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Surface-enhanced Raman scattering of phenazine. Large intensities of overtones and combination bands
CHEMICAL
Volume 121, number 4,s
SURFACE-ENHANCED LARGE INTENSITIES Machiko
TAKAHASHI,
RAMAN
SCATIERING
OF OVERTONES Masatoshi
GOT0
15 November 1985
PHYSICS LETTERS
OF PHENAZINE.
AND COMBINATION and Masatoki
BANDS
IT0
Deporrmenr of Chemisq, Facuhy of Science and Technology, Keio Unive&iry, H(wshi3-11-l. Kohoku-X-w Yokohama 223. Japan
RLwL‘ivcd21 May 1985
SERS spectra of phenazine adsorbed on a silver electrode are observed. They are characterized by unusually large intensity and the clear appearance of cwertones and combination bands exactly at the expected kquencies. These phenomena are explained by the CT mechanism.
of these bands in SERS spectra has not been reported
1. Introduction
[51In recent years, the importance of the charge transfer (CT) mechanism in surfaceenhanced Raman scattering (SERS) has been emphasizes and many experimental results and theoretical investigations have been reported. Proposed CT mechanisms are broadly divided into three types. One is resonance Raman scattering which involves the CT level as an interrnediate state of Raman scattering [ 1,2]_ In this case, the CT level is constructed by an unoccupied molecular orbital (MO) of the adsorbate, and electron transfer from the metal to this unoccupied MO is considered. The second one is the many steps scattering mechanism, in which absorption and emission processes are treated as separate stages [3.4]. In the third type, charge transfer in the ground state, resulting in vibronit coupling
of the ground
state
to another
state,
is
The existence of a new electronic transition state has been shown by EELS of adsorbed pyridine on an Ag film [6], which was assigned to be a charge transfer transition of the electron from the metal to the adsorbate- The relation between Baman excitation frequency and applied voltage which gives the largest SERS intensity also shows the importance ofthe CTmechanism [4,7] _Meanwhile,it was insisted that if the resonance Raman effect occurred by an appropriate excitation wavelength, overtones and combination bands could be observed, but the appearance
considered
458
[5].
In this paper, we report the SERS of phenazine adsorbed on an Ag electrode surface, which indicates the clear existence of overtones and combination bands. The importance of the CT mechanism in SERS of adsorbed phenazine is emphasized_
2. Experimental The solutions were prepared with distilled water and commercially available phenazine, methanol, and HCl without further purification. The supporting electrolyte was IiCl for the methanol solution and KCl for the aqueous solution. The pH of the aqueous solution was adjusted by adding HCl, and the total concentration of Cl- ion in the solution was 0.1 mole/P. An Ag working electrode was polished with alumina and washed with distilled water. The reference electrode was Ag/AgCl. The detection systems for Raman spectra excited by an Ar+ laser were the same as those reported previously 183. When a Coherent model CR-599 dye laser (DCM; 647 nm) was used, Raman scattering was detected by an RCA C31034 photomultiplier connected to a JASCO CT-1000D monochromator and counted by a PAR model 1112 photon counter.
