Chakraborti, P K

IACS Institutional Repository

Indian J. PAy'. S4B, ~U-4M TI9RO) Gas dynamic flow and laser spectroscopy of molecules l' K Chfll'rabo)"t.i Bhabha Atomic R ••• "",h Centre, M.D.R.s., Bombay·SO Abstract: A novol teOhl'uquCl for ovorooming tho difficulties involved lQ high l'eeolution gas phWle Bpeotrosoopic studies of oomplex moleoules i~ des-cribed. A desirablo speotroscopic 8t\mplo would be an "nsemblo of molecules, all in 0. part.icular well dofined quantum state (usua.lly the lowest eneTgy stlLte) travelling in freo spn.co with 0. na.rrow v~locity distriLut.ion, Bnd at 8 auiRoiontly Jow denlillty ftO that intermolecula.r interactiona aro unimport+nt. A supersonic tlxpansioll of molecttles in 0. monGtomc carrier gBR saams, ~o 0. large degree, satisfy tho l'equu'cm(mt of an ldeal sppcf,rosoopio sample. ~me applma.tions ~f this technique to tho study of both .table !IDd unstable tolcoulo. and It, usafulnesFl in relation to 18l3sr isotope separation will be disou sod. , 1. Introduction An extnmlely high l~vol 01 occupancy of tllli low 1ylJlg Icv"IH of cmnplox heavy moleouloB makes them una~tractiv" spccios fOl· high resolution spoctroscopic studioR. The diffioult.ios aro very mU(lh ILggl'BoVo.ted WIth inoI'casing number of atoms in tho moloculo and with increasing molecular woight Theoretically the solution to this problem iR to supercool thb vapour ttl very low' temporaturo without oondensation and WIthout loosing vapour phaso chflTactoristics. Isol'l· tion of t,hc molecules in an incrt gas crystal lattice (Lowis and KElSha 1944. Robinson et al 1957, MeChlre 1954, Paino ct al 1976) Bolvo tl'6 problem only l,artially. This tcclmiquc can not. determine all the energy I>tai.llB of tho froo moleoule, since the inkrmolllcullir mtcractiol1 n'~uJt," in fI loss of infm·mu.tion on the rotation of the molecule, and t,h" geomet.ry of the molecule Cfln bocome Rtrongly distortod. Thus, it, IS neeessa.ry to havo Il moro offootiyo method 01 rodueing the vilJtll.tionfll and rotu.tional stmchtre of tho Spectra. Use of hydrodynamic flow ~ourcu for high lllten8ity molecula.r beams WIIH first suggested in 1951 (Kantrowitz and Groy 1951). Sineo then great advancos ha.vo boon mllde in tho design a.nd operation of nozzle beam sources ami they EIre a.dequately described in the litoratltro (Andorson et al 1966, Pauly and ToellI1is 1968, Fluendy and Lawloy 1973). In conventional moleoular be!l.m studieS fiuch BourceR lire now routinely used (Pllrson et al 1972, Ng et al 1974, Vehmoyor et al 1976). Hydrodynamic souroe has a.lso been used for studying unstllble species e.g. van der Waals' complexes (Milne et al 1970, Golomb et al 1972, Gordon et a! 1971, V~n Deurs6n et a11975, Vehmoyer et al 1976). The forma~ion 444 Laser spectroscopy of molecules 445 of V/W dOl' WallIe comp\llx lD Q llozzlo beanl i~ (Iuo t tl t· Ii • • 0 1\1 ox CII~IV(l COO ng pro. ,hlood in tho Jot. ThiA wdl·lwuwu 1J1'on j' . . • 01l1U11011 () Rllpl'l'!'Oohng of gaR6B u.n,1 va.pourR 111 a. IIIIP"1'1I01110 Htl'BIIIIl und i.hoh· 11",· Ill"P 1 Ii 1 • " • 0 Ill' .r·o~cl)py 'H\'~ 11'~1. I!uggustr( by Milno a.nd GrOl'll (10119) '£h, 't·" HI " I " 1 ""n a,rono. t011ll!uruiurc I\S low nH ().03"1i: Olin be .U.OhlOV~d 111 I!om~ c~~o~ tlll'ough nozzl" UXpILIIHion. I"uiyntomio muim,uip-s 1l~ed.6d lllt~ t.hl~ Ol[po.