Langmuir 1995,11, 711-715
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Surface Charge Dependence of Second Harmonic Generation from a Brass Electrode G. Nagy and D. Roy* Department of Physics, Clarkson University, Potsdam, New York 13699-5820 Received October 7, 1994. Zn Final Form: January 30, 1995@ Optical second harmonic generation from a polycrystallinebrass (63%Cu, 37%Zn) surface is detected with fundamental wavelengths of 1064 and 532 nm. The sample is electrochemicallycontrolled in the absence of faradaic processes. Effects of the solution environment are separately studied in 0.5 M aqueous electrolytes of NaC104 and NazS04. The second harmonic response of the electrode to surface charge variations is probed by combining optical experiments with differential capacitance measurements. The results are compared with those for a polycrystallineCu electrodetreated under similar conditions. From this comparison, it is demonstrated that the surface charge effects for the brass electrode closely follow the behavior of its main constituent Cu, displaying an apparently strong role of bound electron mediated surface optical response. 1. Introduction Optical second harmonic generation (SHG) is a promising in situ probe of solid-liquid interfaces that can be applied to a wide variety of electrochemical systems.l However, a large number of the published electrochemical SHG studies have focused on noble metal surfaces.1,2In comparison, significantly fewer reports are found for the more applied metals, such as those relevant in corrosion r e ~ e a r c h . ~Alloys, -~ despite their particular importance in this latter area, have remained even less explored in SHG experiments.6 In this work, we present SHG results for a polycrystalline a-brass electrode, using a typical commercial alloy species C U ~ . ~ ~ ZLike ~ ~ .its ~ ,metal . counterparts, an alloy surface is also usually charged when it is in contact with an ele~trolyte.~ This surface charge varies with changes in the interfacial voltage that is commonly used to control the electrode. Subsequently, these charge variations can become a governing factor in the SH response of the electrode. Our present work concentrates on these surface charge effects on SHG from brass. Faradaic processes and specific adsorption (SA) of ions are avoided here by using NaC104 and NazSO4 electrolytes and by appropriately choosing the applied voltages. Voltage-dependent surface charge variations are detected with a differential capacitance technique and SHG signals are measured by using fundamental wavelengths of 532 and 1064nm. A polycrystalline Cu electrode is also subjected to similar measurements. The results for the two electrodes are compared to test for possible electronic signatures of Cu in the SHG characteristics of brass.
2. Experimental Section The experimental setup for this work is described elsewhere.8-11 In brief, 7-ns light pulses at 10 Hz and 532 nm or 1064 nm wavelength from a Nd:YAG laser are incident on the sample
* To whom correspondence should be addressed. e-mail:
[email protected]. * Abstract published inAduance ACS Abstracts, March 1,1995. (l)Corn, R. M.; Higgins, D. A. Chem. Rev. 1994,94,107. ( 2 ) Richmond, G.L. Electroanal. Chem. 1991,17,87.
(3) Biwer, B. M.; Pellin, M. J.; Schauer, M. W.; Gruen, D. M. Surf. Interface Anal. 1989,14,635. (4)Biwer, B. M.; Pellin,M. J.;Schauer,M. W.;Gruen, D. M. Langmuir 1988,4,121. ( 5 ) Nagy, G.; Roy, D. Langmuir 1993,9,1868. (6)Joseph, M.;Klenerman, D. J.Electroanal. Chem. 1992,340,301. ( 7 )Kaiser, H. In Corrsion Mechanisms; Mansfeld, F., Ed.; Marcel Dekker: New York, 1987;p 85. (8)Hewitt, T. D.; Gao, R.; Roy, D. Surf. Sci. 1993,291,233. (9)Hewitt, T.D.; Roy, D. Chem. Phys. Lett. 1991,181,407. (10)Nagy, G.; Roy, D. Chem. Phys. Lett. 1993,214,197. (11)Nagy, G.;Roy, D. J. Phys. Chem. 1994,98,6592.
