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Some measurements of the resonance raman effects in β-carotene
Volume 6, number 4
CHJZMICAL PHYSICS
SOME
MZASUREMENTS EFFECT
OF IN
LETTERS
THE
15 August 1970
RESONANCE
RAMAN
P-CAROTENE
M. C. HUTLEY * and D. 3. JACOBS University
of York, Heslington.
York,
UK
Received 5 June 1970
The intensity of Raman scattering of B-carotene has been measured for a wide range of incident wavelengths, and an approximate proportionality between this and the absorption coefficient has been observed.
1. INTRODUCTION
The intensity of Raman scattering is given t.heoreticaIIy by the Kramers-Heisenberg dispersion relatton I
27 .5 mn= &Jlz2
I N( v,,f vmJ4 0
IMPl/nbQlrnY vmvoii
y
photomultiplier
where m, Y, n refer to the initial, intermediate and final states of the scatterer, vo is the frequency of the exciting radiation, y is a damping constant, and the rest of the symbols have their usual meaning [l, 21. When the frequency of incident radiation v. is close to an (electronic) absorption frequency ) there is a considerable enhancement (vo -) vY72 of the scattered intensity. This is known as the rescnance Raman effect. When y. lies actually within an absorption band, the intensity is determined largely by the damping constant y, and scattering under these conditions has been termed “Rigorous Resonance Raman Scattering” [S]. It has been pointed out by Behringer [3,4] that if one may restrict the sum over states Y in eq. (1) to :%single term (for a single excited vibronic state) and if y is large compared with the spacing of the vibrational levels of the excited electronic. state, then one would expect the scattered intensity to be proportional to the absorption coefficient E. An approximate proportionality has indeed been observed experimentalIy [5] but due to *Present Address: National Physical Laboratory, Teddington.
Middlesex.
the extreme difficulty in measuring Raman spectra in highly absorbing media there is not a great deal of experimental data avaiIaLle. We report here the results of an experiment in which the intensity of Raman scattering from p-carotene was measured for a variety of exciting frequencies lying both within and outside of the electronic absorption hand. A IGW PGWered argon/krypton laser aad a He-Ne laser were used to provide 5 widely spaced wave!angths for excitation, and the zgectra were recorded using a Hilger and Watts E612 prism spectrograph,
England,
and phase sensitive detector.
2. EXPERIMENTAL The experiment is complicated by several factors which introduce uncertainties into the results and require that certain precautions be taken [3,6]. It may be of interest to briefly list some of these precautions here. 1) Both the incident and scattered light are absorbed by the sample, and ,SOthe signaI intensi:? is reduced with higher concentration. On the ot?.t?r hand, the intensity is prt$o%oral t0 the number of molecules present, so to Gbserve the spectrum it is necessary to find an optimum concentration at each wavelength. In order to minimise the absorption, the length of path of light through the sample was kept as small as aostible. 2) B-carotene decomposes on exposure to light and this is perhaps the greatest source of error. Measurements were therefore taken on a large number cif freshly made sampies, the order of scanning the lines was varied and the sa.mplt! was ~exposeqto the laser beam onIy during the ~scanning of an actual tian line. As far as possible the spectra were recorded for a range of
269
Volume 6. number 4
CHEMICAL PHYSICS LETTERS
sample concentrations and the laser power was kept as low as possible consistent with observing a reasonable quality spectrum (1 to 3 mW). 3) Since most of the power in the beam was absorbed by the sample, therm-al gradients were set up and this gave rise to a divergence of the beam. Effects similar to those described by Craddock and Jackson [7] were observed. 4) .In addition to the Raman spectrum there was a broad background continuum due to fluorescence. This was particularly troublesome at the shorter wavelengths. The use of a laser in these experiments offers two-advantages over the mercury arc lamps used in previous work. Firstly, a wide range of wavelengths is available, and a particular one may quickly and easily be isolated without the need for filters. Secondly, the simple scattering geometry usually associated with laser F&man experiments permits a straightforward determination (bo’th theoretical and empirical) of the optimum sample concentration. The integrated intensities of the 1155 cm-l and the 1521 cm-l Raman lines were measured for e?citation -with the ofollowing wav$engtins: 4765 A, 4880 A, 5145 A (Ar), 5681 A (Kr) and 6328i
(He-Ne).
The sample
was dissolved
in
carbon tetrachloride and the 313 cm-l Raman line of tine solvent was used as an internal standard of intensity. In view of the strong dependence of intensity on the absorption and concentration of the sample the use of an internal standard is very advisable.