0 009-2614/85/$ (North-Holland
03.30 0 Elsevier Science Publishers B.V. Physics Publishing Division)
Volume 121, nknber 4,s
CHEMICAL PHYSICS LETTERS
3. Results
Phenazine adsorbed on an Ag electrode exhibited a remarkably strong SERS spectrum in methanol solution. The enhanced spectrum appeared immediately after an ORC treatment, but much of the intensity disappeared rapidly. Fig. la shows the timedependent SERS intensity of the Raman line at 1408 cm-l which corresponds to a totally symmetric vibration of phenazine molecule at 1405 cm-l_ The half life period, the time needed for intensity reduction to half maximum, was less than 1 min. It varied with the number of the times of ORC. As ORC was repeated, the intensity maximum decreased and the half life period increased. Fig. lb shows the SERS spectrum of phenazine in the region about 300 to 2000 cm-l at V app = -0.1 V. Because of its rapid increase and decrease in intensity with time, this spectrum was measured immediately after the ORC and the whole spectral region was sweeped within 2 min. Therefore, relative intensities are not correct. But it is clear that the signal of the solvent methanol at 1033 cm-l is very weak in intensity and an enormously large SERS effect occurred in this system. SERS of adsorbed phenazine was stabilized by acidifying the solution. Moreover, in the aqueous solu-
a)
15 November 1985
tion ofpH = 1 to 2, SERS spectrum was fully observed by just immersing an Ag electrode in the solution without applying a potential and ORC treatment _ The SERS spectra observed at -0.1 V with or without ORC treatment were exactky the same as that obtained by the above method, but the absolute intensity was greatly enhanced by connecting an Ag electrode to a potentiostat. Large intensity enhancement without ORC treatment has scarcely been reported for heterocyclic organic molecules in an electrochemical system. The SERS spectrum obtained at VWP = -0.1 V in acidic solution is given in fig. 2d. The spectral feature is almost the same as that obtained in methanol solution except for a clear appearance of a line at 1608 cm-l which indicates protonation of an N atom in the six-membered ring. The dependence of SERS of adsorbed phenazine on applied voltage in acidic solution is given in fig. 2. At vapp = -0.1 V, all intense Raman lines at 409, 606,1167, 1408 and 1571 cm-l are assigned to totally symmetric vibrations of phenazine because of their small frequency changes from those of the bulk spectrum. Spectral features changed remarkably at more negative applied voltages. At Vam = -0.6 V, the totally symmetric vibrations decreased in intensity and it seemed difficult to assign the Raman bands
I
2cm
16CO
Raman
Shift
12co (cm-‘)
Boo
4;
Fig. 1. (a) Time depetient SERS titentity change of the 1408 cm O-1band of adsorbed phenazine in methanol solution of low3 M phenazine. The applied voltage was sweeped fmm -0.1 to +O.ZV and back to -0.1 Vat a rate of 50 mV/s simultaneously with lime. Voltage sweep started at time zem, after 12 s the voltage was hold at Vapp = -0.1 V. (b) SERS spectrum of phenazine on an Ag electrode surface adsorbed from lo5 M methanol solution, taken immediately aft= 3 ORCs.
459
Volume 121,
CHEMICAL PHYSICS LETTERS
number 43
15
November
1985
the applied voltage, which gives the largest SERS intesity, is given in fig. 4a for a typical Raman band at 606 cm-l. Changing the laser excitation wavelength from 514.5 to 647 run caused a shift in the voltage of maximum intensity from +0-l 1 to -0.08 V. On the other hand, the prominent Raman line at 1023 cm-l observed at Vapp = -0.3 and -0.6 V exhibited a completely different behaviour as shown in fig. 4b. Excited at 5145 nm, another maximum is located around -0.8 V. The band could not be observed at any applied voltage when 647 nm excitation Light was used.
4. Discurrion
>T -5
5t
1600
1400
1200
I300
600
600
LCI
Raman shifl(cm-‘1 Fig. 2. Dependence on applied voltage of SERS of phenazine (pH 1.3) of 104 M phenaaine; (a) -0.6 V, fb) -0.3 V. (c) -0.2 Vantl (d) -0.1 V. from acidic aqueous solution
with large intensities at 1023, 1248 and 1597 cm-’ to the normal vibrations of the phenazine molecule. The most noteworthy feature of phenazine SERS is the distinct appearance of overtones and combination bands at Vapp = -0.1 V. In fig. 3, almost all the vibrations except for the nine totally symmetric fundamentals are assignable to overtones and combinations of those species. The band at 1212 cm-l is the overtone of the yls vibration at 606 cm-l, and the 1s the combination of the ug at band at 1355 cm749 cm-1 and the yIo, and so on. These frequencies completely agreed with the calculated ones and the deviations of observed frequencies from calculated ones for other overtones and combination bands were also either zero or less than 10 cm-l _The observed frequencies are summarized in table 1 with their assignments. The relation b.etween excitation wavelength and 460
A shift in the resonance voltage caused by an energy change in the Raman excitation wavelengths is proof of the importance of the CT mechanism [4,7] for the SERS of phenazine. In the case of photon-induced charge transfer from metal to molecule, the applied voltage should shift to more negative potentials to supply the energy loss with decreasing excitation frequency_ Phenazine on an Ag electrode showed this bheavior. However, the voltage difference, 0.19 V, seems too small to supply the energy loss of excitation. In an Ag-KCl system, oxidation of the Ag electrode begins at about +O.OS V. Moreover, the surface concentration of organic molecules decreases on a positively charged surface. These facts combined with the shift of the related molecular energy level by
changing the applied voltage resulted in a decrease in the SERS intensity excited at 5 14.5 run above +0-l 1 V, and the true resonance voltage must be more positive. A main molecular orbital which plays a role as an accepter CT level is thought to be the lowest unoccupied a* of phenazine. Therefore, protonation of an N atom lone pair in acidic solution affects very little the SERS of phenaaine, and the SERS spectra in methanol and acidic aqueous solution resemble each other. There have been several theories of CT resonance. The single step resonance Raman process has not generally been accepted so far. One of the reasons [5] is the absence of overtones and combination bands in spite of the resonance to the CT level. There have been few observations of overtones and combination bands [9] except for the adsorbed dye.which shows surfaceenhanced resonance Raman scattering (SERRS). For
15 November 19B5
CHEMICAL PHYSICS LEl-l-ERS
3600
isal
Ran-tan ShZl
zwo
lwo
loo0
500
(mr?)
