The Vibrational Pumping Mechanism in Surface-Enhanced Raman

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The Vibrational Pumping Mechanism in Surface-Enhanced Raman Scattering: A Subpicosecond Time-Resolved Study V. Kozich† and W. Werncke* Max-Born-Institut, Max-Born-Strasse 2A, D-12489 Berlin, Germany ReceiVed: February 08, 2010; ReVised Manuscript ReceiVed: April 21, 2010

The origin of vibrational pumping in surface-enhanced Raman scattering is investigated applying subpicosecond Raman spectroscopy. We observe selective excitation of distinct fingerprint modes resulting in a pronounced nonequilibrium population distribution followed by a decay with a common time constant for all vibrations of about 1 ps. In particular, the Raman-active mode with the highest frequency in the fingerprint region is strongly excited. Vibrational excitation is delayed with respect to the excitation pulses. Our findings clearly rule out Stokes Raman scattering, but point to electronic excitation as the main source of vibrational pumping. After electronic excitation, energy is transferred to the vibrations by fluorescence and ultrafast internal conversion within about 0.8 ps. Since its discovery in 1974 surface enhanced Raman scattering (SERS) has attracted an ever-growing interest.1 This is due to the enormous enhancement of sensitivity often accompanied by quenching of the fluorescence, which opened up new applications that are inaccessible for conventional Raman techniques.2-4 Concomittantly, these findings initiated investigations addressing the underlying and accompanying physical mechanisms of SERS. One of the phenomena, which is still not understood, is the unusual high anti-Stokes to Stokes SERS intensity ratio appearing with increasing excitation power. The first observation of this phenomenon has been reported by Kneipp et al. for dyes (crystal violet and rhodamine 6G) adsorbed in silver colloidal solutions using excitation far off electronic resonances (i.e., far off the electronic resonance that is observed for the corresponding molecules in an aqueous solution).5 Later on, it was shown that such enhanced ratios also occur for preresonance and for strict electronic resonance conditions.6 Kneipp and co-workers rationalized the increasing anti-Stokes SERS intensities by vibrational excess populations generated by spontaneous Stokes Raman scattering (Stokes Raman pumping model).5 This explanation demands, however, extremely high SERS cross sections of σ ≈ 10-16 cm2 or SERS enhancement factors F ≈ 1014-1015. The significance of these huge values initiated a controversial, still ongoing discussion. Other explanations for the unusual phenomenon were put forward, for instance, conventional heating of the compounds, dispersion effects due to molecular as well as plasmon resonances, or fluorescence pumping.7-10 The relevance of a vibrational pumping mechanism leading to a nonthermal vibrational population distribution was confirmed by measurements at low temperatures taking advantage of the drastically decreased thermally induced vibrational populations.6 It was observed that the excess populations of the vibrations do not scale linearly with the Stokes SERS cross sections.6 On the basis of the Raman pumping model, vibrational relaxation times were derived, however, without any experimental proof. Recently, SERS cross sections determined under single molecule detection * To whom correspondence should be addressed. E-mail: werncke@ mbi-berlin.de. † Present address: Institut fu¨r Experimentalphysik, Freie Universita¨t Berlin, Arnimallee 14, D-14195 Berlin, Germany.

conditions again emerged as too small to account for the observed excess populations.11 It was proposed that this discrepancy is caused by an underestimation of the SERS pumping cross section because of the neglection of scattered photons absorbed by the metal and/or by the action of other pumping processes. Up to now, SERS pumping experiments have been carried out with cw lasers only. Time-resolved investigations on the other hand should allow us to distinguish between the different possible pumping mechanisms based on their different timedependent characteristics.12-16 Here we report on the first SERS experiments with subpicosecond time resolution using Rhodamine 6G (Rh6G) diluted in a colloidal silver solution. Comparing pulsed with low-power cw excitation we estimate the vibrational excess populations accumulated for the different modes within the pulse duration. Applying a single color pump-probe scheme we monitor their kinetics. Our results exclude Raman scattering as the main mechanism of vibrational pumping. Instead, excitation of the electronic system followed by fluorescence and ultrafast internal conversion appears to be most effective. Experiments Rh6G (c ) 10-6 mol/L) was prepared in an aqueous silver colloidal solution and activated by adding NaCl according to literature procedures.17 The shape of the broad absorption band of this solution with a maximum at 437 nm coincides with the spectrum of the neat colloidal solution.2,3 Increasing the Rh6G concentration a band at 520 nm appears, which is probably due to the nonadsorbed Rh6G molecules which are dissolved in water. Stokes and anti-Stokes SERS spectra of the solution were recorded at 633 nm either with cw radiation or subpicosecond pulses. Cw radiation of 0.1-1 mW was delivered by a HeNe laser. Subpicosecond pulses centered at 633 nm with a pulse duration of ∼0.6 ps and a spectral width of 35 cm-1 were generated by an optical parametric generator/amplifier, which was pumped by a 1 kHz picosecond Ti:Sapphire regenerative amplifier. Cw or pulsed radiation, respectively, with beam diameters of about 1 mm was directed onto a sample cell of 1 mm thickness without focusing. In contrast to cw radiation pulsed excitation results in photodecomposition (photobleaching)