~(tHlg ]ollotU11lu\lical.o with t.hi~ I'oirigemtion nlodi .. through bmo.l'Y collwoll CIloU~Ulg uonsideTahl" ""u1iug of tho aifltl'ent degroes of freedom 'Froc, uollillionlMII dow iH lIoohloved vOl'r qUICkly. jlermitting tho highly unst"bl" l·old ~p.,cie~ 1." ~U1'\"iv(l for 1\ pln·joll <luLol'minH<l hy Ow Biz" of tho ~ppa.I'a.tnA. Threo borly l'ullision Jel\llin~ to cOudellAIlt.ioll is mmuniztld ,lun t" tit" Ruadell n Ilot1U'O of th (' cooling. OVer the lu,st. ftlW yOIlI'~ thi~ tuuhu'quH of HUl'0l'HOlliU (Jxpl!.nHiml 11llR boo11 uaed by a numbor of works fur vifublo ftuoroHconCll atlldi(J~ (Smllollcy et 0.1 1974, 197G, 1976, 1978). cloctronic a.bROI·ptiou Ht.Uf\icR (Ka.taov 1976) a.ud inft·a. rOll .. haorptlOn atudioH (Jonsen ., /I! ]076, Travis et ,tl )076, MIller 1977). In thlll a.ccount wo will disous" Bom" of tho oh o.ra.ctori~tlCR of u, Rupm'Bonic jut flow !Ioml .. Iso tr!\(10 vary briody tho p"ogreMR of r080801'011 in tho .. pplica.tioll of auporsonic hea.ms o,nd jots towards the solution of a numb~r of prolJlomg in molecular optical Bpactros. oOPY n.nd to iHU'Itrate t·ho promise BUt'll (lJ(1pl1l'lmOn~S hold 2. Cooliag in superloDi~ beams and jets A molt.eula.r beam somco consiRts of IL high·proRKUle gOos roam'voir, l\J.1 orifieo ill the lllsorvoir to IIoll0w tho ellca.po of g1lo9, oollimOoting apertures to KhOop!I tho down· stroo.m 1low pattem. Aond auffi.~io\lt pumpmg spoed to maintain low dowllstroam prOSSl1l'O. [f collima.tillg aplIrtul'Cs 0.1'0 a.bsont and tIlero is 110 Iltt'llmpt tu control the downst.l'ou,m dow p'll.tern. .. SIlPOTSOllW source iR co.lIod a. free jet. For supoI'donie opero,t.ion the ROlU'ee pl'l!RRUrO must KRtisfy t·he cllmlit.ioll D:> ).0' whero J) is tho OI-ifico din.l1loter o.nd ).0 st,agna.t.ion moan freo pa.th. 'fhe mOAt impol·tOont. rua.turc~ 01 a. hydrodynll11lio boa.m dllrlVC from the flloct tha.t thll nozzlo eonvortH tho Tllndom kinetic energy of the source go.R into direoted mll/!S dow with volocit.y U Illld wit.h a. lowor loullol gl\~ temperature T (dofinod ill torms of the width of tho "olooity distribution). Urldllr a.tbabat.ic r~vorso,bltl dow condition (no sllo(,k waVeS, no Rheol' for COli, no hoa.t Ronren 01' ~ink and llO Iwu.t oonductivity) UlO oxpansioll ill i~'mtrOl)ic. CO\l!lorvat.ion (If "nergy along Uo flow stroa.m·li1l6 I'equiro t.ha.t lit a.ny point in the dow, (1) whero H = O,T+Const. iij tho IIlltha\py par uwt mo,R~ anrl tho subscript 0 rofor to stagna.tion eOlu\ition in the source. 