C. R. Quick, Jr. and C. Wittig Department of Electrical Engineering University of Southern California University Park LOS Angeles, California 90007
Molecular Multiple Photon Absorption An undergraduate physical chemistry experiment in laser isotope separation
T h e use of lasers in the research areas of chemical physics has been widespread during the past decade. Many clever experiments have been carried out that would not have been oossible were it not for the laser. and areas such as eas ohase kinrtim have been advanced markedly asa direct co&&enre of the laser.'l'hi, use of lasers in undergraduate laboratories, huwver, has not bern partirularly suggesti\.e of the enormous out+:ntial thur these deviws offt-r for c a r r.v i n- ~out basic research. I n this paper, we describe experiments that can be carried . . out using r a r h t ~standard lalxm~toryapparatus in nmjuncr~nn with the simplest of d l lasrri tu ronstrurt, a CO? laser. The experiments are very easy to perform and allow fora great deal of innovation, a t all levels of thought, without requiring additional equipment. In addition to a COz laser, the only necessary major equipment items are an infrared spectrophotometer and a vacuum manifold for the oreoaration of eas . . sampli:~.'l'hrse items are ct~mrnunplarein senior level underrradtinte laburatories in ohvsical rhemistrv or the like. 'fhe notion of multiple phiton molecular"absorption is rather new and deserves some studv. For a detailed descriotion of this phenomenon, the intereked reader is referred t o the literature (1-8). What happens in this phenomenon is as follows. In the presence of a strung laser field, certain molecular species are able to absorb the laser radiation, and these species are promoted to a level of excitation that is of the order of the dissociation energy. This excitation occurs in the absence of collisional proL&ses. The laser frequency must, of course, be coincident with one of the species' characteristic absorption frequencies. For several reasons, molecules coutaining only 2 or 3 atoms are bad candidates, and larger molecules (SFs, SiF4, CzH2C12, etc.) are more amenable to being excited by multiple photon absorption. Since the excitation process occurs in the absence of collisions. we have now identified n means of preparing mdecules in very highly excited states. from which t h w can either dissociate. or undereo chemical reaction. Since oily those species whosk characteristic absorotions are coincident with the laser freauencv can he exrited, it is possiblr to excite a single rhrmical species in a CHS mix. while leaving other suecies unaffected. Also \\hen isotope shifts are sufficiently large, it is possible to selectively excite a particular isotooic species. Thus, there exists the opportun'ity to carry out iaser induced chemistry andlor laser isotope separation experiments using multiple photon molecular absorption as a means of selective excitation. T h e only laser required for these experiments is a pulsed CO* TEA laser.' This laser is rather routine to construct, and there are several papers in the literature describing the construction of such devices (9). Commercially available COz TEA lasers are also readily available and these are versatile, convenient to use, and of more than adequate output energy. Prices begin a t $4000, and suppliers will gladly furnish information concernine the safetv . asoects . and the noeration of the device. Rest sources for these lasers are Lumonics Research Ltd. and Tachisto Inc. A homemade CO>TEA laser ran ~ is very he constructed for -$2000. The output from a 6 0 laser intense, and extreme caution should be exercised in manipulating the laser beam. One must NEVER allow the laser beam to become "lost." This is a ~ r o b l e mcharacteristic of infrared lasers, and the most practical way to handle this
problem is with a large (-100 cm2) sheet of carbon, which can be obtained for a few dollars from a " elassblower's suoolv .. .store. When the COz laser beam is incident on the carbon, the carbon absorbs the laser enerm and ~ r o d u c e sa visual image of the h~.am.ctmvenimtly a l l k i n g ihe location tli the be& to he established. 1'rotrctis.e safet\ylassri should also he worn at all times while the laser is on. ?he most dangerous aspect of the CO? laser is the high voltaae that is used to produce a discharge in the gas, and if a homemade ('0:.lasir is run. structrd, a safety interlock, identical 10 those required in mmmiwial lasers. shwld iw included in the design. The output from the laser should he at least sewral 100's of mJ's ocr nulse..and the nulit! duration shoukl lw not much more than 1-2 @.If a grating is used as one of the reflectors in the laser, it is possible to tune the laser to many different wavelengths in the 9-11 fim region, thus improving the chances of findine a coincidence between a laser line and a molerular al~sorptim.With no frrquency selectwe ekment in the ourical carit\.. the h e r will ntminall\r iscillate at 1'1201 of the (601.100) baLd, which liesat 944 cm". There are many molecules which absorb this laser line. and a laser with no dispersive elements in the optical cavity is quite adequate for initial experiments. The output from the laser is focussed into a gas cell containing the molecular vapor under study. The focussing element can he either a front surface reflector or a lens. A lens is most convenient to use and can be purchased commerciallyZ or made in the lab by polishing an NaCl disc in a curved dish. The gas cell is nominally constructed of Pyrex (2.5 cm dia. X 10 cm long) with NaCl windows of the type commonly used with an ir spectrophotometer. In focussing the laser radiation into the pas cell. care should be exercised in order to avoid damaging surfaces with the intense, focussed radiation. This can he most easily accomplished using focussing optics with a focal length -10 cm since this allows the radiation to be focussed into the center of the cell, thus keeping the energy incident on NaCl surfaces relatively diffuse. Under no circumstances should the beam be focussed in an indiscriminate manner. The intensity in the focal region is likely to be -lo9 W cmV2,and this intense focussed beam will damage almost any object placed in its path. The gas cell can be evacuated and filled with a molecular vapor using a conventional vacuum system. There are mans illustrative experiments which can be performed with the apparatus ilesc~ibedabuve. Hatber than outlininfi at length the different experiments. one partkularly easv and derno&rative e x ~ e r i m i n will t be described here. xie ens ions of this experimknt are left to the reader's ingenuity. Actually, the reader need not even be very ingenious, but simply follow the published literature (1-8).
