Raman Spectra of Small Solid Samples Including Lattice Vibrations D. C. NELSON and W. N. MITCHELL Applied Physics Corp.,. Monrovia, Calif.
b Valuable lattice vi,bration information for solids has heretofore been difficult to obtain, either by the Raman effect or from infrared investigations, because of experimental difficulties. The proposed technique for pressed pellets and single crystals provides complete spectra which exhibit much less background interference. Spectra can be recorded milch closer to the exciting frequency with as little as 20 mg. of material in favorable cases. Spectra of various types of compounds are discussed with particular attention to the lattice vibration region.
A
LTHOUGH COhfPLE,TE UNDERSTANDING of the vibriitional spectra of
liquids cannot be attained by present methods ( I ) , the nterpretation of spectra of gases and solids is fundamentally simple. Gas spectra have been extensively studied, but the lattice vibration fundamentals which are characteristic of solids have been directly accessible only in the far infrared, and because of the difficulties of the technique and the small number of laboratories equipped for such work, they are unrecorded even for many common and interesting substances. It is true that they cE,n be examined in the near infrared as c3mbination bands through their interxtion with the higher frequency vibrations, but assignments are often uncertain and interpretation is confused by band interactions. Except for experimental difficulties, the Raman effect would be a powerful aid for study of solid s,pectra. The high symmetry of lattice vibration is favorable to easy Raman excitation. Until now, however, the problem of discrimination against exciting radiation and the accompanying inefficiency of illumination have mitde i t practically impossible to obseripe solids a t the small frequency shiftai characteristic of lattice fundamentalri, especially in photographic instrunients (for which the tremendous improvement of the double monochromator is unavailable), Powdered materials are particularly troublesome, but single crystals of adequate size are se dom available or easily grown. The background (and
noise with a photoelectric instrument) upon which the Raman spectrum must be superimposed is accentuated in the very region, 20 to 300 Acm.-l, where lattice fundamentals occur. The most recent and pertinent Raman spectroscopic work on solid samples includes reports by Schrader, Nerdel, and Kresze (6) and Tobin (6), working with photographic instruments, and Ferraro (3) with a photoelectric instrument. It is almost certain that Schrader would have completely anticipated the present work if he had employed a photoelectric, double monochromator instrument. [-4dditional references may be found in the recent review by Jones and Tunnicliff (4)1. This paper describes a solid-materials technique which affords spectra with much lower background levels. The improvement yields two major dividends: complete spectra with smaller quantities of material, and spectra providing information as close as 40 A cm.-' to the exciting frequency. The work was done on the Cary Model 81 (8) which combines a double monochromator of high light-gathering power
and an unusually sensitive photoelectric photometer. The relative significance of front-face illumination-e.g., direct illumination of the sample surface closest to the monochromator-in contributing to scattered radiation apparently has not been realized before. Recently Tobin (6), in reviewing the alternatives of solid-sample mounting, described the almost universally accepted conical configuration identical to the acceptance cone of the entrance optics of the monochromator, where the conical surface, illuminated by the excitation energy, faces the monochromator. The assumption is that, if the sample is between the source and monochromator, its opacity greatly limits the illumination of the zone of sample effectively viewed by the monochromator. The second author of this paper found empirically that the reverse is true. The simple expedients of shielding the front sample face from direct illumination, and using a sample whose thickness is in inverse relation to its opacity to the exciting radiation, produce spectra exhibiting much reduced back-
0 0.00 0 0 0 n
J
a
n
0 0 0 0 0 0 0 Figurs 1.
