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decays from semiconductors, this practice must be avoided. These results also bear on the use of hqh injection radiative decay profiles in the determination of interfacial charge-transfer kinetics. The high injection condition is often employed in order to minimize effects due to space charge fields under open circuit conditions. According to our results, such studies will tend to underestimate surface trap state populations due to the saturation. We note that measurements of the STV performed on clean GaAs ~urfaces”J~2~ compare most favorably with the highest power results presented here. The lower power results indicate clearly, however, that high power STV’s cannot be indiscriminately used as data for analysis of kinetics of charge transfer under low photon fluxes. This caution applies most notably to the analysis of semiconductor photoelectrcchemical systems under solar irradiation. Conclusions The luminescence decay of n-GaAs held at the flat band potential has been measured as a function of laser power. The nonexponential decays observed can be simulated by assuming a time dependent STV, which is justified by recognizing that the activation barrier to trapping will change as the low barrier traps are filled. The STV’s at t = 0 that were determined from the decays were also found to be dependent on laser power. This effect has been attributed to trap saturation occurring within the duration of the laser pulse. A similar power dependence has been observed in epitaxially grown GaAs, but at excitation levels several orders of magnitude greater than those used here, and has been attributed to saturation of bulk trap states.21 This illustrates that the excitation level necessary for saturation depends on the number of trap states available. For the study of surface trapping in high quality epitaxially grown materials, this implies that saturation of surface traps should occur at very low excitation levels. Effects due to saturation of surface traps were observed at all three laser powers used in our experiment. This finding implies that the excitation power is a critical parameter in comparing the results of various experiments. Finally, the observation of saturation effects indicate that the surface minority trapping is fast compared with processes which remove minority carriers from the traps. Acknowledgment. This work was supported by DOE Grant DEF G06-86ER45273. Registry No. GaAs, 1303-00-0; Na,S, 1313-82-2.
References and Notes (1) (a) Chang, K. C.; Heller, A,; Schwartz, B.; Menzes, S.; Miller, B. Science 1977,196, 1097. (b) Nozik, A. J. Annu. Rev. Phys. Chem. 1978,29, 189. (2) (a) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J . Am. Chem. SOC.1977, 99, 2839. (b) Ellis, A. B.; Bolts, J. M.; Kaiser, S. W.; Wrighton, M. S. J . Am. Chem. Soc. 1977, 99, 2848. (c) Gerischer, H. J . EIectroanaL Chem. 1983, 150, 553. (d) Lewis, N. S. J . Electrochem. Soc. 1984, 131,2496. (e) Rosenbluth, M. L.; Lewis, N. S. J . Phys. Chem. 1989, 93, 3755. (3) (a) Parkinson, B. A.; Heller, A.; Miller, B. Appl. Phys. Lett. 1978,33, 521. (b) Harzion, Z.; Huppert, D.; Gottesfeld, S.; Croitom, N. J. Electroanal. Chem. 1983, 150, 571. (c) Evenor, M.; Huppert. D.; Gottcsfeld, S. J. Electrochem. Soc. 1986, 133, 296. (d) Gmitter, J. J.; Yablonovitch, E.; Heller, A. J . Electrochem. Soc. 1988, 135,2391. (e) Benjamin, D.; Huppert, D. J . Phys. Chem. 1988,92,4676. (f)Bcssler-Podorowski,P.; Huppert, D.; Rosenwaks, y.;Shapira, Y. J . Phys. Chem. 1991, 95, 4370. (4) This quantity is usually referred to as the surface recombination ve-
locity. However, optical experiments at moderate excitation powers only measure the loss of minority carriers since the trapping of such carriers at the surface will result in nonradiative decay, regardless of the kinetics which govern the subsequent equilibration of the occupied surface traps. Therefore the terminology used here is more appropriate to describe the surface nonradiative decay process which we observe. (5) Kasinski, J. J.; Gomez-John, L. A.; Faran, K. J.; Gracewski, S. M.; Miller, R. J. D. J . Chem. Phys. 1989, 90, 1253. (6) Gomez-Jahn, L. A.; Min, L.; Miller, R. J. D. Mol. Cryst. Liq. Cryst. 1991, 194, 181. (7) OConnor, D. V.; Phillips, D. Time-Correlated Single Photon Counting Academic Press: London, 1984. ( 8 ) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes: Plenum Press: New York. 1980. (9) Smandek, B.; Chmiel, G.; Gerischer, H.ber. Bunsen-Ges. Phys. Chem. 1989. 93. - .-., . - , -1094 -. ..