10.1021/jp101219e  2010 American Chemical Society Published on Web 05/19/2010

The Vibrational Pumping Mechanism in SERS

Figure 1. Stokes and anti-Stokes SERS spectra of Rh6G in a silver colloidal solution recorded with 633 nm cw excitation of 1 mW. Upper spectra: anti-Stokes SERS spectra enhanced by a factor of 6 and 60, respectively. Lower spectrum: Stokes SERS spectrum.

of the sample. This was, however, largely avoided by attenuating the pulse energies below 50 nJ. After passing a notch filter, scattered light from the sample was imaged onto the slit of a double spectrograph (f ) 320 mm) equipped with a 300 and 600 L/mm grating for the first and second spectrometer stage, respectively, and monitored by a nitrogen-cooled CCD camera. We applied a 135° backscattering geometry. For monitoring the temporal evolution of vibrational populations we applied a onecolor pump-probe SERS scheme. Pulsed radiation was split into beams of equal intensities which were recombined with variable delay times in an approximately collinear pump-probe geometry. Pulse energies were about 15 nJ and each spectrum was typically accumulated for 5 min. SERS spectra with delay times between -10 and +15 ps were accumulated several times. For recording these spectra only the first stage of the double spectrometer with the 300 L/mm grating was used. Taking advantage of its sufficiently low dispersion we monitored Stokes and anti-Stokes SERS spectra simultaneously. Anti-Stokes SERS spectra were normalized to the Stokes SERS intensities to account for drifts and fluctuations of excitation intensities, photobleaching of the sample, and changes of the SERS cross section upon vibrational excitation.18,19 Furthermore, it enabled us to average over several series of measurements. After subtraction of the broad SERS background, SERS lines were approximated by Voigt profiles. Experimental Results A first series of experiments was carried out to estimate the excess population originating from vibrational pumping. For this purpose we recorded Stokes and anti-Stokes SERS spectra of Rh6G at 633 nm either with pulsed radiation (50 nJ) from the OPG/OPA or with cw radiation (1 mW), the latter with an intensity small enough to neglect any vibrational pumping. Although the average energy for pulsed excitation is even lower than that for cw excitation, pulsed excitation increases the photon flux by a factor of about 105. Other dependencies influencing the anti-Stokes and Stokes SERS intensity ratios, as for instance dispersion of the resonances, spectral sensitivity of the CCD camera, and throughput of the spectrograph, etc., remain unchanged under both experimental conditions. In Figure 1 we present Stokes and anti-Stokes SERS spectra recorded with cw radiation. We checked by decreasing the excitation intensity to 0.1 mW that both Stokes and anti-Stokes SERS signals scale linearly with intensity, i.e., the anti-Stokes to Stokes SERS intensity ratio (IaS/IS)cw is independent of

