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ELECTROMAGNETICALLY INDUCED CHANGES IN INTENSITIES, SPECTRA, AND TEMPORAL
BEHAVIOR OF LIGHT SCATTERING FROM MOLECULES ON SILVER-ISLAND FILMS
S. Garoff, D. Weitz, M. Alvarez, J. Chung
To cite this version:
S. Garoff, D. Weitz, M. Alvarez, J. Chung. ELECTROMAGNETICALLY INDUCED CHANGES
IN INTENSITIES, SPECTRA, AND TEMPORAL BEHAVIOR OF LIGHT SCATTERING FROM
MOLECULES ON SILVER-ISLAND FILMS. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-
345-C10-348. �10.1051/jphyscol:19831069�. �jpa-00223527�
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Colloque CIO, supplément au n°12, Tome kk, décembre 1983 page C10-345
E L E C T R O M A G N E T I C A L L Y INDUCED C H A N G E S IN I N T E N S I T I E S , S P E C T R A , A N D T E M P O R A L B E H A V I O R OF L I G H T S C A T T E R I N G FROM M O L E C U L E S ON S I L V E R - I S L A N D F I L M S
S. Garoff, D.A. Weitz, M.S. Alvarez and J.C. Chung
Exxon Research and Engineering Company, P.O. Box 45, Linden, Dew Jersey 07036, U.S.A.
Résumé - Les interactions électrodynamiques conduisant à la diffusion Raman exaltée de surface (SERS) conduisent aussi à des modifications dramatiques des intensités, des spectres et du comportement en fonction du temps de l'émission de fluorescence des adsorbats sur des surfaces métalliques rugueuses. Des déplacements spectraux de la fluorescence d'adsorbats sur des couches granulaires ont été observés par différentes méthodes. On peut rendre compte des modifications observées dans le cadre de la théorie électromagnétique du SERS, généralisée pour inclure les détails des résonances moléculaires et des processus d'émission relaxée.
Abstract The electrodynamic interactions which lead to surface-enhanced Raman scattering (SERS) also lead to dramatic changes in the intensities, spectra, and temporal behavior of the fluorescent emission of adsorbates on rough metal surfaces. Spectral shifts of the fluorescence of adsor- bates on island films have been observed (a) by examining the emission from adsorbates on different substrates, (b) by a "photochemical holeburn- ing" method, and (c) by measuring the temporal evolution of the fluores- cent spectrum at very short times after laser excitation. The observed changes can be accounted for within the framework of the electromagnetic theory of SERS generalized to include the details of molecular resonances and relaxed emission processes.
Beyond the initially observed enhancement of normal Raman scattering, a number of other optical properties of molecules absorbed on rough metaJ surfaces also,exhibit unusual behavior: absorption , resonance Raman scattering , fluorescence ' > > , and photochemistry'"8. The dominant mechanism underlying these phenomena is the electrodynamic interaction among the optical fields, the vibrational and electronic states of the adsorbate, and the plasma resonances localized on the roughness fea- tures of the surface. A model based on these electrodynamic interactions has been used to explain many observations of the optical behavior of adsorbates on island films including: the hierarchy of enhancements of normal Raman, resonance Raman and
relaxed fluorescence3; excitation profiles of Raman scattering9; phosphorescent lifetimes ; and photochemical degradation rates .
In this paper, we discuss the intensity, spectral, and temporal characteristics fluorescent emission from adsorbates on island films. Our measurements show the the molecules with the strongest electromagnetic coupling to islands have altere<
emission characteristics compared to their characteristics in an electromagnetically inert environment. The emission rate of these molecules is substantially increased;
their photochemical degradation rate is substantially decreased; and both the peak wavelength and shape of the fluorescent emission spectrum are shifted. Each of these effects can be understood when the details of the molecular resonance and of
relaxed emission processes are included in the picture of the electrodynamic coup- ling with the islands. Furthermore, the observed changes can be used to probe the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831069
ClO-346 JOURNAL DE PHYSIQUE
s p a t i a l dependence o f t h e electromagnetic i n t e r a c - t i o n s of t h e inhomogeneous
-
r-
environment i n h e r e n t i n t h ea s t r u c t u r e o f i s l a n d f i l m s .
-
0 c We f i n d t h a t t h e temporal-
and s p e c t r a l behavior com-P -
-
pliment each o t h e r as a:
probe o f t h e s p a t i a l de-Z pendence, and b o t h a r e
z c o n s i s t e n t w i t h r e s u l t s of
experiments which employ d i f f e r e n t s u b s t r a t e s t o emphasize fluorescence f r o m
500 600
,,,,,
molecules i n various posi-Wavelength (nm) t i o n s on t h e f i l m s .
F i g u r e 1. Normalized fluorescence soectrum from Experimental procedures RuTBP on (
. . . . . .
. ..
. (. .
) SiO,. ) Ag/Si02, ( - - --
- ) v ~ / A l , t e r i z e i s l a n d f i l m s , used t o prepare and chgrgc- 3'apply3 ,8 molecular coat- i gs, and measure f l u o - rescence spectra and photochemical degradation r a t e s 3 are r e p o r t e d i n d e t a i l elsewhere. The adsorbate used i n t h e present study i s ruthenium t r i s - b i p y r i d i n e (RuTRP). We use s i l v e r and g o l d i s l a n d f i l m s w i t h i s l a n d s about 2008 i n diameter and separated by s i m i l a r distances. L i f e t i m e measurements are made u s i n g a time- c p r r e l a t e d photon counting techniquelo, and t e m p o r a l l y resolved spectra a r e measured u s i n g a v a r i a t i o n o f a i f c h n i q u e which has been suggested f o r fluorescence r e j e c t i o n f o r Raman spectroscopy.