Fig. 3. Overtones and combination bands in the SERS spectrum observed at Vapp = -0.1 tionbands.
V. vindicates the overtone and mmbina-
Table 1 Raman frequendes of phenazine adsorbed on an Ag electrode at -0.1 V wrsus SCE from the aqueous solutionof pH 1.3 Observed
Axignment
409
y11
606 629 657 737 749 801 822 965 1020 1149 1167 1212 1292 1355 1408 1478 1503
"10
&lculated
(v4s) (b4) @33) "9 had 2Ull
818
"17 ~EI~~~lo+~ll u9+"11
1015 1158
"7
1212
=I0 V6 v9
+"lo
1355
y5 "4 2-J
1498
Observed
Assignment
1571 1608 1772 1817 1895 1981 2012 2175 2418 2523 2576 2694 2734 2807 2849 2965 3066 3142
y3 W23) Yg+u9oru7+v~O "5 tv11 v4+y11 "3+Yll vs +y10 "3+"10 4hO y7++9+y10
or3y10
Calculated
1769.1774 lB17.1118 1867
1980 2014 2177 2424 2523
2vs
2576 2700 2729 2816
y3+"6 2u40rv3+v6
2849 2956 ~ 2979
%5
+"7
YS*uYg "3+y9+yll
Y1 2y3
OIY2
3142
461
CHEMICAL
Volume 121, number 43
(a)
516Snm
-a?
0
I
(b)
-cu
Applied
-0.6
-0.4
*Q2
0
-02
Voltage (VI
514.5nm
1..
- 0.4
PHYSICS LETTERS
.
-0.6
,
-Q8 Applied
.
~~tas;p”c
VI
-cl2
.
.
0
.
.
*02
Fig. 4. Raman scattering intensities as a function of the applied voltage_ (a) 606 cm-l band excited at 514.5 and 647 nm, (b) 1023 cm-l band excited at 514.5 nm, in the aqueous solution of pH 1.3 and lo4 M phenazine.
phenazine, however, there appeared obviously a lot of overtones and combination bands in the SERS spectrum. Phenazine has no electronic absorption band in the present excitation wavelength region. Therefore there is no possibility of SERF& Furthermore, all the large bands in fig. 3 correspond to the totally symmetric vibrations of phenaiine and the relative intensities are completely different from those of the bulk spectrum. In ordinary Raman scattering, a totally symmetric vibration gains its intensity from a Franck-Condon contribution [lo], i.e. displacement of the equilibrium position of the excited electronic state from the ground. state. Adrian [2] indicated that the same mechanism is important in CT resonance SERS. Under rigorous resonance conditions, the relative intensities of Raman bands change drastically by excitation with light of a different wavelength within the electronic absorption 462
15 November
1985
band, and overtones and combinatioti bands are able to.have compar&le i&e&ties to the fundamental bands. For adsorbed phenazine, rigorous resonance conditions to the C? state may be successfully achieved by using 5 14.5 run laser light. This leads to an unusually large SERS intensity and the appearance of overtone and combination bands of totally symmetric vibrations. The relatively small intensity ofnon-totally symmetric vibrations is explained by the necessity of another higher electronic state to gain Raman scattering intensity from the Herzberg-Teller mechanism [lo]. We can conclude that CT mechanism plays an important role for the SERS of phenazine. On the other hand, another enhancement factor, i.e. SPP. is also very important. We have also found SERS of phenazine adsorbed on Ag sol, and confirmed that the Raman signals of adsorbed phenazine were enhanced by aggregation