J . Phys. Chem. 1993,97, 1678-1683
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Photochemical Decomposition at Colloid Surfaces J u g Sang Sub
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Department Qf Chemistry Education, Seoul National University, Seoul 151 742, and Center for Molecular Science, 373- 1 Kusung-dong, Taejon 305- 701, Korea
Mar$in Moskovits' Department of Chemistry and Ontario Laser and Lightwave Research Centre, University of Toronto, Toronto, Canada M5S IAl
Jalal Shakhesemampour Department of Chemistry, University of Birjand, Birjand, Iran Received: November 18, I992
The kinetics of the photodecomposition of 2-pyrazinecarboxylic acid adsorbed on colloidal silver is investigated using SERS as a probe. Analysis of the rate of formation of surface carbon as a function of laser fluence and wavelength indicates that the rate-determining step in the photoprocess involves two-photon excitation of the arene to a r* excited state. At high laser fluences, both the SERS signal and the apparent photodecomposition rate appear to decrease with time. This is ascribed to thermal fragmentation of the colloidal aggregates responsible for SERS. The energy barrier to aggregate fragmentation is found to be approximately 15 kcal mol-'. Photodecomposition is observed only with those substituted benzenes that bind with the ring perpendicular to the silver surface. This is explained in terms of the direction of the transition dipole describing the two-photon process and the relative magnitudes of electromagnetic field components tangential and normal to the metal surface.
Introduction The study of photochemistry at surfaces and interfaces has become a burgeoning enterpribe engaging the efforts of more than a dozen groups. Experimental and theoretical studies involving infrared' and visibleZexcitation of adsorbed molecules resulting both in photodesorption and in light-induced chemical transformations of the adsorbate have been reported. Recently, several groups have reported photochemical events at surfaces which involve transient anions produced by metal-to-adsorbate charge transfer.3 Clearly, early fears that rapid nonradiative relaxation of excited molecules at metal surfaces4would preclude the observation of surface photochemistry have been allayed. Enhanced surface photochemistry of molecules adsorbed at surfaces capable of producing surface-enhanced Raman was first considered by Nitzan and Brus5 and expanded by Metiu6 and George7 and their co-workers. These workers concluded that the same field enhancement responsible for SERS is also expected to enhance photochemistry at surfaces. Moreover, they determined that the most efficient place for carrying out surface photochemistry in such systems is not at the metal-vacuum interface but at a little distance above it where the competition between field enhancement, which decreases as one moves away from the surface, and excited-state lifetime, which decreases on approaching the surface, strikes the best compromise. An early report of this competitiveeffect*observed by following thedecompositionof pyradine tographiticcarbon when separated from a rough silver surface by spacer layers was challenged as being really due to the pyridine diffusing through the ~pacer.~qI~ According to that work,9J0the apparent maximal photochemical rate some distance above the surface was due to coherence effects at the metal surface induced by the increasing thickness of the overlayer. If, as implied in refs 9 and 10, spacer layers are not essential to observeenhanced photochemistry,then one can use aggregated silver or gold colloid, known from SERS experiments to sustain large electromagnetic fields when illuminated with light of the 0022-3654/93/2097-1678$04.00/0
proper wavelength, as a suitable medium on which to carry out such experiments. In this paper, we report the photodecomposition of 2-pyrazinecarboxylicacid to graphitic carbon. Kinetic studies are carried out by following the temporal evolution of the increasing SERS signal from the carbonaceous photoproduct.
