scientificamerican0565-58

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MOLCUAR BES

Studies involving cohrt beams of t and 10leul ave had

a pofoun inn on dvelopent o oer pyis. An0n

hir eoloicl idns ae ae, asers and aoi lok

Te atoic pyicit, like evey-body ee, eed ai o betig,but otewie it i i i way At

oal atopeic pee eac aiolecue i ivolved i ealy a biliooliio a ecod Oy i gae t auc lowe peuei wat i ooelycaled a vacuudo olece atti aeaoabe degee o pivacy e eue c ow pee i it caledto," ate te Itaia ivetigatoEvgelita Toicei, wo devied aetod o obtaiig ily good vacu i 1643 Atoeic peue iabot 60 to oe ta 00 yeate, i 1855, te ea gbowe

eiic eie ed Toicei etod to evacate ga te dow toabout 01 to ti cieveet ed di-ecty to te dicovey o catode y,wic wee evety idetied abea o ee eecto by J J Tooi 189 At uc pee eectoca tavel ay ice, but at te epee te e ee pat o a aioecue betwee coiio i oyabout a iiete To acieve a coeet bea o ay kid o ato o oecle oe eed c ette vacu

at eat 0000 10

to o betteTecique o obtaiig vey owpeue wee developed pidly duig te t decade o ti cetuy, adi 1911 te Fec yicit L Duoye ucceeed i poducig tewod' t olecua bea e deotated tat i a good vacuu te odi ato tat copied i beaoved i taigt lie Doye' ele-gatly iple appaatu coited o aevacuated gla tube wit two co-tictio [ op o o p

60] We e eted oe odi ioe o te ed coptet, a dak de-poit o odi ppeaed i te copatet at te oppoite ed Te de-poit coveed oly te aea tat coud

O R F

be eced by ato tt d paed itaigt ie toug te cotictio

I eece Duoye' appaatu wa

te pototype o al ode olecula-bea device Te tee eceay eleet o ay uc device ae ( 1 oce o olecule uualy caled aove, eve toug it i ot alwayeated wit a apetue tougwic te olecule c ecape ito avac, a ecod, coiatig ap-ete, ic o te be by toppig o deectig al te olecule tatdo ot pa diecty toug it, ad 3 a etod o detectig te eect ovaiou iece o te bea I co

tat to ea o ubatoic patice,ic c be acceeated to peed appoacig te peed o igt, oeculabea oay tvel t teal"peed: ie a ecod o o Tegeeic te oecua bea" appliebot to goup o to o exapeeutl ydoge, eac oecule owic coit o two ydoge atoad to ige to o exaple o-di vapo)

oecua bea ca be de to patog agetic o eectic eld tey

ca be diected at vaio obtacle,uc a cyta o ga olecule teyca be iliated wit itee ligt,bobaded wit electo o coedby ote oleca bea Fo ucexpeiet uc a bee leaedabot te baic tucte o atte tatcod ot ave bee leed i oteway Te ivetio o ae ad la-e aoe out o a oeculabea expeiet, d a bea o ceiu o y-doge to i toic clock" i teot accute tiekeee eve built

e eel ment

Te idea tat ga oecule ae iapid otio goe bck to a teatie

witte i 138 by te Swi ateticia Daiel Beoulli e poited ottat bobadet by t oecule

would exet a eue o te w oa cotaie, d tat te peue oudie i popotio to te ube oolecule pe uit vole, o te de-ity o te ga a Robet Boye ad obeved oe 80 yea ealie Beouio expaied te ie o peue witiceaig tepeate by ig tatte olecue ove ate we te gai eated O coe Beouli w igti evey epect, ut i ggetioaied at let ay quetio a teyaweed

a olecue ove a t iebuet wy, te, doe it tke iuteo a el to pead o oe ed oa oo to ote? \e ow kow teawe i tat te olecue keep col-idig ad teeoe ake little eadwy I act, o te te t wic aell doe pedto pt it oe geeally, te ate at wic oe ga di-ue ito aoteit i poibe to calcuate ow ote te olecue i aga collide evey ecod Suc calcua-tio wee ot ade util te iddle o

te 19th cetuy d oly te didpyicit begi to egad ato adolecule a eal ad ot eely a c-tio coveiet o ceit Sevealetod wee devied o coputigte peed o olecule ad tei eaee pat betwee coliio I geealte eult ageed ad uppoted teew kietic teoy o gae, wic wly etablied duig te tte ao te 19t cetuy by Je lekMaxwel o Bitai, Ludwig Boltzao Autia ad ote teoit Noete-

le, te evidece o te ietic teoyeaied idiecTe t diect eaueet o te

peed o g olecue a ade i190 by Otto Ste, wo wa te

1965 SCIENTIFIC AMERICAN, INC


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wokig t te iveity o Fkut;it w o te t eueet iwi oeu be eoyedItd o a ove Ste ued a otiu ie ted wit ve adoitioed i te idde o a evutedbe Sive to evoted ote wie i a dietio, but oy toe

tt ed toug ow it weebe to ga dteto ate, oi tey et deoit i te o o ow ie A og a te aatueied ttioy te eed o teto dd ot atte; at o ow, tey oowed te ae tgt t

But Ste d outed te ait o atute ivetedwe, t it d te g teo vetia x tat oud be de tootte at oe 1500 tu e iuteiide te be j [see boom illustra-

tion o

xpage

At tt tie, witvuu teiue ti i tei iy,

ti w uite tou de oe et tu w ottig, te deoiwa od i igty dieet e,beue te g te w ovig te to wee edig o it Fote out o it Ste w abe oetite te vege ed o te atot 58 ete e eod; ti w wit-i ew eet o te gue editedby te eti teoy o ge Moe-ove, e oeved tt te deoiod duig ottio w it uzzy;ti owed tt ot te ive atod te e ed, wi xwe adoeee

About ye ate Ste ubied,togete wit i oegue atere, te eut o aote oeu-be exeiet Ti exeied ooud iuee o te deveoet o quatu ei; it u-otd te ate iedibe ouio,dw o etooi obevatio,tt te obit o eeto ovigbout te ueu o to i a ag-eti ed ut wy be t igt ageo te lie o oe i te ed, atoug te eeto obit i oedietio o te ote Su tiaqutiztio oetey t oddwit i yi, odig owi geti ed woud eeke te eto obit ee, orwobe, aoud ed ie te ike iig to, witout ioig iit o it oiettio