(IO) Pankove, J. I. Optical Processes in Semiconductors; Dover: New York, 1971. (1 1) ’t Hooft, G. W.; van Opdorp, C. J . Appl. Phys. 1986,60, 1065 and references therein. (12) Wilson, T.; Pester, P. D. J . Appl. Phys. 1988, 63, 871. (13) Yablonovitch, E.; Sandroff, C. J.; Bhatt, R.; Gmitter, T. Appl. Phys. Lett. 1987, 51, 439. (14) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: Orlando, FL, 1985. (15) Archer, M. D.; Bolton, J. R. J . Phys. Chem. 1990, 94, 8028. (16) Sze, S. M. Physics oJSemiconductor Devices, 2nd ed.; Wiley: New York, 1981. (17) Kauffman, J. F.; Richmond, G. L. Appl. Phys. Lett. 1991.59, 561. (18) Kauffman, J. F.; Richmond, G. L. Unpublished results. (19) Beck, S. M.; Wessel, J. E . Appl. Phys. Lett. 1987, 50, 149. (20) Min, L.; Miller, R. J. D. Chem. Phys. Lett. 1989, 163, 55. (21) Fouquet, J. E.; Burnham, R. D. IEEE J . Quantum Electron. 1986, QE-22, 1799.
Power Dependent Effects in Photoluminescence vs Voltage Scans of GaAs/Electrolyte Junctlons Uslng Plcosecond Pulse Excitation J. F. Kauffman,t B. A. Balko, and G.L.Richmond* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403 (Received: January 24, 1992; In Final Form: April 9, 1992)
Photoluminescence from n-GaAs has been measured as a function of applied voltage under excitation with a picosecond laser at three excitation power levels. A large increase in t h e photoluminescence intensity at the flat band potential is observed as the excitation power is increased. Analysis of the data with the modified dead layer model shows that the surface minority trapping velocity decreases as the laser power is increased. We attribute this to a saturation of surface minority carrier traps resulting from picosecond pulse excitation and compare the results with a companion study in which surface minority trapping velocities at the flat band potential are determined from luminescence decay profiles.
Introduction Over the past few years there has been a growing interest in the use of picosecond laers to study carrier dynamics at the semiconductor/liquid interface.’ In the previous paper we observed Permanent address: Department of Chemistry, University of Missouri, Columbia. MO 6521 1.
saturation effects in picosecond luminescence decays which can influence the interpretation of experimental results.* Specifically, we show that surface minority carrier traps in the n-GaAs/Na2S photoelectrochemical system can be saturated by the use of picosecond laser pulses of moderate power. This saturation results in a reduction of the surface minority trapping velocity (STV) with increasing excitation power, as evidenced by longer lu-
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GaAs/Electrolyte Photoluminescence vs Voltage Scans
l
o
VI
c C
z
o 0 Low Power Picosecond
minescence intensity at the flat band potential assuming infinite STV,a is the absorption coefficient for the semiconductor electrode at the excitation wavelength, and W is the width of the depletion layer. The width of the depletion layer is given by
3
Excitation
w = (2eeo(Arr)/qNd)'/'
0 0 P)
I \\
C
Yh
I
0
.-E 0
C
3 ' 00 50 -2.500 -2.000 -1.500 -1.000 -0.500 0.000 Applied Potential
0.500
Figure 1. Luminescence intensity versus applied potential. The lower solid curve was obtained using a cw HeNe laser for illumination (0.280-mW average power). The bpper solid curve was the result of using low power picosecond pulses (3 X 10" (photons/cm*)/pulse; 0.017mW average power). The open circles and triangles show the intensity expected in the depletion region using the dead layer model for the 633- and 532-nm experiments, respectively.
minescence decay profiles. In the present work we wish to report the influence of saturation effects on photoluminescenceintensity measured as a function of applied voltage. Analysis of the data with the modified dead layer model3" confirms that the STV decreases with increasing photon flux under picosecond pulse illumination. The results of this study are consistent with STV's recovered from the luminescence decay profiles.