of Ag sol [I I]. In general, it is very difficult to satisfy simultaneously the condition of SPP excitation and the rigorous resonance condition for each vibrational mode. Adsorbed phenazine exhibits the CT energy level at the frequency of SPP excitation, and the SERS spectral feature is mainly governed by the CTmechanism. Further investigations of the excitation wavelength dependence of SERS should be necessaj to clearify the details. There remain two other interesting but inexplicable phenomena in the SEFG of phenazine. One is the unusually short lifetime but large SERS intensity in methanol solution. If the phenazine molecule has a poor sticking probability to the surface, it may-give a poor SEF&S signal. We confirmed that addition of water in methanol solution stabilized the SERS of phenazine. Therefore, the enormously large solubility of the surface complex in methanol may be the reason. The second is the occurrence of a completely different SERS spectrum at vpp = -0.6 V. The spectrum given in fig. 3a was obtained both in acidic aqueous solution and in a mixed solvent of methanol and water (3: 1). Since phenazine is known to react photochemically with the solvent, some kind of phoeochemical reaction may occur in this electrochemical system. References [l] B.NJ.‘Persson,
Chem. Phys. Letters 82 (1981) l& Ueba, J. Chem. Phys. 73 (1900) 725.
561;
Vo@me
121;number
4;5
CHEMICAL
(21 FJ. Adrian, J. &em. Phys. 77 (1982) 5302. [3] E. Burstein. YJ. Chen, C-Y_ Chen. S. Lundquist and E. Tosatti. Solid State Commun. 29 (1979) 567. [4] A. Otto, J. Billman. J. Eickmans, U. Ertiirk and C. Pettenkofer, Surface Sci. 138 (1984) 319. [5] ME. Lippitsch and F.R. Akenegg. in: Surface studies with lasers. Springer series in chemical physics. Vol. 33, eds. F.R. Aus.%negg, A. Leitner and ME. Lippitsch (Springer, Berlin, 1983) p_ 41. [6] JE. Demuth, K. Christmann and PN. Sanda, Chem. Phys. Letters 76 (1980) 201.
PHYSICS LETTERS
15 November
1985
[7] TE. Furtak and SH. Macomber, Chem. Phys. Letters 95 (1983) 328. [8] hi. Takahashi and M. Ito. Chem. Phys. Letters 103 (1984) 512; M. Takahashi. M. Fujita and M. Ito, Chem. Phys. Letters 109 (1984) 122. [9] B. Pettinger. Chem. Phys. titters 78 (1981) 404. [IO] A.C. Albrecht, J. Chem. Phys. 34 (1961) 1476; J. Tan8 and AS. Albrecht, in: Raman spectroscopy, Vol. 2. ed. H.A. Szymanski (Plenum PresS, New York, 1970) p- 33. [ll] M. Takahashi, M. Fujita and M. Ito. to be published.
463
Volume 121, number 4,s
SURFACE-ENHANCED LARGE INTENSITIES Machiko
TAKAHASHI,
RAMAN
SCATIERING
OF OVERTONES Masatoshi
GOT0
15 November 1985
PHYSICS LETTERS
OF PHENAZINE.
AND COMBINATION and Masatoki
BANDS
IT0
Deporrmenr of Chemisq, Facuhy of Science and Technology, Keio Unive&iry, H(wshi3-11-l. Kohoku-X-w Yokohama 223. Japan
RLwL‘ivcd21 May 1985
SERS spectra of phenazine adsorbed on a silver electrode are observed. They are characterized by unusually large intensity and the clear appearance of cwertones and combination bands exactly at the expected kquencies. These phenomena are explained by the CT mechanism.