I tikig about te Steeaexeiet it i eu to tet teato a toug it wee a a brget wit a ageti oe {Te ageti oet o a obitigge tveig wit te eed v o ie o du r, i give by te equa

TPICAL MOLECULARBEAM PPARATUS phoographed in he laboraor of or

man F. Ramse a Harvard niversi is used for udes of nuclear magneic resonance

A beam of hdrogen molecules eners he apparaus a he far end passes hrough severamagnec elds and s deeced a he near end (see schematic diagrm at bottom of page

OURCEEN VIE of he molecularbeam resonance apparaus shown above reeas heend of he rs or ed deecng magne The beam of hdrogen moecues asses

beween he wo semicircularshaped pole piees a he cener The source euipmen and

he bulkheads separaing he various chambers have been removed for eas iewin



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SOURE H A BE R

OAORHABER

O PP

OBSERVAION HABER

J

FIRST MOLECLR-BEM RTS s strted y L Dyer 191 1 Whehe heted sme sdm i e f the ed cmprtmets evted glss tube left),

drk depst f sdm ppered the mprtmet t the ppste ed ight_ The

depst cvered ly the re tht culd e reched y sdm tms tht hd pssedin strght ls thrugh the t cstrcts The te s ut eght ches l

tio = evr/2c, beig te peed oligt e uc a aget i ietedito a agetic eld at a agle wit epect to te ed lie, it eegyotet i caged by a aout eqalto tie te coie o [see bottostrtion on opposite pge I we co

ide a lage be o uc aget,oieted at ado, te te coie o i equally likely to ave ay vlebetwee 1 ad + 1 accodigly teagetic eegy ould ave ay valuebetwee ad + If patialqaizatio exited, oweve, te e

B JAR

AN . - = /."' ISOU R

ASS DOR PA

FIRST DIRECT MESMENT f the speed f gs mlecules s mde i 1 2 0 y OttSter; this s ls the rst mesuremet hh mleclr em s emplyed The

mi prts f the pprtus shown fom bove in this shemati view ere muted vertcl xis tht culd e mde t rtte t sme 100 trs er mute sde evcu

ted ell jr Whe the pprtus s rttg depst s frmed slghtly deret

plce the glss detectr plte t he the pprtus s sttry Frm the mutf sift Str estmted the verge speed f the tms He ls sered tht the depstfrmed drg rtt s fzzy; ths shed tht t ll the ms hd the sme speed

6

egy could ave oly te two exteevlue ad + ad otig ibetwee

Fo ealy a ecae beoe te teelac exeiet te claicl cocet o doly oieted electo obit bee i coict wit teZeea eet, te act t oted byte Dtc yicit Pete Zeea i

186 tat te lie i a petu aeote plit ito two o oe lie wete eittig ato i laced i a agetic e t t te ao fo teeakble ae o tee ectallie w ukow Te t tep towada detig o ti eoeoce i 10 wit te itoductio oMax Plck tu otlate, wicae te te tatlig auptiott ligt i eitted i quata, o dicete pacel o eegy I 113 Nielo ae te te auptio tat

te eegy otet o ato t alobe uatize, ad tat te eiio oectal lie i caed by te ato'taitio fo oe eegy tate to aote ccodig to Bo' odel teeegy dieece betwee te twotte i caied away by te eittedligt t a tu eteie tewvelegt o te eitted ligt Tepe o te eegy tte guatee te ae o te etal lie

I ato wee oieted at ado ia agetic eld, tei eegy cotet

would, a we ave ee, be blued"by a aout eqal to twice TeZeea eect, oweve, deotatedtt petal lie ae ot i act bluedbut itead ae plit ito two o oeap lie aalyi o te Zeealittig o te ectal lie o ilveowe tat i tei oal tate teilve ato ould be apable o olytwo oietatio, coeodig i teo agetic eegy to te value ad +

Now, te agued a ollow: If a

getic ato wee placed i a ioogeeou agetic eld a eldwoe iteity vaie o oit tooit, te te ato' eegy cotetwould alo vay I ode to ove teato o poit to ot wok wouldave to be doe I ote wod, teagetic ed would exet a oce ote ato a a wole te oce wouldo coue deed o te oetatio ofte ato wit epect to te ageticeld I te ato wee oieted aoly, te oce would vay cotiu

ouly o to + beig teeld gadiet, o te cage i eld iteity e cetiete If, o te otead, atial uatizatio obtaie, teoce would alway be eite o



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+ y aking e orce arge enougo caue obervabe deecion in abea o iver ato Sern reaoneda i ig be poibe o decide experientaly ow ao orien teeve a agneic eld

Quie by cance Gerlac ad beeneauring te gradient o a agneticeld near te arp edge o a agne

Stern copued a uc a agneould deec iver ao enougo i purpoe o ey oined orceeir experien wa a copete ucce: e ne line o iver a ad beendepoited in e abence o e agneic eld wa ditored ino a uzzyayetric oop wen te agnet waurned on [see illustration on page 63]e uzzine wa caued by te Maxwelian diribution o e peed o eilver ao e ayetry by eact a e eld gradien wa greater

near te knieedged agnetic poe anaway ro i e crucia nding wata e depoi wa a loop and no ablur: near e idde tere wa pracically no iver I te iver ato adbeen oriented at rando in e agneticeda caica pyic deandedere oud ave been no ewer atwi al deection tan ato witlarge one Te experiental reul wain perect agreeen wit te concepo paial quantization