15 August 1970
photometer. The positions of the laser lines are also shown, and from this it may be seen that the three argon laser lines lie within the absorption band while the other two fall outside. The intensities of the 1155 cm” and 1521 cm-I Raman lines of B-carotene are listed in table 1 along with the approximate optimum concentrations and absorption coefficients. Data for excitation with 5461 A are included from Behringer’s work using a mercury source [8]. In view of the dif?erences in experimental arrangements their agreement with the present results is fairly good. It must be borne in mind that these intensity values are all relative to a carbon tetrachloride standard, and must therefore be corrected to allow for the wavelength dependence of the intensity of this standard. The correction factor shown has been based on the theory of Shorygin*
PI-
The final columns of table 1 list the values between Z and E is seen to be reasonable, especially when it is noted that E varies over 3 orders of magnitude. (See fig. 2). There is, however, some deterioration of the proportionality at longer wavelengths. This is consistent with observations made by other of I/E. The proportionality
workers [5], and indeed, one would not expect
the theory to hold far away from resonance. We note, however, two interesting features of these results. * The absorption bands in CC& are sufficiently far away from the visible (17OOA) that the theories of Shorygin [9] and Albrecht [2] yield effectively the same result.
.
3. RESULTS Fig. 1 shows the absorption spectrum of /Icarotene in the range 4000 to 6500 A, as mea-
sured on a Coleman Hitachi EPS-ST snectro-
Fig. 1. Absorption spectrum of p-carotene tetrachloride.
270
in carbon
Fig. 2. Logarithmic plot of Rsman Intensity of fl-carotene ( in frequency corrected intensity units) against absorption coefficient E (in cm2/mole). o, 1521 cm-l; A, 1155 cm-l; x. 1521 cm-l with resolved values of e.
Volume 6. number4
CHEMICAL PHYSICS LETTERS
25 August 1970
Table 1 Raman intensities and absorption coefficients of b-carotene. Intensity units based on a scale of 313 cm-l Iine of CCI* = 100 I.U. fcv) ccl4 is 1(~/~,)2+1~2/[~~~~2-~]4, xa = i700 A. I(corr) = ‘QJJ.) XfOCCL4 Optimum concentration cm2/mole (aPPrw E
x (tz
D ft
4880 D (4880t 5145 4 54614 56814 6328 A
I 1155
‘1521 (I.U.)
(I.U.)
Correction factor fW cc14
(F)
(F)l52, 1155
f"i x;
5 x105 :I
2.0 X108%20%
1.23X108 *20%
3.65 XlO-2
75.5
2.5x108=20%
2.4 x108 zt20%
3.07 x10-2
82.6
pi; m;, 1:20~103 4.12 X102 4.35 x 10
5x105:1 5 x105 :1 8190 : 1 104:l 5 x103: 1
1.2 x108 *20% s x106*50% 3.1x106+30%
1.2 1 4.8 1.6
2.33 X10-2 1.64 X10-2 1.38 X10-2 0.81x10-2
1.2~106~20%
1) For the 1521 cm-1 line, there is a definite in intensity in going from 4830 A. This may be due to the fact that radiationless transitions are occurring and robbing the scattering of decrease
~108~20% X107 *5Ot$ x106*30% X106+20%
784 68 106 223
.46.5 (202) 7l3.e (123) 76.0 136 160 300
the separation of the vibrational levels in the excited state. This is not completely true in this case since vibrational structure is clearly visible in the absorption spectrum.
its intensity. Alternatively, it may simply be due to the fact that 4765 A is further away from the
first absorption maximum (4975 A) than is 4880A.
If this is the case, then it suggests that the first excited state has more influence on the scattering process than has the second. This in turn might imply that the intensity should be propcrtional to only that part of the absorption spectrum arising from states actually involved in the Raman scattering process. If we therefore, resolve the absorption spectrum into its components we obtain a new set of values for E (shown in parenthesis
in table 1).