Experimental Section Silver sols were prepared as described previously.\\ Briefly, 60 mL of a 2 X le3M sodium borohydride solution was mixed with 22 f 2 mL of a 1 X l t 3 M silver nitrate solution. 2-Pyrazinecarboxylic acid was introduced into the colloid as an aqueous solution. Sufficient carboxylic acid solution was added to the colloid, dropwise, to cause it to turn blue, indicating colloid aggregation. Poly(vinylpyrro1idone) (pvp, MW 40 OOO) was added to the solution as a stabilizer, preventing further aggregation and eventual flocculation of the colloid. The final concentration of pvp in the solution was approximately 0.00196 by weight. Experiments were also carried out in the absence of pvp in order to ascertain that the polymer did not noticeably affect the spectroscopy or the kinetics. Kinetics runs were carried out on samples introduced by syringe into standard 1.5-1 d-mm Pyrex capillaries. Although spectra recorded with samples in 1-cm cuvettes were superior to those obtained with capillaries, kinetic runs carried out in cuvettes produced very slow spectral changes presumably because convection constantly replenished the material in the laser beam with fresh reagent. This effect was minimized greatly in the capillary. Experiments were carried out on equal volumes of fresh sample each contained in a new capillary. Both photochemistry and surface-enhanced Raman were excited by focused Ar ion laser light (Spectra-Physics Model 171 or Model 2016) traversing the capillary. Spectra were recorded using a SPEX 1400series monochromatorequippedwith a photon counter and interfaced to a Tektronix 4052 computer or an IBM personal computer. Kinetic runs were carried out by observing
0 1993 American Chemical Society
Photochemical Decomposition at Colloid Surfaces
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1679 1
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Figure 1. SERS spectrum of 2-pyrazinecarboxylicacid adsorbed on silver colloid.
the time evolution of the light scattered 1440 cm-I to the red of the exciting laser line. This corresponds to a point on the broad Raman band that grows in during sample irradiation free from interference by the Raman spectrum of the reagent. In a few experiments,the intensity decrease of the Raman lines belonging to the reagent was observed in order to satisfy ourselves that the rates of product appearance and reagent consumption were complementary. Kinetics were recorded as a function of laser power at 488 nm and as a function of laser wavelength, for the seven intense Ar+ laser lines in the visible (457-514 nm) at 200 mW.
Results The SERS spectrum of 2-pyrazinecarboxylicacid adsorbedon the surface of colloidal silver contained in a 1-cm cuvette is shown in Figure 1. The strongest band (1031 cm-I) is the YI ringbreathing vibration. The bands at 1055, 1523, and 1589 cm-l are assigned to 4 2 , YE,, and Y~L,, respectively, while Yl9a and Yl9b occur at 1469 and 1409 cm-I, respectively. The strong band at 1373 cm-1 is assigned to us(C02-). This indicates that the carboxylic group is ionized at the surface. The band at 851 cm-l is probably the C-COz- vibration. The observation of a strong CH stretching vibration at 3068 cm-l suggests that the molecule is bonded to the surface through the carboxylate group with the aryl group either 'standing up" on the surface or gently tilted with a substantial normal component.lZ The surface-enhanced Raman spectrum of 2-pyrazinecarboxylic acid adsorbed on silver colloid contained in a 1-cm X 1-cm cuvette is shown in Figure 2 (lower curve) together with the analogous spectrum for a sampleplaced in a 1.5-1.8-mm capillary (upper curve). (The baselines have been displaced for clarity.) The latter shows two broad bands centered at approximately 1350and 1530cm-l along with those assigned to the carboxylate. These bands have been previously assigned by several authors to disordered graphite.13 When very weak laser radiation is used, the two broad bands are initially very weak, but they grow in progressively with continued irradiation. Clearly, the carbon is a product of decomposition of the adsorbed acid by the laser. Its apparent absence in SERS spectra obtained from cuvettes is likely due to the increased convection possible with the larger samplecontainer which causes reagent-bearing colloid to flow into the rather narrow region of the focused laser beam. This process is greatly reduced in the capillary where the laser beam occupies a much larger portion of the sample's width. The time evolution of the SERS intensity at 1440 cm-I (which corresponds to carbon signal only) is shown in Figure 3 for a series of laser powers ranging from 100 to 200 mW (the low-
800 1000 1200 RAMAN SHIFT (cm-l)
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Figure 2. SERS spectra of 2-pyrazinecarboxylic acid adsorbed on silver colloid and recorded from a cuvette (lower curve) and from a capillary (upper curve) showing the appearance of carbon bands.