Experimental Section All experiments were performed on Si-doped n-GaAs (100) - ~ Spehaving a carrier concentration of 2 X 10'' ~ m (Crystal cialties). Sample preparation and electrolyte composition are identical to that in ref 2. Photoluminescence was measured as in ref 2 with the following alterations. Luminescence intensity was monitored by gated photon counting using a 100-nsgate synchronized to the 152-Wz pulse train. This was necessary to assure that we only observed photoluminescence due to excitation with the main pulse of the modulated laser pulse train. Photon count rates were kept below 2000 counts/s over the entire potential range for all scans in order to avoid pulse pileup. The count rate was monitored as a function of time with a multichannel scaler using a 1-s dwell time as the potential was scanned. The scan rate was 20 mV/s. The luminescence intensity vs the applied potential was measured over the potential range of -2.5 to +O.S V. In all cases, at least three complete scans were performed in succession to assure that the electrode surface was stable over the time scale of the experiment. No systematic changes in photoluminescence intensity from one scan to the next were observed under the conditions described. Flat band potentials were determined as described in ref 2 to be -1.1 V. Using this value, we observe excellent agreement between experimental luminescence intensities and the behavior predicted by the modified dead layer model under HeNe laser excitation and low power picosecond pulse illumination, as discussed below. Reeults Figure 1 shows scans of luminescence intensity vs potential from +0.5 to -2.5 V using low power picosecond pulse illumination (3 X 10" (photons/cm2)/pulse) and using a cw HeNe laser for illumination. The potential range covers electrode behavior from inversion to accumulation regimes. The modified dead layer has been used to calculate the expected luminescence intensity, and the results of these calculations are also shown in Figure 1. The dead layer model predicts that, in the depletion regime, the luminescence intensity will depend on the applied potential according to the expression Ip1= I" exp(-aW) (1) where Zpl is the measured luminescence intensity, I" is the lu-
(2)
where t is the dielectric constant of the material, to is the permittivity of the vacuum, q is the elementary charge, Nd is the doping density of the semiconductor, and AU = V, - V , is the difference between the applied potential and the Eat band potential. Substitution of eq 2 into eq l gives the dependence of Iplon the applied potential. The resulting equation (which is only valid in the depletion regime) indicates that a maximum in the photoluminescence is expected at the flat band potential and implies that the flat band potential can be determined from the ph~toluminescence.~ Equation 1 is based on the assumption that the surface minority trapping velocity, s, is much greater than D/L, the ratio of the diffusion coefficient to the diffusion length of the minority carriers ( L is defined as where r is the bulk carrier lifetime). This condition may not hold near the flat band potential, and deviations from this condition will cause an increase in the photoluminescence intensity as the flat band potential is approached. (This phenomenon is further discussed below.) For this reason, analysis of the experimental result and determination of the flat band potential from the photoluminescence intensity must be performed as follow^.^ First a potential is chosen where a very large effective surface recombination velocity is expected. This condition is usually fulfilled under strong depletion conditions, and we have chosen to use +0.5 V for the present analysis. Equation 1 is then used to predict the photoluminescence intensity as a function of potential. The values of P and AU are then adjusted until the mean square deviation of the experimental result from the predicted result over the depletion region is minimized, and the resulting value of AU is used to determine V,, the flat band potential. Though two parameters are simultaneously varied, the use of eq l and the measured intensity at a known reference voltage assure that only one informational degree of freedom is being adjusted in this analysis. In the case of the low power picosecond pulse excitation using a value6of a = 78 421 cm-I, we obtain excellent agreement between the model and experimental data when the flat band potential is chosen to be -1.1 V. A small deviation in the photoluminescence intensity is observed at the flat band potential in the cathodic sweep. We attribute this to recombination currents which make the potential somewhat difficult to control near the flat band potential and introduce small shifts which depend on the extent to which shallow traps are filled. This is expected to result in a shift between results of cathodic and anodic sweeps, as observed in our measurements. Similar effects are observed in flat band potential measurements based on Mott-Schottky analysis.' These considerations make it difficult to assign flat band potentials to an accuracy better than 100 or 200 mV, regardless of the method used to make the measurement. Nevertheless, the results of our photoluminescence analysis are in good agreement with other measurements and predictions described above. Similar results are obtained in the analysis of the photoluminescence intensity using cw HeNe laser excitation. We note here that the average power of the HeNe laser incident upon the sample is 0.280 mW, compared with 0.017,0.070,and 0.280 mW average powers used for the low, medium, and high power picosecond pulse excitation experiments, respectively. In this case a value of a = 39 000 cm-I was used.6 Again, agreement is good over most of the depletion region, with some deviation being observed near the flat band potential. The complete expression for the luminescence intensity at the flat band potential is
P=-[ kI0 1
- a2L2
1-
aL(aL
]
+ S)
l + s
(3)
where S is the reduced surface minority trapping velocity ( S = SLID),Io is the intensity of the exciting radiation, and k is the
6376 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992
Kauffman et al.