With these values there is some improvement in the proportionality between 11521 and E (dashed line in fig. 2). So it may be that for 1155 cm-l line both the first and second excited states contribute to the scattering whereas for the 1521 cm-l line the second state has much less effect than the first. 2) The ratio of Z/E steadily decreases as the incident wavelength becomes shorter. This may be evidence that the approximations involved in predicting that Z is proportional to E, are not entirely valid. Firstly, all terms except the first in the summation over y in eq. (1) were neglected. The inclusion of such terms would add to the intensity for all wavelengths, but it is not obvious which way this would affect the variation of Z/E. We note that it has been suggested by Albrecht [2] that it is in fact necessary to take account of at least one second excited &ate, even when describing Scattering at resonance. Secondly, it was assumed that the damping constant was large compared with
4. CONCLUSIONS The proportionality between E&man intensity and absorption coefficient hzs been qualitatively confirmed in the case of /l-carotene. Indeed, considering the range of values of these parameters and the experimental difficulties involved in measuring F&man intensities, it may be said that the quantitative agreement between theory and experiment is also good. At this stage it is not possible to determine the cause of the reduction of the parameter Z/E for wavelengths within the absorption band. Both the existence of non radiative processes and the necessity of including the effects of other excited states in the theory, could account for this observation. It may certainly be seen that much useful information concerning the nature of the Raman scattering process can be obtained from experiments such as those described here. Furthermore, with laser technology in its present state, such experiments forward.
should be comparatively
straight-
The authors would like to thank Professor J. Behringer for his frequent encouragement and helpful discussion concerning this work. REFERENCES (11G. Placzek, Haadbuchder Radiologie (Leipzig. 271
1,\&me-,6.-‘-&mher
4
:
-CI@MItiAL
PHYSICS
LETTERS
_, ‘.-
-__ U. S,kbmic Energy Conitiission UCBL 526 (L) 0962). [Zi 2, C.Albrecht. J. Chem; Phys. 34 (1961) 1476. -[3] J.-Behringcr,in< R+man SpectrosCopy. ed. H. Szymanski (L~li?num-Press. New York, 1967) ch. 6. [S] J.Behringer. Z.Elektr~hem. $2 (1958) 906. 15) ,M; M. Suschinsky and V. A. Zubov, .Otp. Spectry. : ., 13 (1962) 434. ‘ ._
‘_
.- 1934):. EngIiih &mslarion,
--
[6] M. C. Hutley. Thesis. University [7] FL C. Craddock and D. A. Jackson, Phys. (J. Pbys. D) 1 (1968) 1575. 181J. Behringer and J. Brandmllller. 4 (l359) ‘7, 234. 191P. P.Shorygin. Pure Appl. Chem.
“_I5 August 1970 .of York (1968). Brit. 3. Appl. J. Ann. Physik 4 (1962) 1.87.
CHJZMICAL PHYSICS
SOME
MZASUREMENTS EFFECT
OF IN
LETTERS
THE
15 August 1970
RESONANCE
RAMAN
P-CAROTENE
M. C. HUTLEY * and D. 3. JACOBS University
of York, Heslington.
York,
UK
Received 5 June 1970
The intensity of Raman scattering of B-carotene has been measured for a wide range of incident wavelengths, and an approximate proportionality between this and the absorption coefficient has been observed.
1. INTRODUCTION
The intensity of Raman scattering is given t.heoreticaIIy by the Kramers-Heisenberg dispersion relatton I
27 .5 mn= &Jlz2
I N( v,,f vmJ4 0
IMPl/nbQlrnY vmvoii
y
photomultiplier
where m, Y, n refer to the initial, intermediate and final states of the scatterer, vo is the frequency of the exciting radiation, y is a damping constant, and the rest of the symbols have their usual meaning [l, 21. When the frequency of incident radiation v. is close to an (electronic) absorption frequency ) there is a considerable enhancement (vo -) vY72 of the scattered intensity. This is known as the rescnance Raman effect. When y. lies actually within an absorption band, the intensity is determined largely by the damping constant y, and scattering under these conditions has been termed “Rigorous Resonance Raman Scattering” [S]. It has been pointed out by Behringer [3,4] that if one may restrict the sum over states Y in eq. (1) to :%single term (for a single excited vibronic state) and if y is large compared with the spacing of the vibrational levels of the excited electronic. state, then one would expect the scattered intensity to be proportional to the absorption coefficient E. An approximate proportionality has indeed been observed experimentalIy [5] but due to *Present Address: National Physical Laboratory, Teddington.
Middlesex.
the extreme difficulty in measuring Raman spectra in highly absorbing media there is not a great deal of experimental data avaiIaLle. We report here the results of an experiment in which the intensity of Raman scattering from p-carotene was measured for a variety of exciting frequencies lying both within and outside of the electronic absorption hand. A IGW PGWered argon/krypton laser aad a He-Ne laser were used to provide 5 widely spaced wave!angths for excitation, and the zgectra were recorded using a Hilger and Watts E612 prism spectrograph,
England,
and phase sensitive detector.