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Figure 4. Time evolution of the SERS intensity at 1440 cm-l excited with 488-nm Ar+ laser power ranging from 100 to 500 mW in 100-mW intervals (bottom to top along r = 0). Jagged lines are experimental data; smooth lines are recalculated according to the model described in the text.
power series) and in Figure 4 for a range of laser powers from 100 to 500 mW (the higher power set). The rate constants obtained by fitting the time evolution of the SERS intensity at 1440 cm-1 for a series of laser wavelengths while keeping the power constant at 200 mW are shown in Figure 5. For the lowpower set, the intensity of the carbon SERS signal rises gently with increasing time to saturation. At higher powers, the carbon SERS signal increases to a maximum and then decreases, and
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Figure 5. Portion of the UV-visible absorption spectrum of aqueous 2-pyrazinccarboxylic acid. Points at left are the photodecomposition rate constants determined as described in the text and arbitrarily scaled to bring them within numerical range of the measured absorptionspectrum. The rate constant values are plotted against one-half the wavelengths used to excite the photochemistry in order to emphasize the two-photon process involved. The different point shapes indicate the results of the various runs. The circular points a t the right indicate the relative SERS enhancement, in arbitrary units, plotted against exciting wavelength. The line drawn through them is a guide to the eye.
with 500 mW, only a decrease of the SERS intensity at 1440 cm-I with time is observed. For both sets (Figures 3 and 4), the initial SERS intensity (at 1440 cm-1) scales with laser power within experimental error. With changing excitation wavelength, the t = 0 value of the SERS intensity at 1440 cm-I (or indeed that of the entire SERS spectrum) increases gently toward the red (when the trivial dependence of the Raman intensity is factored out) from 476 to 514 nm, as has been reported pre~iously.’~ With 457-nm laser excitation, however, the SERS crosssection increasesdramatically (Figure 5 ) . The relative Raman cross-section values shown in Figure 5 are averages of several runs. The run-to-run reproducibility was very good. The photodecomposition rate constants (extracted from the data in the manner described in the Discussion section) are found to increase toward the blue. This is shown in Figure 5 , where the relative photodecomposition rate constants (scaled so as to bring them within numerical range of the measured absorption spectrum) are plotted as a function of one-half the excitation wavelength in anticipation of the subsequent discussion in terms of a two-photon initial photoexcitation step. These values are plotted over a portion of the UV-visible spectrum of aqueous 2-pyrazinecarboxylicacid,showing that the rateconstants follow the band centered at approximately 230 nm quite well. Photodecomposition was attempted with several other related reagents such as pyrazine and the three isomers of phthalic acid. Of these, only phthalic acid (as opposed to isophthalic and terephthalic acids) was found to photodecompose, even though the electronic absorption spectra are very similar for the entire series.
Diseurrcion The observed time evolutionof the SERS band intensity arising from the graphitic product (Figures 3 and 4) may result from several processes. First, the Raman signal of the photoproduct may increase due to photochemistry. Second, because photoreaction takes place only in the laser beam, colloidal particles carrying the photoreagent can continuouslydiffuse into and colloid carrying the photoproduct can diffuse out of the reaction zone. Third, since most of the SERS signal originates from colloidal particles that have aggregated into fractal assemblies, often consisting of thousands of particles,Is the absorption of the laser
radiation might cause the clusters to fragment, possibly resulting in the loss of SERS signal intensity. Last, the signal will depend on the intensity and wavelength of the laser excitation used. The model that we use to account for the observed time evolution of the increase in the Raman signal of the carbon photoproduct (or alternatively of the decrease of the Raman lines of the pyrazinecarboxylic acid photoreagent) includes the possibility of contributions from all of these effects. If we let the concentration of colloidal particles carrying photoreagent and photoproduct be A and B, respectively, then the differential equations describing the rates of decrease of A and increase of B are given by dA/dt = ri - kA dB/dt = kA - k@
(1)
The quantity ri expresses the rate at which reagent is brought into the reaction zone during the course of the reaction. Since the reaction normally consumesa very smallfraction of the reagent available, we assume ri to be a constant. Likewise, k@ is the rate at which colloid carrying photoproduct is removed from the reaction zone, presumablyby diffusion, although that assumption need not be made. The rate constant, k,is a function of the laser power, I. We consider two possibilities. In the first, carbon forms in a sequence of processes, the first of which is an n-photon absorption. In that event, k = UP. In the second, the carbon is a pyrolysis product which forms within the colloidal clusters at the elevated local temperatures resulting from their strong absorption of visible light. A reasonable form for k in the latter case is which assumesthe local temperature to be linear with laser power, I, and TOis the ambient temperature. Equations 1 are solvable, trivially yielding the solution
