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Concentration-dependence studies of Raman spectra of water by the method of self-deconvolution
Volume
103. number
CHEMICAL PHYSICS LETTERS
1
16 December
19S3
CONCENTRATION-DEPENDENCE STUDIES OF RAMAN SPECTRA OF WATER BY THE METHOD OF SELF-DECONVOLUIION G.M. CEORCIEV, Department
T.K. KALKANJIEV,
of Quantum
electronrcs,
Faculty
V.P. PETROV. of Physics,
Sojii
Zh. NICKOLOV Uniuem’ty.
I1 26 Sojk,
Bulgaria
and
M. MITEVA Faculty
of Chemrsrry,
Sofm
Unrvenity.
1126 Sofii,
Bulgana
Received 4 August 1983. III final form 29 September
1983
The method of selfdeconvolutlon ISapphed m Raman spectroscopy. The advantages pronded by the polarized nan~re of the scattered light are dlscussed. The behaviour of the components in the compovte Raman spectrum of water (OHstretching region) versus the concentration of dissolved Nai and N&104 salts IS mvesrigated. Effects which are lmposnble to observe by the widely used fitting procedures are also reported
I. Introduction
tion as a method of resolution enhancement when spectral lines overlap due to their mtrmslc halfwidth,
It has long been recognized that the OH-stretching region in the vibrational spectra of electrolytes in water is a superposition of several components, but their origm is a subject of debate [ l-81. The Raman iineshape in water evolves in a complex manner when the concenlration and the type of dis: solved salts are changed. The usual approach in analysing this evolutron is to decompose the contour Into gaussian or lorentzian components and to study the concentration dependence of their parameters [l-6] _ However, any iterative decomposition procedure re-
i.e. self-deconvolution
[13-181.
When applying this
method, no preluninary assumptions are necessary. The process of resolution enhancement IS lunltcd however by the random noise III the spectrum. So far, the capablhtles of the method of selfdeconvolution have been demonstrated in Infrared [ 13,161 and ESR spectroscopy [ 171. In this work, we expand the area of application
into Raman spec-
troscopy.
quires prehminary assumptions concernmg the num-
2. Experimental
ber and shape of the components and these assumptions lead to ambiguous results [9]_ Moreover, some authors recognize the neglect of asymmetric component shape when decomposing the Raman contour of water to be a weak point of their investigations [IO,1 I]. Most commonly, the application of the deconvolurion technique in spectroscopy is associated with the removal of the smearing effect due to the finite resolution of the experimental apparatus [ 121. Recently, considerable attention has been paid to deconvolu-
The experiments were carned out on our computercontrolled Rarnan spectrometer. The excitation source was an argon laser (Spectra Physics model 165 09) with an output power stability of 0.1% emitting at 488 nm. A photon-counting system and a stepping motor which serves to position the gratmgs were interfaced to the microcomputer. The spectra were recorded at selected wavenumbers under an effective resolutron of 2.5 cm-l (full width at half maximum), the mterval between two experimental points bemg 5.3 cm-l,
0 009-2614/83/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
83
t.h~ avoiding the convolution effect of the apparatus slit width. Corrections for the photomultiplier dark count and the spectrometer sensitivity were performed simultaneously with the accumulation of the spectral data. The fluorescence background was esti-
mated by changing the laser wavelength and was then removed from the spectrum. Both parallel Y(ZZ)X and perpendicular Y(ZY)X polarized spectra were recorded for each solution sample. From these spectra, isotropic -and anisotropic spectra were derived. The frequency shift of each experimental point is associated with a channel numbered %” in which the accumulation of the srgnal S(n) takes place. The number of channels is 256, so that the fast Fouriertransform algorithm developed by Cooley and Tukey i 191 can be applied directly_
It is well known that deconvolution by an appropriate trial function results in considerable resolution enhancement of the input spectrum [13,16,18]. In order to make some quantitative estimates of this effect, we shall assume that the lineshape of the components can be approximated by a lorentzian curve. In this case, the composite spectrum 1s M
ax)= mqlA, 11+ [(x - &J/um]2]-1 _ = (1 /o,a)[
1 + (x/o,)3
]- l .