2. EXPERIMENTAL The experiment is complicated by several factors which introduce uncertainties into the results and require that certain precautions be taken [3,6]. It may be of interest to briefly list some of these precautions here. 1) Both the incident and scattered light are absorbed by the sample, and ,SOthe signaI intensi:? is reduced with higher concentration. On the ot?.t?r hand, the intensity is prt$o%oral t0 the number of molecules present, so to Gbserve the spectrum it is necessary to find an optimum concentration at each wavelength. In order to minimise the absorption, the length of path of light through the sample was kept as small as aostible. 2) B-carotene decomposes on exposure to light and this is perhaps the greatest source of error. Measurements were therefore taken on a large number cif freshly made sampies, the order of scanning the lines was varied and the sa.mplt! was ~exposeqto the laser beam onIy during the ~scanning of an actual tian line. As far as possible the spectra were recorded for a range of
269
Volume 6. number 4
CHEMICAL PHYSICS LETTERS
sample concentrations and the laser power was kept as low as possible consistent with observing a reasonable quality spectrum (1 to 3 mW). 3) Since most of the power in the beam was absorbed by the sample, therm-al gradients were set up and this gave rise to a divergence of the beam. Effects similar to those described by Craddock and Jackson [7] were observed. 4) .In addition to the Raman spectrum there was a broad background continuum due to fluorescence. This was particularly troublesome at the shorter wavelengths. The use of a laser in these experiments offers two-advantages over the mercury arc lamps used in previous work. Firstly, a wide range of wavelengths is available, and a particular one may quickly and easily be isolated without the need for filters. Secondly, the simple scattering geometry usually associated with laser F&man experiments permits a straightforward determination (bo’th theoretical and empirical) of the optimum sample concentration. The integrated intensities of the 1155 cm-l and the 1521 cm-l Raman lines were measured for e?citation -with the ofollowing wav$engtins: 4765 A, 4880 A, 5145 A (Ar), 5681 A (Kr) and 6328i
(He-Ne).
The sample
was dissolved
in
carbon tetrachloride and the 313 cm-l Raman line of tine solvent was used as an internal standard of intensity. In view of the strong dependence of intensity on the absorption and concentration of the sample the use of an internal standard is very advisable.
15 August 1970
photometer. The positions of the laser lines are also shown, and from this it may be seen that the three argon laser lines lie within the absorption band while the other two fall outside. The intensities of the 1155 cm” and 1521 cm-I Raman lines of B-carotene are listed in table 1 along with the approximate optimum concentrations and absorption coefficients. Data for excitation with 5461 A are included from Behringer’s work using a mercury source [8]. In view of the dif?erences in experimental arrangements their agreement with the present results is fairly good. It must be borne in mind that these intensity values are all relative to a carbon tetrachloride standard, and must therefore be corrected to allow for the wavelength dependence of the intensity of this standard. The correction factor shown has been based on the theory of Shorygin*
PI-
The final columns of table 1 list the values between Z and E is seen to be reasonable, especially when it is noted that E varies over 3 orders of magnitude. (See fig. 2). There is, however, some deterioration of the proportionality at longer wavelengths. This is consistent with observations made by other of I/E. The proportionality
workers [5], and indeed, one would not expect
the theory to hold far away from resonance. We note, however, two interesting features of these results. * The absorption bands in CC& are sufficiently far away from the visible (17OOA) that the theories of Shorygin [9] and Albrecht [2] yield effectively the same result.
.
3. RESULTS Fig. 1 shows the absorption spectrum of /Icarotene in the range 4000 to 6500 A, as mea-
sured on a Coleman Hitachi EPS-ST snectro-
Fig. 1. Absorption spectrum of p-carotene tetrachloride.
270
in carbon
Fig. 2. Logarithmic plot of Rsman Intensity of fl-carotene ( in frequency corrected intensity units) against absorption coefficient E (in cm2/mole). o, 1521 cm-l; A, 1155 cm-l; x. 1521 cm-l with resolved values of e.
Volume 6. number4
CHEMICAL PHYSICS LETTERS
25 August 1970
Table 1 Raman intensities and absorption coefficients of b-carotene. Intensity units based on a scale of 313 cm-l Iine of CCI* = 100 I.U. fcv) ccl4 is 1(~/~,)2+1~2/[~~~~2-~]4, xa = i700 A. I(corr) = ‘QJJ.) XfOCCL4 Optimum concentration cm2/mole (aPPrw E
x (tz
D ft
4880 D (4880t 5145 4 54614 56814 6328 A
I 1155
‘1521 (I.U.)