446 P K Ohakraborti From thiA wo derive ( 1-1 ]-1 T/To = 1+-2- MB (2) It tlloreforu follows that cooling can he quito oxtenRiv6, foI' example for a jIlt with M - 10 and 1 = 5/S, TITo = SXIO-I. Fo!" c()ntlTlUOU~ 11011' the Ma.c}1 numher dependonce on diijtanco from the nozzle (Aahkenas a.nd Sllarma.n 1966) i~ given by M = A(XID)'-1 (3) where X iR the distance from tho nozzlc a.nd D is the nozzle di~tre The quantity A is a conRtaIlt whieh depond on 'Y 11000 is 3·26 for a. monatop.ic gas Thero aro practical limitations to tho usofumes8 of equation (3) \nd to tho I."xtflnt of cooling that C&I1 be a.chieved. Those a.re : \ a) Rare/action. '. Binary oollision rate decreases rapidly with progreBB of expansion and vcry ~oon tbo collision rate becomes too low to cha.nge the velocity distribution apprecia.bly. Anderson and Fonn (1965) predict. fOI· the &symptonic terminal Ma.ch numb~l' l.:=L Mf' = Const. (Ao/D) 1 = 133(PoD)o.t (for argon) (4) wher~ Po is the ~ta.gnation pressure in atmosphere and D is the nozzlo dia.meter in centimetres Substitntion of equation (4) in equation (2) give for the termina.l temperature for argon. ~f' = rl+5896(PoD)0'8]-1 o (5) Partiei1)ation uf the in~ernal dogrec~ of freedom of a polyatomic molQculr slows down tho cooling rate, thus expansion of pUle polyatomic gas produoos much higher torminal temperature. Polyatomic molecUles must therefore be used in low concontration in a monatomio ca.rricr ga~ to achieve lowest possible internal tempera.turos. In goneral wo expect Po >T.,t> TrfI, > T". b) Pumping 8peed : Inadeqqate pumpiDg speed or obstacles in the flow pattern Dan oreate ato.ndins shock waves w~ioh must be a.voided if experimentll "Te to be performed LaBtr apeetr0800py oj molecules 447 un oold molaou.los. Caomparguu (Ca.mpa.rguo et liZ 1976) disculisod t.he li.m.i.ta.tlOns a.ud importam1i pra.ctioa.l molloDJ! of mitiga.ting tho ",!toots of finito pumping speeds. c) Oondensation: The oondenslIotlOn l'a.to lij VOl'y low in a fast expa.nding ~PCl'HOllic flow NeYol'ih81e~s, at aODlo poilli in thQ How thll lOl'lIla.tion of dlmora, polymu,rs a.lId oYontulUly oryli1;1Us Ilond dl'Oplots wul begin a.ud this will limit 1,110 ult.imate tr®s1u,-LlOna.l tempera.l,uro a.i,t/Aina.blo. In Fignl'l' 1 Wo presont tho mellosurod intonsity 1D'l ~Ar' -10~ 1'._" 1. lnt.enaity VB ~tasWltion pre~U1'" at IItllj!naLion temperature, To" 808"K. da.tu, for IIoI'gon a,II obta.inod by tho Pl'll~Ullt .. u~ho]'. it iK olJ~ol'v(ld tllu,t, tho lU~1l0-mOl' ini,onsUiY passes tJtrough u, pOllok. 'rillS i,~ in (IIJ)lfol'mit,y with tJIO ohSCrva.tllJD.1I 448 P K Oka1r:rahorti ma.de by other worker iu the field (Leckenby et at 1964., Golomb ee at 1970, 1972, Vim. D~ursan 6t al 1974). Tntcnaity dllor~a.so for tho monatomic speoies could prima.rily 1)0 a.ttr!butad to IOBR of pa,rticleR duo to condonsa.tion. However, a.ddi~Hlna,lioRs of monomer particlos from thu co\liml\ted beam CHon ho attributell to u,(lditiollU,1 momontum ga.in from huat of condonsation Propor assignment of til" dctoctod Kignlloi is (lompliOlloted by dis~oollltivo ioniz .. ~ion of higher elusterR !Jl t,ho !OlUz(ll' of tha doteotion Sytltem. SeverILI workors (Van DeuRsun Bt a1197G. Vohmoyor 61 at 1976) hHove utilized the measured dimcr collision crOSB suction M a iilnctwn of stagnatio)J preJ!)lure a.lld stagna.tion tompera.ture to locato tho initillotioll POint for tho f01'lllation of highor (lhIHt.erH Figure 2 !!how~ tho dHotU. ~:I 0.3 0.2 O'SI S 0.5 :::104 ~ 0.3 ~ 0.2 2 0,4 ;; 03 0.2 \ ., .. :> . , • J .. PI. ,j...