(2)
This is equivalent to drviding the Fourier transform of S(x) by the Fourier transform of Z(x) and perforrning an inverse Fourier transform. Choosing the Fourier transform pair as f%(x)
= S(V) = J
exp(-2rriyx)S(x)
dx ,
(3)
-DD F-Q-y)=S(x)=
J exp(25riry)sCy) dr . -DD we obtain in succession that
FT(x)= f(y) = exp(-2au.JA).
(6)
M
X expt--2n(um- uT)lA] ,
F-l =
(4)
(7)
=S’(x)
WYWI
m$l _tqIum [l+(&$),I_,. 1
m
(8)
T
In order to perform the rnverse Fourier transform of (7). the trial function halfwidth should satisfy the inequality aT < urn
in
= min(ut , 02, .. . . uM) .
(9)
The deconvolved spectrum S’(x) (or self-deconvolved, as the trial function shape is sunilar to that of the components) is again a sum of lorentzrans with unchanged peak positions em. Resolution enhancement is a consequence of the component halfwidth decrease and amphtude mcrease. Similar results can be obtained analytically assummg gaussian or Vorgt convolution lineshape of the components. Eq. (8) was derived using the continuous Fourier transform (3) and (4). The discrepancy between continuous and discrete Fourier transforms (realized in practice) is the reason for the discrepancy between S’(x) and the computer-deconvolved
Let us deconvolve S(x) by a trial function nx)
16 December 1983
CHEhllCAL PHYSlCS LETTERS
Volume 103. number 1
spectrum.
This
results in the appearance of spurious peaks - pods * The character of these pods can be modtfied by means of various low-pass frequency
filters [ 141.
These filters are called apodization ’ functions. The pods can be divided into two groups: (1) Pods arising from the apodization, from overdeconvolution (when UT > Umin) and from inappropriate shape of the trial function (shape pods). These pods may appear even when deconvolving simulated spectra. (2) Pods arising from the random noise superimposed on the spectrum, from incorrect background removal and from fluctuations of the fluorescence
* For the origin of pods and apodization refer to ref. [ 161.
84
Volume 103. number 1
CHEMICAL PHYSICS LETTERS
16 December 1983
caused by floating orgamc rmpurrties in the sample (fluorescence pods). As in thus work we put emphases on the frequency shifting of the components versus concentratron of dissolved salts, we did not avord over-deconvoluuon. On the other hand, over-deconvolution makes it possible to apply a restoratron procedure (refs. [ 13 ,181, see also section 4). We achieved the best resolution enhancement with minimum apodlzatron pods by a gaussian apodlzation and a lorentzran trial function.
4. Results and discussion Comparison of the parallel polarized spectra of Nal solutrons (fig. 1 A) and the same spectra after deconvolution (fig. 1 B) clearly illustrates the effect of resolution enhancement. The weak pods at both ends of the deconvoluted spectra (below 3000 and above 3640cm-l) are fluorescence pods. Evidence for this comes from a comparison of the deconvolved spectra of solutions prepared with different degrees of purification. Ihe lowfrequency fluorescence pods become stronger as the fluorescence background decreases with a frequency shaft m the OH-stretching region. The two strong negative pods at about 3290 and 3550 cm-l are over-deconvolution and/or shape pods. The high-frequency negative pod has an apparent modulation at ~3620 cm-l. This represents vibrations of non-hydrogen-bonded water molecules which form a shoulder at the same frequency in the pure water spectrum [ 10,ll). This shoulder cannot be detected visually in concentrated alkali halide solutions - fig. 1 A (see also fig. 9 of ref. [7]). We shall neglect tlus component because of the uncertainty of detection. Thus, the parallel deconvolved spectra consist of two components, the stronger of these having a visible structure for concentrations of 2 and 4 M. Further over-deconvolution by increasing the hallfwidth of the trial function does not result in further decompo. sition of this component as the fluorescence pods increase severely in amplitude and destroy the whole spectrum. Additional resolution can be achieved by deconvoiution of the isotropic spectra using the same trial
2620
2960
3300 FREQUENCY
3640 SHIFT
3980
C~I-1
Fii. 1. (A) Parallel po-hrked Raman spectra of water versus concentrahon of dissolved Nal. (B) The spectra from (A) deconvolved by using a lorentzhn t.rlal function and gaussian
apoduatron. and apodization fun:tions (fig. 2A). The deconvolved isotropic spectra consrst of threee components whose intensitres and posiaons have the followingconcentration dependence: (1) The low-frequency component decreases in intensity (al 5% per M) and increases in frequency (~2.6 cm- 1 per M 1. (2) The middle-frequency component is approximately unchanged. (3) The high-fre auency component, which is assigned to I--wate: interactron, increases in intensity (=1..5 tunes per M) and decreases in frequency (~7 cm- 1 per M). 85
Volume
103. number
CHEMICAL
1
PHYSICS
16 December
LETTERS
1983
.
