Double-beam Raman difference spectroscopy Martin Moskovits and Kirk Michaelian University of Toronto, Lash Miller Chemical Laborato- ries, Toronto, Ontario M5S 1A1. Received 29 January 1977. Double-beam techniques have been routinely used in ab- sorption spectroscopy for almost three decades. An impor- tant feature of these experiments is the reference beam, which makes possible the elimination of spectral features common to both samples and facilitates accurate measurement of rel- ative wavelength or frequency shifts in the spectra of the two samples. The comparison of sample and reference beams also minimizes the effects of temporal source intensity fluctuations on the spectra. Although the same needs occur in Raman spectroscopy, Raman spectra are commonly obtained by single-beam methods because it is more difficult to produce the emissive reference beam required for double-beam Raman spectroscopy. With one exception, the Raman difference techniques which have been previously proposed, 1-5 are all single-beam experiments. In this Letter we describe a new Raman difference technique which has several important advantages over these earlier methods, including the fact that it is a double-beam technique. We also present some illus- trative spectra obtained with our system. The apparatus is shown schematically in Fig. 1. An argon ion laser beam passes first through an electrooptic modulator (Pockels cell), which causes the plane of polarization of the light to alternate from the direction perpendicular to the scattering plane to that parallel to this plane at a frequency of 200 Hz. This is accomplished by adjusting the modulator voltage at λ/4 and by tilting the cell slightly to produce a static λ/4 retardation. The beam next impinges on a 2° Wollaston prism, which causes the bursts of conjugate polarization to diverge in space. A 45° polarization rotator restores the equivalence of these beams before they strike the samples, which are contained in ordinary 1.5-mm capillaries. The scattered light is focused onto the entrance slit of the mono- chromator and is detected by a photomultiplier connected to a synchronous photon counting system, which directs counts originating in each of the half cycles of modulation (and hence from each sample) to two different memories, computes their sum and difference, and finally plots and prints the data. Fig. 2. Polarized sum and difference spectra for a 1:10 v/v solution of CHCl 3 in CCl 4 vs neat CCl 4, uncompensated for CCl 4 concentration difference. Excitation source was the 488.0-nm line of an Ar + laser, and the output power was 0.7 W. The sensitivity of the difference spectrum is five times as great as that of the sum spectrum. Fig. 3. Polarized sum and difference spectra for a 1:10 v/v solution of CHCl 3 in CCl 4 vs neat CCl 4, with CCl 4 bands nulled in difference spectrum. Other details are the same as in Fig. 2. Fig. 1. Apparatus used to obtain Raman difference spectra, a, Monochromator; b, photomultiplier tube; c, synchronous photon counter; d, Pockels cell driver; e, Ar + laser; f, Pockels cell; g, Wollaston prism; h, polarization rotator. After a preset accumulation time, the monochromator grating is stepped to a new spectral position; and the above cycle is repeated. The system is balanced by positioning the capil- laries and collection optics to give an intensity difference of less than 1% when the capillaries contain identical samples; a null of 0.1% can be obtained with care. Since the capillaries are spatially separated, slight defocusing of the collecting lens is required to equalize the intensities obtained for the two samples; this defocusing produces only a small loss in over-all intensity. Once the optics are optimized in this way, small differences in the Raman spectra of the two samples, one of 2044 APPLIED OPTICS / Vol. 16, No. 8 August 1977
Transcript
Double-beam Raman difference spectroscopy
Martin Moskovits and Kirk Michaelian University of Toronto, Lash Miller Chemical Laboratories, Toronto, Ontario M5S 1A1. Received 29 January 1977.
Double-beam techniques have been routinely used in absorption spectroscopy for almost three decades. An important feature of these experiments is the reference beam, which makes possible the elimination of spectral features common to both samples and facilitates accurate measurement of relative wavelength or frequency shifts in the spectra of the two samples. The comparison of sample and reference beams also minimizes the effects of temporal source intensity fluctuations on the spectra. Although the same needs occur in Raman spectroscopy, Raman spectra are commonly obtained by single-beam methods because it is more difficult to produce the emissive reference beam required for double-beam Raman spectroscopy. With one exception, the Raman difference techniques which have been previously proposed,1-5 are all single-beam experiments. In this Letter we describe a new Raman difference technique which has several important advantages over these earlier methods, including the fact that it is a double-beam technique. We also present some illustrative spectra obtained with our system.
The apparatus is shown schematically in Fig. 1. An argon ion laser beam passes first through an electrooptic modulator (Pockels cell), which causes the plane of polarization of the light to alternate from the direction perpendicular to the scattering plane to that parallel to this plane at a frequency of 200 Hz. This is accomplished by adjusting the modulator voltage at λ/4 and by tilting the cell slightly to produce a static λ/4 retardation. The beam next impinges on a 2° Wollaston prism, which causes the bursts of conjugate polarization to diverge in space. A 45° polarization rotator restores the equivalence of these beams before they strike the samples, which are contained in ordinary 1.5-mm capillaries. The scattered light is focused onto the entrance slit of the mono-chromator and is detected by a photomultiplier connected to a synchronous photon counting system, which directs counts originating in each of the half cycles of modulation (and hence from each sample) to two different memories, computes their sum and difference, and finally plots and prints the data.