Stern could oreee any oter po-

enia ue or olecular bea andin 1923 wen e becae proeor andead o te departen o pyicalceitry a te Univerity o aburg e et out to develop te etodyteatically During te next 10year e and i tudent publied aerie o 30 paper reporting any ipan ndinga aive deontration o te power o intelligent planningI ad te good ortune to work witStern ro 930 to 193 and it wawonderul training Our wole ca-

paign ad been outlined in te rt otoe 30 paper; in due coure ot ote original probem were attacked andolved a we a ome probem tatad not been oreeen but tat grew outo our work Noting wa let to canceor conjecture Every piece o apparatuwa deigned wit care and it beaviorwa cecked out in advance; in ti wayany ortcoming wa quicky revealedand i poible reedied beore tepiece wa put into ue Wit ti orto training Stern' tudent were wel

prepared to go out in te word andtart teir own ucceul coo owork and oe o te did

One o te experiment Stern adpropoed in ta r paper wa a tet

c

J 1

J T! f

. 1 DEVELOPMENT OF ELECTRON THEORY is eected in these thee drawings, hchepresent three derent conceps of the behavo of the electron in a magnetc eld Accord

ing to classcal, o pequantummechancal, physics op the magnetic eld would merelymake the electron's orbit precess, or wobble, around a eld line, whout imposng any

lmaton on its orientaon. Accodng to the Bohr model of the hydogen atom mddle

the electron's orbt in a magnetc eld must always be at rght angles to the eld lines, withthe electron orbtng n one drecton o the other. Ths concep, caled spatal quantiza

ton," was suppoted by the obseved Zeeman splitig o spectal lnes and also by theSernGerlach moleculaream experiment. Snce 192 the spn of the electron has been

known to contbute to he total angular momentum of the aom that undegoes spatia

quanzaon. In he case of hydrogen or sver atoms n ther ground state bottom thee so ora momenum hence eecron spn s agned parae or anparael to magnetc eld

MGNET NOGY s helpful n thnkng aout the SernGerach expermen; accord

ng o the anaogy, he atom is reated as though t were a sma bar magnet wh a manetcmoment When suc a magnet is nserted no a agnetc ed at an ange with espect o te ed lnes, s energy content s changed y an amount equal to tmes thecosne of . If e consder a large number of such magnets orented at andom, then hecosne of s equay lkey o have any vaue eteen 1 and +1; accordingy he energcold have an vaue etween and + If spatal quantaton exsed, however,the energy coud hae nly he to exreme values and + and nong in between.

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HU AT D, sed o deectingbeams o magnetic atoms and molecles, can be set in seea

ways The oleiece aangement in is simila to tha sed in heoiginal teneach exeiment. The aangement in b is a lateesion o which the eld can be moe eadily calclated om he

dimensions o the ole ieces n the beam is diected thogthe edge o a homogeneos eld the deection deends ony n

SOUREHAMBER

SEPARATIONCHAMBER

b

d

the angle o incidence and the stength o the ed between andotside the ole ieces. n d an inhomogeneos ed simia o thain b is set by two cylindical condcos thog whic a stngelectic cent is assed in oosite diections he absence o

ion in this aangement acilitates the calcation o he ed n

all o examles a encillike beam o atoms is shown being si

into two comonent beams aing oosite magnetic moments

MANCHAMBER

SOURCE -COLLMANG SLIOILMANOMETER L-= FLOW METER

ANKS

YDROGEN DEUTERIUM

UAA A AAAU, sed o measing e ncea magnetic moments ydge moeces, i

s in is schematic diagam Te aaats conain mogenes magneic eds: a oaie (an ed) secmoeces o a aicla oientation and an aaye (a B ed) es o eiened moeces The ansiis m ne e

taion e oe ake ace in an inemediae id ed (a ed), wic ms e bo mogeneos and accae

RADOFRQNCY LOOS

The cental egion aso cnains w oos, thg wic a sci

ating cen om a sma adieenc genea is assed

wo inomogees eds ae aanged a amseeced and smewa sead te ed a ggete agin B ed t detco is a ec good actio e igina eam esses insid he cambs

a e scien ms meces ae eie

engt aaat wito eing sbected cisis



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f \IoLIIUM UOR Y1AL

2

/'

20/ . 1\ 1+\ 1\ 1\ /

1\ I--j/------

DFFRACTO

ATTR

DIFFRACTIO OF HELIUM ATOM by a cystal sufac o

lithium uoid was acivd by tn and is collagus in 1929

Thei appaatus was aanged in such a way that th cystal coub tilted in od to dict th ected and diactd atoms into

a small tub (uppr rgh which was connctd to a vy snsi

iv manomt. Th gap at fa ight shows a typical esult The

hap pak at tilting angl zo is causd by t ctd atoms

sometims called th spcula bam. As th tilting angl is incasd, th numbe of tctd atom st dops shaply but thn

iss again whn th diactd atoms bgin to nt dtcto. Ths

ed Such atoms are always reecteddiusel as light s from white papersometimes with a slight enhancement nthe specular drecton as from slightshny paper Not much work has beendone in this area but such studies mabe of some s in the future

ertain anomalies were found nStern's draction experiments that n-dcated a very weak attraction betweenthe helium atoms and the surface ofthe crystal In addtion slight irregular-ties n the shape of the reected beamfound later by Stern and Estermannoered a clue to the nature of subm-croscopic crystal imperfections In bothcases further research is now dormantbt perhaps t s not defunct

Magne.Momen Expermens

y far the most fertile ine of workn molecular beams has grown out ofthe original SternGerlach experimentand deals with the deection of mag-netic atoms in nhomogeneous magneticelds The interpretation of the Stern-Gerlach experment changed soon aftert was completed; the magnetism of thesilver atoms was now believed to becaused not by the orbtal moton of tsoutermost electron as n ohr's modeof the hydrogen atom but by the rota-tion of the electron around ts own axisWhen the dea of a spnning electron

6

was rst published b Samuel A Goud-smit and George E Uhlenbeck n 1925a number of puzzlng spectrographicfacts fell neatly nto place In som

atoms the magnetic moment was apparently due to a combination of elec-tron spn and electron orbital motionand n 192 Alfred Land had alreaddevised an empircal formula for computing the magnetc moment of theseatoms to account for the observed ee-mn splitting of the spectral lines of theatoms in a magnetc eld Stern hadnduced a couple of hs students toverify Land's formula for the elementsbismuth and thallium but on the wholenterest in the magnetic moments ofatoms seemed to be on the wane

In 192 however Stern pointed othat the molecularbeam method oughtto be capable of detecting and perhapsmeasuring magnetic moments 1times smaller than any previouslmeasured One might expect suchmagnetic moments to be created b therotation of molecules or to be ntrinsicto atomic nuclei Interest in such smalmoments quickened two years laterwhen A . Dirac published hiscelebrated mathematical theory of the

electron Dirac had tred to mprove onSchrdnger's wave equation whch wasnonrelativistic (that s valid only forparticles moving fairly slowly comparedwth the speed of light) Dirac succeed

ed n designing a wave equaton thawas remarkably simple and he wasthrilled to nd that the electron f tobeyed hs equation had to spin ust asGoudsmit and Uhlenbeck had predctedt would The magnetic moment of theatom was also given by Dirac's equation

