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Resonance Raman effect in MnO−4 with 5145 Å laser excitation
Volume 8. number 5
CHEMICAL PHYSICS LETTERS
RESONANCE WITH
RAMAN
5145
A LASER
EFFECT
L March 1971
IN MnO3
EXCITATION
W. KIEFER and H. J. BERNSTEIN Division
of Chemistry,
National Research Ottawa, Canada
Council of Canada,
Received 4January1971
Resonance Raman scattering of the MnOi ion in Hz0 and D20 solutions is reported. Several overtones of the vI(A~) mode of this ion (Td symmetry) and combination tones nvI(A*) + %(F.$ are observed. From the wavenumbers of the overtones of vl the dissociation energy of the MnOJ ion has been estimated.
1. INTRODUCTION
2. EXPERIMENTAL
Recently Holzer et al.[l] studied the Resonance Raman Effect (RRE) in-halogen and interhalogen gases. A strong RRE with the appearance of many overtones of the symmetric vibration was observed, when the exciting line falls into a region of continuous absorption. A theoretical explanation of this behavior has been given by Behringer [2] and Mortensen [3,4]. The latter author found also a similar effect in iodinechloroform solutions. Due to the great difficulty in measuring Raman spectra in strong absorbing media there are not many experimental data available. In this paper we report experimental results on the RRE in water and heavy water solutions of potassium permanganate. The 5145 d ion laser line falls in the strong visible absorption region of the MnO,j ion. Excitations with higher frequency argon laser lines show different results. But this will be reported in a following paper, in which an attempt will be made to correlate the F with the visible absorption band around 5000 A, which shows structure even in the liquid phase. The non-resonance Raman spectra of this highly coloured compound is given by Hendra 151, who used the He-Ne laserline at 6328 A for Raman excitation. These results were confirmed by one of the authorst with excitation by a qua&continuous ruby laser [6].
The RRE spectra were excitzd with an argon ion laser (x 1 W power at 5145 A). The spectra were recorded at room temperature with a Jarrell-Ash 25-100 double monochromator, a thermo-electrically cooled ITT FW-I30 photomultiplier with a dark current of about 0.2 Hz; and photon counting detection. The laser beam was focussed into a single pass quartz cell with anf= 75 mm lens. The laser beam power at the sample was chosen to give maximal Raman signal, without local boiling in the solution. The highly absorbing solutions produced a strong so-called thermal lens effect. The beam was kept as cIose as possible to the sample celt window to minimize self-absorption of the scattered Raman light. The spectra of MnOi were taken in H20 and D20 solutions. The optimum concentration of KMn04 wa6 found experimentally to be = 1.0x 10-3 mole/L MnOz decomposes on exposure to light. Measurements were therefore taken on a large number of freshly made samples which were exposed to the laser beam only during the scanning of Raman lines. Fig. 1 shows for that reason only the spectral ranges, in which Raman lines of the Mn04 vibrations are to be expected. The KMnO4 used was the analytical reagent and the water was double distilled. The purity of the D20 was 99.7%. All measurements were made with tie direction of observation perpendicular to the laser polarization. Depoiarization ratios, as well as
‘# W.Kiefer,
unpublished data. obtained in the iaboratory of Professor J.BrsndmUller, Munich, Gcrmsny.
361
Volume 8. number
cmrv1n2AL PHYSICS GETTERS
5
1
MnQin
Hz0
“20
Band
4060
March 1971
3060
Iot)o~cfn”
2060
I 0
Fig. 1. Resonance Raman spectra of lUnOi in H20 (upper spectrum) and D20 (lower spectrum); Concentration 1.0 Y 10-3 mole/l: Slits 7.5 cm-l; time constant 1 set; 26 urnin; 5146 A excitation. For missing bands at abcut 3500 cm-l (H20) and 2500 cm-l (D20) see text. Frequencies (y). depolarization ratios (a),
i VI
Assignment nv1 +v3
vi v3
‘- .2v1 ..
in H20
““1
2v1+v3.
Ps
in H20
:837.3*0.5
837.0*0.5
co.01
co.01
903*2
909*2
dp < 0.02
dp< 0.02
dp
dp
1745*5
1672.0*1 1742*5
2505.3 * 1
in D20
relative intensities in H20 in D20 1.oo
1.00
0.63 .-
< 0.02
0.52
,2580*10 3337+-2
4Vl 3y +v3 ‘5y
V(cm-l)
in D20
1672.3f l-
v1 f v3
Table 1 and relative intensities of the overtones for the Raman spectrum of MnOi (5145 A excitation)
_:
3415*10 416455
4167*5
-
.