Fig. 2. Polarized sum and difference spectra for a 1:10 v/v solution of CHCl3 in CCl4 vs neat CCl4, uncompensated for CCl4 concentration difference. Excitation source was the 488.0-nm line of an Ar+ laser, and the output power was 0.7 W. The sensitivity of the difference
spectrum is five times as great as that of the sum spectrum.
Fig. 3. Polarized sum and difference spectra for a 1:10 v/v solution of CHCl3 in CCl4 vs neat CCl4, with CCl4 bands nulled in difference
spectrum. Other details are the same as in Fig. 2.
Fig. 1. Apparatus used to obtain Raman difference spectra, a, Monochromator; b, photomultiplier tube; c, synchronous photon counter; d, Pockels cell driver; e, Ar+ laser; f, Pockels cell; g, Wollaston
prism; h, polarization rotator.
After a preset accumulation time, the monochromator grating is stepped to a new spectral position; and the above cycle is repeated. The system is balanced by positioning the capillaries and collection optics to give an intensity difference of less than 1% when the capillaries contain identical samples; a null of 0.1% can be obtained with care. Since the capillaries are spatially separated, slight defocusing of the collecting lens is required to equalize the intensities obtained for the two samples; this defocusing produces only a small loss in over-all intensity. Once the optics are optimized in this way, small differences in the Raman spectra of the two samples, one of
2044 APPLIED OPTICS / Vol. 16, No. 8 August 1977
which may be a solution while the other may be the pure solvent, or those which occur because of intermolecular interactions, concentration differences and so on, are easily detected.
Our method offers several advantages. It requires no mechanical movement of either optics or the sample. The latter may be important when differences due to temperature changes are sought. It offers a true difference on-line, unlike computer-based systems; and it is capable of adjustment and nulling without resorting to unequal count periods. It can also be added to existing Raman systems with only minor modifications. Once it has been aligned, we have been able to use it for hours without realignment.
To illustrate, we show the sum and difference spectra of 1:10 v/v solution of CHCl3 in CCI4 subtracted from or added to that of pure CCl4. One of two strategems is used in doing difference spectroscopy. In the first, one wishes to measure the true difference between the spectra obtained by nulling the apparatus with two identical samples of solvent, then replacing one with a solution without further adjustment. Such a spectrum is shown in Fig. 2, where the CHCl3 bands at 262 cm– 1 , 366 cm– 1 , and 668 cm" 1 produce positive intensity differentials; and the CC14 bands at 218 cm– 1 , 314 cm– 1 , 459 cm - 1 , and 790 cm – 1 result in negative differences, since in the solution there is less CCl4 per volume than in the solvent. The 761-cm -1 CHCl3 band, which is coincident with that at 762 cm – 1 in CCl4, is almost canceled out in this spectrum. Note that there is a fivefold increase in the sensitivity with which the difference spectrum was recorded, as compared with that for the sum.
In the second method one assumes the solvent to be parasitic and one aligns the system with both solution and solvent in place until the solvent bands are minimized. This assumes, of course, that the presence of the solute does not shift or broaden the solvent lines. Such a spectrum is shown in Fig. 3, which shows the bands belonging to CCl4 reduced by two orders of magnitude, compared with those of CHCl3, making the 761-cm-1 line of the latter clearly visible under the tenfold more intense bands of CC14 at 762/790 cm– 1 .
One area of research currently being investigated with our Raman difference method is the study of ion-ion and ion-water interactions in electrolyte solutions. As an example of this work, we present in Fig. 4 sum and difference spectra obtained when a LiCl solution is compared with pure water. Although we are currently studying the entire spectrum, only the OH stretching region of the spectrum Will be shown.
Figure 4 shows the reduction in v(OH) intensity near 3200 c m - 1 and 3650 cm– 1 , as well as the intensity increase at 3450 cm - 1 , which occur in the polarized Raman spectrum of water on addition of LiCl. The use of the difference technique has enabled us to study these changes as a function of solute concentration with considerable accuracy. Moreover, the 3200 -cm–1 and 3450-cm–1 difference bands are readily seen to be asymmetric, implying that each of these bands is a superposition of two or more peaks. The weaker negative band at 3650 c m - 1 identifies yet another component of the broad v(OH) envelope. Thus, the difference spectrum aids considerably in the decomposition of this extremely broad spectral feature by quantifying the intensity changes which occur in its components under certain conditions.
In conclusion, the results presented in this Letter illustrate the capabilities of a new double-beam Raman difference technique. The ability to measure accurately small intensity changes with this method makes feasible the study of a number of interesting systems, of which electrolyte solutions are but a single example.
Fig. 4. Polarized difference and sum spectra for aqueous 9.18-M LiCl solution vs water. Experimental conditions for these spectra were
the same as described for Fig. 2.
References 1. J. S. Bodenheimer, B. J. Berenblut, and G. R. Wilkinson, Chem.
Phys. Lett. 14,533(1972). 2. W. Kiefer, Appl. Spectrosc. 27, 253 (1973). 3. J. W. Amy, R. W. Chrisman, J. W. Lundeen, T. Y. Ridley, J. C.
Sprowles, and R. S. Tobias, Appl. Spectrosc. 28, 262 (1974). 4. D. J. Gardiner, R. G. Girling, and R. E. Hester, J. Chem. Soc.
Faraday Trans. II, 71, 709 (1975). 5. A. K. Covington and J. M. Thain, Appl. Spectrosc. 29, 386