surface at an angle of 45". The reflected SH signal at 266 nm or 532 nm is collected in ap-in,p-outpolarization configuration, detected with a cooled photomultiplier tube, and processed with conventionalelectronics. This SH signal showsits expectedsharp spectral character, as well as a quadratic dependence on the incident light intensity. The brass (1.00 cm diameter)and the Cu (0.95 cm diameter) samples are used in a standard threeelectrode configuration with a R counter electrode and a saturated calomel electrode (SCE)reference. Neutral aqueous electrolytes of 0.5 M NaC104 and 0.5 M NazSO4 (pH 7, thoroughly purged with ultrahigh purity Ar) are used with a voltage range of -1.2 V to -0.8 V for brass and -1.0 V to -0.4 V for Cu. In the solutions chosen, these voltages represent the experimental ranges where faradaic processes and SA of anions can be minimized on brass12J3 and Cu,889 respectively. The cell voltage is scanned with a potentiostat at arate of 2 mV/s between these nonfaradaic limits. The differential capacitance is measured with phase sensitive techniq~es.~J~-l~ The electrochemical and SH data are recorded and stored on a microcomputer.
-
3. Results The cell currents Z (dashed lines) and the differential capcities Cd (solid lines) as functions ofthe applied voltages V for Cu in NaC104 and Na2S04 are shown in parts A and B of Figure 1, respectively. Corresponding results for brass are presented in parts C and D of Figure 1, respectively. The negligible levels of the currents in all four cases indicate the absence of any detectable faradaic processes within the voltage ranges used. The c d data of Figure 1 are related to the relative surface charge densities Aq as14J5
where q and qr are absolute charge densities, the latter determined at a reference voltage V,. When V, is set to the potential of zero charge (PZC) of a n electrode, qr = 0 and Aq = q. For Cu, the PZC has been measured to be -0.96 V and this value can be utilized to obtain q.19 To our knowledge, the PZC for brass is unknown at this time (12)Chenyakov, V. N.;Pchelnikov, A. P.; Losev, V. V. Sou. Electrochem. 1991,27,1457. (13)StevanoviC, J.; Skibina, L. J.;StefanoviC, M.; DespiC, A.;JoviC, V . D. J. Appl. Electrochem. 1992,22,172. (14)Gao, R.; Hewitt, T. D.; Roy, D. J.Phys. Chem. Solids 1993,54, 685. (15)Hupp, J.T.;Larkin, D.; Weaver, M. J. Sruf. Sci. 1983,125,429. (16)JoviC, V. D.; JoviC, B. M.; DespiC, A. R. J. Electroanal. Chem. 1990,288,229. (17)Cooper, I. L.;Harrison, J. A.Electrochim.Acta 1984,29,1165. (18)Nagy, G. Ph.D. Thesis, Clarkson University (in progress). (19)Henning, H.; Batrakov, V.Electrochiniya 1979,15, 1833.
0743-7463/95/2411-0711$09.00/00 1995 American Chemical Society
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712 Langmuir, Vol. 11, No. 3, 1995
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V (vs. SCE) Figure 1. Differential capacity cd (solid lines) and dc cell current I (dashed lines) as functions of applied voltage V for polycrystalline Cu (in A and B) and Cuo.63Zno.37brass (in C and D) electrodes. The electrolyte is 0.5 M NaC104 in parts A and C and 0.5 M Na2S04 in parts B and D. The voltage is scanned at a rate of 2 mV/s in the positive potential direction for both cases. Only nonfaradaic voltage ranges, which are different for the two electrodes, are explored in these experiments.
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V (vs. SCE) Figure 2. Voltage dependencies of the SH signal SP and the relative surface charge density Aq for Cu in 0.5 M NaC104 (in A) and 0.5 M NazSOl (in B). The dashed lines represent Aq and the dotted and solid lines represent Sn produced by incident beams of 1064 nm (hw = 1.17 eV) and 532 nm (hw = 2.34 eV) wavelength, respectively. All other experimental conditions in A and B are the same as in parts A and B of Figure 1, respectively. The SH intensity is normalized with respect to its value at a reference voltage of -0.96 V (PZC of Cu). The Aq plots of Figure 2 are obtained by using eq 1 with the c d data of parts A and B of Figure 1, respectively.