(I.U.)
Correction factor fW cc14
(F)
(F)l52, 1155
f"i x;
5 x105 :I
2.0 X108%20%
1.23X108 *20%
3.65 XlO-2
75.5
2.5x108=20%
2.4 x108 zt20%
3.07 x10-2
82.6
pi; m;, 1:20~103 4.12 X102 4.35 x 10
5x105:1 5 x105 :1 8190 : 1 104:l 5 x103: 1
1.2 x108 *20% s x106*50% 3.1x106+30%
1.2 1 4.8 1.6
2.33 X10-2 1.64 X10-2 1.38 X10-2 0.81x10-2
1.2~106~20%
1) For the 1521 cm-1 line, there is a definite in intensity in going from 4830 A. This may be due to the fact that radiationless transitions are occurring and robbing the scattering of decrease
~108~20% X107 *5Ot$ x106*30% X106+20%
784 68 106 223
.46.5 (202) 7l3.e (123) 76.0 136 160 300
the separation of the vibrational levels in the excited state. This is not completely true in this case since vibrational structure is clearly visible in the absorption spectrum.
its intensity. Alternatively, it may simply be due to the fact that 4765 A is further away from the
first absorption maximum (4975 A) than is 4880A.
If this is the case, then it suggests that the first excited state has more influence on the scattering process than has the second. This in turn might imply that the intensity should be propcrtional to only that part of the absorption spectrum arising from states actually involved in the Raman scattering process. If we therefore, resolve the absorption spectrum into its components we obtain a new set of values for E (shown in parenthesis
in table 1).
With these values there is some improvement in the proportionality between 11521 and E (dashed line in fig. 2). So it may be that for 1155 cm-l line both the first and second excited states contribute to the scattering whereas for the 1521 cm-l line the second state has much less effect than the first. 2) The ratio of Z/E steadily decreases as the incident wavelength becomes shorter. This may be evidence that the approximations involved in predicting that Z is proportional to E, are not entirely valid. Firstly, all terms except the first in the summation over y in eq. (1) were neglected. The inclusion of such terms would add to the intensity for all wavelengths, but it is not obvious which way this would affect the variation of Z/E. We note that it has been suggested by Albrecht [2] that it is in fact necessary to take account of at least one second excited &ate, even when describing Scattering at resonance. Secondly, it was assumed that the damping constant was large compared with
4. CONCLUSIONS The proportionality between E&man intensity and absorption coefficient hzs been qualitatively confirmed in the case of /l-carotene. Indeed, considering the range of values of these parameters and the experimental difficulties involved in measuring F&man intensities, it may be said that the quantitative agreement between theory and experiment is also good. At this stage it is not possible to determine the cause of the reduction of the parameter Z/E for wavelengths within the absorption band. Both the existence of non radiative processes and the necessity of including the effects of other excited states in the theory, could account for this observation. It may certainly be seen that much useful information concerning the nature of the Raman scattering process can be obtained from experiments such as those described here. Furthermore, with laser technology in its present state, such experiments forward.
should be comparatively
straight-
The authors would like to thank Professor J. Behringer for his frequent encouragement and helpful discussion concerning this work. REFERENCES (11G. Placzek, Haadbuchder Radiologie (Leipzig. 271
1,\&me-,6.-‘-&mher
4
:
-CI@MItiAL
PHYSICS
LETTERS
_, ‘.-
-__ U. S,kbmic Energy Conitiission UCBL 526 (L) 0962). [Zi 2, C.Albrecht. J. Chem; Phys. 34 (1961) 1476. -[3] J.-Behringcr,in< R+man SpectrosCopy. ed. H. Szymanski (L~li?num-Press. New York, 1967) ch. 6. [S] J.Behringer. Z.Elektr~hem. $2 (1958) 906. 15) ,M; M. Suschinsky and V. A. Zubov, .Otp. Spectry. : ., 13 (1962) 434. ‘ ._
‘_
.- 1934):. EngIiih &mslarion,
--
[6] M. C. Hutley. Thesis. University [7] FL C. Craddock and D. A. Jackson, Phys. (J. Pbys. D) 1 (1968) 1575. 181J. Behringer and J. Brandmllller. 4 (l359) ‘7, 234. 191P. P.Shorygin. Pure Appl. Chem.
“_I5 August 1970 .of York (1968). Brit. 3. Appl. J. Ann. Physik 4 (1962) 1.87.