What about the electron's big brotherthe proton? It was 18 times heaviethan the electron but Drac's theormposed no weight limts Indeed therewas no reason why Dirac's theoryshould not apply to the proton as well;if t did the spn or angular mo-mentum of the proton woud be thesame as that of the electron but tsmagnetic moment would be 1 8 timessmaller than that of the electron

Here was a fresh challenge fo SternThe diraction experments had givenhim experience with hydrogen beamsand the hydrogen molecule was par-ticularly suted for ths new test: ts twoelectrons wer known to spin n op-posite directons bt ts protons usuallyhad their spns pointing n the samedirection Hence the hydrogen moleculewould possess a magnetc moment twicethat of a single proton The sensitivtyof the SternGerlach experiment wouldhave to be mproved about a thousand-fold but Stern was condent ths couldb done The nhomogeneous magneticeld was made narrower (to provide a

higher magnetc gradient) and longe



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IZ

L . .

3

5 8oL

-

4

58

000 0,00 500 ,000 ,50NNS AN F D (RSDS) N N S I A N D (RSDS)ENERGY OF A SODIUM ATOM, ploted as a funcon of mag

netc-eld ntnsty /, appears to ncrease along two sraght

lnes, correspondng to the parallel and antparalel orentatons of

the atom's spnnng ecrons wth respec o the magnetc eld In

acualty the magnetc moment of he nucleus also makes a mne

conbuton to the total magnec energy of the aom Thus the tw

lnes on the graph are actualy each a set of four lnes, the lnes

beng very close ogher because he magnec moment of he

nucleus s very sma In a weak magnec d th neracon ofthe eecron spn and he magnetc momen of the nueus gves

rse o an even more complcated hyprne" sructure right. The

arrows ndcae th transtons beween the closely spac energy

staes hat can be sued by he moecuar-beam reonane mehod

Tranon s coparavey easy to suy because the eergy dfference and henc the requncy s much owr than for he ohe

tranons Transton s sutabe for use n an atomc cock"

on o gp e tll e set of

fo lines, te lines being ve losetogete bese te mgneti momentof te nles is ve smll n mesol minte ontibtion to te totlmgnetic eneg of te tom.

n we mgnetic el te sittionis fte omplite b te ft ttbot te eleton n te nles mbe togt to pecess in omplex

mnne. S peqntmmenilws of speing e of little se, btqntm menis enbles s to oot te llowe eneg vles [see

ision Tigh boveJ t zeoel tee e jst two eneg vle,oespoding to te eleton sinbeing ligned pllel o ntipllel tote nle spin. Tese two sttes venet sins of I eqls +1/2 n I els1/2 espetivel. Fo soim tiswos ot to two n one net sinsesetivel; ts we el splits teeneg vles into ve nd tee lines,oesponding to ve n tee llowenle oienttions. s te intesit ofte eld ineses, tee is gl

tsition to sittion in wi temtl intetions of te eleton spinn nle spin beome insignint.

ll te foegoig penomen ee

66

nown fo some time befoe Rbis ex-

eiments, tog te st of spec-toso. s te st de spet-soes wee impove lte in te 19tent it ws fon tt some of tespetl lines ee omplex. Fo in-stne, te ellow soim line wsfo to be i of lines ieing inwvelegt b bot . 1 peent; similne stte ws fon in mn otelines. Te stnd explntion, givenb osmit n Ulenbe in 1925,ws tt te eleton n spin eite inte ietion of its obit o in te op-

posite ietion, te two possible sttespossessig enegies tt ie b bot.002 eleton volt.

s te spetosopists tools bememoe elite, e of te sodim linesws itself seen s gop of sevellines, n mn ote spectl linesevele simil pene" stte.Sometimes tis stcte is meel nisotope eet te element i qestiononsists of sevel isotopes wose spe-tl lines o not extl oinie. Usl-l, oweve, te ene stte is

se b te intetion of te electonsin n te mgnetic moment of tenles, s ve exline bove.Colete onmtion of te tte

view ws ilt bese setl lines

e often not s enog to llow tele esoltion of pene stte.olel bems wee ble to elp

ee in two ieent ws. n te stle te old be sed to oespe spetl lines. n me o neletic isge te toms tt emitligt e ing bot wit spees p to mile seon o so. Wen n obsevet est eeives ligt fom movingsoe, e eceives moe wve ests eseond if te soce is moving towim n fewe wve ests if te soe

is moving w. Tis penomenon(nown s te Dopple eet fteCistin Dopple, wo st peiteit in 1842) ses spe in wvelengts monting to sevel pts in

million n obses some etils ofene stte. f te soe on-sists of toms in bem, oweve, nte ligt tt is emitte is obseve p-oximtel t igt ngles to te dietion of te bem, te Dopple boen-ing of setl lines beomes msmlle. Tis ti ws se sessfll

in 1928 b te Soviet investigtos L.Dobezov n . Teenin; in ft, tewee te st to obseve n mesete pene stte of te ellow



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oter Te ony net mgnetic momentof sc moece is e to its nclesin n molecl ottion (s in tecse of yoge). s conseqenceey ong n e bems d to be sedin ode fo te bem to be slit eciby in n inomogeneos A e Onte ote nd, te C eld cold be

m qite stog witot getting excessiely ig esonnce feqencies swe e seen, in stong mgnetic edte mtl intection of te nclesin moments te moleclottion moments is negigibe.

Toy, 27 yes fte Rbis stesonnce exeiments, es dozebotoies e still sing is metod toetemine nce mgnetic moments,n tee is no sign tt its ictionse been exste. In ecent yessome cometitos e eed foexme, it is now ossibe, tns to

metos deised by Feix Bloc tStnfo niesity nd Edwd M.Pcel t H iesity, to mese nce mgnetic moments in sois. Te moecbem metod re

-.