I
2620
FREQUENCY
CONC. ._ . . . . . . .
_
_.-
_an
3640
3300
2960
stiIF1
6960
2620
2960
IIll-’
CONC.
NPI
NAcLa4
. . . . . . . . . Ill
ZH
3300
FREQUENCY
-. -: .r .,*
3640
SHIFT
3960
CR -1
I
611
:
Fig. 2. Deconvolved isotropic (A) and anisotropic (B) Raman spectra of Water versus concentration of dissolved Nal.
The middle- and high-frequency components are not resolved in the deconvolved anisotropic spectra (fig.-2B). This can be explained by the larger haifwidth of these components in the anisotropic spectrum [lo], and by a highly depolarized component, appearing in the anisotropic spectrum, and/or the low depolarization ratio of the middle-frequency component in the isotropic spectrum. To investigate the middle component we deconvolved the isotropic and the anisotropic spectra of NaC104 solutions. As the ClO;-water interaction is weaker than the l--water interaction [S], it is reasonabie to expect that the component which is assigned to the ClOi-water interaction be shifted to high86
Fig. 3. Deconvolved isotropic (A) and anisotropic spectra of water verszs concentration of dissolved
(B) Raman NaCl04.
er freqtiency, thus resolving the middle-frequency component. Fig. 3 shows that ad&tional resolution is indeed achieved. In addition, two new effects can be observed : (1) The middle-frequency component decreases in intensity with concentration_ (2) Abnormal negative sinking at the high-frequency end of the ClOi-water interaction component. To investigate the concentration dependence of this component, we separated it from the deconvolved spectrum and then restored it (fig. 4) by a convolution using
CHEMICAL PHYSICS LETTERS
Volume 103. number 1
with a sum of subcomponents seems to us unjustified.
16 December 1983 (as obtained in ref. [6])
5. Conclusion The method of self-deconvolution plied to investigate the concentration
has been apdependence of
the components In the composte Raman spectra of water. In previous work [ 181 ,we developed the deconvolution method as a decomposition technique. In the present work, due to the high degree of over-
2620
2960
3300 FREOUENCY
-128
0
3640 SHIFT
3980
CH -1
CHRNNELS
128
Fig. 4. The restored component corresponding to the ClOiwater interaction versus concentration of dissolved NaUOa. The intrinsically asymmetrtcal shape is the reason for the negative smkmg at the high-frequency end of fg. 3.
the same trial function (the effect of the apodization function is compensated for by an inverse operation). As a result we obtained an excessively asymmetric character of the component. Of course, some part of the asymmetry 1s due to an inaccurate choice of splitting points. So far, there has not been any theoretical treatment of the method of self-deconvolution in the case when the spectrum is an envelope of asymmetric overlapping spectral lines. However, two circumstances led us to consider the ClOi-water interaction component as being intrinsically asymmetric. Ftrst, if we deconvolve the spectrum increasmg successively the halfwidth of the trial functton, then the negative sinking will increase linearly in amplitude. If this sinkmg is interpreted as a pod, it should show a threshold character [ 131. Secondly, having in mmd the weak electrostatic interaction ClOi-water, it is natural to expect that the shape of the corresponding component (fig. 4) should resemble the shape of the Raman .speCtNm of water, in which the hydrogen bonds are to a great extent broken; for example, see the spectra at the critical temperature presented m fig. 6 of ref. [20]. This is why the replacement of this component
lappmg and the different halfwidths of the components, we base our analysis on the deconvolved spectra. We consider the peak positions of the compo-
nents to be the most reliable Information that can be derived from the deconvolved spectra. For correct mterpretation of the components, It is necessary to compare the results obtained with deconvolved spectra of water recorded at different temperatures, as well as with the deconvolved spectra of the uncoupled OH-stretching vibrations (i.e. dilute solutions of H,O in D,O) recorded at different salt concentrationsand temperatures. The main conclusion that can be made 1s that the method of self-deconvolution combined with the advantages provided by the polarized nature of Raman scattered light 1s a powerful tool for probmg existing water structure models, especially those that predict the behaviour of the components m the OH-stretchmg region.
