The Raman Effect Applications and Present Limitations in Petroleum Clhemistr y J h n a ~ sH.
BEN, Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C.
a light quantum and amolecule. HE utility of the Raman A brief discussion of the ntilure of the Raman A certain can be drawn effect as a nieans of obeffect and of its application to the study of carious between the mechanism for elicittaining fundamental insystems is giuen. I t is to be particularly eming Raman spectra and ordinary formation in regard to petroleum phmized that the applications and limilations the case of collihydrocarbons hears the same mentioned pertain not only to the petroleum insion between two molecules it is relation to the petroleum indusquite conceivable that one moletry as it does to any other indusdwlry but also to any other industry or scientiJir cule may bounce Off after an intry or special branch of science research which seeks to utilize this new research elastic impact with less velocity %shore iundamental information tool. The Raman spectra method of intestiguand eonseauelitlv less enerer is per se of advantage. -" tion is one which m y proce particularly usefur than it poises& p r e v i o u s l y , The p a r t i c u l a r efficacy of under conditions where other methods of approach imparting some of its energy to the Raman effect lies in the fact the other moIec& as kinetic or that it. is a p h y s i c a l method are not applicable. There are, hozuecer, certaiu nhich neither shifts an equilimitutwns such as the complexity of the system ~~~~~~~~~~~r librium nor c a u s e s chemical to be examined, color, and lack of Raman lines change, and is independent of the atoms I uI) . the of suflcient intensity. may be set into 1-ibration. state of aggregation of the material to bo examined and, within wide limits, its chemical constitution. It can, in certain cases, RAXAKPmcrit.% by empirical analysis give a picture of the constitution OF cerIn the case of Itanian spectra a gas, a liquid, a crystal, or an tain complexliquidswhich are otherwise so complex as to defy amorphous solid is illuminated with monochromatic radiation. other methods of approach. It provides information in reWhen the quanta of this radiation interact with one of these molecules, part of the energy may go toward displacing the equilibrium vibration of the atoms which constitute the molecule or toward increasing the rotation of the molecule as a whole. The resultant light which is scattered as a result of this collision has generally less energy than the original quantum. If, however, the molecule is in an energy state higher than its normal state, then the scattered light may he of higher frequency than the incident radiation, since it contains the additional energy of transition from the higher level to the normal level. If the original monochromatic radiation, which is termed the "exciting radiation," is viewed with a spectroscopc in the direction of the beam, one single line will be seen, or recorded on the photographic plate. If, however, the light scattered by an organic liquid is photogard to the frequency of vibration of the atoms within the graphed, for convenience, at right angles to the source OF molecule, the strength of the bond between atoms in their monochromatic radiation, morc than one liny: will appear. normal states, their amplitude of vibration and, in simple The experimental arrangement for such recordmg is given in cases, their spacial configuration. &om this information it simplified form in Figure 1, and typical Raman lines are has been possible to calculate the specific heats of certain com- shown in Figure 2. The strongest line will be that correpounds and their normal stability, and to gain an insight into sponding to the exciting light, but, in addition, there may the oomnlexitv of solutions ~8 both organic and inorganic c 2
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Raman i i s c o v e r e d t h i s effect there have been more than a thousand puhlications on this subiect.
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be one or more lines corresponding to light scattered by those molecules which have taken up a c e r t a i n a m o u n t of energy f r o m the incident radiation. These lines, consequently, represent smaller frequencies than the exciting r a d i a t i o n a n d a p p e a r shifted to the longer wave-length side, depending on how much of this energy has been removed. For exampls, if the exciting r a d i a t i o n is the blue 4358 A. mercury line, the scattered lines appearing on a photographic plate will consist of the 4358 8. mercury line plus lines of a greenish blue or green, or in exceptional cases p e r h a p s e v e n yellow. If a molecule is in one of its higher energy states, a c o r r e s p o n d i n g shift in the opposite direction is possible but comparatively rare. These shifted lines are Raman lines. The term "shift" is used to indicate the fact that the Raman line is a weaker part of the original MtTh'ANE exciting radiation which has b e e n degraded, CXANE a n d c o n s e q u e n t l y t h e modified radiation CARBON ~~~~i.~s EFFECT
The Raman effect has certain limitations which should be thoroughly understood beiore applying it to a particular problem. Perhaps the greatest deterrent is in the actual weakness of the Raman lines t.hemselves. The total amount of enerev ,... scattered or emitted in the form of Raman lines mav approach a fair percentage OF that part of the exciting radiation which is also scattered. IF, however, there is a large number OF lines, this reduces the intensity of an individual line to 8 rather sinall magnitude, and it may take long exposures under Fairly ideal conditions to record tho Raman scattering. IF the photographic plate is fogged owing to fluorescence, to continuous radiat.ion from the exciting souroe, or to direct scattering of the exciting radiation in the case of small crvstals., for examule. the faint Raman lines are difficult to distinguish. In actual practice there is a certain element of confusion owing to the presence OF more than one exciting line, as each exciting wave length gives rise to its own individual set of %man tines. A Further such element is introduced in the overlapping of Raman lines themselves. .
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If only one plate is taken in a given study, the attributing of lines to a given source of exoitation may be entirely erroneous. It is necessary, therefore, that the illumination be reduced to as near a monochromatic sonrce as possible by choosing a proper source of light and by the interposition OF proper filters.