+
B = (r,/ko)( 1 - e-kof)
[(kA, - ri)/(ko - k)] (e-kf- e-k“‘) (2) where A0 is the value of A to t = 0. (Actually one should average all of the rate constants over the exponentially decreasing light intensity due to the beam’s traversal of the absorbing solution and over the beam’s lateral intensity profile. It can be shown that for nonlinear processes, such an average can affect the apparent value of exponent n. For a very short path length, as is the case with the capillary, we calculate that the effect of averaging the laser power is not serious, however.) Although for proper choice of parameters, B(t) as given by eq 2 can monotonically increase with time or increase to a maximum and then decrease (over a finite time interval), it cannot monotonically decrease with time, as is observed with 500 mW of illumination and as shown in Figure 4. One might argue, however, that for k >> ko,the initial increase, before B ( t ) begins todiminish with time, isofsuchshort duration that it isunobserved in the experiment. This is unlikely, however. The initial value of the Raman intensity at 1440 cm-I (Le., the baseline value) reported in Figure 4 scales exactly with laser power for all of the experiments performed. It is unlikely that in every case where a monotonic decrease was observed, there was an unobserved initial increase followed by a decrease which restored the Raman intensity to precisely the value expected for the intensity of the background scattering at that laser power. Moreover,all attempts t o account for the observations in terms of eq 2 alone failed or produced unrealistic values of the parameters. It is, therefore, necessary to modify eq 2 further in order to account for the data of Figure 4. Accordingly, the monotonic decrease in Raman intensity at high laser power is ascribed to photofragmentation of the colloidal aggregates from which most
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1681
Photochemical Decomposition at Colloid Surfaces of the SERS signal arises. That photofragmentation of the colloidal aggregates is taking place is corroborated by the fact that at higher laser powers the Raman background signal is found to decrease with time more or less in step with the decrease of the carbon Raman signal. The background signal observed with SERS-active systems is known to be enhanced. It is believed to arise from enhanced electronic Raman originating from the meta1.17 Stockman et a t i 6 have shown that for fractal colloidal aggregates large enough so that scaling may be invoked, the Raman enhancement is independent of the number of colloidal particles in the aggregate. Hence, the Raman intemity will be proportional to the number of colloidal particles in the aggregate, assuming that the adsorbate responsible for the SERS signal is more or less uniformly distributed on the surfaces of the wlloidal particles. Under such circumstances, cluster fragmentation would result in a decrease in Raman signal only if individual colloidal particles or very small clusters are ejected from the larger aggregates. In that event, the signal from adsorbate or photoproductcarried by those small clusters would be essentially unobserved owing to the low enhancement predicted for them. If, by contrast, the photofragmentationprocess produced, more or less, qual-sized fragments so that the size of each remained in the scaling region, then no decrease in Raman ligna1 would be expected. The possibility of cluster division into fragments of more or less q u a l size has been p r o p e d previously'8 to account for hole burning in the plasmon absorption bands of aggregated colloid. Although such a proccss is not impossible, in principle, it cannot account for our observations. Photofragmentation of the former kind would result in an exponentially decreasingRaman signal, assuming that the SERS signal arising from the smaller photofragments can be ignored. In that event, the time evolution of the Raman signal from the carbon photofragment would be of the form s(t) = aZS(t)e-kf'
(3)
where kr is the rate constant describingthe fragmentation of the colloid, Zthe laser power, and u a collection of constants including the Raman scattering cross section. Except at very high laser powers, the colloidal clusters remain large and the rates of diffusion in and out of the reaction zone remain approximatley qual; Le., ri E k d o . In that event, the effect of diffusion can be ignored and B(r) takes on the simpler form
B(t) = A,(I
- e-&')
(4)