of these bands in SERS spectra has not been reported
1. Introduction
[51In recent years, the importance of the charge transfer (CT) mechanism in surfaceenhanced Raman scattering (SERS) has been emphasizes and many experimental results and theoretical investigations have been reported. Proposed CT mechanisms are broadly divided into three types. One is resonance Raman scattering which involves the CT level as an interrnediate state of Raman scattering [ 1,2]_ In this case, the CT level is constructed by an unoccupied molecular orbital (MO) of the adsorbate, and electron transfer from the metal to this unoccupied MO is considered. The second one is the many steps scattering mechanism, in which absorption and emission processes are treated as separate stages [3.4]. In the third type, charge transfer in the ground state, resulting in vibronit coupling
of the ground
state
to another
state,
is
The existence of a new electronic transition state has been shown by EELS of adsorbed pyridine on an Ag film [6], which was assigned to be a charge transfer transition of the electron from the metal to the adsorbate- The relation between Baman excitation frequency and applied voltage which gives the largest SERS intensity also shows the importance ofthe CTmechanism [4,7] _Meanwhile,it was insisted that if the resonance Raman effect occurred by an appropriate excitation wavelength, overtones and combination bands could be observed, but the appearance
considered
458
[5].
In this paper, we report the SERS of phenazine adsorbed on an Ag electrode surface, which indicates the clear existence of overtones and combination bands. The importance of the CT mechanism in SERS of adsorbed phenazine is emphasized_
2. Experimental The solutions were prepared with distilled water and commercially available phenazine, methanol, and HCl without further purification. The supporting electrolyte was IiCl for the methanol solution and KCl for the aqueous solution. The pH of the aqueous solution was adjusted by adding HCl, and the total concentration of Cl- ion in the solution was 0.1 mole/P. An Ag working electrode was polished with alumina and washed with distilled water. The reference electrode was Ag/AgCl. The detection systems for Raman spectra excited by an Ar+ laser were the same as those reported previously 183. When a Coherent model CR-599 dye laser (DCM; 647 nm) was used, Raman scattering was detected by an RCA C31034 photomultiplier connected to a JASCO CT-1000D monochromator and counted by a PAR model 1112 photon counter.
0 009-2614/85/$ (North-Holland
03.30 0 Elsevier Science Publishers B.V. Physics Publishing Division)
Volume 121, nknber 4,s
CHEMICAL PHYSICS LETTERS
3. Results
Phenazine adsorbed on an Ag electrode exhibited a remarkably strong SERS spectrum in methanol solution. The enhanced spectrum appeared immediately after an ORC treatment, but much of the intensity disappeared rapidly. Fig. la shows the timedependent SERS intensity of the Raman line at 1408 cm-l which corresponds to a totally symmetric vibration of phenazine molecule at 1405 cm-l_ The half life period, the time needed for intensity reduction to half maximum, was less than 1 min. It varied with the number of the times of ORC. As ORC was repeated, the intensity maximum decreased and the half life period increased. Fig. lb shows the SERS spectrum of phenazine in the region about 300 to 2000 cm-l at V app = -0.1 V. Because of its rapid increase and decrease in intensity with time, this spectrum was measured immediately after the ORC and the whole spectral region was sweeped within 2 min. Therefore, relative intensities are not correct. But it is clear that the signal of the solvent methanol at 1033 cm-l is very weak in intensity and an enormously large SERS effect occurred in this system. SERS of adsorbed phenazine was stabilized by acidifying the solution. Moreover, in the aqueous solu-
a)
15 November 1985
tion ofpH = 1 to 2, SERS spectrum was fully observed by just immersing an Ag electrode in the solution without applying a potential and ORC treatment _ The SERS spectra observed at -0.1 V with or without ORC treatment were exactky the same as that obtained by the above method, but the absolute intensity was greatly enhanced by connecting an Ag electrode to a potentiostat. Large intensity enhancement without ORC treatment has scarcely been reported for heterocyclic organic molecules in an electrochemical system. The SERS spectrum obtained at VWP = -0.1 V in acidic solution is given in fig. 2d. The spectral feature is almost the same as that obtained in methanol solution except for a clear appearance of a line at 1608 cm-l which indicates protonation of an N atom in the six-membered ring. The dependence of SERS of adsorbed phenazine on applied voltage in acidic solution is given in fig. 2. At vapp = -0.1 V, all intense Raman lines at 409, 606,1167, 1408 and 1571 cm-l are assigned to totally symmetric vibrations of phenazine because of their small frequency changes from those of the bulk spectrum. Spectral features changed remarkably at more negative applied voltages. At Vam = -0.6 V, the totally symmetric vibrations decreased in intensity and it seemed difficult to assign the Raman bands
I
2cm
16CO
Raman
Shift
12co (cm-‘)
Boo
4;
Fig. 1. (a) Time depetient SERS titentity change of the 1408 cm O-1band of adsorbed phenazine in methanol solution of low3 M phenazine. The applied voltage was sweeped fmm -0.1 to +O.ZV and back to -0.1 Vat a rate of 50 mV/s simultaneously with lime. Voltage sweep started at time zem, after 12 s the voltage was hold at Vapp = -0.1 V. (b) SERS spectrum of phenazine on an Ag electrode surface adsorbed from lo5 M methanol solution, taken immediately aft= 3 ORCs.