K / l
and is difficult to measure with simple Cd techniques.18,20 In this work, we discuss our experimental results in the general framework of eq 1. In parts A and B of Figure 2, we plot Aq (dashed lines) for Cu in NaC104 and Na2S04, respectively, using eq 1 with the corresponding Cd data from parts A and B of Figure 1. In Figure 2, we also plot the SH signals SQ (Q - 2w) generated by fundamental wavelengths of 532 nm (dotted lines, hw = 1.17 eV) and 1064 nm (solids lines, hw = 2.34 eV) for Cu. The relative charge densities (dashed lines) and the SH intensities (dotted lines for ho = 1.17 eV and solid lines for h o = 2.34 eV) for brass are shown in Figure 3A (NaC104)and Figure 3B (Na2S04). In both Figures 2 and 3, we choose VI = -0.96 V to obtain Aq. With this VI, for Cu, Aq represent the absolute charge density of the surface, while for brass, A q represents the deviation of the charge density from its (possibly nonzero) value a t -0.96 V. The SQdata of Figures 2 and 3 are typical of single voltage scans and their variations with V are reproducible with respect to both direction and number of successive scans. In Figure 2, for Cu with hw = 1.17 eV, we observe a correlation between q and SQ through their similar voltage responses. We also observe similar, although less drastic, correlations between q(V) and SQ(V) for hw = 2.34 eV, on Cu (Figure 2) and on brass (Figure 3). There is, however, no detectable similarity in the behaviors of q(V) and SQ(V)with hw = 1.17 eV on brass in Figure 3. By comparison of the data presented in A and B of Figures 2 and 3, we also find that these behaviors of Cu and brass are largely preserved in going from the NaC104 to the Na2S04 environment. For further clarifications of the surface charge effects seen in Figures 2 and 3, it is necessary to examine the
Figure 3. Voltage dependencies of Sn and Aq for brass in 0.5 M NaC104 (in A) and 0.5 M Na2SO4 (in B). The dashed lines represent Aq and the dotted and solid lines represent Sn for 1.17- and 2.34-eV incident photons, respectively. All other experimental conditions in A and B are the same as in parts C and D of Figure 1, respectively. The Aq plots of Figure 3 are obtained by using eq 1 with the c d data of parts C and D of Figure 1, respectively. As in Figure 2, the SH intensity here is also normalized with respect t o its value at -0.96 V.
(20)Pchelnikov,A. P.; Sitnikov, A. D.; Marshakov, I. K.; Losev, V. V. Electrochim. Acta 1981,26, 591.
explicit forms of the SQ(V)-Aq(V) relationships implied in these figures. To address this issue, we refer to our
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Langmuir, Vol. 11, NO.3, 1995 713
previous work where we analyzed the surface charge dependence of SQfrom polycrystalline C U . ~In that work, we found that for ho = 1.17 eV, the experimental SH signal for Cu followed the relationship
S&)
1l(A -
(2)
where A is a constant such that [AI > Iq I. Previously, we normalized this expression with respect to the PZC of Cu to obtain
(3)
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W
In the present case, however, while consideringthe general form of eq 1, we normalize eq 2 as follows
cnc
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(4) where B A - qr. When the PZC is known, we can use Vr = PZC with qr = 0 in eq 1; then B =A, and eq 4 reduces to eq 3. In the following, for each electrolyte of our present studies, we examine eq 4 for four cases: (I) Cu a t h a = 1.17 eV; (11)Cu a t hw = 2.34 eV; (111)brass a t hw = 1.17 eV; (IV)brass a t hw = 2.34 eV. In parts A and B of Figures 4, we show how eq 4 (solid line) fits with the experimental S&) data (open circles) for case I. For these fits, we use V, = PZC of Cu = -0.96 V, qr = 0, B = A = 47 pC/cm2in Figure 4A, and B = A = 35 pC/cm2in Figure 4B. These values are similar to those found in our earlier work on C U . ~Next we examine if such phenomenological fits to eq 4 can be obtained in the remaining three cases of our present study. Experimental SH data for cases 11,111, and IV are shown with the solid circles in Figure 4A,B, open circles in Figure 4C,D, and solid circles in Figure 4C,D, respectively. In cases 11and IV,we do get reasonable fits as indicated by the solid lines. For these latter fits, we use: B = A = 162 pC/cm2, B = A = 97 pC/cm2,B = 66 pC/cm2,and B = 95 pC/cm2, in parts A-D of Figures 4, respectively. However, no appropriate values of the parameter B are found with which eq 4 can be reasonably fitted to the solid circles for case I11 in Figure 4C,D. Thus for both NaC104 and NazSO4 electrolytes, cases I, 11, and IVagree with eq 4, while case I11 exhibits a deviation from this formulation. 4. Discussion As indicated by Figure 4, the same type of mechanism (represented by eq 4) seems to be responsible for the observed SQ- q behaviors for Cu a t both incident photon energies used, as well as for brass at the higher photon energy. Moreover, it is not impossible that with a proper explanation, the relatively passive behavior of S&) for brass with the 1.17 eV incident photons can also fit in with the same scenario. In order to test these possibilities, we briefly review the findings of our previous SHG studies of Cu with hw = 1.17 eV.8 There, SHG from Cu is found to be predominantly bound electron-supported, with a n interband transition between the d and Fermi levels of Cu activated by the SH photons ( h a = 2.34 eV).21 This predominance of the bound electron effects is viewed as a result of a n efficient quenching of the free electron component of S Q ~by ~the, oxygen ~ ~ adsorbates expected (21) Tom, H.W. K.; Aumiller, G. D. Phys. Rev. B 1986, 15,8818. (22) Bradley, R. A.;Friedrich, K.A,; Wong, E. K. L.;Richmond, G. L. J.Electroanal. Chem. 1991, 309, 319. (23) Rudnick, J.; Stern, E. A. Phys. Rev. B 1971, 4, 4272.