.
-
.. .the;ihtensity.
data which were~orr&ed
onli for
the gpe’ctral sensitivity of the spectrometeri . were d&ived.froq &ea m_eaFrements. .. Fbr. Accurate frequendJi~det~kmic&ions neon .: an~&g&‘disclkge lamps were used s‘imultanaously du&g the’r&brding.~of th& Raman.l&es; . . ,.,‘.. .-..___ ,,_ . I, : : : ‘_:__ I y2;. ’ ‘._., : . ., ,_ ‘. :_ -_
3.. RESULTS
-
.-
k.fi‘g; 1 the RRE-spectra of -04 in I320 (upper @$&urn) and D2Q_ (lower spektrum), _ excited wi& &&.5145 A line .of the akgon ion ;- hei ai’9 tdiom~. The .,redi for fF,equenci& : .’ ‘: -’ ,I. ~ : , .;,. : 1:_.1, : -1. _ I ~ __ .’ : : ” :.--,: -_
Volume 8, number 5
CHEMICALPHYSICSLETTERS
(v), depolarization ratios (&) and relative intensities for the observed Raman lines are listed in table 1. The highly polarized Raman line at 837 cm-1 is assigned to the totally symmetric vibration vi(A1) and the weaker line at 908 cm-l to the IJ~(F.$ vibration in agreement with the assignment of Hendra [5]. The wavenumber for the degenerate stretching mode at 908 cm-l seems to us to be more accurate than Hendra’s value of 921 cm-l. In our case, the frequency was measured, using the analyzer in crossed position to the laser polarization, so there was no overlapping by the highly polarized Raman line of 837 cm-l. From the symmetric stretching mode at 837 cm-l, four overtones could be observed with decreasing intensity. They were all highly polarized (ps C 0.02). In the H20 solution spectra the third .(in the D20 solution the second) overtone could not be recorded , because of the strong water band at 3500 cm-l (at 2500 cm-l for D20). In the region of the first overtone of the H20 solution spectrum there is also an interference with the weaker H20 line s 1656 cm-l. No attempt was therefore made to measure the overtone intensity in this solution. The intensity values in the table are normalized to that for the fundamental. The Flaman line at 908 cm-l was found to be depolarized, as well as the line at 1745 cm-l. Depolarization ratios and frqencies of the sequence 1745, 2580 and 3415 cm-1 support the assignment to the combination tones of VI and u3 listed in table 1. From the measured vibrational frequencies we and nv1 (n=l,Z,... , 5) we could determine wexe for the y vibration. The values obtained are We= 839~ 0.5 cm-l
and we% = 1.0 f 0.2 cm-l
.
These values allow us to derive the dissociaticn energy D, of the MnOi ion into its components
.’ -_
:
-
.
I March 1971
(Ma- +O+O+O+O) potential function: 9 D
z-z
e
4w&
with an assumed
176000 5 35.600 cm-l
MOrEe
type
.
From the values for the heats of formation of MnO& O(gas) and Mn(gas) one finds the heat of atomizatkn of MnOz in solution into Mn(gaz.) + 40 (gas) to be > 18.8 eV OF > 152 000 cm-l. 4. CONCLUSION The high intensity of the ul and v3 fundamentals of the permanganate ion m very dilute water solutions (only very weak bending modes were found in contrast to the non-resonance spectra [5, see also footnote on p.3811 as well as of the overtones TZV and combination tones no + v3, indicate that 1here is a strong RRE in this system. The very low depolarization ratios of the symmetric stretching mode and-of its overtones emphasize this interpretation and make it distinguishable from resonance fluorezcence [7], even though the exciting line (5145 A) falls inside the vibrational levek of an excited that further experiwork is necessary this system, using the lower wavelength lines of au argon laser are in progress. REFERENCES W.Hblzer, W.F.Murphy and H.J.Berxtein. Chem. Phys. 52 (1970) 399. [Zi J. Behringer, Z. Physik 229 (1969) 209. [lj
J.
[3] O.S.Mortensen. Chcm. Phys, Letters 5 (L970) 515. 1410. S.Mortensen. to be oublished. 25j P. J. Hendra. &ectro&im. Acta 24A (L968) 125. -161 . W.Kiefer. Z. Angew. Physik 25 (19681 236: Cbem. Inst-entation. b be published. [ 71 P. Pringsheim, Fluorescence and phosphorescence (Interscience. New York. L949).