459
Volume 121,
CHEMICAL PHYSICS LETTERS
number 43
15
November
1985
the applied voltage, which gives the largest SERS intesity, is given in fig. 4a for a typical Raman band at 606 cm-l. Changing the laser excitation wavelength from 514.5 to 647 run caused a shift in the voltage of maximum intensity from +0-l 1 to -0.08 V. On the other hand, the prominent Raman line at 1023 cm-l observed at Vapp = -0.3 and -0.6 V exhibited a completely different behaviour as shown in fig. 4b. Excited at 5145 nm, another maximum is located around -0.8 V. The band could not be observed at any applied voltage when 647 nm excitation Light was used.
4. Discurrion
>T -5
5t
1600
1400
1200
I300
600
600
LCI
Raman shifl(cm-‘1 Fig. 2. Dependence on applied voltage of SERS of phenazine (pH 1.3) of 104 M phenaaine; (a) -0.6 V, fb) -0.3 V. (c) -0.2 Vantl (d) -0.1 V. from acidic aqueous solution
with large intensities at 1023, 1248 and 1597 cm-’ to the normal vibrations of the phenazine molecule. The most noteworthy feature of phenazine SERS is the distinct appearance of overtones and combination bands at Vapp = -0.1 V. In fig. 3, almost all the vibrations except for the nine totally symmetric fundamentals are assignable to overtones and combinations of those species. The band at 1212 cm-l is the overtone of the yls vibration at 606 cm-l, and the 1s the combination of the ug at band at 1355 cm749 cm-1 and the yIo, and so on. These frequencies completely agreed with the calculated ones and the deviations of observed frequencies from calculated ones for other overtones and combination bands were also either zero or less than 10 cm-l _The observed frequencies are summarized in table 1 with their assignments. The relation b.etween excitation wavelength and 460
A shift in the resonance voltage caused by an energy change in the Raman excitation wavelengths is proof of the importance of the CT mechanism [4,7] for the SERS of phenazine. In the case of photon-induced charge transfer from metal to molecule, the applied voltage should shift to more negative potentials to supply the energy loss with decreasing excitation frequency_ Phenazine on an Ag electrode showed this bheavior. However, the voltage difference, 0.19 V, seems too small to supply the energy loss of excitation. In an Ag-KCl system, oxidation of the Ag electrode begins at about +O.OS V. Moreover, the surface concentration of organic molecules decreases on a positively charged surface. These facts combined with the shift of the related molecular energy level by
changing the applied voltage resulted in a decrease in the SERS intensity excited at 5 14.5 run above +0-l 1 V, and the true resonance voltage must be more positive. A main molecular orbital which plays a role as an accepter CT level is thought to be the lowest unoccupied a* of phenazine. Therefore, protonation of an N atom lone pair in acidic solution affects very little the SERS of phenaaine, and the SERS spectra in methanol and acidic aqueous solution resemble each other. There have been several theories of CT resonance. The single step resonance Raman process has not generally been accepted so far. One of the reasons [5] is the absence of overtones and combination bands in spite of the resonance to the CT level. There have been few observations of overtones and combination bands [9] except for the adsorbed dye.which shows surfaceenhanced resonance Raman scattering (SERRS). For
15 November 19B5
CHEMICAL PHYSICS LEl-l-ERS
3600
isal
Ran-tan ShZl
zwo
lwo
loo0
500
(mr?)
Fig. 3. Overtones and combination bands in the SERS spectrum observed at Vapp = -0.1 tionbands.