This expression can now be substituted for that of q 2 in q 3 to describe the time dependence of the Raman signal. Three models relating kr, the cluster fragmentation rate constant, to the laser power were considered. (i) The fragmentation process was assumed to be photochemical and kr taken to have a form, kr = a", similar to k, except that the values of n and n' need not be q u a l . (ii) Alternatively, the colloid fragmentation process was assumed to be thermal and kr was considered to depend on I through the effect of the local temperature. In particular, if the heating of the colloidal aggregatesresults from the excitationof localized surfaceplasmon modes followed by degradation into heat, then the local temperature is given, to first order, by T(I) = TO+ bl, and the rate constant, kr, will have the form kr= kmexp[-AHr/k~T(I)], where TOis the ambient temperature and AHr is the activation energy for cluster dissociation. (iii) Finally, the possibility was considered that the local heating results from the heat released by the surface photoreaction so that T ( 0 = TO+ b T . The data shown in Figures 3 and 4 were fit to eq 3 using a nonlinear least-squares program based on a Fletcher-Powell search algorithm. Almost qually good fits were obtained by
using either eq 2or eq4 todescribeB(r). However,q2produced slightly better fits to the data only with unrealistic values of ko and ri. In particular, the weighted sum of the residuals decreased by approximately 20% when q 2 was used in place of q 4, but the parameters returned implied that the rate of reagent arrival in the reaction zone was 4 times greater than that of product departure. This result is unrealistic since it implies that after fragmentationthe clusters diffuse more slowly than before. When the fit was restricted to a region of parameter space corresponding to ri 4 kdo, the equality was always returned, implying that q 4 describes our conditions adequately. In carrying out these fits, eqs 3 and 4 were modified to include two baseline parameters, one independentof time, expressing the fact that there is an unenhanced Raman background originating from the aqueous solvent, and a time-dependent background signal resulting from the colloid itself. As the colloidal aggregates fragment, the background signal is expected to diminish along with the Raman signal. This point was tested by following the time evolution of the baseline during laser irradiation. An exponential decreasewas observed. The two baseline parameters were introduced as an additive constant to eq 2 or q 4 for the time-dependent baseline and an additive constant to q 3 for the time-independent baseline. Of the three models considered relating kr to 1, the third produced significantly poorer fits to the observed data when the same value of n was assumed in the expression for k(Z) and T(I). The first and second produced almost equally good fits to the data; however, the best fit using form i was obtained for a value of n' = 5.7. It is unlikely that the colloidal clusters fragment through a six-photon process. Accordingly, only kr = km e x p [ - m f / k ~ T ( I ) ] and T(I) = TO bl are found to produce both a good fit to the data and physically realistic values for the parameters. On theother hand, both modelsrelatingk tol, that is,describing the manner in which theadsorbed molecules decompose, produced equally good fits. Therefore, one cannot distinguish, on the basis of the goodness of the fit alone, between an n-photon photoprocess and thermal decomposition as the primary cause of the conversion of adsorbed 2-pyrazinecarboxylicacid to carbon. The solid lines in Figures 3 and 4 were, therefore, calculated using form ii for kr and using eq 3 and 4. For both figures, all of the curves in a given set were fit simultaneouslywith a single set of parameters (a, n, km, AHf,and b when photodecomposition was assumed or km, AHf, E,, kd, and b when thermal decomposition was considered). For the data of Figure 4, individual values of the time-independentbaseline were allowed to account for the effect of baseline drift from run to run. For the data of Figure 3, a common value of the baseline sufficed. In both cases, the appropriate value of the laser power was incorporated into the equations. The fitsobtainedarequitegood. Whilebetter fits wereobtained when individual members of the two sets were fit to q s 3 and 4, it is the results of the simultaneous fitting that are reported since they represent a more stringent test of the model. Assuming the crucial step in the surface decomposition process to be photochemical, best fits were obtained with n = 1.8 and AHr/kB = 7790 K for the higher power set (Figure 4) and n = 2.5 and AHf/kB = 5024 K for the lower power set (Figure 3). The values of a, km. and b are not useful without precise knowledge of other experimentalfactors such as the concentrationof colloidal clusters, the surface adsorbate coverage, and the laser beam size. The value of a obtained from the lower power set (Figure 3) (2.5 x 10-7 s-I mW-1.8) was approximately half that obtained from the other set. Similarly, the value of km returned by the fit to the lower power set was approximately 10 times larger than the value obtained with the other set (Figure 4). The value of b obtained with the higher value set implied a local temperature of approximately 845 OC when 500-mW illumination was used.