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Figure 4. Correlation between the normalized SH signal and the relative surface charge density on Cu (in parts A and B) and brass (in parts C and D). The electrolyte is 0.5 M NaC104 in parts A and C, and 0.5 M NazSO4 in parts B and D. The open and solid circles represent data collected with 1.17-and 2.34eV incident photons, respectively. For each electrode and photon energy, these data points are obtained by averaging several Sn(V) plots such as those shown in Figures 2 and 3.The solid lines here represent data fits with eq 4. No reasonable fit with eq 4 could be obtained for 1.17eV incident photons on brass.
on C U . Furthermore, ~ ~ ~ the physical phenomenon associated with eq 4 is interpreted as a voltage (charge) induced Stark shifi of the threshold of the interband t r a n s i t i o n ~ . ~This J~,~ latter ~ effect is considered by writing the bound electron component of the second-order surface susceptibility of Cu as27928
(uiI' $2'
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l'jl v>(uk'l I4
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WJ+Jti2(s2 ww,)(o - ouu) where u , u , and w denote the initial and two successive intermediate states, respectively, of a three-step SHG process; ri, rj, and rk indicate Cartesian components of the dipole operator for the quantum transitions involved. Here, surface charge effects on x(2)are considered in terms of the Stark shifted values of hwwu and/or hwuu. In the following, we examine the different cases of o w present (24) Materlik, G.;Schmiih, M.; Zegenhagen, J.; Uelhoff, W. Ber. Bunsen-Ges. Phys. Chem. 1987,91, 291. (25) Vilche, J. R.;Juttner, K. Electrochim. Acta 1987, 32, 1567. (26) Katz, R.; Kolb, D. M. 2.Phys. Chem. (Munich) 1978, 112,69. (27) Urbach, L.E.;Percival, K. L.;Hicks,J. M.; Plummer,E. W.; Dai, H.-L. Phys. Rev. B 1992, 45, 3769. (28) Jiang, M. Y.;Pajer, G.; Burnstein, E. Su$. Sci. 1991,242,306.
Letters
714 Langmuir, Vol. 11, No. 3, 1995
numerator of this equation remain relatively insensitive to variations in q. With these considerations, eq 5 takes the form
4
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Figure 5. Schematic energy level diagrams, showing possible mechanisms for bound electron supported SHG from Cu and brass. The Fermi level is denoted as EF and the energy scale is normalized with respect to EF for Cu. For each electrode, three-step SHG transitions involving the d and p bands are considered in (a) and (b)with incident photon energies of 1.17 and 2.34 eV, respectively.