AcknowIedgement We are particularly grateful for helpful dscusnons with H.H. hlantsch from the National Research Council of Canada. We also wish EO thank K.V. Stamenov from Sofia Umversity for his constant terest in Gus work.
in-
References [l]
W.R. Busing
and D.F. Hornig. 1. Phys Chem. 65 (1961) 284. [ 2] J.W. Schultz and D.F. Horn&, J. Phys. Chem. 65 (19611 2131.
87
CHEMICAL PHYSICS LETI-ERS
Volume 103. number 1
131 G.E. W&&en,& Chem Whys. 36 (1962) 1035. f4j T.T. Wall and D F. Hormg, J. Chem. Phys. 47 (1967) 784. IS] G.~.W~afen,J.Chem.Pfiys.52(1970)4li76 f6 J G E Wairafen, 3. Chem. Fhys. 55 0971) 768. 171 N. Abe and hi. Ito, J. Raman Spectry. 7 (1978) 161. (81 F. Ahotta, M.P. Fontana, G. Mauano, P. M@ardo and F Wanderkngh, Opt. Acta 27 (1980) (1975) [lOJ
[ 131 J-K. Ilauppinen. Cameron, Appl. 1141 J.K. Kauppmen. bwtsch, Appf. 1151 J.K. Kauppinen, Cameron, Anaf. [ 16 J R.N Jones and
931_
19 j B G.M. Vandeginste and L. de GaIan, An&
(1983) Chem. 47
2124.
W-F. Murphy and H.J. Bernstein, J. Phys. Chem. 76 (1972) 1147. [II j J R. Scherer, M.K. Go and S. Kmt, J. Phys Chem. 78 (1974) 1304. [I21 H. Yakowtz and J.W. Colby, in. Practical scannmg electron mrcroscopy, eds. J.I. Goldstein and H. Yakowz (Pienum Press, New York, 1975) p. 479.
16 December 1983
[I?]
K.
D.J.
hioffatt,
Spectry.
H H. Mantsch and
D.G.
35 (19811271.
DJ. Moffatt. D G. Cameron and H H. Opt. 20 (19811 1866. D J htoffatt, H.H. Mantsch and D.G. Chem. $3 (1981) 1454. K. Shimokoshi, Appl. Spectry. 37
59.
S~~~kos~i and R.N. Jones, Appl. Spectry. 37 (1983) 67 [ 181 G.M. Georgtev, T.K. Kalkanjev and V.P. Petrov, Spectry. Letters, to be pub~~ed. 1191 J.W. Cooley and J.W. Tukey, hiath.Comput. 19 (196s) 297. f2Ot PC. Cross, J ~urnbam and P A. Lerghton, J. Am. Cl-tern. sot. 59 (1937) 1134.
103. number
CHEMICAL PHYSICS LETTERS
1
16 December
19S3
CONCENTRATION-DEPENDENCE STUDIES OF RAMAN SPECTRA OF WATER BY THE METHOD OF SELF-DECONVOLUIION G.M. CEORCIEV, Department
T.K. KALKANJIEV,
of Quantum
electronrcs,
Faculty
V.P. PETROV. of Physics,
Sojii
Zh. NICKOLOV Uniuem’ty.
I1 26 Sojk,
Bulgaria
and
M. MITEVA Faculty
of Chemrsrry,
Sofm
Unrvenity.