-: T:l01·K ~ 0'51 P" ;~l : : :::::.~""., ~2'L_~o--------~---~--'-~-----~~~----~~~" 10 101 10' '0' p. IN TORR FIc ..... 2. Total oollioion Dl'o •• ·seotion 410"" (without velooity ..,Jaotion) for Ar.-Ne (el and for Ar-Ne( xl as • funotion of atapatioD prwsure for <hffcrent; .ourae temperatures. The limiting pre88UJe p£ ror eaoh tempera~ ... i. indicated by an arrow. (From VahJneyer H .. ,.z 1976). for aJ'gun dimOI'H !'Ou.ttoI'CcI hy IlUOlJ (VolUlll'yul' ~(/611976). Thu poillt~ lUlIol'kull by PL mflork the boginning of the formation of cialsterR high(ll' tha.n dimotR Tht: frequency of thrae hody coUisionA nl1COJjHllry ~o initia.tu condenaation depend UpOll t.hr: lUlL~S lInd ItItnrflotornio l,otuntial of u.j,nIllB in 1l1lostiOll. CondOnRatio!l Lasr:r spectroscopy of molsculfJ8 449 IS minimizod by .using Iighter apecise 0.1!l. He or H. m particular helium whiuh h808 very WflU Intormolecular force. Th&e is no bound Hoa moleculo (Van Deursen and Reusa 1975), thll formatioll of puro helium olustor requires at least four body colli~ioll for initl&tion, Uond.thus Lho growth of Rucll clusters iR a very ~low prOc6111!. In fUoct, the growth of clusters of helium with molecules of spoctro. scopic intarelll, is muoh fastor tban tbe growl,h of helium cluat&a Such cluster formation is minimizod by limiting tho concentration of molecules to a. low valuc. 6100 6000 11900 !800 WAVEL EtotGT'H f..c) Figure 3. A portioll of the f1uores..,nc •• x .. t"tion "p.otrum or NO. for (top) .. convenLlOn,,1 ,'oom temper&ture a&mple of pure NO, at ()'04 torr pr ... oro, (mldcJle) " supersonIC be""" of put ... NO., and (bottom) .. supersonic boam of 5% NO, III Ar. All .poctr .. wero takoD usmg .. CW dye laser. La.ser band. Width "'IlS 0 6 A. for tho lower two 8pootr&. (From Smalley n E " al 1974). Prol..lllolJility of binary colbHJOII l~ pl'oportlOllal to DI>'o and tha.t· of trinllyt· oolltsion iH prtlpol'tlollUoI to VIA-oB. l'ho ra.tw of throo body to two body collisiou :s PI'opOI'tional to thu numbor d~nsity o[ atoDlB 1'0, I:linw thu tormina.l tempoI'D,' tura is a funotion of 7'oD, a fixod terminal tODlpol'atnrc with minimum complex fOl'lllation 1~ achiovod by docroa~hlg 1/0 and increasing D, koeping ?loD constllnt. MaSll throughput iH proportional to 'linD!, thUR increasing noz~le dia.Dletol' incroa.scs pump ~jzo. 25 450 P K Ohakraborti 3. Optiea1 spectroscopy with •• per.ODic beams a.d jets Tho availability of tunable lasers along with the prospect of isolating supor. coolod molecules, oomploxos and free radiclos in a. jet or a beam has givon a new momentum to moleoular speotroscopists. It is now within our means to separato tho individual vibronio bands and mako proper rotaUonal assignment of tim bands even for tho most oomplex molocules. Isotopio fes,tures in the i.l". absorption spectra of oomplox molocules has c1ollorly been rosolvod ua.ing flow oooled mot)l()(1. All thoRO havo fundamental importanoo as woll as pract,ical implicatiol1s. Horo We will project ~omo solooto<l optical studies oarried out with sll})(Jrcoolod beaIDli ~l1d jot/:! to dOlllonstrato tho oapability of this teohniquo. lL) .Fluoresoenc6 st'udies : Siuoa 1974 Luovy and Willuton group at Chicago ha.