Medium Power Picosecond
v11000
Y
High Power Picosecond Excitation
Y
C 3
0
0 0
W
W
W C
W C Lo v
50 -1 -2.500
I
I
U
I -2.000
-1.500
-1.000
-0.500
0.000
0.500
Applied Potential
-2.500
-2 000 -1.500
-1.000
-0.500
0.000
0.500
Applied Potential
Figure 2. Luminescence intensity versus applied potential. The solid curve was obtained using medium power picosecond pulses ( 1 X 10l2 (photons/cm2)/pulse;0.070-mW average power). The triangles show the intensity expected in the depletion region using the dead layer model.
Figure 3. Luminescence intensity versus applied potential. The solid curve was obtained using high power picosecond pulses (4 X loi2(photons/cm2)/pulse; 0.280-mW average power). The triangles show the intensity expected in the depletion region using the dead layer model.
photoluminescence quantum yield which is assumed to be con~ t a n t . "This ~ expression includes effects of both surface trapping and diffusion. When the surface minority trapping velocity, s, is effectively infinite (Le., S >> D / L ) ,
carriers are not being generated at a rate sufficient to saturate the surface traps, whereas the high peak power of the picosecond pulse excitation generates enough free carriers to occupy a large fraction of the traps before nonradiative recombination with electrons can occur. Saturation of surface traps results in a reduction of the effective surface minority trapping velocity under picosecond pulse excitation of sufficient power, giving rise to photoluminescence intensities in excess of that predicted by the dead layer model. We have used eq 6 in the analysis of the data in Figures 2 and 3 in order to determine the surface minority trapping velocity of GaAs at the flat band potential. The analysis follows the procedure outlined above. The scans at various excitation powers are normalized at +0.5 V, where the surface minority trapping velocity approaches infinity. The ratio of the measured photcluminescence intensity at the flat band potential to that predicted by the modified dead layer model is used as the parameter 0 in eq 6. We use the values 7 = 6.4ns and a = 78 421 cm-' in the analysis. Unlike the luminescence decay measurements, the STV values recovered from the dead layer model analysis are extremely sensitive to the value of D. Because the diffusion coefficient has not been measured independently, we have performed these calculations for D = 3.5 and 5.5 cm2/s, which allows us to assign STV values in the range of 105000-170000 cm/s for high power excitation and 170000-270000 cm/s for medium power excitation. The low power result indicates that the surface minor trapping velocity is effectively infinite according to this analysis. These results are in agreement with the values of s = 105000,s = 170000, and s > 500 OOO cm/s recovered from the luminescence decays for high, medium, and low power excitation, respectively?