1126 Sofii,
Bulgana
Received 4 August 1983. III final form 29 September
1983
The method of selfdeconvolutlon ISapphed m Raman spectroscopy. The advantages pronded by the polarized nan~re of the scattered light are dlscussed. The behaviour of the components in the compovte Raman spectrum of water (OHstretching region) versus the concentration of dissolved Nai and N&104 salts IS mvesrigated. Effects which are lmposnble to observe by the widely used fitting procedures are also reported
I. Introduction
tion as a method of resolution enhancement when spectral lines overlap due to their mtrmslc halfwidth,
It has long been recognized that the OH-stretching region in the vibrational spectra of electrolytes in water is a superposition of several components, but their origm is a subject of debate [ l-81. The Raman iineshape in water evolves in a complex manner when the concenlration and the type of dis: solved salts are changed. The usual approach in analysing this evolutron is to decompose the contour Into gaussian or lorentzian components and to study the concentration dependence of their parameters [l-6] _ However, any iterative decomposition procedure re-
i.e. self-deconvolution
[13-181.
When applying this
method, no preluninary assumptions are necessary. The process of resolution enhancement IS lunltcd however by the random noise III the spectrum. So far, the capablhtles of the method of selfdeconvolution have been demonstrated in Infrared [ 13,161 and ESR spectroscopy [ 171. In this work, we expand the area of application
into Raman spec-
troscopy.
quires prehminary assumptions concernmg the num-
2. Experimental
ber and shape of the components and these assumptions lead to ambiguous results [9]_ Moreover, some authors recognize the neglect of asymmetric component shape when decomposing the Raman contour of water to be a weak point of their investigations [IO,1 I]. Most commonly, the application of the deconvolurion technique in spectroscopy is associated with the removal of the smearing effect due to the finite resolution of the experimental apparatus [ 121. Recently, considerable attention has been paid to deconvolu-
The experiments were carned out on our computercontrolled Rarnan spectrometer. The excitation source was an argon laser (Spectra Physics model 165 09) with an output power stability of 0.1% emitting at 488 nm. A photon-counting system and a stepping motor which serves to position the gratmgs were interfaced to the microcomputer. The spectra were recorded at selected wavenumbers under an effective resolutron of 2.5 cm-l (full width at half maximum), the mterval between two experimental points bemg 5.3 cm-l,
0 009-2614/83/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
83
t.h~ avoiding the convolution effect of the apparatus slit width. Corrections for the photomultiplier dark count and the spectrometer sensitivity were performed simultaneously with the accumulation of the spectral data. The fluorescence background was esti-
mated by changing the laser wavelength and was then removed from the spectrum. Both parallel Y(ZZ)X and perpendicular Y(ZY)X polarized spectra were recorded for each solution sample. From these spectra, isotropic -and anisotropic spectra were derived. The frequency shift of each experimental point is associated with a channel numbered %” in which the accumulation of the srgnal S(n) takes place. The number of channels is 256, so that the fast Fouriertransform algorithm developed by Cooley and Tukey i 191 can be applied directly_
It is well known that deconvolution by an appropriate trial function results in considerable resolution enhancement of the input spectrum [13,16,18]. In order to make some quantitative estimates of this effect, we shall assume that the lineshape of the components can be approximated by a lorentzian curve. In this case, the composite spectrum 1s M
ax)= mqlA, 11+ [(x - &J/um]2]-1 _ = (1 /o,a)[
1 + (x/o,)3
]- l .
(2)
This is equivalent to drviding the Fourier transform of S(x) by the Fourier transform of Z(x) and perforrning an inverse Fourier transform. Choosing the Fourier transform pair as f%(x)
= S(V) = J
exp(-2rriyx)S(x)
dx ,
(3)
-DD F-Q-y)=S(x)=
J exp(25riry)sCy) dr . -DD we obtain in succession that
FT(x)= f(y) = exp(-2au.JA).
(6)
M
X expt--2n(um- uT)lA] ,
F-l =
(4)
(7)
=S’(x)
WYWI
m$l _tqIum [l+(&$),I_,. 1
m
(8)
T
In order to perform the rnverse Fourier transform of (7). the trial function halfwidth should satisfy the inequality aT < urn
in
= min(ut , 02, .. . . uM) .