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10485 excitation intensity. Wavenumbers, relative SERS cross sections of the strongest vibrations, as well as IaS/IS for both cw and pulsed excitation are summarized in Table 1. In accordance with a thermal vibrational population distribution, (IaS/IS)cw ratios decrease with increasing wavenumber of the modes. We are unable to measure the corresponding antiStokes Raman spectra of high-frequency fingerprint modes for conventional Raman scattering of nonabsorbed molecules because their Raman cross sections are too small. Instead we compare (IaS/IS)cw for SERS at ambient temperature with (IaS/ IS)calculated ) k(ω){n/(n + 1)}300K, i.e., with the calculated intensity ratios for Raman scattering far off the electronic resonance at 300 K (cf. Table 1). Here, ωL is the wavenumber of the radiation used for excitation, Ω is the wavenumber of the vibration, k(ω) ) {(ωL - Ω)/(ωL + Ω)}4, and n is the population number, n )(epΩ/kT - 1)-1. The measured ratios (IaS/ IS)cw are significantly higher than (IaS/IS)calculated. As can be derived from Table 1, (IaS/IS)cw/(IaS/IS)calculated rises from 4.4 to 18 with increasing Ω from 604 to 1643 cm-1. In Figure 2 we show Stokes and anti-Stokes SERS spectra recorded with pulses of 50 nJ. Taking into account that the line widths in the SERS spectra are broadened because of the spectrally broad excitation pulses, the relative Stokes SERS intensities (after convolution) do not differ significantly for pulsed and cw excitation. Lowering the pulse energy to 5 nJ results in an about 10-fold decrease of the Stokes SERS intensity, whereas the anti-Stokes SERS intensities are too small for detection. To elucidate the influence of the different photon fluxes on vibrational populations we now compare IaS/IS for cw and pulsed excitation (cf. Table 1). Under both excitation conditions these ratios are similar for the vibrations at 600 and 780 cm-1. Above 1000 cm-1, however, they differ significantly. In particular, (IaS/ IS)pulsed for the vibration at 1643 cm-1 exceeds the corresponding ratio (IaS/IS)cw by a factor of 20. Furthermore, the (IaS/IS)pulsed ratios do not decrease monotonically with increasing vibrational frequencies. For instance (IaS/IS)pulsed of the vibration at 1643 exceeds (IaS/IS)pulsed of the adjacent vibration at 1510 cm-1 by a factor of 3, whereas (IaS/IS)pulsed for the vibrations at 1355 and 1305 cm-1 show just the opposite behavior (factor 0.7). We estimated populations of individual modes n/(n + 1)pulsed for pulsed excitation from the relation n/(n + 1)pulsed ) [(IaS/IS)pulsed/ (IaS/IS)cw][n/(n + 1)300K] (cf. Table 1). In this way the influence of the electronic resonance conditions on the anti-Stokes Raman intensities for the different vibrations is taken into account. n/(n + 1)pulsed are enhanced with respect to n/(n + 1)300K at room temperature. As will be discussed later on, enhanced populations strongly deviate from any thermal population distribution indicating mode-selective vibrational pumping. To identify the mechanism responsible for mode-selectivity in the pumping process that is observed with pulsed excitation, we monitored the temporal characteristics of vibrational populations applying pump-probe techniques with pairs of pulses in a second series of experiments. In Figure 3 we present pump-probe anti-Stokes SERS spectra, which were recorded for different delay times. An antiStokes Raman spectrum recorded at a delay time of 15 ps was always subtracted from these spectra. We observe modes at 1178, 1510, 1643 and the doublet at 1305/1355 cm-1, which is spectrally not fully resolved. Intensities increase from 0 ps and reach a maximum around 1 ps followed by a fast decay. It is important to note that the relative intensities of the three most prominent bands around 1330, 1510, and 1643 cm-1 remain

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TABLE 1: Comparison of SERS of Rhodamine 6G (10-6 mol/L) in a Silver Colloidal Solution for Pulsed and Cw Excitationa Ω (cm-1) σ n/(n + 1)300K k(ω){n/(n + 1)}300K (IaS/IS)cw (IaS/IS)pulsed n/(n + 1)pulsed

604 0.75 ( 0.02 0.052 0.075 0.33 ( 0.02 0.37 ( 0.03 0.058

765 0.64 ( 0.02 0.025 0.037 0.17 ( 0.02 0.21 ( 0.02 0.031

1178 0.42 ( 0.01 0.0035 0.0064 0.045 ( 0.005 0.12 ( 0.02 0.095

1305 0.70 ( 0.02 0.0019 0.0037 0.027 ( 0.003 0.15 ( 0.02 0.011

1355 0.92 ( 0.03 0.0015 0.0030 0.027 ( 0.003 0.11 ( 0.02 0.006

1510 1 0.0007 0.0017 0.015 ( 0.003 0.10 ( 0.02 0.005

1643 0.32 ( 0.02 0.0004 0.0009 0.016 ( 0.003 0.31 ( 0.02 0.008

a Ω: wavenumber of the vibration; σ: SERS cross section normalized to the cross section of the strongest vibration at 1510 cm-1; n/(n + 1)300K: population number calculated for T ) 300 K; k(ω) ) {(ωL - Ω)/(ωL + Ω)}4; ωL: wavenumber of excitation; (IaS/IS)cw and (IaS/IS)pulsed: anti-Stokes to Stokes SERS intensity ratios measured for cw and pulsed excitation, respectively; n/(n + 1)pulsed: population number estimated for pulsed excitation.