work in this formulation of eq 5. To facilitate this discussion, we use the schematic energy level diagrams for Cu and brass surfaces shown in Figure 5. These simple diagrams are constructed by following refs 28 and 29 for Cu, and refs 30-32 for brass. In both cases, noting the polycrystalline makeups of our samples, we consider only the bulklike states and neglect the details of band structures, as well as possible roles of intrinsic surface states. With hw = 1.17 eV, the SHG steps for Cu (case I) are considered to involve8,28(1) a n intraband transition from a p-band electron below the Fermi level to an empty p-state just above the Fermi level, (2) a n interband transition of a d-band electron to the vacated state in the p-band below the Fermi level, and (3) the relaxation of the excited electron above the Fermi level to the vacated state in the d-band. These steps are indicated in (a) of the energy diagram for Cu in Figure 5. With these transitions, we have in the denominator of eq 4: oUv wpprand o,, 5 wp'd, where the prime refers to the active p-state above the Fermi level. While the states in different energy bands show relative Stark shifts, all states within a given band are expected to shift a t comparable rates.26 Thus, a charge induced Stark effect would not affect wppr(which involves only the p-band),but it would change the d-EF transition threshold dependent U p % This stark shift in Wp'd has been calculated to be I A o p , d l = (G1/R)q.8,14Here G1 is a "screening"parameter related to the decay of the dc electric field inside the metal and the unperturbed wavefunctions ofthe transition-active states. This parameter represents the strength of electrostatic screening experienced by the single electron states participating in a resonant transition.14 In this formulation, the nearly resonant denominator term [Q - ( o p ' d f AWp'd)] in eq 5 becomes q dependent. The nonresonant denominator term, as well as the (29) Hummel, R. E. Electronic Properties of Materials; SpringerVerlag: New York, 1985; p 167. (30) Bansil, A.; Ehrenreich, H.; Schwartz, L.; Watson, R. E. Phys. Rev. B 1974,9,445. (31) Staines, M. Phys. Rev. B 1981,24,7143. (32) Sasovskaya, I. I.; Korabel, V. P.Phys. Status Solidi B 1986,134, 621.
where C1 is a constant containing the matrix elements of the SH transitions. Since SQ \x(2)12, a comparison of eqs 3 and 6 yields A = [h(Q - o p , d ) / G 1 l . In this treatment of the empirical parameter A, the earlier mentioned oxygen adsorbates on Cu are assumed to participate in the Stark effect only by altering the value ofq.8 This Stark shifting of SHG suggests a possible physical mechanism for the observed bihavior o-fthe open circle data of Figure 4A,B. As expected in this description, the values ofA are similar in the two electrolytes. Recently, Petrocelli et al. have reported wavelength-dependent SHG results for Cu that are consistent with the above described scenario.33 These authors also observe that SHG from Cu is dominated by bound electron effects involving interband transitions. In addition, they find that oxygen adsorption on Cu reduces the overall level of SQ (by quenching the free electron polarization) but does not have significant effects on the spectral profile of interband transitions for SHG. By noting this consistency between our proposed mechanism for Stark affected SHG and the wavelength dependent SH data of Petrocelli et al., next we examine if such a mechanism is possible for the solid circle data in Figure 4A,B. With the 2.34-eV photons incident on Cu (case 111,the three-step SHG p r o c e ~ s ~ may ' , ~ ~involve , ~ ~ (1)an interband transition of a n electron a t the top of the d-band to a n empty state just above& (mu" = wdp'), (2)further excitation of this electron to another empty statep" higher up in the p-band, and (3) the relaxation of this electron back into the d-band (owu = 0 p " d ) . These transitions are shown in (b) for Cu in Figure 5 . Here, only step 1 involves the d - EFthreshold and is expected to be Stark affected. This effect would cause a charge dependent shift in Wdp'. However, the single electron d- and p'-levels involved in this transition can be different from those associated with eq 6. Therefore, ifwe assume the earlier mentioned linear q dependencey of Stark energies, a different screening parameter Gz should be considered in the presence case. With this consideration, the Stark shift of cudp' for 2.34 eV incident photons can be taken as IAwdp,I (Gz/h)q. For this case, eq 5 takes the form
(7) where C2 contains the relevant transition matrix elements and the constant A in eq 3 now becomes equal to [h(o wdp,)/G2].In terms ofthis definition,A should be different for SH transitions in Cu with 1.17 and 2.34 eV incident photons-a prediction consistent with the different values of A found for these two photon energies. According to the above discussion, the Stark shift mechanism provides a reasonable explanation for the observed S&) from Cu for both incident photon energies employed in this work. Next, we investigate whether the brass data of Figure 4C,D can be explained with this same mechanism. The predominantly Cu sites of the brass surface are expected to adsorb residual oxygen from the e l e ~ t r o l y t e . ~ As ~ , ~in6the case of pure Cu, this should lead (33) Petrocelli, G.;Martellucci, S.; Francini, R. Appl. Phys. A 1993, 56,263.
