its inherent noise corresponds to atemperature of a few degrees absolute, instead of several thousanddegrees for the best type of electronic receiver. A low-noise-temperature receiver is invaluable in radio astronom y or satellitecommunication, where the aerial is directed at the opensky whose radiation background also corresponds to a lowtemperature.

ConclusionIt is, of course, impossible to d o justice in a survey of this kindto all branches of radio-spectroscopy. For example, I havemade no mention of ferromagnetic resonance, discovered byGriffiths in 1946, or similar electron-spin resonance phenomenain other substances where the spins act together in co-operativefashion under the action of exchange forces. Nuclear magneticresonance has also played its part in such substances, acting as aprobe to measure the internal magnetic field. Another impo rtantbranch is cyclotron resonance, where the helical motion of afree charged particle in a magnetic field is detected by its inter-action with an oscillatory electric field at the resonant frequency.

With free electrons and protons, this has yielded the mostaccurate values at present available for the ratio of charge tomass for these fundamental particles. In semiconductors andmetals, cyclotron resonance shows how the electronic motion ina m agnetic field is modified through interaction with th e periodicfield of the lattice.A lecture which included reference to every important aspectof radiospectroscopy would be reduced to a mere catalogue.Instead, I have tried to give an outline of the chief lines alongwhich the subject has developed, and to show how importantpractical applications have ultimately resulted. It should beemphasized th at in every case the first steps in the various fieldswere taken as part of science of the purest kindfew aspects ofphysics could seem more remote from practical application thanthe determination of nuclear magnetic moments, but a direct

descendant of this work is an atomic standard of frequency.An interesting feature of the history of radio-spectroscopy isthat, by and large, the possibilities of practical applications havebeen realized by the same scientists as were responsible for theinitial developments for 'pu re' research work. Finally, the ideaswhich have come forward in radiospectroscopy have stimulatedprogress in other fields; I mention only the laser, suggested bySchawlow and Townes, in which coherent stimulated emissionof light is produced, whose restricted spread in range of wave-length and angle of beam surpasses by orders of magnitude anyprevious achievements.Acknowledgments

I am indebted to Dr. R. E. Richards, Physical ChemicalLaboratory, Oxford, for Fig. 8, to Dr. M. J. Aitken, ResearchLaboratory of Archaeology and History of Art, Oxford, forFig. 9 and to Dr. W. Hayes, Clarendon Laboratory, Oxford,for Fig. 10.

VOL. 109, PART B, N O. 48. 19

bleaney, b. -- the fifty-third kelvin lecture. radiospectroscopy

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