(4) and depends only on the bulk parameters of the semiconductor and the intensity of the excitation. Smandek, Chmiel, and Gerischer3q4have suggested that deviations from the luminescence intensity at the flat band potential predicted by the dead layer model can be used to determine s. Atcording to the above equations, the ratio of the measured intensity at the flat band potential to the intensity predicted by the dead layer model is given by the expression
P -=@=I"
+
aL(aL S) (1 - CUL) [I1+s
]
(5)
When s is infinite, 0 = 1, and the dead layer model as given above holds. When s = 0, j3 = (1 + aL) > 1. Thus as s decreases, an increase in photoluminescenceintensity at the flat band potential over that predicted by the dead layer model is expected. Rearranging eq 5 gives
s = (1
+ CUL - @)/(@- 1)
(6)
which is the relationship between the reduced surface minority trapping velocity and 0. Figures 2 and 3 show the photoluminescence intensity as a function of applied potential using picosecond pulse excitation at higher incident powers than that used in Figure 1. The results of the dead layer model analysis have been included in each figure for comparison. The intensities have all been normalized to a common value at + O S V in order to highlight intensity differences as the flat band potential is approached. This is appropriate for analysis of the results, since the intensity at +0.5 V is used as a point of reference in the dead layer model. The dramatic increase in the luminescence intensity at the flat band potential as power is increased indicates the dependenceof the surface recombination velocity on excitation power. In our previous report, we observed a decrease in s with increasing excitation power by measuring the luminescence decay from GaAs held at the flat band potentiaL2 We attributed this phenomenon to increased saturation of surface trap states as the excitation power was increased. Comparison of the high power picosecond excitation result (Figure 3) with the luminescence intensity using cw HeNe laser excitation of the same power (Figure 1) indicates further that the reduction of s responsible for the increased photoluminescence is a consequence of the pulsed nature of the excitation. Surface trap saturation can only occur when the traps are filled more quickly than they are emptied. In the case of cw HeNe excitation, free minority
Discussion Qualitatively the results of this dead layer analysis are in agreement with our previous measurements of surface minority trapping velocity based on photoluminescence decay measurements. Most importantly, both sets of data confirm that surface traps are being saturated under moderate picosecond pulse excitation. Quantitatively the STV's recovered from luminescence decay profiles are in agreement with the values from the present analysis when a value of the diffusion coefficient which is lower than would be expected from the doping density of our samples is used. Here we argue that this effect is also attributable to the picosecond pulse excitation. The dead layer model is based on the assumption that a steady state has been achieved. That is, the model assumes that cw illumination is used and that the excitation is present long enough before the measurement begins so that carriers are distributed to a depth on the order of the diffusion length of minority carriers in the material. In experiments using picosecond pulse excitation of relatively short wavelength (compared with the diffusion length), such a steady state is never attained. Carriers are generated over only a small fraction of the diffusion length (about 20% in these experiments),
J. Phys. Chem. 1992,96,6377-6381 and the initial flux of carriers toward the surface opposes the flow of carriers toward the bulk. Because carriers in the bulk are more likely to undergo radiative relaxation, a lower luminescence intensity can be anticipated under picosecond illumination than for cw illumination, given that all other parameters are unchanged. Examination of eq 6 indicates that a lower measured intensity associated with this non-steady-state condition will result in calculated surface minority trapping velocities which err to larger values. The degree to which this condition influences the results is unknown and will require a modification of the dead layer model which includes dynamic effects. However, a simple correction to the model can be made by using a small value for D. This has the effect of holding the steady-state model carrier distribution closer to the surface, as is the case in the experiment under picosecond pulse excitation. (A similar approach has been applied to diffusion length measurements in order to account for selfabsorption. Self-absorption of luminescence photons has been shown to increase the effective diffusion length in GaAs and requires the use of a large diffusion coefficient in order to model experimental resultss.) In fact when we use a value of 3.5 cm2/s for the diffusion coefficient in our calculations, the extracted STV's are in agreement with the values obtained from the luminescence decays. The assumption that the surface minority trapping velocity is infinite at +0.5 V may not hold under the higher power illumination. The dead layer model as it is presented here provides no means for correcting for this. Qualitatively, however, we expect the luminescence intensity at +0.5 V to increase as the STV at +0.5 V becomes smaller, and the entire scan should then be scaled to greater intensity. Normalizing the data to the model at +0.5 V therefore also introduces a condition which may result in STV's which err toward values which are too large.