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The 10-cm cell should be filled with 2.0 ton of a 10:lmix of H&Fs. SFo is an inexpensive, non-corrosive, easily handled gas. It is not necessary to purify the gases. SF6 has a very strong absorption in the ir near 950 cm-', and a spectrophotometer scan will reveal this absorption. Now focus the laser output into the center of the gas cell. 1 The acronvm TEA refers to the transverse electric discharee at atmospheric pressure that excites the lawr. ?The Hawshau Chemical Compnn)
Volume 54, Number 11. November 1977 / 705
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NUMBER OF LASER
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NUMBER OF LASER PULSES (units of
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Figure 1. Spectrophatometer traces of the SF. absorption near 950 cm? taken at intervalsof lo3 pulses. In W s e experiments,me wtput froma COn TEA laser (Lumonics944 crn-', 350 mJ, 1 Hz) is focussedwith a 28-crn f.1. NaCl lens into the gas cell containing 2 tarr of a 10:l mix of H2:SFe.
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Fiaure 2. traces of the 950 cm-' SF. absarDtian taken at - Soectroohatameter ~.~~ ntsrvals of 10' p~lses n mese expedmenls,the output from a nomeme "pm TEA laser 1944 cm '. 250 mJ. 10 rlzl is focusseawth an 8-cm 1.1. NaCl lens Into the gas cell ~
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What is occurring, is as follows. In the focal region, the intense laser beam is pumping SFs molecules to a very high level of excitation nhv + SF6
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SFstt
(1)
where tt denotes a high degree of vibrational excitation. Since some of theSF6" molecules can containmoreenergythan is necessary for dissociation, a fraction of the SF6moleeule~will dissociate. The precise nature of the photodissociation is still not clearly understood, but for the purposes of the present demonstration this is unimportant. The role of the Hzis that of a chemical scavenger, such as in the removal of fluorineatoms (2) F+Hz-HFtH The H F produced by eqn. (2) reacts rapidly with the walls of the cell, producing SiF4. It is possible, and quite illustrative, to monitor the concentration of SiF4 via its ir absorption near 1030 cm-1. The most fundamental point of the photodissociation process is that only species whose characteristic absorptions arenear the COzlaser frequency are dissociated. The remaining species do not interact with the laser beam and these snecies are left relativelv unaffected. This feature is clearly revealed in the case of SF6, since sulfur has two naturallv occurine isotooes: 32S(95%)and 34S(4.2%).The isotope shift near 950 em-' is 17 cm-I, and the two isotopic species can be distinguished with an ir spectrophotometer, as shown in Figure 1. Figures 1-3 show the results of several such simple experiments carried out with both a commercial laser (Lumonks) and a homemade "pin" type TEA laser. These experiments serve not only to demonstrate the specificity of the photodissociation, but also they underline an imoortant avolication for this vhenomenon. Laser isotope separation has been little more thin a conjecture since the first definitive work on the suhiect appeared in the literature (8). It is now possible, with uncompliiated lahoratory equipment, to actually achieve photodissociation and isotope separation using asimple COn TEA laser. The student carrying out these experiments is in a very unique position. Because of the experimental simplicity, the physical process of performing the experiment is deceptively simple. These experiments arestate of the art and are presently being pursued in a number of very serious research atmospheres. It would not be a t all surprising ~~~~
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706 / Journal of Chemical Education
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crn l e n s 8 cm l e n s 4 I
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NUMBER OF LASER PULSES (units of lo4)
Figure 3. Spenraphotometertraces illustrating the effect of me focussing lens on the photodissociation efficiency.
if students wrrr to uncover signifirant new information runcerninr fundamental proressrs during the course of thcie experiments! Literature Cited (1) Amhsrtaumyan,R V.,Gomkov.Yu.A.. Lefokhw,V. S.,andMakamv,G.N.,JETPlnrl., 21.171 (19751. (2) Ambartaumyen, R. V., Gomkhov, Yu. A., Lotokhav. V. S..Mahrou. 0.N., Ryabav. E. A.,andChekslin, N. V.,Soo. JQE, 5.1196 (1976). (3) Lyman, J. L.,Jonsen. R. J.,Rink, J., Robin~on,C. P.,snd Rackwmd. S. D.,Appl. Phys. Left., 27.97 11975). (4) Lyman,J. L.,andRaekwwd,S. D., J . Appl Phys.. 17,595 (1976). (5) Hsncock,G.,Campbell,J.D.,andWo1p.K. H.,Opt. Cornm., 16.177 (19761. (6) Ambartaumyan, R. V.,Gorokhov,Vu. A.. Letokhou,V. S..and Purotakii.A. A.. JETP Lett., 22.177 11975).
(71 Ambartsumym.R. V..Dolzhikov,V. S.,Lefokhov,V.S.,Ryehov,E. A.,andChekalin,
N.V.,Sou.Phys JETP, 42.36 119761. V. S.,and Moore, C. B.. Sou. JQE, 6,259 11976).
(91 Lefokhov, (8)
Wood,0. R., IEEEProc., 62.355