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The values of km, AHf,and b obtained from the data of Figure 3 are not as trustworthy as those obtained from thedata of Figure 4 since the downturn in the SERS intensity that is attributed to laser-induced thermal fragmentation of the colloid clusters is not observed at low laser powers. Nevertheless, the value of AH1 obtained from that set is within acceptable reach of the one obtained with the data of Figure 4. On the other hand, the values of a and n obtained from the data of Figure 3 are expected to be more trustworthy than those extracted from the data of Figure 4 since the low power data are less perturbed by the effect of laser-induced fragmentation. When thedata of Figure4 were fit using theformofkconsistent with thermal decomposition, AHfIkB and EJke were found to be 9064 and 2193 K, respectively, while the value of the local temperature was calculated to be 1700 K with 500-mW illumination. Thelowvalueoftheactivationenergy,E, (4.3 kcalmol-I), the unrealisticallyhigh values of the local temperature, exceeding the melting temperature of silver, and the observation (discussed below) that the action spectrum of k closely follows an absorption band of 2-pyrazinecarboxylic acid makes the thermal decomposition model a much less likely explanation than photodecomposition to account for the observed conversion of the adsorbed photoreagent to surface carbon. We will, therefore, continue the discussion on the assumption that we are observing surface photodecomposition of 2-pyrazinecarboxylic acid in which (at least) the first step is a two-photon absorption and simultaneous thermal fragmentation of colloidal aggregates, for which the barrier is estimated to be 15.5 kcal mol-’. Plausible arguments for the production of carbon in the photodecomposition reaction have already been presented in the context of similar processesobserved on cold rough silver surfaces prepared in UHV.899 Two-photon excitation with visible photons can access the lowest singlet mr* state of 2-pyrazinecarboxylic acid, whichisconnected to thegroundstate by a transitioncentered at 230 nm (Figure 5). This state likely corresponds to the IBlu state of pyrazine, the more symmetric parent of the molecule used here. That state has been located at 260 nm in the gas phase.19 A lBlu IA, absorption is formally two-photonforbidden. This restriction is weakened for the substituted pyrazine used in this study whose center of inversion is removed. Alternatively, the two-photon absorption may involve an excited state analogous to the IBzg (nr*) state of pyrazine that has been postulated to lie near IB2u19avc but not yet observed definitively. Photodecomposition of pyrazine excited to its ?y* state does not normally lead to carbon. Irradiation with 160-nm light is reported to lead to HCN and acetylene.z0 The appearance of carbon suggests one of several plausible mechanisms. The first is that two-photon excitation of one of the ?r* states of pyrazinecarboxylic acid is merely the rate-determining step followed by facile multiphoton absorption by the molecule or one of its photoproducts leading to decomposition to carbon. Alternatively, the excitation of the molecules occurs by charge transfer of an electron from the metal, leading to a transient anion that decomposes to carbon as one of its products.3 Finally, the possibility exists that the metal is inert toward ground-state azabenzenes but is a powerful catalyst toward the photoexcited molecules leading to their decomposition. Two-photon absorption can alsoaccessseveralsinglet and triplet n r * states as well as a triplet m* state, all of which can be connected to the ground state by transitions in the 320-370-nm range.19 The striking agreement between the action spectrum of the photodecomposition rate constant and the absorption band to the m* state of the molecule which correspondsto the lBzuof pyrazine leads one to believe that the crucial first step in the photodecomposition process is intramolecular excitation, rather than a metal-to-molecule charge transfer, and although it is difficult to narrow down the mechanism any more specifically based on the