~ illoll~uroU tho vl~lble fluoresconoe ~peotl"o, of a number of molecules and comploxeH usi~ ~lIpol's(lllio jots and beams and II l,unlloble OW dye lasor. 1'Iru aystem illost thoroughly atudiO(l by tllom iucludo molecules 0 g., NO., L., S·tetra,zino fIond thoir oomploxas WIth holium .A~ a typica,l exu.mple wo havo tu.kon tho ea~o Ilf NO. ill l,ho WlIovo-!ongth region 6700 til 5700!. Tho mu.gnitudo of RIDlpliiicfIotioll hL tho obsorvoll flow coolod spoctra for NO. (Smo,lloy et a! 197J) i8 Ul,m()B~t!I'lIotod ill l!'igllre :I. Tho pou.k detected fluoroscollce iutonsi~y is _10" photon/~oc In l,ho ~ood(ld hCfIom-dato. vibrollie ba.nds Ooru ll0W c1lOarly iIDplloru.tcd aud the rotatlOual Htl'UctUrll appou.r til lie dOIlllnllotod by a triplet. The ratio of in~on8ity of tho contral mombtl1 of a tl'iplot to that of tho othor two componollt~ proviLle flo moasuru of rota.tIollllol temperature, and this givc~ a rotOotiOllllo1 temperaturo ui 3:1:.1 UK for tho /!lllodl,d beam. Subsequontly using II> diluto mixturo of NO. iu holium u.t lOti Ootm. t.!wy wore ablo to Iloohieve a, rotational tomperu.turo (If 0'7" K. III tho ulloSO of S-tctrazCIIO a rotatIOnal tomporatut'o of 0'5"K was achioved and this peIlnlttcd u. comploto Iloualysis of the rotllotional structul'o. h) Infrared .db/lOrpli~1t : Apa.rt from bllosio inture~t ill high ro~olution ~p6etroscopy, absorption studioa with suporcoolod moillou!es has prllctiea.1 impliollotion in laser isotope sopa.ro,tiOll. For any feasible mudel for lasor isotopo 8opa.rOotion, tho mmlt fundo.meutllol roqull"(J-mout is tho oX!istanoo u.nd idontifiou.tion of isola.ted isotopio spoetra] feo,turo. Room tompnrllotUl"O spoctl'a of polya.tomic molocultts o.g, SF., UFo otc. Iloro vf)ry cOl.llplox duo to 'hot band' traHsitions. Suporcooling tho moleculos ill a super-sunic flow CU,lI produco tho dosirod IIp(lch'obeopic rosult. Ulling Ilo tlUla.ble diodo las~r IIond flow coolmg, high rosolution spoctra of SFu IIolld VF. molocules hu.vo Laser IJpectr08copy of m OZet)Ule8 451 been roportod (Jonson et al 197(;, Travis et a! 197") Tn Fi r ~ J "t-" . go a ... Wn lavo "110Wn tho room tomperllot,nrc "poel.l'o, of tho "3 hand of SF. which IIo,n ho compllred witJl 1'1 r--------------.... 1·0 0.9 o·a SOOK O~ _________ ~~~--------~--------~~ Figan 4. Infrared absorption 8pINltrum of SF, at lOom temperature. (From JOIIlIOII R. J. 01 tJl19'161. tho spootra of SF. (Jensen et a11976) in Figure I) lIow-coolod to -oIlDK. Tho cooling makes the Q·branoh muuh ahllilll\lr than tll" room tomporlloture spectra, indiclltina the 6Xtent of rodllotion in tho '·hot·b .. nd" I\bsorption from t.hat at room tempel·ature. A composite high resolutIOn •• ·blllld apootra. of flow foolod U'F B (Jensen et al 1976) is shown in Figure 6. The isotope offoots a.to very clelU"ly resolved. This spectre mllY bo compared with the corresponding room·tlllllperature speotra Figure 7. Supersonic Bow lind its re16vBonOo to high resolution laser speotroscopy hIlS boon oIelU"ly dooumented in the oited roaulta. 