Figure 2. Stokes and anti-Stokes SERS spectra of Rh6G dissolved in a silver colloidal solution recorded with 633 nm pulsed excitation. Upper spectrum: anti-Stokes SERS spectrum enhanced by a factor of 6. Lower spectrum: Stokes SERS spectrum.

Figure 4. Temporal evolution of the anti-Stokes to Stokes intensity ratios IaS/IS for the modes at 1643 and 1510 cm-1 measured in a onecolor pump-probe scheme. Solid line: Approximation by a three level model with a rise time τ1 ) 0.8 ps, decay time τ2 ) 1.0 ps convoluted with the cross correlation function of a width (fwhm) of 0.8 ps.

varying the rise (τ1) and decay times (τ2) of the intermediate level (the population of which is observed in anti-Stokes Raman scattering) and convoluting its population kinetics for δ-pulse excitation with the cross correlation function of the experiment, the optimum fit is obtained. We approximated the kinetics of both vibrations with τ1 ) 0.8 ( 0.3 ps and τ2 ) 1.0 ( 0.3 ps. Similar kinetics (not shown here) is also observed for the doublet 1305/1355 cm-1. Discussion

Figure 3. Spectral evolution of anti-Stokes SERS difference spectra of Rh6G.

unchanged indicating that both the rise times of these vibrations as well as their decay times are close to each other. In Figure 4 we present the temporal evolution of the (IaS/ IS)pulsed ratios for the modes at 1510 and 1645 cm-1. The temporal evolution of the vibrations is symmetrical with respect to zero delay time. This is because in the single color experiment each of the pulses, which are of equal intensities, act likewise as pump and probe pulse.20 The kinetics shows a minimum at zero delay time followed by a fast rise and subsequent decay. Our approximation of the temporal evolutions presented in Figure 4 is based on rate equations for a three-level model. By

The ratios (IaS/IS)cw are independent of excitation intensity. Consequently, we can safely assume that they are due to the thermal vibrational population distribution n at ambient temperature, which is given by n )(epΩ/kT - 1)-1. Nevertheless, (IaS/IS)cw for Rh6G excited at 633 nm is significantly higher than (IaS/IS)calculated )k(ω){n/(n + 1)}300K, i.e., than one would expect for conventional Raman scattering far off electronic resonance at room temperature. The strong deviations between measured and calculated values are mostly due to the electronic resonance of the anti-Stokes Raman transitions with respect to plasmonic and electronic transitions of Rh6G. Both the maximum of the plasmonic resonances of the colloidal solution and the known maximum of Rh6G dissolved in water are located at shorter wavelengths than the excitation wavelength and, therefore, it is reasonable to assume that the resonances of the Rh6G molecules adsorbed at the colloids which are relevant for SERS are located at shorter wavelengths too. This is in accordance with (IaS/IS)cw/(IaS/IS)calculated > 1 as well as with its rise with increasing wavenumber of the vibrations. Under these conditions the anti-Stokes Raman transitions of high-frequency modes are closer to electronic resonance than low-frequency modes.7-9