3

U >zUwwZw

I Ij ________

mins nbeen, owee, wen itcomes to mesing te mgnetic moments of e secies of ncleifoistnce sotlied dioctie ncei,of wic only minte monts cn beobtined t time. Moeoe, mnyoeties of fee toms n molecles,edicted by qntm teoy ndogly tested by te metods of oticl sectoscoy, e been inestigtein mc ne detil by te molecem esonnce metod. oemostmong tese ltte lictions ws tembRetefod exeiment, wicws fomed t Colmbi in 1947.

The Lab-eherford xperet

Te sectm of te ydogen toms been te tocstone of tomic ysics fom te ery beginning. Te eglrity wit wic te lines in te sectm e sced d been noted byte Swiss scooltece Jonn JobBlme in 1885 Bo's st model ofte ydogen tom ccoted foBmes obsetions nd edict

S ' / P ' i i

;

S

P/

P ' /

FRST EXTED STTE f te yge tm esps t te se eegy

level te dgm t left s expeted t te D tey t e splt t

tee lsely djet egy stes cener f te slle S level ed xtly t te P leve s g te mleulemese met Wlls Lm d R

Retef t lum fud tt tese t levels t ft e ut eesepted y ut 1000 megyles rht Ts sep s k s te m t

6

itionl sectl ines n bot te infed n te tiolet egions of teydogen sectm te existence oftese ines ws eied sot timeter. Scinges we eqtion ge new mening to Bos moe, emoig its bitriness nd some of its inconsistencies. t is moe, Scinges eqtios mde it cle ow Bosyogen model cold be lied toote toms. Dic moied te weetion to big it into cco witeltiity teoy fon tt te sinof te eecton folowed ntlly fomis modiction. He ws ble to comte te ne stcte of te sectlines of ydoe witot ny d hossmtions, bt it ws iclt to decide wete o ot te teoy ellygee wit te fcts. Te e stctre of ydogens sectl lines is ticy e, te ines e stonglyboene by te Doe eect (becse ydogn toms e so igt ndence fst. o ney 20 yes teeidence f Dics eictions eine inconcsie. Some sectosco

rlwZwe J

o _ _ . _ -I N T E N S T Y O F M A G N E I C F E L

MGNET FELD te D they

eted tt te tee eegy levels f te

st exted stte f hydge suld esplt fue t ttl f egt sttes.

g 10 utm tey 8 f te 28 eJe tsts etee te egt stts

8



12/20

psts foud small devatos from thetheory whereas others thought all waswell.

Whe World War II eded 1945may physcsts retued to ther laboratores kee to take up pure researchoce more ad to se the new tools thewar had forced hem to desg. I partcular the developme of radar ha

produced ew mcrowave techquesand creased he scope of the resoace method for studyg atomctrastos. olumba Wlls Lambdecded to use the resoance method tostudy transtos betwee he threeclosely adacet eergy sates to whchhe Drac theory had spl he rs ected sate of the hydrogen aom. Inhe presece of a magetc eld thesestates should be spl furher to atoal of egh states. ccordg to theselecto rules" of quatum theory 1

of the 2 cocevable rastos bewee the egh states should be forbdde leavg 10 trastos to be observed of whch ve were epected obe very feeble.

o oba hydroge the desredstate Lamb ad . . etherford rs

had to break up he hydroge moleculesto aoms. hs was doe by usg atungsen ove at tese whte hea.he escapg atoms were the bombarded by electros wth eough eergyto rase them to the rs ected sae.oms tha were rased to oe of the

socalled

sates would retu almosta oce o ther groud stae emttg

a photo or qatum of lght. he Ssate o the other had s measable;hat s t lass a much loger tmeong eough to le he atom complete tsouey through the apparatus. oreover a metasable atom s easy o deect: whe strkes a metal plate eds to lberate a electron ad thusbetrays ts presece. Only oe hydrogeatom several mllon was rased tothe metastable state bu sce a farly

broad beam cold be sed there wereeough atoms to perform the measurements. o detec the trastos wasalso easy a atom hat made a rasto from the S state o one of he

15 ,000

saes would he mmedately radatea photo ad drop to the groud satelosg ts ablty to release a electrofrom the metal plate.

he procedure adopted by Lamb adetherford was smlar to ha of ab'srs eperme. he rado frequencywas se successvely a a number ofvalues and each tme he magetc eld

was vared utl a dp n he elecrocurre dcated ha a transo wastakg place. he results were strkg dsagreeme wth Drac's theory;o eplan them oe had o assume thathe S level dd ot cocde wth he evel as epected bu was abou1000 megacycles hgher. hs shftow generally called he Lamb shfsoly about half the ypca Dopplerbroadeg of a optcal le of hydroge. No woder he specroscopsts hadbee uable to p dow!

he theoretca physcsts ralledquckly from ther surprse ad whna few weeks Has ethe produced acomplete eplaato of he Lamb shf.cually they had o been unprepared

l a

--l 2 _ w 10 000[:

w

----- ---- - ---4 w5 8

T N S T O M A N E T C I D -

should be fobidden, leaving 10 tansitio

to be obseved, of which ve ee expected

to be vey feeble The emaining ve ashown hee in the positions pedicted by th

Diac theoy (ft) and in the positionfound by amb and Rethefod (rght)

:Lo

5,000---

"- o ,:

e- - -o 000 2,000 3000

T N S O A T C D ( AU S S)

RESONANCES ERE OBSERVED by amb and Rethefod at the positions maked bythe small open cicles on this gaph accoding to the Diac theoy, esonances should have

een oseved along the boken lins. To explain the discepancy theoists wee foced topusue Diac's calclations beyond a st appoximation. Th best theoetical gue fo

h amb shift is now 1,057.2 megacycles, wheeas expeiments indicate 1,057.8. These

alculations also sucessfully pedited a slightly lage magneti moment fo the electon.