383
CHEMICAL PHYSICS LETTERS
RESONANCE WITH
RAMAN
5145
A LASER
EFFECT
L March 1971
IN MnO3
EXCITATION
W. KIEFER and H. J. BERNSTEIN Division
of Chemistry,
National Research Ottawa, Canada
Council of Canada,
Received 4January1971
Resonance Raman scattering of the MnOi ion in Hz0 and D20 solutions is reported. Several overtones of the vI(A~) mode of this ion (Td symmetry) and combination tones nvI(A*) + %(F.$ are observed. From the wavenumbers of the overtones of vl the dissociation energy of the MnOJ ion has been estimated.
1. INTRODUCTION
2. EXPERIMENTAL
Recently Holzer et al.[l] studied the Resonance Raman Effect (RRE) in-halogen and interhalogen gases. A strong RRE with the appearance of many overtones of the symmetric vibration was observed, when the exciting line falls into a region of continuous absorption. A theoretical explanation of this behavior has been given by Behringer [2] and Mortensen [3,4]. The latter author found also a similar effect in iodinechloroform solutions. Due to the great difficulty in measuring Raman spectra in strong absorbing media there are not many experimental data available. In this paper we report experimental results on the RRE in water and heavy water solutions of potassium permanganate. The 5145 d ion laser line falls in the strong visible absorption region of the MnO,j ion. Excitations with higher frequency argon laser lines show different results. But this will be reported in a following paper, in which an attempt will be made to correlate the F with the visible absorption band around 5000 A, which shows structure even in the liquid phase. The non-resonance Raman spectra of this highly coloured compound is given by Hendra 151, who used the He-Ne laserline at 6328 A for Raman excitation. These results were confirmed by one of the authorst with excitation by a qua&continuous ruby laser [6].
The RRE spectra were excitzd with an argon ion laser (x 1 W power at 5145 A). The spectra were recorded at room temperature with a Jarrell-Ash 25-100 double monochromator, a thermo-electrically cooled ITT FW-I30 photomultiplier with a dark current of about 0.2 Hz; and photon counting detection. The laser beam was focussed into a single pass quartz cell with anf= 75 mm lens. The laser beam power at the sample was chosen to give maximal Raman signal, without local boiling in the solution. The highly absorbing solutions produced a strong so-called thermal lens effect. The beam was kept as cIose as possible to the sample celt window to minimize self-absorption of the scattered Raman light. The spectra of MnOi were taken in H20 and D20 solutions. The optimum concentration of KMn04 wa6 found experimentally to be = 1.0x 10-3 mole/L MnOz decomposes on exposure to light. Measurements were therefore taken on a large number of freshly made samples which were exposed to the laser beam only during the scanning of Raman lines. Fig. 1 shows for that reason only the spectral ranges, in which Raman lines of the Mn04 vibrations are to be expected. The KMnO4 used was the analytical reagent and the water was double distilled. The purity of the D20 was 99.7%. All measurements were made with tie direction of observation perpendicular to the laser polarization. Depoiarization ratios, as well as
‘# W.Kiefer,
unpublished data. obtained in the iaboratory of Professor J.BrsndmUller, Munich, Gcrmsny.
361
Volume 8. number
cmrv1n2AL PHYSICS GETTERS
5
1
MnQin
Hz0
“20
Band
4060
March 1971
3060
Iot)o~cfn”
2060
I 0
Fig. 1. Resonance Raman spectra of lUnOi in H20 (upper spectrum) and D20 (lower spectrum); Concentration 1.0 Y 10-3 mole/l: Slits 7.5 cm-l; time constant 1 set; 26 urnin; 5146 A excitation. For missing bands at abcut 3500 cm-l (H20) and 2500 cm-l (D20) see text. Frequencies (y). depolarization ratios (a),
i VI
Assignment nv1 +v3
vi v3
‘- .2v1 ..
in H20
““1
2v1+v3.
Ps
in H20
:837.3*0.5
837.0*0.5
co.01
co.01
903*2
909*2
dp < 0.02
dp< 0.02
dp
dp
1745*5
1672.0*1 1742*5
2505.3 * 1
in D20
relative intensities in H20 in D20 1.oo
1.00
0.63 .-
< 0.02
0.52
,2580*10 3337+-2
4Vl 3y +v3 ‘5y
V(cm-l)
in D20
1672.3f l-
v1 f v3
Table 1 and relative intensities of the overtones for the Raman spectrum of MnOi (5145 A excitation)
_:
3415*10 416455
4167*5
-
.