The Vibrational Pumping Mechanism in SERS Now we discuss the vibrational population distribution observed for pulsed excitation. As pulsed and cw excitation are applied at the same wavelength, i.e., under comparable resonance conditions for the different modes, the about 20-fold increase of (IaS/IS)pulsed/(IaS/IS)cw at 1643 cm-1 is mainly due to vibrational populations. For thermal equilibrium an increase of population by a factor of 20 for this mode corresponds to an increase of the temperature from 300 to 485 K. For a such a jump of the temperature we calculate an about 3-fold increase of population for the mode at 604 cm-1. In contrast, its intensity increase amounts to a factor of 1.1 only. In general, we observe a stronger enhancement of population of high-frequency modes relative to low-frequency modes compared to calculated thermal population distributions. For modes with frequencies close to each other, i.e., with almost the same electronic resonance conditions, pronounced deviations from thermal population distributions are clearly seen too. The excess population of the 1645 cm-1 mode is twice the population of the mode of lower frequency at 1510 cm-1 (cf. Table 1), which is in sharp contrast to any Boltzmann population distribution. Just the opposite behavior is obtained for the doublet 1305/1355 cm-1 where for pulsed excitation the population of the high-frequency component at 1355 cm-1 is only half the population of the low-frequency component at 1305 cm-1 (cf. Figure 2 and Table 1). Finally, we note that during the decay of the modes within a few picoseconds (cf. Figure 3) there is no significant redistribution of the intensities between the modes. As such change of relative intensities is indicative for the cooling of thermalized molecules, this means that even during the few picoseconds accessible to our observations thermal equilibrium is not achieved.25,26 We conclude that we do not observe conventional heating of Rh6G, which could arise from energy transfer from the colloids,27 and/or if the energy redistribution between the modes is completed on a subpicosecond time-scale already. Instead, we observe a pronounced nonequilibrium vibrational population distribution within the first few picoseconds. We will now discuss possible models of vibrational pumping taking mode-selectivity of pumping as well as population kinetics into account. If Stokes Raman scattering excites the vibrations, their population should respond to the excitation pulse extremely fast. For conditions of strict resonance with a broad electronic transition, Raman scattering may be delayed due to dephasing of the intermediate electronic level, however, with a time below 100 fs. For detuning from resonance the response will be even faster. In contrast, population of the vibrational levels observed experimentally builds up with a comparatively slow rise time of ∼0.8 ps. Furthermore, the pumping rate for the Stokes Raman pumping model scales linearly with the Stokes SERS cross section σ.5 Population accumulated within the pulse duration is approximately proportional to the product of σ and τ2. For the ratio of the Stokes SERS cross sections at 1643 and 1510 cm-1 we obtain σ1510/σ1643 ≈ 3. Correspondingly, for equal vibrational decay times one expects from the Raman pumping model an approximately three times higher excess population for the 1510 cm-1 than for the 1643 cm-1 mode. This is just opposite to the observation that the vibration at 1643 cm-1 gains about twice the population of the 1510 cm-1 mode (cf. Table 1). For achieving this population distribution by Raman pumping, the lifetime of the 1510 cm-1 should be six times shorter than that of the 1643 cm-1 mode. Instead, our time-resolved experiments show that both vibrations have about the same decay time of 1 ps (cf. Figure 4). It should be noted that for cw excitation of

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10487 Rh6G for the modes at 1510 and 1643 cm-1, relaxation times of 0.4 and 1.3 ps, respectively, have been estimated assuming that the Raman pumping model is valid.6 We summarize that the comparatively slow rise times of populations as well as the observation that mode populations do not scale linearly with the product of σ and τ2 rule out the Raman pumping model. After excluding Raman pumping we now consider electronic excitation as the source of vibrational pumping. Vibrational pumping scales linearly with excitation intensity indicating a one-photon absorption process.5,6 Radiation at 633 nm can be absorbed in the wing of the electronic absorption band of Rh6G adsorbed at the silver particles or by the silver particles themselves.21,27 Furthermore, new charge-transfer states due to chemisorption are created.22 They are assumed to absorb in a very broad range in the visible spectral region leading to the chemical enhancement mechanism of SERS. They occur for dyes, such as Rh6G, which are adsorbed on silver colloids associated with anions as for instance Cl-.23,24 These are just the systems where vibrational pumping has been observed. Once the system is excited electronically, vibrations are populated by both fluorescence and internal conversion. Both processes are in accordance with a delayed rise of vibrational populations. We note that;similar to the present observations;modeselective excitation and energy redistribution have been monitored after intramolecular electron transfer excitation and subsequent ultrafast internal conversion (IC).25,26,28 Vibrational population rise times of a few picoseconds of distinct highfrequency fingerprint modes comparable to the present SERS experiments have been observed in these experiments too. As SERS of Rh6G is accompanied by strong fluorescence quenching, this suggests that the vibrations are excited predominantly via ultrafast IC here as well. In conclusion, we monitored nonequilibrium vibrational excess populations for excitation of SERS and their kinetics. For the first time this allows us to distinguish between possible models of vibrational pumping based on their different temporal characteristics. We observed mode-selective vibrational excitation due to vibrational pumping which is delayed with respect to the excitation pulses and followed by a decay of vibrational populations with a common time constant of 1 ps. These findings rule out Stokes Raman pumping as a main contribution. Instead, vibrational pumping in SERS originates from electronic excitation followed by fluorescence and ultrafast internal conversion. Acknowledgment. W.W. gratefully acknowledges fruitful discussions with K. Kneipp, Department of Physics Technical University of Denmark. We thank T. Elsaesser for critically reading the manuscript. This work was supported by the Deutsche Forschungsgemeinschaft, project WE 1489/6-2. References and Notes (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163166. (2) Dieringer, J. A.; McFarland, A. D.; Shah, N. C.; Stuart, D. A.; Whitney, A. V.; Yonzon, C. R.; Young, M. A.; Zhang, X. Y.; Van Duyne, R. P. Faraday Discuss. 2006, 132, 9. (3) Surface-Enhanced Raman Scattering-Physics and Applications; Kneipp, K., Moskovits, M., Kneipp, H., Eds.; Springer: New York, 2006. (4) Kneipp, K. Phys. Today 2007, 60, 40. (5) Kneipp, K.; Wang, Y.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1996, 76, 2444. (6) Maher, R. C.; Galloway, C. M.; Le Ru, E. C.; Cohen, L. F.; Etchegoin, P. G. Chem. Soc. ReV. 2008, 37, 965. (7) Haslett, T. L.; Tay, L.; Moskovits, M. J. Chem. Phys. 2000, 113, 1641. (8) Brolo, A. G.; Sanderson, A. C.; Smith, A. P. Phys. ReV. B 2004, 69, 045424.