13/20

3D2 I3P{ 2 n3D23 2 3P' / l 1a c e 9P/ 1

9

'/ 2 P' /2C OF AMB SHFT in the second enegy leel o heli is isible in te seto

ga at let, ade by Hezbeg o the ational Reseach Concil o Canada_ The setallines designated by lettes ae podced by tansitions between the hypene enegy leels

indicated in the chat at ight The lines designated and d wold coincide i the ab shit(whic is the splitting between the leels aked and wee zeo_ ines and

bae not as well sepaated, becase thei sepaation is the dieence o the splitting in te e and lowe states, wheeas the sepaation between ad d is the s o the ab shit inte e and lowe states_ The othe sectal lines ae not aected by the ab stfo discepancies since Diacs calcu-aions had o been pusued beyoda s appoximaion The eason asha evey aemp o go o highe ap-poximaions had poduced onsenseeven he enegy conen of a fee elecon came ou innie Lambs expei-men no foced heoiss o acke

hese inniies in eanes f he enegyof a bound sae of he elecon cameou innie one had o deduc fomi he enegy of a fee elecon a eshich as also innie Tha is haehe did and ha ohes did afe

him ih geae enemens To compue he dieence beeen o innieuaniies seemed a someha unsoundpocedue bu i oked emakablyell A ace fo pecision beeen hheoiss and he expeimenes ededih excellen ageemen he bes he

oeica ge fo he Lamb shif is0572 megacycles heeas he expeimens indicae 0 57 8 megacycles adieence of ess han pecen ohgoups may ell be poud of heiachievemen The heoiss gained agea dea of condence in hei echniues and much has been done sinceo pu hose mahemaically doubfulsubacion pocedues on a soundefooing migh be added ha hesecacuaions aso indicaed a slighyage magneic momen fo he elec-

on and ha oo has been vey econmed by aomicbeam expeimensin paicua by usch

n his kind of ok he eed fopecision is obvious an impoan as

7

pec of he heoy had o be esed ada chage of a e pas in a milion ohe magneic momen of he eeconould have indicaed ha he heoyhad boken don Someimes h vaueo pecision is no so appaen Thknoedge of nucea spis and mageic momens has been a gea hep in

consucing nuclea models ha is ideciding ha mahemaical appoximaions ae usefu in acking he appaling complexiy of aomic ucei ispecisey because of his compexiyha no one has ye ied o pedic a

nuclea magneic momen o anyhinglike he expeimenes vegue accuacy Al he same aiming a higpecision is usually good policy and iseaded i he end

Th udamena imi o he accuacy ih hich a feuecy ca be

measued is he ime duig hich aoscilaing sysem ca be kep udeobsevaion f one ee o compaeo ains of aves ih ealy hsame feuency i ould be evideha hey can be uie accuaemached fo a ime bu ha hey veualy ge ou of phase The cose hfeuencies ae he longe i akes ohe phase dieence o sho up Duing he ansiion beeen o enegysaes an aomic sysem has a avfuncion ha oscillaes ih a feuency

ha is popoiona o he enegy diffeence beee he saes Tasiiosi be caused by a exena oscilaing ed if he o feuencies ae easonably mached duing he ime he

aom spends in he eld Hence ege a esonance hose peak unde opimum codiions can be ocaed pehaps a hundedh of is idh

A ypica hydogen molecule akesabou 0 second o ave an inch inode o measue is feuecy o ihin0 cycles pe second one eeds a ed a leas 0 inches ong his ceaes

a poblem because i is uie dicu okeep he magneic eld sucienly consan ove such a disance Noman FRamsey ho s oked ih Rabiand hen saed his o schoo aHavad Uivesiy found an ingeniousay aound his dicuy He eaizedha he o ends ee he mos impoan paces fo he compaison ofo ave ains so insead of exposi

his moecules o adio euency aalong hei pah hough he e he

meely placed oe sho coi a o ed

ad a secod coi a he ohe is aangemen shapened h esoanc byas much as i he had eghened he ed by 0 pecen oeove smaieguaiies in he mageic ed oge maeed

Aomic Clocks

Ramseys mehod as an impoasep oad he deveopm o aomiccocks Evey cock excep o aecocks and simia pimiive devices

depeds o a feuency sandad suchas a pendulum o a balance heecoupled o some kid of gea ha dis

AMMOA MASR, inented by hales

H Townes in 954, was based on a iciple ennciated by instein i 99, aely

that the esence o a light wae cold as

an ato to lose enegy athe than gai t

the light wae, on assing thogh sc



14/20

plays the number of oscillations thathave occurred Since the frequencystandard is under continuous observation, the diculty have ust mentioned does not arise ut a pendulum,left to itself, will lose much of itsenergy in a few hundred oscillations;there must be a driving mechanism toreplace the energy loss, either continu

ously or from time to time, and if tatmechanism is not carefully designed, itis liable to alter the frequency of thestandard Clearly for high precision itis desirable to use oscillators that conserve a high percentage of their originalenergy

ith the advent of electronic devicesit bece possible to use standards thatoscilLe at much higher frequencies;tunin fors that could oscillate everal

hundred times a second were soon succeeded by quartz crystals capale of

oscillating at frequencies 1000 timeshigher Such crystals hang on to theirenergy with remarale tenacity, andfor ome years uartz clocks held therecord fr precision, with errors of nomore than a few seconds a year Theymade it possible to detect small irregularities in the rotation of the earth,wich greatly interested geophysicists,who began to as for still higher precision That was not easy, however; notwo quartz crystals are exactly alike,nd even a given crystal changes light

ly with timeAn ideal frequency standard should

depend only on the properties of a

B E A M O F A M M O N I A M O E C E S

welldened atom or molecule andshould not be aected by external conditions Most of the transitions hownin the graph at the right on page 66occur at frequencies that depend strongly on the magnetic eld, but there is onein the region where the eect of a wea

magnetic eld is very small All alalimetals have such a transition, ut

cesium seems to be most suitable Itsatoms are heavy and will evaporate atquite a low temperature; hence they areslow and spend a long time in the Celd Their frequency is high, about

nine illion oscillations per second,wich provides a igh fractional precision for a given change in frequencyCesium can be detected eciently byits ionization on contact with a hottungsten wire, and all its atoms arestrictly alie because cesium has onlyone stable isotope

The oscillations in a cesium cloc aremade by a small microwave generatrwose frequency peak is ept adusted,by a ind of electronic servomechanism,to within 1 percent or so of te widthof the resonance pea The best valuefor the cesium frequency is probablythe one found by L. Essen in ritain,namely 9,129,6 31 ,830 10 oscillationsper second; te 10 is due minly tothe diculty in obtaining te astronomical oservations by whic the length ofthe second is measured A cesium clock

is a fairly complex assembly of electronic equipment in wich the cesiumbeam merely serves to ee a cec on

te frequency Even so, a commercialversion hs been produced and is atpresent te most accurate cloc in general use