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data presented here, the concept of a rapid multiphoton process beyond an initial two-photon step challengesthe imaginationeven in a system benefitting from surface electromagneticenhancement. That a two-photon initial step occurs is, therefore, strongly implied by our results. The facility of such a process even at rather low laser fluences is a consequence of surface electromagnetic field enhancement. The involvement of an excited r* state of the molecule in the photodecomposition may also explain the observation that this reaction was only observed with azabenzenes that “stand up” on the surface. The transition dipole matrix element connecting the IBzu of pyrazine to the ground state is polarized in the plane of the ring and perpendicular to the N-N axis.I9 It is well-known21 that the tangential component of the electromagnetic field at an illuminated metal surface is reduced, while that normal to the surface is increased. Although for silver this is particularly so in the red and infrared, there is a substantial increase of the normal component over the tangential component even in the visible. Since pyrazinecarboxylic acid is bonded through the ionized carboxylate group, the pertinent transition dipole would have a significant component normal to the surface. Similar arguments can be made for essentially all substituted benzenes and azabenzenes. Molecules such as 2-pyrazinecarboxylic acid.and phthalic acid that bind in such a way that the plane of the arene is normal to the metal surface12 (and the inplane transition dipole is not parallel to the surface) are capable of intense mr* excitation and therefore of photoreaction, due to the higher intensity of the appropriately oriented electromagnetic field component. On the other hand, molecules such as pyrazine and isophthalic and terephthalic acids that bind with the ring parallel to the surface12 are excited by a weaker component of the em field, absorbing more weakly in the m* region with a consequent reduction in the photodecomposition rate. (With pyrazine, the transition dipole of the lBzu IA, transition would be parallel to the surface even if the molecule is bound with one of its nitrogens to the surface). Finally, the increase in the SERS cross section on going toward the red has been explained previ0us1y.l~~Briefly, it reflects the field enhancement characteristics of colloidal particle aggregates. Recently, an analytic expression was reported for the wavelength dependence of the SERS cross section of a fractal aggregate, assumingdipolar coupling among its monomers.16 The expression agreed adequatelywith the experimentallydeterminedwavelength dependence. The roughly 2-fold increase in SERS cross section on going from 476-nm to 457-nm excitation, however, is unusual and mysterious. There are no intramolecular absorptions sufficiently near that wavelength to make a resonant Raman contribution on top of SERS a likely explanation. It is possible, however, that the increased cross section at 457 nm signals the presence of a charge-transfer transition from metal to molecule or vice versa that adds its contribution to surface enhancement by modifying the Raman polarizability of the surface species. Such contributions have been proposed previouslyz2and discussed widely.23
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Achwkdgment. M. M. is indebted to NSERC, the University of Toronto’s Connaught fund, and to the Centres of Excellence for Molecular and Interfacial Dynamics for financial support. Discussionswith Professor Vladimir Shalaev and Dr. Constantine Douketisaregratefully acknowledged. J.S.S. thanks the Daewoo Research Fund of S.N.U. for financial support. References and Notes (1) For review see: Chuang,T. J. SurJSci. Rep. 1983,3,1and referencts therein. Heilweil, E. J.; Casassa, M. P.; Cavanaugh. R. R.; Stephenson, J. C. Annu. Rev. Phys. Chem. 1989,40, 143. ( 2 ) For review see: Ho, W . In Desorption Induced by Electronic Transirions, DIET IV; Betz, G., Varga, P., Eds.; Springer: Berlin, 1990; p 48. Dynamics of Gas-Surface Inreractions; Rettner, C. T.; Ashford, M. N. R.; Eds.; Royal Society of Chemistry: London, 1991.