Solid sample source optics showing
A. B. C. D. E. F. G.
Blackened metal optics holder Lens Entrance aperture, 8.5 mm. 4 mm. diameter slit-slicer image Sample Mask Filter iacket H. Toronto arc 1. Standard Model 81 solid sample holder VOL. 34, NO. 3, MARCH 1944
555
ground. The technique permits the low frequency shift region to be examined readily for most samples. Both single crystals and powders show improved spectra. EXPERIMENTAL
Compounds used t o make pellets were Analytical Reagent Grade materials. Single crystals were either grown from saturated solutions or supplied through the courtesy of associates. Pellet Preparation. Pellets were prepared by the familiar infrared techniques of making KBr disks, with one significant departure. The amount of solute used varied from 10 to lOOyo instead of the usual 1% or less. I n most cases the pellets were 100% solute. The procedure results in an opaque white tablet, not unlike an aspirin tablet in appearance. A Wig-LBug (Crescent Dental Mfg. Co., Chicago, Ill.) was used for grinding and mixing, and a conventional laboratory press (Wabash Hydraulics, Wabash, Ind.) in forming the pellets. The inch die used in the press was homemade but commercial dies would doubtless produce equivalent results, Die evacuation was not necessary. Sample Mounting and Optics. Figure 1 is a scale drawing of the space inside the filter jacket of the Model 81 Raman Instrument (a). A low-pressure mercury arc of the Toronto type, H , surrounds a cylindrical filter jacket, G, which contains a solution designed to pass the 4358 A. line and absorb the other mercury lines. A set of entrance optics is mounted in the source compartment so that a 4 mm. diameter slitslicer image (8) is formed about 2.5 mm. in front of the entrance aperture. The entrance aperture, C, is 8.5 mm. in diameter. Pressed pellets are mounted with one face flush with the tip of the solid hollow optic cone, A , so that this face of the pellet is not illuminated by the excitation energy. The sides and rear of the pellet are exposed. The pellet is held by three stainless steel prongs inch diameter with a slip ring) to provide the necessary tension around the pellet. Pellets as thin as 0.2-0.3 mm. can be mounted without breaking. A pellet die 6.4 mm. in diameter was available and gave pellets which proved efficient to use with the existing solid sample optics. Although the exact shape of a pellet can be controlled, crystals have to be mounted as they come. The ideal shape for a clear crystal is a rod about 7 to 8 mm. in diameter with a polished face. I n practice, all shapes and sizes are used. An attempt is made to get a flat surface of the crystal flush with the entrance aperture, filling as much of the aperture as possible. As noted before, care is taken not to illuminate the face of the crystal toward the entrance aperture. Lamp Ghosts. I n some cases two very weak peaks, one a t 145 A cm.-' and one a t 172 A cm.-', can be observed. They form a part of the arc background, varying in intensity relative to the exSamples.
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0
ANALYTICAL CHEMISTRY
Figure 2.
Naphthalene pellet, 95 mg.
citation line with electrode temperature and arc current. They appear when such diverse substances as MgC08, cellulose, and glass are used to scatter the exciting radiation into the monochromator. Because of their relative intensity and shape, they are easily recognizable and can be differentiated empirically from sample Raman shifts. Instrument Settings. I n most cases the zero suppression circuit provided in the Model 81 was not required because background was naturally low. Scanning speeds were no faster than one spectral slit width per fullscale pen period. I n most cases, the speed was one wavznumber per second, the pen period 5 seconds, and the spectral slit width 5 cm.-l. 4358 A (22,938 crn.-l) is defined as zero A cm.-l. Stokes lines were measured in all cases, so that all vibrations discussed hereafter refer to Raman shifts in wavenumbers from 22,938 cm.-' to lower frequencies. Signal to Background Noise Measurements. The instrument used is
limited by the shot noise due to the
Figure 3.
discrete random nature of the radiation and of the photoelectric effect, except in cases where background is negligible and the Raman energy is extremely low. Under these conditions, significant phototube darkcurrent noise may occur. I n the work described, the dark-current effect is negligible. In the: shot-noise limited case, the base line noise is proportional to the square root of the background energy falling on the phototube. The background arises principally from excitation energy which is reflected into the monochromator from the optical inhomegeneities of the sample. The intensity of the exciting frequency at zero cm.-l is a relative measure of the square of the amount of noise that will occur a t a given frequency for a given sample mounting configuration. The base line and hence noise superimposed on it will, in general, decrease rapidly as the instrument scans farther from the excitation frequency (the assumption is made here that samples do not fluoresce). Thus, the square root of the exciting line is taken as the noise, and the peak
Sodium chlorate-KBr
mixture, 75 mg. each
Table 1.
Form Pellet Pellet Pellet Pellet Pellet Crvstal Crystal Crystal Crystal Crystal
Figure 4.