Mae and ae

n a resonance experiment of theRabi type one can select either te

higher or the lower energy state forpassing through the C eld f thehigher state is selected, the radiofrequency eld will ten cause transitionsto the lower state Te asorption ofradiation, wich raises an atom from alower to a higer state, is a familiphenomenon, ut Einstein had shown,as early as 1909, that the oppsitetransitions are ust as liely n the latter case the presence of a light wavewould cause the atom to lose energyrather than gain it; the light wave, on

passing through such a material, wouldsuer negative asorption" and getstronger, ot weaker Einstein used teprse stimulated emission" for thisphenomenon, thus setting it apart frmthe spontneus emission by which anatom in a high energy state will eventually lose its energy if left to itself

Normally a light wve will be asored rther than causing stimulatedemission, because in normal atomicpopulations tere are many more atomsin the lower state than in the higer

one n an atomicbeam apparatus, however, one can create an inverted population in which the molecules in te

R E S O N A N C AV I Y

materal would suer negate asorpton" and get stronger noteaker The maser used an nhomogeneous elecrc eld set up y

our parallel meal ars o ocus the hgher-energy ammona mole

cules color toward the end of he eld whereas those n the lower

sate black were pued ou o he eam When the hghenergy

eam entered a tuned cavty right n whch a mcrowave osclla

ton o he correct requency was presen the eam had the epected eect o srengthenng these oscllatons_ When the pressure ohe ammona was ncreased the oscllatons could e kept at the

same nensty wh less nput o mcrowave power nally he

power could e swched o alogether and he eam hen aced as

a generator o mcrowave power_ A cross secton s shown a let



15/20

\1

1\ ,0 \ "\ "I

/

\" \ ',\

)

I

J ," r


16/20

Want a portable eld facility

that sets up in 30 minutesan provides all the comforts

of a modern oce or lab?Jut ask Garrett.

When you as k, hr are somof the answrs you'll gt:

Garrett's porable gas turbin

p o w e r p a c k a n d i n f a t a b l e o r

xpandable shelr giv you alth comforts of a compltly cont r od nvi r on mn t - whe hr

our base is in the midde of theSahara one wk or mov to th

Arcic C ircl th nxt.

I t works this way :

Th power pack provids lctricity, rerigeraion, air haing,

air pressur, an watr hating

and pumping.You simp ly connc w er and

fuel lines. Sart he urbin whichinats and das h srucursp i c t u r e d h e r , o r s t u p t h expandabl shlr-and you ha

a modrn, welllighd, mratureconrolld facility - flly protctd from sun, wind, rain, snow, c e , h e a t , h u m i d t y , i r a n

nscs.

You n d just ve mn ad halfan hour to gt th units up andoperating.

How do you transport h unis?Easy.Thy com packagd in con

tainers hat t nicly on the bd ofa truck or in a helicoptr slng orin th cargo hold of an airplan.

ood exampe of ths nwd units is th MUST complexthat GarrettAiRsarch is buil

ng for the U. S. rmy MicalServic. MUST means MdicalUnit, Slfcontaind, Transpor

b, and comprises a compl

400 bd vacuaion hospital.Ouputs from a typica MUST

powr pack includ: 90 kw 400

cycle and 10 kw 60cycl elcricalpower, 20 tons of refrigration,5 2 0 , 0 0 0 B T U / h r. a i r h a t i n g ,

60,000 BTU/hr. water heating.he har of th powr pack is

a small gas urbin - h only tyof powr that can giv yo somany oupus from a single light

wight uni.

O n l y G a r r e t t h a s s o m u c hxperic in dsigning and apply

ing small gas rbines to this typof sysm.

Th componnts are designda n d p r o d u c e d b y o u r t w o

ARsarch Manufacuring Divisions and Air Cruisrs Division. In

adition, w also or total systemanagemnt rsonsibility as well

as ingrain g th comon n sinto he compl sysm.

ypical applications of th nwed units ar: chmica and bo

ogical protcion; raar insallation, commnicions bases, civildfne shltrs; and oil, mining,

and construction oprations.P l e a s e d i r c t i n q u i r i s t o

iResearch Manufacturing Division Phonix Arizona

arett lS exerIence

AR Mf Dv L A Px

7 1965 SCIENTIFIC AMERICAN, INC


17/20

EDP

y?E D P s y s t em s des i gn er s , bu i l der s an d

us er s wh o ar e f i gh t i n g t h e t r ad i t i o n a l l y

h i gh co s t o f co m p ut i n g . Th at ' s wh o

They specify system components thatd o t h e j o b ee for less mne Fo rexam p l e t h ey s p ec f y co m p ut er m ag

n et i c t ap e u i t s f r o m D at am ec E i t h er

t h e o r t h e . Bo t h ar e wel l know for sett ing ew idustry stand

a r d s i g r e a t e r a l l - a r o u n d e c o o m y :

o wer i t i a l co s t r educed m ai t e a ce

exp e s e gr eat er up t i m e h i g h er p er

f o r m an ce r e l i ab i l i t y

Th e s an att r acti v e l y p r i ced u i tf o r co m p ut er a o f f - l i n e ap p l i cat i o swh er e m o der at e s p eed p er o m a ce i s

h i gh l y p r act i ca l ( dat a t r a s er r at es up

t o ch ar act ers p er s eco d) .Th e o er s t h e s am e u p r eced e t e d ecnmy and e i a i l i y forh e a v y d u t y o n l i e u s e w i t h d i g i t a lco m p ut er s a d o t h er d i g i t a l E s y s

e m s r e q u i r i n g h i g h e r d a t a t r a s f e r

r a t e s ( u p t o c h a r a c t e r s p e rs e c o n d

S o m e e a d i g c o m p a n i e s a l r e a d yus e D at am ec co m p ut er m ag et i c t ap e i t s t h e i r dat a s y st em s . L i ke t o r ead

ver the l ist of people who have the

an s wer o E D eco n o m y ? W r i t e To m

T r a c y a t D a t a m e c C o r p o r a t o n M ddl ef e d R o ad M o un t a i n V ew Cal i f

575@ederhp n w-c/hgh-ry

dg mgnec pe hndng

7

boucg aroud for a secod or so before tey escape aga troug tesmall ole by wc tey etered. Teask tself s a tued cavty ad tssde s pated wt Teo, a ertmateral tat reects mpgg ydroge atoms wtout ay sgcat eecto ter trasto frequecy. At rstt may seem strage tat a frequecy

stadard ca bouce aroud a bottle,udergog 100000 collsos or so asecod, but te proto sp sde aydroge atom s well potected romoutsde dsturbaces, except for magetc oes, ad Teo s ert precselybecause all ts valeces are saturatedad so ts molecules develop practcallyo magetc eld.

ecause te atoms stay so log tecavty, ts tug as oly a muteeect o te frequecy tat s geerated (less ta oe part a bllo).