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of spectrogralns is taken, it is pmsible, by diininisi1ing the intensity of various exciting lines, to dimiuisli also the concomitant. Raman liiies and so attribute eacll Raman line
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to its proper source of excitation. In ti similar manner Auorescence may be reduced by eliminating the light of higher frequency and by quenching the fluorescence-throiigh collisions of the second kind---by the addition of proper substances to the solution under examination. Examples of this quenching process are given in Figure 10. These are Ilaman plates of water-white oil; B contains one per cent or less of nitrobenzene. Actual Raman lines are discernible in B. Raman lines corresponding to an atomic vibration of low frequency, of tbe order of 200 wave numbers or less, fall close to the exciting radiation and are difficult to distingnisli from slit ghosts and halation of the exciting line. In general, scattering from the source of the light may be reduced by operat.ing a t a suficiently low temperature and, in the case of small cryxtals, by using great precantions to obtain a monochromatic radiation and by using plates which are antihalo in the extreme. Another deterrent is in the color of the solutions themselves. Since the intensity of the Raman scattering varies as a fourth paver of the wave length, it is advisable to use as short a wave length as possible for excitation pur-
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puses. If tile wliitiona arc coivrcd, t.his is not p w d h , since there will he a11 absorption of both the exciting radiation and of tlio Raman lines themselves. Conseqnently it is necessary to use, in this case, as a soiirce of
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green, yellow, or even red. This reduces tho intensit.y of the Raniaii scattering and diminishes the actual distance between the Iiamnn lilies and the incident line on tlie photographic plate because of the lower dispersion in tlie red or yellow of tlie a v e r a g e prism,
lines more difficult. Last of all in this cunnection, there arc few practical sources of monochromatic radiation in the yellow and red parts of the spcctnini. In tlie case of studying the Haman spectra of compounds having a highly coniplex constitution, the Raman lines are su numerous that it hecumcs next to inipossihle always to assign a particular vibration to its "wromr . source. Kevertlielean, in spite of these limitations, as has been stated previously, much progress bas been made and many of these limitations may be reduced with the development of improved technic. ~~ITERATUlU!C'WED'
( I ) Andrewa, NLW he.,34, ID26 (19%): 36, 644 (1930) (2) Uhagnrantnrn, Jnd. J . Physics. 5, 49 (1930): Signer anti Weiler, Hab. Chim. Acta, 15. 649 (1932): Dadieu and Kohlrausrii, ~~tiLiioisrcrLsc/~/tia/irn, IS, 154 (1930) (31 I3lineav:~ni.ani.Jnd. J . I ' h y s i w 5, 73 (11330); Trurnpy, Z.P/ilisih, 62. 806 (1930). (4) Uonino and Rnill, IhCl., 58, 194 (11329) Dudicu, Poorratu. and Kohlrsiisch. Sitiber. Aicnd. TVisr. K i I'. Moth. natvrw. K l a s s a , Aht. IIa. 1 4 0 , 3 5 8 , 6 4 7 (1931). ( 5 ) Bouraucl. Compt. vend., 194. 1736 (1032): Collins, Phgs. Rm..40, 829 (1932). (6) Daurc, Ann. phya., 12. 375 (1929): Miolieinnow-aka, %. Phuriir. 6 5 . 1 2 4 (1930). (7) Dupont. Dame. Allsril. iuid IAvy, Chimie & induutrie, Special No.. p. 630 (1932): Krislmemurti, Jb'dure, 128, 639 (1931): Crigiier. J . Am. Chert.. Soc., 54, 4207 (1932); Whiting snd Martin. Tranr. Roy. Soc. Can., 2 5 8 7 (1931): Andant, Chimie Le industrie.Spooia1 No., P. 480 (.June, 19331. (ti! Ganesan. J n d . J . Phusics. 4, 281 (1929): Bhsgavsntam, 16id., 5, 257 (1930): Kohlrsusch, "Der Smekal-Raman Effekt," .Julius Springer, Berlin, 1931: Sirkar, Jnd. I . Physics. 7, 431 (1932): I-iibhen, Clicm. Ben., 1 3 , 3 4 5 (1933). (9) Hibhen, J . Am. C h m . Soc., 53, 2418 (1931). (10) Hibben, PTW.NaU. Aced. Sei., 1 8 , 5 3 2 (1932). (11) Lespiaeu. Bourguel, and Wakeman, Cornpit. rend.. 193. 108: (1931). (12) Rao. Nature, 124, 762 (1929). (13) Vsnkateswaran and Uhagavilntam. Proc. Roy. Soc. (London). 128.i. 252 (1930). 2txcsrveu February 8. 1934 Presented BL~ Pert of the Sympmium on Physical Propertiea o! Hydrooaibon Mixtures before the Division 01 Petroleum Chemistry st the 85th Xeeting a! the American Chemical Sooiety. lvaahiwton. D. C.. Marah 20to31, 1833.
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1 The references cited are only B small i i a e t i o n o! tim published r a i k s . 'The bibliographies iscluded in tlie references cited under ( 6 ) cover a !air total of the publirhed material.
lux. In the paper on "Sulfunic Aoi& from Petroleum," IND.EX. CHSM.,26, 395-6 (1934), we should like to correct an error. Beto. sulfonates are obtained by the action of imcentrated sulfuric acid on mineral oils and not when oils are treated with iurning acid, asstated in the tahular material in COIL umn 1, page 395. Another point which has heen omitted i s that alpha and gamma k'iGURE 10. hM.4N SPECTn.4 OF OILS, SHOWING DECRXASE aulfonio acids form, eont,rary to common opinion, some 25 to 40 IN F~.UORESCENCE RESlJUrIXG PROM TEE AnorTroy O F A SMALL QUANTITY OF NITROBENZENE per cent of the bulk of the aoid sludge resulting from the treatA , above: B. below. ment of lubricating oils with concent,rzted sulfuric acid. Thk Some Raman linea are visible on the original negative of E. ixct seems to be of commercial interest. E. N E Y MAND . ~ 8. Pii&