Photochemical Decomposition at Colloid Surfaces (3) Dixon-Warren, St.-J.; Jensen. E. T.; Polanyi, J. C. Phys. Reu. Leu. 1991,67,2395. Ying. Z. C.; Ho, W. Phys. Rev. Lerr. 1988,60,57; 1990,65, 741. Marsh, E. P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. Phys. Reu. Lrr. 1988, 61, 2725. Gilton, T. L.; Dehnbostel, C. P.; Cowin, J. 0.J. Chem. Phys. 1989, 91, 1937. Hatch, S. R.; Zhu, X.-Y.; White, J. M.; Campion, A. J . Chem. Phys. 1990,92,2681. Jo, S. K.; White, J. M. J. Phys. Chem. 1991, 94, 5761. (4) Chance, R. R.; Prock, A.; Silbey, R. Adu. Chem. Phys. 1978,37, 1. ( 5 ) Nitzan, A.; Brus, L. E. J . Chem. Phys. 1981, 74, 5321; 1981, 75, 2205. (6) Das, P.; Metiu, H. J . Phys. Chem. 1985, 89, 4680. (7) Leung. P. T.; George, T. F. J . Chem. Phys. 1980,85, 4729. (8) Goncher, G.M.; Parsons, C. A.; Harris, C. 8.J . Phys. Chem. 1984, 88, 4200. Goncher, G.M.; Harris, C. B. J . Chem. Phys. 1982, 77, 3767. (9) Wolkow, R. A.; Moskovits, M.J. Chem.Phys. 1987,87,5858. Blue, D.; Helwig, K.; Moskovits, M.; Wolkow, R. J . Chem. Phys. 1990, 92,4600. (10) Blue, Douglas; Helwig, Kate; Moskovits, MartinJ. Phys. Chem. 1976, 93, 8080. (11) Suh, J. S.; DiLella, D. P.; Moskovits, M. J . Phys. Chem. 1983, 87, 1540. Creighton, J. A. p 315 of ref 17. (12) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984, 88, 1293. Woollman, J. A.; Katkanat, V.Phys. R w . B 1980,30, (13) Dillon, R. 0.; 3482. Wada, N.;Gaczi, P. J.; Solin, S. A. J. Non-Crysr. Solid 1980, 35/36, 543.
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1683 (14) (a) Creighton, J. A.; Blatchford, C. G.;Albrecht, M. G. J . Chem. Soc., Furuduy Truns. 2 1979,75,790. (b) Suh, J. S., Moskovits, M. J . Phys. Chem. 1984,88, 5526. (15) Weitz, D. A.; Oliveria, M. Phys. Rev. Leu. 1984, 52. 1433 and ref 14b. (16) Stockman, M. I.; Shalaev, V. M.; Moskovits, M.; Botet, R.; George, T. F. Phys. Rev. B 1992, 46, 2821. (17) Surfuce Enhanced Roman Scutrering, Chang, R. K.,Furtak, T. E., Eds.; Plenum: New York, 1982. (18) Karpov, A. V.;Popov, A. K.; Rautian, S. G.;Safonov, V. P.; Slabko, V. V.; Shalaev, V. M.; Shtokman, M. 1. JETP Leu. 1988, 48, 571. (19) (a) Canuto, C.; Goscinski, 0.;Zerner, M. Chem. Phys. Lerr. 1979, 68, 232. (b) Kong, H. K.; Robinson, G.W. J . Mol. Spectrosc. 1974,52, 1. Suzuka, I.; Mikami, N.; Ito, M. J . Mol. Specrrosc. 1974, 52, 21. Thakur, S. N.; Innes, K. K. J . Mol. Specrrosc. 1974, 52, 130. (c) Esherick, P.; Zinsli, P.; El-Sayed, M. A. Chem. Phys. 1975, IO, 415. (20) Calvert, J. G.;Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966, p 460. (21) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205. Moskovits, M. J. Chem. Phys. 1982, 77,4408. (22) Schmeisser, D.; Demuth, J. E.; Avouris, P. Chem. Phys. Leu. 1982, 87, 324. (23) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J . Phys. Condens. Murrer 1992, 4, 1143 and references therein.