Moreover, ts eect becomes less aste beam s made stroger, ad t cabe mmzed by tug te cavty utla cage beam testy s see to

make o derece Ramsey as sowtat two suc ydroge clocks, afterbeg adjusted depedetly, wll keeptme to better ta oe part a trllo; eter wll ga over te oter by

more ta a tousadt of a secod 30 years!

Some Reen Innovaons

Tere are a umber of less spectacular uses of molecular beams. For stace, geergy pyscsts are curretly terested usg beams of polarzed protos (tat s, protos wtter sps potg pedomatly oe drecto). Fast protos ca bepolarzed by selectg tose tat avebee deected troug a certa agleby, say, carbo ucle. Ts s a mostecet metod, owever, sce olya mute racto of te protos te

orgal beam are deected a gvedecto. A molecularbeam metodca do muc better, ad sources of slowpolarzed protos, ready for accelerato watever mace s avalable, avebee developed several laboratores.

Collsos betwee molecules ca bestuded uder cotrolled codtos bylettg a molecular beam pass trouga dlute gas or, eve better, by crossg t wt aoter beam It s possble ts way to study te cemcal reactos betwee gases ad to observe te

sgle steps tat are dcult to separatewe te reacto takes place a mxture of gases. Of course, gases ote doot react uless tey are ot eoug,

tat s, uless te relatve speed of tecolldg molecules s g eoug. Atrgug tecque for gvg te

molecules extra speed was developedrecetly by Plp . Moo of te Uversty of rmgam. He foud ecould cause a spdlesaped steel rotorto rotate at more ta a mle a secod a vacuum f a small amout of gas was

admtted, tose molecules tat got te way o te rotor tps were slappedto rapd moto, ad some couldescape troug a small ole to aadjacet evacuated space, producg amolecular beam wt a g ad approxmately uorm speed, fast eougto make tem react cemcally wtoter molecules wt wc tey wereallowed to collde.

A deret metod, wc gvesbeams of g testy as well as gspeed, s te use of a gpressure

ozzle stead of te usual ove slt.Usually te pressure te ove s keptso low tat te molecules escape wtout muc jostlg, eac wt te velocty t ad te ove. Oly te slowest

molecules ted to get pused from bed, ad deed oe observes ewerslow molecules ta oe would expectfrom Maxwells formula. If te pressures rased, more molecules get pused toa ger speed, ad evetually oe getsa gas jet tat bot expads ad cools.y te tme te molecules emerge from

te ozzle tey are several tmes fasterta ormal beams, but te spread ofveloctes as become less. We suca ozzle beam strkes a sold surface,te surface s subjected to a bombardmet somewat lke tat to wc aspacecraft or te ose coe of a rockets subjected o reeterg te atmospere. Te laboratory study o teeects of suc bombardmet as attracted te terest of desgers ofspace vecles.

Molecularbeam researc started out

as a amly aar more ta 40 yearsago. For may years t was all coducted, wt a few exceptos, by Ottoter ad s studets ad ter studets. ter as every rgt to be proudof te eterprse e started te m

medate famly aloe ts success asresulted ve Nobel przesto termsel 1943 to Rab 1944 toLamb ad Kusc 1955 ad to Towes 1964 Te applcatos of molecularbeam tecques ow exted to allbraces of pyscs ad ca o loger be

sad to be strctly a famly aa Newprojects are beg udertake laboratores all over te world, ad ew surprses may well be just aroud te corer.



18/20

Rio de Jan iro

SCIENTF

AERIC

A n n o u n c e s a s i g l e - t op i c i s s u eco ce rne d wit h the u rb aizat o n o f the wo r ld p o p t o n

u e t e

CITIEST b p ub sd Se p e mb e , 1 965



19/20

Somems e pooadvlo righ lon with he cu.

Photography, we admit, is the awkward stepchild of e visual arts. And undrstandaby so.

Because photography, if i s an ar form, sthe only one tha isn' a continuous crativeprocss Oils, watr coors, scupture - oname it-all lt you crat one sp at a time.If you make a ms you lan from t rhhn If ou're insird you an take advantae of your inaon immdiatly. Yourtchnique an iprove and your satsfationcan inrase wihout nterruption.

ut t phoorahr who mu wat to sehs piu don hav t adana of

First try, too fa away and not vey we composeAn so iesonal it coul be anyboy's famiy.

tha onuous xprin H's praing asopando a fom Wn the average prson aks a iue h don know how iurns out i das ar Evn if s nrd

in pong b his msaks 's too late. Theloaion and ing a drnt The mood gone. The ave oninuiy s aterd(Evn gting invovd in rosin your ownicturs doesn' do much expt sortn th

minte ate? geting beter but sti not pe/ect.Now get a ie owe a wait o he j ght moment .

watng iod) Smal wondr thre an'more "Sunda pooa

The on xon o a s : a oarodLand amra W ik is k o an normous saisfn wa o xprs oursf

If ou ak ak d w piurs ou hem in

onds. I ou fr oor one

mnu s al t aks nd bau ou s ourrsus so a o oon an atcvaus of n phooa Inrpraionoposion ighng and oa et ar aapan and ana on h so hhan and asfaion bcome a litsharpr w v iu o ak

Whr or a ama u or no wuggs hat ou borow a oaod Land amra and ry i out W an aot aran

tha your sond ur w b br thanour rst. And tat our hrd w b rtha our scond.

And tha's wa Polaroid Land photorahis all about.



20/20

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