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(9) Maher, R. C.; Cohen, L. F.; Le Ru, E. C.; Etchegoin, P. G. Faraday Discuss. 2006, 132, 77. (10) Xu, H.; Wang, X.-H.; Persson, M. P.; Xu, H. Q. Phys. ReV. Lett. 2004, 93, 243002. (11) Galloway, C. M.; Le Ru, E. C.; Etchegoin, P. G. Phys. Chem. Chem. Phys. 2009, 11, 7372. (12) Elsaesser, T.; Kaiser, W. Annu. ReV. Phys. Chem. 1991, 42, 83. (13) Qian, J.; Schultz, S. L.; Bradburn, G. R.; Jean, J. M. J. Phys. Chem. 1993, 97, 10638. (14) Matousek, P.; Parker, A. W.; Toner, W. T.; Towrie, M.; Faria, D.L. A.; Hester, R. E.; Moore, J. N. Chem. Phys. Lett. 1995, 237, 373. (15) Reid, P. S.; Doig, S. J.; Wickam, S. D.; Mathies, R. A. J. Am. Chem. Soc. 1993, 115, 4754. (16) Nakabyashi, T.; Okamoto, H.; Tasumi, M. J. Phys. Chem. A 1997, 101, 3494. (17) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (18) Kozich, V.; Dreyer, J.; Werncke, W. Chem. Phys. Lett. 2004, 399, 484.

Kozich and Werncke (19) Kozich, V.; Werncke, W. J. Mol. Struct. 2005, 735, 145. (20) Gunaratne, T. C.; Milliken, M.; Challa, J. R.; Simpson, M. C. Appl. Opt. 2006, 45, 558. (21) Zhao, J.; Jensen, L.; Sung, J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2007, 129, 7647. (22) Hildebrandt, P.; Stockburger, J. M. J. Phys. Chem. 1984, 88, 5935. (23) Doering, W. E.; Nie, S. J. Phys. Chem. B 2002, 106, 311. (24) Maruyama, Y.; Futamata, M. J. Raman Spectrosc. 2005, 36, 581. (25) Hogiu, S.; Werncke, W.; Pfeiffer, M.; Elsaesser, T. Chem. Phys. Lett. 1999, 312, 407. (26) Hogiu, S.; Werncke, W.; Pfeiffer, M.; Dreyer, J.; Elsaesser, T. J. Chem. Phys. 2000, 113, 1587. (27) Varnavski, O. P.; Goodson, T., III; Mohamed, M. B.; El-Sayed, M.-A. Phys. ReV. B 2005, 72, 235405. (28) Kozich, V.; Werncke, W.; Dreyer, J.; Brzezinka, K.-W.; Rini, M.; Kummrow, A.; Elsaesser, T. J. Chem. Phys. 2002, 117, 719.

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