V. vindicates the overtone and mmbina-
Table 1 Raman frequendes of phenazine adsorbed on an Ag electrode at -0.1 V wrsus SCE from the aqueous solutionof pH 1.3 Observed
Axignment
409
y11
606 629 657 737 749 801 822 965 1020 1149 1167 1212 1292 1355 1408 1478 1503
"10
&lculated
(v4s) (b4) @33) "9 had 2Ull
818
"17 ~EI~~~lo+~ll u9+"11
1015 1158
"7
1212
=I0 V6 v9
+"lo
1355
y5 "4 2-J
1498
Observed
Assignment
1571 1608 1772 1817 1895 1981 2012 2175 2418 2523 2576 2694 2734 2807 2849 2965 3066 3142
y3 W23) Yg+u9oru7+v~O "5 tv11 v4+y11 "3+Yll vs +y10 "3+"10 4hO y7++9+y10
or3y10
Calculated
1769.1774 lB17.1118 1867
1980 2014 2177 2424 2523
2vs
2576 2700 2729 2816
y3+"6 2u40rv3+v6
2849 2956 ~ 2979
%5
+"7
YS*uYg "3+y9+yll
Y1 2y3
OIY2
3142
461
CHEMICAL
Volume 121, number 43
(a)
516Snm
-a?
0
I
(b)
-cu
Applied
-0.6
-0.4
*Q2
0
-02
Voltage (VI
514.5nm
1..
- 0.4
PHYSICS LETTERS
.
-0.6
,
-Q8 Applied
.
~~tas;p”c
VI
-cl2
.
.
0
.
.
*02
Fig. 4. Raman scattering intensities as a function of the applied voltage_ (a) 606 cm-l band excited at 514.5 and 647 nm, (b) 1023 cm-l band excited at 514.5 nm, in the aqueous solution of pH 1.3 and lo4 M phenazine.
phenazine, however, there appeared obviously a lot of overtones and combination bands in the SERS spectrum. Phenazine has no electronic absorption band in the present excitation wavelength region. Therefore there is no possibility of SERF& Furthermore, all the large bands in fig. 3 correspond to the totally symmetric vibrations of phenaiine and the relative intensities are completely different from those of the bulk spectrum. In ordinary Raman scattering, a totally symmetric vibration gains its intensity from a Franck-Condon contribution [lo], i.e. displacement of the equilibrium position of the excited electronic state from the ground. state. Adrian [2] indicated that the same mechanism is important in CT resonance SERS. Under rigorous resonance conditions, the relative intensities of Raman bands change drastically by excitation with light of a different wavelength within the electronic absorption 462
15 November
1985
band, and overtones and combinatioti bands are able to.have compar&le i&e&ties to the fundamental bands. For adsorbed phenazine, rigorous resonance conditions to the C? state may be successfully achieved by using 5 14.5 run laser light. This leads to an unusually large SERS intensity and the appearance of overtone and combination bands of totally symmetric vibrations. The relatively small intensity ofnon-totally symmetric vibrations is explained by the necessity of another higher electronic state to gain Raman scattering intensity from the Herzberg-Teller mechanism [lo]. We can conclude that CT mechanism plays an important role for the SERS of phenazine. On the other hand, another enhancement factor, i.e. SPP. is also very important. We have also found SERS of phenazine adsorbed on Ag sol, and confirmed that the Raman signals of adsorbed phenazine were enhanced by aggregation of Ag sol [I I]. In general, it is very difficult to satisfy simultaneously the condition of SPP excitation and the rigorous resonance condition for each vibrational mode. Adsorbed phenazine exhibits the CT energy level at the frequency of SPP excitation, and the SERS spectral feature is mainly governed by the CTmechanism. Further investigations of the excitation wavelength dependence of SERS should be necessaj to clearify the details. There remain two other interesting but inexplicable phenomena in the SEFG of phenazine. One is the unusually short lifetime but large SERS intensity in methanol solution. If the phenazine molecule has a poor sticking probability to the surface, it may-give a poor SEF&S signal. We confirmed that addition of water in methanol solution stabilized the SERS of phenazine. Therefore, the enormously large solubility of the surface complex in methanol may be the reason. The second is the occurrence of a completely different SERS spectrum at vpp = -0.6 V. The spectrum given in fig. 3a was obtained both in acidic aqueous solution and in a mixed solvent of methanol and water (3: 1). Since phenazine is known to react photochemically with the solvent, some kind of phoeochemical reaction may occur in this electrochemical system. References [l] B.NJ.‘Persson,
Chem. Phys. Letters 82 (1981) l& Ueba, J. Chem. Phys. 73 (1900) 725.
561;
Vo@me
121;number
4;5
CHEMICAL
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PHYSICS LETTERS
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1985
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