.
-
.. .the;ihtensity.
data which were~orr&ed
onli for
the gpe’ctral sensitivity of the spectrometeri . were d&ived.froq &ea m_eaFrements. .. Fbr. Accurate frequendJi~det~kmic&ions neon .: an~&g&‘disclkge lamps were used s‘imultanaously du&g the’r&brding.~of th& Raman.l&es; . . ,.,‘.. .-..___ ,,_ . I, : : : ‘_:__ I y2;. ’ ‘._., : . ., ,_ ‘. :_ -_
3.. RESULTS
-
.-
k.fi‘g; 1 the RRE-spectra of -04 in I320 (upper @$&urn) and D2Q_ (lower spektrum), _ excited wi& &&.5145 A line .of the akgon ion ;- hei ai’9 tdiom~. The .,redi for fF,equenci& : .’ ‘: -’ ,I. ~ : , .;,. : 1:_.1, : -1. _ I ~ __ .’ : : ” :.--,: -_
Volume 8, number 5
CHEMICALPHYSICSLETTERS
(v), depolarization ratios (&) and relative intensities for the observed Raman lines are listed in table 1. The highly polarized Raman line at 837 cm-1 is assigned to the totally symmetric vibration vi(A1) and the weaker line at 908 cm-l to the IJ~(F.$ vibration in agreement with the assignment of Hendra [5]. The wavenumber for the degenerate stretching mode at 908 cm-l seems to us to be more accurate than Hendra’s value of 921 cm-l. In our case, the frequency was measured, using the analyzer in crossed position to the laser polarization, so there was no overlapping by the highly polarized Raman line of 837 cm-l. From the symmetric stretching mode at 837 cm-l, four overtones could be observed with decreasing intensity. They were all highly polarized (ps C 0.02). In the H20 solution spectra the third .(in the D20 solution the second) overtone could not be recorded , because of the strong water band at 3500 cm-l (at 2500 cm-l for D20). In the region of the first overtone of the H20 solution spectrum there is also an interference with the weaker H20 line s 1656 cm-l. No attempt was therefore made to measure the overtone intensity in this solution. The intensity values in the table are normalized to that for the fundamental. The Flaman line at 908 cm-l was found to be depolarized, as well as the line at 1745 cm-l. Depolarization ratios and frqencies of the sequence 1745, 2580 and 3415 cm-1 support the assignment to the combination tones of VI and u3 listed in table 1. From the measured vibrational frequencies we and nv1 (n=l,Z,... , 5) we could determine wexe for the y vibration. The values obtained are We= 839~ 0.5 cm-l
and we% = 1.0 f 0.2 cm-l
.
These values allow us to derive the dissociaticn energy D, of the MnOi ion into its components
.’ -_
:
-
.
I March 1971
(Ma- +O+O+O+O) potential function: 9 D
z-z
e
4w&
with an assumed
176000 5 35.600 cm-l
MOrEe
type
.
From the values for the heats of formation of MnO& O(gas) and Mn(gas) one finds the heat of atomizatkn of MnOz in solution into Mn(gaz.) + 40 (gas) to be > 18.8 eV OF > 152 000 cm-l. 4. CONCLUSION The high intensity of the ul and v3 fundamentals of the permanganate ion m very dilute water solutions (only very weak bending modes were found in contrast to the non-resonance spectra [5, see also footnote on p.3811 as well as of the overtones TZV and combination tones no + v3, indicate that 1here is a strong RRE in this system. The very low depolarization ratios of the symmetric stretching mode and-of its overtones emphasize this interpretation and make it distinguishable from resonance fluorezcence [7], even though the exciting line (5145 A) falls inside the vibrational levek of an excited that further experiwork is necessary this system, using the lower wavelength lines of au argon laser are in progress. REFERENCES W.Hblzer, W.F.Murphy and H.J.Berxtein. Chem. Phys. 52 (1970) 399. [Zi J. Behringer, Z. Physik 229 (1969) 209. [lj
J.
[3] O.S.Mortensen. Chcm. Phys, Letters 5 (L970) 515. 1410. S.Mortensen. to be oublished. 25j P. J. Hendra. &ectro&im. Acta 24A (L968) 125. -161 . W.Kiefer. Z. Angew. Physik 25 (19681 236: Cbem. Inst-entation. b be published. [ 71 P. Pringsheim, Fluorescence and phosphorescence (Interscience. New York. L949).
383