(9)
The deconvolved spectrum S’(x) (or self-deconvolved, as the trial function shape is sunilar to that of the components) is again a sum of lorentzrans with unchanged peak positions em. Resolution enhancement is a consequence of the component halfwidth decrease and amphtude mcrease. Similar results can be obtained analytically assummg gaussian or Vorgt convolution lineshape of the components. Eq. (8) was derived using the continuous Fourier transform (3) and (4). The discrepancy between continuous and discrete Fourier transforms (realized in practice) is the reason for the discrepancy between S’(x) and the computer-deconvolved
Let us deconvolve S(x) by a trial function nx)
16 December 1983
CHEhllCAL PHYSlCS LETTERS
Volume 103. number 1
spectrum.
This
results in the appearance of spurious peaks - pods * The character of these pods can be modtfied by means of various low-pass frequency
filters [ 141.
These filters are called apodization ’ functions. The pods can be divided into two groups: (1) Pods arising from the apodization, from overdeconvolution (when UT > Umin) and from inappropriate shape of the trial function (shape pods). These pods may appear even when deconvolving simulated spectra. (2) Pods arising from the random noise superimposed on the spectrum, from incorrect background removal and from fluctuations of the fluorescence
* For the origin of pods and apodization refer to ref. [ 161.
84
Volume 103. number 1
CHEMICAL PHYSICS LETTERS
16 December 1983
caused by floating orgamc rmpurrties in the sample (fluorescence pods). As in thus work we put emphases on the frequency shifting of the components versus concentratron of dissolved salts, we did not avord over-deconvoluuon. On the other hand, over-deconvolution makes it possible to apply a restoratron procedure (refs. [ 13 ,181, see also section 4). We achieved the best resolution enhancement with minimum apodlzatron pods by a gaussian apodlzation and a lorentzran trial function.
4. Results and discussion Comparison of the parallel polarized spectra of Nal solutrons (fig. 1 A) and the same spectra after deconvolution (fig. 1 B) clearly illustrates the effect of resolution enhancement. The weak pods at both ends of the deconvoluted spectra (below 3000 and above 3640cm-l) are fluorescence pods. Evidence for this comes from a comparison of the deconvolved spectra of solutions prepared with different degrees of purification. Ihe lowfrequency fluorescence pods become stronger as the fluorescence background decreases with a frequency shaft m the OH-stretching region. The two strong negative pods at about 3290 and 3550 cm-l are over-deconvolution and/or shape pods. The high-frequency negative pod has an apparent modulation at ~3620 cm-l. This represents vibrations of non-hydrogen-bonded water molecules which form a shoulder at the same frequency in the pure water spectrum [ 10,ll). This shoulder cannot be detected visually in concentrated alkali halide solutions - fig. 1 A (see also fig. 9 of ref. [7]). We shall neglect tlus component because of the uncertainty of detection. Thus, the parallel deconvolved spectra consist of two components, the stronger of these having a visible structure for concentrations of 2 and 4 M. Further over-deconvolution by increasing the hallfwidth of the trial function does not result in further decompo. sition of this component as the fluorescence pods increase severely in amplitude and destroy the whole spectrum. Additional resolution can be achieved by deconvoiution of the isotropic spectra using the same trial
2620
2960
3300 FREQUENCY
3640 SHIFT
3980
C~I-1
Fii. 1. (A) Parallel po-hrked Raman spectra of water versus concentrahon of dissolved Nal. (B) The spectra from (A) deconvolved by using a lorentzhn t.rlal function and gaussian
apoduatron. and apodization fun:tions (fig. 2A). The deconvolved isotropic spectra consrst of threee components whose intensitres and posiaons have the followingconcentration dependence: (1) The low-frequency component decreases in intensity (al 5% per M) and increases in frequency (~2.6 cm- 1 per M 1. (2) The middle-frequency component is approximately unchanged. (3) The high-fre auency component, which is assigned to I--wate: interactron, increases in intensity (=1..5 tunes per M) and decreases in frequency (~7 cm- 1 per M). 85
Volume
103. number
CHEMICAL
1
PHYSICS
16 December
LETTERS
1983
.
I
2620
FREQUENCY
CONC. ._ . . . . . . .
_
_.-
_an
3640
3300
2960
stiIF1
6960
2620
2960
IIll-’
CONC.
NPI
NAcLa4
. . . . . . . . . Ill
ZH
3300
FREQUENCY
-. -: .r .,*
3640
SHIFT
3960
CR -1
I
611
:
Fig. 2. Deconvolved isotropic (A) and anisotropic (B) Raman spectra of Water versus concentration of dissolved Nal.