Sodium chlorate single crystat,
height above background a t a chosen frequency (1380 cm.+ for naphthalene) as the signal in calcilating signal to noise ratios discuss:d later. This method of evaluating; noise is faster and more accurate than estimating pen excursions from base line noise. Of course, the actual noise at the peak Raman intensity is thl: sum of the shot noise from background plus Raman signal. However, we have used the peak height to base 1 ne noise because i t simplifies mathematical considerations and does not (tppreciably alter usefulness. Figure 2 is an example of a spectrum of a pellet, in this case 95 mg. of pure naphthalene. Similar spectra have been obtained with as little as 19 mg. of naphthalene. Figure 3 is a spectrum illustrating the use of KBr to form a pellet. The pellet is a 50-50 mixture of KBr and sodium chlorate, with a total weight of 150 mg. Figure 4 is a spectrum of a large NaC103 crystal. ?Sole that with the exception of the lat1,ice vibration a t 63 A cm.-', the other lattice vibrations (7) are present (125, 175 A crn.-') in the KBr diluted pellet (Figure 3). Figure 5 is spectrum of a single crystal of lanthanum trichloride. This material is of interest as host material in laser research. Because of its hygroscopic nature, this crys tal was contained in a closed glass tube. The orientation of the crystal in the tube was not ideal for minimizing exciting energy scattered into the monochromator. One end of the glass tube was flat and mounted flush with the entrance aperture of the source optics. No special precautions were taken with the fliLt end of the glass tube, but it was suff ciently large and flat to cover the ent-ance aperture of the source optics. The peaks occurring in this spectrum have 3een corroborated as lattice vibrations through private communication with another laboratory, This spectrum points up that solid materials that must b: grown andlor handled in inert atinospheres, using conventional glasswaIe, can be easily managed in obtaining; a Raman spec-
71/2
grams
trum, a t least if available as a single crystal of adequate size. RESULTS AND DISCUSSION
Table I summarizes some of the crystal lattice vibrations observed using this method. Lattice vibrations cited were either corroborated by comparison spectra of an aqueous solution of the material, or through private communication with associate in other laboratories. Schrader, Nerdel and Kresze (6), have shown that when a cylindrical pellet is illuminated on the back surface only by the exciting energy, the resulting Raman intensity (IR) emerging from the front side of the pellet a t a given frequency is given by Equation la: ZR
=
&lob exp (
- 2.303 kb)
(la)
Crystal Lattice Vibrations
Compound Ys hthalene
KPPo~
Urea KCNS NaCIOa Potassium biphthalate Potassium azide Rubidium azide Cesium azide Lanthanum trichloride
Frequency of lattice vibrations observed, A cm.-' 45, 75, 108 88 59 68, 96, 124 63, 125, 175 84. 105
107, 151 68, 132 43, 96 108, 186, 210
Q is an efficiency constant related to the conversion of exciting energy to Raman scattering, lo is the intensity of the exciting energy on the rear of the pellet, b is the thickness of the pellet in mm., and k is an extinction coefficient. The exciting ( I E ) energy passing unaltered through the pellet is given by Equation l b : IE
=
Zo exp ( - 2.303kb)
(lb)
To a first approximation (Rayleigh scattering has not been considered) for a photographic instrument illuminating the back surface of a pellet only, the signal-to-noise ratio is proportional to b, the thickness of the pellet. The first derivative of Equation l a shows that the Raman intensity has a maximum a t b,, = 0.434/k. However, for a photoelectric shot noise limited instrument with zero suppression, the background can be electrically suppressed in almost all cases. The base line noise will increase as the square root of the background
VOL. 36, NO. 3, MARCH 1964
557
20
B
I
ha
I 2m
I Jmn
1
PELLETW-MI
PELLET THICKNESS
Figure 7. Relative signal to background noise level for 1380 cm.-' Naphthalene peak vs. pellet thickness or weight. A. Back illumination only. 6. Back and cylinder area I I I 2fmn PELLET THICKNESS
PELLET W-Ug
lnvl
Figure 6. A. 1380 cm.-' Naphthalene peak intensity vs. pellet thickness or weight, for a back-illuminated pel6. Background intensity aclet, companying curve A. C. (1 380 cm.-') naphthalene peak intensity vs. pellet thickness or weight, illuminating cylinder area and back. D. Background intensity accompanying curve C
energy striking the detector. The signal-to-background noise level ratio is :
S/N = &/lob (exp -2.303 kb/2)
(2a)
Equation 2a has a maximum such that bmsx = 0.868/k. To check these assumptions, we made use of the 1380 A cm.+ peak of naphthalene. Pure pellets l/h inch in diameter were prepared as described in varying weights (the weight being directly proportional to pellet thickness). Figure 6, curve A , represents the relative peak height of the 1380 cm.-1 peak plotted us. pellet thickness or weight. The curve passes throubh a maximum as predicted by Equation la, when illuminating the back surface only. Curve E is the relative background intensity. If the cylinder area and back of the pellet are illuminated, curves C and D are for the Raman intensity and background intensity, respectively. Figure 7, curve A , shows the ratio of the 1380 cm.-l peak height to the square root of the exciting line peak height for back-illuminated pellets us. pellet weight or thickness. The ordinate is the relative signal-to-noise ratio, determined as outlined in the experiment section. Note that the maximum of curve A , Figure 7, orcurs a t approximately twice the pellet thickness as the maximum of curve A , Figure 6.