452 P K OAakraborU ~ -;I VI SF. in 9..p._ic ....... (Moe .. e) ..,..,. • • .. S· I I '~ \ ~. " • \ I "1140 Figare 5. Speotrum of flow Dooled SF. (From Jensen R .• 1 .• 1 ,,/1976) . • '~CRUSING FREQUENCY. Figare G. Oomposite speotrum of flow·aooled UF. tIIkon with t\JD&ble diode b'Mre. Peaks 2, 4 and 5 are R·branoh linas of ,.sUF •• peak 3 is the Q branoh of··'UF. and peaks I, 6 and 7 are from reference gIlll9ll.' (From Jenson III al 1976). Lfl8er IJpecWOlCopy of molecules '''\IF • 620 5'A&:QUENCY IN eM"' 453 Fipre 7. Speotrum o{"'UF. at room temperll.ture with ground stll.te PQR structure for 'J band •• pectrum "'UF. i. Shown lit bottom for' comparison, Reference. Anderson J B, Andree R P Il.Ud Fonn J B 1986 Adv. OlllJm. Ploy •. Vol 10 J ROM Ed, Juhn Wiley and SOllS, Palo Alto Anderson J B and Fenn J B 1965 Phg •. Fluid. 8 780 A8hkon ... H Il.Ud Sherman F S 1968 in RlJttjierl aru DlInamw. 4tlJ Sypmposlllm)Vol II, J H de Leeuw Ed. Aoademic Press, NY, pp 84 Oampargue R, Lebebot A and Lemonnisr J C 1976 lOth Inl. SymptJlliutll· on Rarefiea aRB ])yMm'ia Aspen USA, pp 19 Fluendy MAD and Lawley K P 1978 Oh./Ilkal Application of Mole.lllar Bram. Ch"pmllll and HllI Publication Co 454 P K Ohakraborti Golomb D, Good R E and Brown R F 1972 J. Ollem. Ph" •. 52 1545 Gordon R J, Lee Y '1' and Herschbe.oh D R 1971 J. Ohem. Pllys. 54 2393 Jenson B J, MarizuOl J G, Robinson C P "",d Bookwood S D 1976IA •• r Fooua 12 G K""trowitz A and Grey J 1961 Ro". Sci. I ... t.22 328 Kamov D 11976 P';'mG Zh. JJJ1csp. TMr. Fi •. 23 152 Low.s G N Wld Kaeha M 1944 J. Am. Ollem. Soo. 66 2100 Me Clure D S 1954 J. allem. Phys. 22 1668 Miller R E 1977 Tile.';', Dept. of Phy', University of Waterloo, Oanada Milno T A, Vo.ndel'grift A E and Groen F T 1970 J. Ohem. PMJ', 62 1552 lIfilne T A, and Groon F T 1969 Adv. High. Temp. OI.em 2 107, L Eyrmg Ed, Aoadenuc Press Ng C Y, Lee Y T IIoIld Barker J 1974 J. OilBm. Pilys. 81 1996 Pauly H and TOBnnies J P 1968 Melh.d. of JJJ:ept. Phy •. 71t. 237 Parson J M, Siska P E and Lee Y T 1972 J. Ohem. PhYB. 56 1511 r .. ine R T, MoDowali B S, A.prey L B 8.Jld Jane. L H 1976 J. Ohom. Phy •. 64 3081 Robinson G W, Me Cart.y Jr M and Keotly M C 1957 J. OI .. m. Phy •. 27 972 I SmaJloy B E, Ramakrishna B L, Lovy D Hand Whanton L 1974 J. Ollem. P\R 81 4363 Smalley R E, Wharton L and Levy D H 1975 J. Ohem. Phys. 63 4977 Smalley B E, Lovy D R Bnd Wha.·ton L 1976 .J. Ohem. Ploy •. 64 3266 SmaJloy B E, Wharton L, Levy D H and Chandler D W 1978 .J. Mol. Speot. 69 ~94 Trav .. D N, McGurk J C, McKeown D and Demung B G 1976 AERE·R 8430, Harwell England Veinneyer H, Felt.gen B, Chakraborti P, Dueker M, Torollo F and Pauly H 1976 Ohem. Phy •. LeUB. 61 1996 van Demson A, van LumlS A and Reuss.J 1975 Int. J. Moa •. 8pectrom. a/1d Ion Phy,w8 18120 

Gas Dynamic Flow and Laser Spectroscopy of Molecules

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Gas Dynamic Flow and Laser Spectroscopy of Molecules

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