Letters to a significant quenching of the free electron component of SHG from brass. In this view, the SH signal from brass should also be dominated by bound electron effects and should be addressed in the framework ofallowedinterband transitions a t the brass surface. As shown in Figure 5, the band structure for Cuo.63Zno.37a-brass is similar to that of Cu, with the following additional features: (i)the Fermi level is shifted up from its value in pure Cu by -1.3 eV;30,34(ii) the top of the Cu d-band shifts downward by -0.2 eV;31(iii)the bottom of the Cu p-band shifts up by -0.6 eV and a t the same time, the entire p-band narrows by -1.0 eV,30-32(iv)the Zn d-band appears -10.5 eV below the Fermi level of pure Cu29332 and therefore cannot affect any interband transitions considered here. The possible SHG transitions are shown in Figure 5. Since EFin brass is about 3.5 eV above the top of the Cu d-band, the three-step SHG process which involves two successive 1.17 eV excitations (case 111) cannot include any d-EF transitions. Hence, changing q and with it the d-EF energy threshold, will not result in a tuning/detuning effect on SHG. This explains the lack of correlation between q and SQthat we observe in the case of the 1.17 eV incident photons in Figure 4C,D. When, however, the three-step transition involves two 2.34-eV excitations (case IV),the SHG process does involve a n interband transition that can be influenced by a Stark shifted d-EF threshold. In that case, the SHG process may be comprised of (1)an intrabandp-p' transition, (2)an interband d-p transition, and (3) a subsequent p'-d relaxation. The last step of this SHG process is associated with the d-EF threshold and hence, should be sensitive to Stark effects in a way similar to the process on Cu found with the 1.17 eV incident photons. In view of our earlier discussion, we assume IAop,dl x (Gd?i)q,where G3 is a n appropriate screening parameter for the p'-d transition involved here. With this assumption, the fitting parameter B in eq 4 represents the quantity B = A - qr = [h(Q- op,d)/G31 - qr. Clearly, this fitting parameter would be different from those found for Cu. At the same time, in the absence of SA, the two electrolytes used in the same concentrations should not have major effects on these vlaues of B. In other words, B found for the two solutions should have comparable orders of magnitudes. These predictions are consistent with the earlier mentioned fitting parameters found for the solid circles in Figure 4. (34)Rubim, J. C . Chen. Phys. Lett. 1990,167,209.
Langmuir, Vol. 11, No. 3, 1995 715 Thus, for both Cu and brass electrodes, the charge dependence of SHG can be explained in terms of a Stark shifted d-EF threshold whenever the experimental photons allow for such an interband transition. This proposed mechanism allows for a single theoretical framework to treat the SHG results for Cu and brass for two different wavelengths. A quantitative analysis of Stark effects would require quantum mechanical calculations of the Gi parameters. Previously, it was estimated that G1 5 meV C ~ ~ / ~ With C . this ~ J value, ~ A 38 pC/cm2, which is comparable to the values of A found here for the open circle data in Figure 4A,B. However, no such estimates for GZand G3 are available a t this time. In addition, the quantity q, for brass is also unknown. Hence, it is difficult to test how the actual values of GZand G3 obtained from the considerations of the Stark effect compare with the fitting parameters of Figure 4C,D. Further work is needed for such quantitative tests. In this regard, it would be particularly useful to study single crystal electrodes with well-characterized symmetry points and surface states. For such surfaces, rigorous quantum mechanical treatments can be applied to calculate the q dependent surface field and the Aw terms in a self-consistent manner.35In addition, experiments involvingcontinuously tunable light sources would be necessary to compare these calculated results with the observed S&p) data. We hope that our present report will invite such SHG analyses of brass electrodes.
-
-
5. Conclusions In this report, we presented results for SHG from a brass electrode and focused on the surface charge dependent behavior of the optical signal. We have shown that this behavior of brass is similar to that of Cu and in both cases, the results can be explained in terms of bound electron supported surface SHG. The results of this work also indicate that with further theoretical and experimental efforts, it should be possible to develop SHG techniques for surface analyses of brass, and probably of other alloy electrodes as well.
Acknowledgment. We thank Mr. M. Love11 for technical assistance and the School of Science of Clarkson University for financial support for this work. LA940784S (35) Aers, G. C.; Inglesfield, J. E. Surf. Sci. 1989,217,367.