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ANALYTICAL CHEMISTRY
This is in agreement with the results of Equations l a and 2a for bmaXproviding some assurance that our assumptions were correct. However, curve E of Figure 7 shows that it is much preferable to illuminate the back and sides of the pellet. This again points up that the most significant part of the technique is to shield the surface of the pellet facing the monochromator from the source energy. The pellets providing the data for Figures 7 and 8 were pure naphthalene. The background decreases somewhat with pellet thickness up to about 100 mg. (2 mm.). For this reason pellets were prepared containing varying percentages of KBr to determine if a better signal-to-noise ratio would result by making the pellets thicker by dilution with KBr, when small amounts of sample are available. Figure 8 shows that some loss is always encountered in dilution. Thus, pellets should be prepared in the pure state, even down to 20 mg. or less, if they can be pressed into a mountable pellet. A small amount of KBr (10 to 15%) can be added as a binder with very little loss if the material resists pelletizing. Many compounds other than naphthalene were studied and some of the spectra have been included here. In all cases we found that 120 to 160 mg. was an optimum amount to use, and that dilution with KBr did not improve matters. It might be expected that the optimum pellet thickness would change from compound to compound. It probably does, but only slightly. The principal effect is the opacity of the pellet which is more a matter of particle size and crystallinity than any compound absorption characteristic. Colored materials exert the same limitations with solids as they do with liquidr, hence KBr dilution may yield improved spectra.
Us. HAPHTHALEE
Figure 8. Relative signal to background noise level for 1380 cm.-' Naphthalene peak vs. pellet thickness or weight for various per cent naphthalene-KBr mixtures. Numbers refer to per cent naphthalene
In summary, small amounts of pure compounds pressed as pellets alone, or with KBr, can yield more nearly complete Raman spectra if mounted as described. The amount of sample required will vary from compound to compound, depending upon the Raman scattering power of the compound. Improvement in single crystal spectra is also achieved by observing the mounting precautions described. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of R. C. Hawes of the Applied Physics Corp. in preparing the manuscript. LITERATURE CITED
(1) Brugel, Werner, "An ;fntroduction to
Infrared SDectroscopv, _ _ Wiley, New York. 1962: (2) Cary, H., Gallaway, W. S., George, K. P., "Design Considerations, Details and Performance of the C a y Model 81 Raman Spectrophotometer, 1961. Reprints available from Applied Physics Corp., Monrovia, Calif. (3)Ferraro, J., Mack, G., Ziomek, J.,
Spectrochim. Acta 17, 802-814 (1961). (4) Jones, A. C., Tunnicliff, D. D., ANAL. CHEM.34, 261R-276R, (April 1962). (5) Schrader, B., Nerdel, F., Kresae, G , Z. Anal. Chem. 170, 43-55 (1959). (6) Tobin, M. C., Developments i n Applied Spectroscopy 1 , 205-214 (1962). (7) Williams, Dudley, Methods of Exp. Phys. 3 , 152 (1962). RECEIVED for review October 7, 1963. Accepted December 16, 1963.