6377
Conclusions The present study provides further independent evidence that saturation of surface traps can occur under modest picosecond pulse excitation. Comparison of results under cw illumination of the same average power also indicate that it is the peak power of the picosecond pulses which saturates the traps, and we reemphasize the cautions stated in the study of luminescencedecay profiles: STV's measured under high injection conditions (or even under milder excitation) may not represent the true rate of surface minority trapping under normal operating conditions for surface barrier devices. Finally, comparison of the STV's recovered from luminescence decay profiles and the dead layer model suggest that the dead layer model alone is inadequate for determination of the dependence of the surface minority trapping velocity on photon flux under picosecond pulse excitation. Acknowledgment. This work was supported by DOE Grant DEF G06-86ER45273. Registry No. GaAs, 1303-00-0; Na2S, 13 13-82-2. References and Notes (1) (a) Evenor, M.; Huppert, D.; Gottesfeld,S.J. Electrochem. Soc. 1986, 133,296. (b) Benjamin, D.; Huppert, D. J. Phys. Chem. 1988,92,4676. (c) Bessler-Podorowski,P.; Huppert, D.; Rosenwaks, Y.; Shapira, Y. J. Phys. Chem. 1991, 95, 4370. (2) Kauffman, J. F.; Balko, B. A,; Richmond, G. L. J. Phys. Chem., preceding paper in this issue. (3) Smandek, B.; Chmiel, G.;Gerischer, H. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 1094. (4) Chmiel, G.;Gerischer, H. J. Phys. Chem. 1990, 94, 1612. (5) Hobson, W. S.;Ellis, A. B. J . Appl. Phys. 1983, 54, 5956. (6) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (7) Li, J.; Peter, L. M. J. Electroanal. Chem. 1986, 199, 1 . (8) Ehenberg, M. Appl. Phys. Lett. 1977, 30, 207.
Photochemistry of Azo Compounds on Silver Island Films Studied by Surface Enhanced Raman Spectroscopy D. Franzket and A. Wokam* Physical Chemistry II, University of Bayreuth, D- W-8580 Bayreuth, Germany (Received: February 5, 1992; In Final Form: April 14, 1992)
Photochemical decomposition reactions of (methoxyphenyl)azosulfonates,of (3-vinylphenyl)azosulfonate,and of the model compound 4-nitrobenzoic acid, are studied by surface enhanced Raman scattering (SERS). The reactants are deposited onto silver island films by spin coating. Subsequent to pulsed excimer laser irradiation at 308 nm,Raman spectra are excited using a continuous wave visible laser. In the case of 4nitrobenzoic acid, the formation of an azodibenzoate radical recombination product is confirmed. For (2-methoxyphenyl)azosulfonate,the photolysis yield has been determined by monitoring the decrease in Raman intensities of the -N=N- stretching mode at 1486 cm-I and of the 1064-cm-I skeletal vibration. When referred to the azosulfonate absorption, the photochemical quantum yield is found to be enhanced by 1 order of magnitude, as compared to the decomposition reaction in aqueous solution.
Introduction
In suitable systems, surface enhanced Raman scattering (SERS)l4 has been established as a sensitive spectroscopic technique for detecting and characterizing adsorbed layers on group IB metal surfaces. Raman signals can be enhanced by up to 6 orders of magnitude for molecules which are close to a rough surface of certain noble metals, in particular Ag, Cu,and Au. On these surfaces, the incident laser field excites localized surface plasmons which give rise to strong local electromagnetic fields Author to whom correspondence should be addressed. Resent addreas: Physical Chemistry Laboratory, Swiss Federal Institute
of Technology, ETH Zentrum, CH-8092 Ztirich, Switzerland.
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close to the ~ u r f a c e . ~ . ~Together .~ with the "chemical contribution": which is due to resonant electron-hole pair creation followed by vibrational excitation of the adsorbate, the local fields give rise to the observed enhancement of the Raman scattering cross section. It has been proposed earlier that the surface fields could also be used to induce photochemical reactions in suitable adsorbates>*6 Nitzan and B m 5have drawn attention to the radiationless energy transfer to the metal substrate, which depopulatesthe excited state of the molecule, and thereby competes with the enhanced absorption. The distance between metal surface and adsorbate is crucial for the radiationless transfer. These concepts have been verified experimentally by Wokaun et al.' and by Leitner et al.,* 0 1992 American Chemical Society