The middle- and high-frequency components are not resolved in the deconvolved anisotropic spectra (fig.-2B). This can be explained by the larger haifwidth of these components in the anisotropic spectrum [lo], and by a highly depolarized component, appearing in the anisotropic spectrum, and/or the low depolarization ratio of the middle-frequency component in the isotropic spectrum. To investigate the middle component we deconvolved the isotropic and the anisotropic spectra of NaC104 solutions. As the ClO;-water interaction is weaker than the l--water interaction [S], it is reasonabie to expect that the component which is assigned to the ClOi-water interaction be shifted to high86
Fig. 3. Deconvolved isotropic (A) and anisotropic spectra of water verszs concentration of dissolved
(B) Raman NaCl04.
er freqtiency, thus resolving the middle-frequency component. Fig. 3 shows that ad&tional resolution is indeed achieved. In addition, two new effects can be observed : (1) The middle-frequency component decreases in intensity with concentration_ (2) Abnormal negative sinking at the high-frequency end of the ClOi-water interaction component. To investigate the concentration dependence of this component, we separated it from the deconvolved spectrum and then restored it (fig. 4) by a convolution using
CHEMICAL PHYSICS LETTERS
Volume 103. number 1
with a sum of subcomponents seems to us unjustified.
16 December 1983 (as obtained in ref. [6])
5. Conclusion The method of self-deconvolution plied to investigate the concentration
has been apdependence of
the components In the composte Raman spectra of water. In previous work [ 181 ,we developed the deconvolution method as a decomposition technique. In the present work, due to the high degree of over-
2620
2960
3300 FREOUENCY
-128
0
3640 SHIFT
3980
CH -1
CHRNNELS
128
Fig. 4. The restored component corresponding to the ClOiwater interaction versus concentration of dissolved NaUOa. The intrinsically asymmetrtcal shape is the reason for the negative smkmg at the high-frequency end of fg. 3.
the same trial function (the effect of the apodization function is compensated for by an inverse operation). As a result we obtained an excessively asymmetric character of the component. Of course, some part of the asymmetry 1s due to an inaccurate choice of splitting points. So far, there has not been any theoretical treatment of the method of self-deconvolution in the case when the spectrum is an envelope of asymmetric overlapping spectral lines. However, two circumstances led us to consider the ClOi-water interaction component as being intrinsically asymmetric. Ftrst, if we deconvolve the spectrum increasmg successively the halfwidth of the trial functton, then the negative sinking will increase linearly in amplitude. If this sinkmg is interpreted as a pod, it should show a threshold character [ 131. Secondly, having in mmd the weak electrostatic interaction ClOi-water, it is natural to expect that the shape of the corresponding component (fig. 4) should resemble the shape of the Raman .speCtNm of water, in which the hydrogen bonds are to a great extent broken; for example, see the spectra at the critical temperature presented m fig. 6 of ref. [20]. This is why the replacement of this component
lappmg and the different halfwidths of the components, we base our analysis on the deconvolved spectra. We consider the peak positions of the compo-
nents to be the most reliable Information that can be derived from the deconvolved spectra. For correct mterpretation of the components, It is necessary to compare the results obtained with deconvolved spectra of water recorded at different temperatures, as well as with the deconvolved spectra of the uncoupled OH-stretching vibrations (i.e. dilute solutions of H,O in D,O) recorded at different salt concentrationsand temperatures. The main conclusion that can be made 1s that the method of self-deconvolution combined with the advantages provided by the polarized nature of Raman scattered light 1s a powerful tool for probmg existing water structure models, especially those that predict the behaviour of the components m the OH-stretchmg region.
AcknowIedgement We are particularly grateful for helpful dscusnons with H.H. hlantsch from the National Research Council of Canada. We also wish EO thank K.V. Stamenov from Sofia Umversity for his constant terest in Gus work.
in-
References [l]
W.R. Busing
and D.F. Hornig. 1. Phys Chem. 65 (1961) 284. [ 2] J.W. Schultz and D.F. Horn&, J. Phys. Chem. 65 (19611 2131.
87
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