The fluorescence spectra from RuTBP a t t h e same adsorbate coverages on d i f f e r e n t substrates i s shown i n F i g u r e 1. These normalized spectra show a progressive s h i f t o f t h e emission t o l o n g e r wavelength i n t h e f o l l a i n g sequence: s i l v e r - i s l a n d s on aluminum ( A g / ~ l ) t o s i l v e r - i s l a n d s on s i l i c a (Ag/SiO ) t o hare s i l i c a (Si02). The i n t e n s i t y o f t h e emission from Ag/Si02 i s enhanced l?y 1.7 over t h e intensity f r o m Si02. This r e s u l t i s i n t e r m e d i a t e between t h e enhance e n t o f fluorescence found f o r very high and very l a quantum e f f i c i e n c y adsorbatesy and suggests t h a t t h e t o t a l emission f o r RuTRP on Ag/Si02 i s made up of emission from molecules b o t h near t h e i s l a n d s and on t h e s i l i c a between t h e islands. h r photochem'cal r e s u l t s f o r RuTRP
B
are s i m i l a r t o those found f o r another adsorbate, Rhodamine 66
.
The r a t e o f photo- chemical degradation o f molecules on Ag/Si02 i s slower t h a n on SiO and t h e spectra again show a progressive s h i f t i n g t o l o n g e r wavelength i n t h e f o f l o w i n g sequence:a f t e r degradation t o b e f o r e degradation t o spectrum o f emission remove by t h e photo- chemical process (equal t o t h e spectrum b e f o r e degradation minus t h e spectrum a f t e r degradation).
The f l u o r e s c e n t decay curves o f RuTBP on s i l v e r and g o l d i s l a n d f i l m s are shown i n Figure 2. In s o l u t i o n , RuTRP emission has an exponential time dependence w i t h a l i f e t i m e o f = 200 nsec. In c o n t r a s t , t h e decay on i s l a n d f i l m s i s much more r a p i d and has a nonexponential character. The temporal e v o l u t i o n o f t h e emission spectrum of RuTBP on i s l a n d f i l m s a f t e r pulsed e x c i t a t i o n i s shown i n Figure 3. As t h e delay time between t h e e x c i t a t i o n pulse and t h e observed fluorescence i s increased, t h e r a t i o o f Raman s c a t t e r i n g t o fluorescence decreases, and t h e f l u o r e s c e n t emission s h i f t s t o l o n g e r wavelength.
Our r e s u l t s can be understood i n terms of a model based on t h e electrodynamic i n t e r - a c t i o n s on i s l a n d f ilms3. The i n f l u e n c e of t h e resonances a f f e c t s t h e o p t i c a l pro- cesses i n two ways. F i r s t , t h e l o c a l f i e l d s i n and around t h e i s l a n d s a r e enhanced and t h e r e f o r e t h e pumping of t h e o p t i c a l process i s increased. Second, t h e o.sci1- l a t i n g d i p o l e o f t h e adsorbed, e x c i t e d molecule induces a macroscopic p o l a r i z a t i o n i n t h e islands. The d i p o l a r component o f t h i s response increases t h e t o t a l emission d i p o l e o f t h e coupled molecul e - i sland system and t h u s increases t h e r a d i a t i v e decay
Log Time (nsec)
F i g u r e 2. Temporal decay o f fluorescence o f RuTBP ( I ) S i l v e r - i s l a n d f i l m w i t h e x c i t a t i o n a t 488 nm.
(11) Gold-island f i l m w i t h e x c i t a t i o n a t 458 nm.
Emission detested a t 600 nm. (--
-- -
) Theoryf o r each c a s e e .
r a t e o f t h e system. Also, n o n r a d i a t i v e decay channels a r e opened t o t h e e x c i t e d molecule by coupl i ng t o l o s s y modes i n t h e island.
The balance between these new s u r f ace-induced r a d i a - t i v e and n o n r a d i a t i v e decay channels as w e l l as those channels i n t r i n s i c t o t h e molecule determine t h e r a d i a t i v e y i e l d o f t h e system. Each o f t h e por- t i o n s o f t h e electrodynamic c o u p l i n g i s a s e n s i t i v e f u n c t i o n o f t h e s e p a r a t i o n between t h e molecule and t h e island. The subset of molecules on t h e f i l m c l o s - e s t t o i s l a n d s a r e most s t r o n g l y coupled t o t h e plasma resonance o f t l l e i s l a n d and t h u s e x h i b i t f a s t e s t decay o f t h e adsor- bate. Molecules between i s l a n d s e x h i b i t decay more 1 i ke t h e f r e e molecule.
The e f f e c t s of t h e increased r a d i a t i v e r a t e and inhomogeneous n a t u r e o f an i s l a n d f i l m are seen d i r e c t l y i n t h e decay curves i n F i g u r e 2. The inhomogeneity o f cou- p l i ngs across t h e f i l m causes d i f f e r e n t molecules t o experience d i f f e r e n t surface- induced increases i n t h e i r r a d i a t i v e r a t e s and t h u s produces t h e observed nonexpon- e n t i a l behavior. I n curve I, b o t h t h e e x c i t a t i o n and emission wavelength a r e tuned t o t h e s i l v e r i s l a n d f i l m resonance. However, i n curve 11, only t h e emission wave- l e n g t h i s tuned t o t h e resonance o f t h e g o l d i s l a n d f i l m . This d i f f e r e n c e leads t o d i f f e r e n t l y shaped decay curves i n t h e two cases. As shavn by t h e dashed 1 in e s i n F i g u r e 2, t e electrodynamic p i c t u r e a p p l i e d t o d e s c r i he f 1 uorescence decay on i s l a n d f i l m s k accounts b o t h f o r t h i s d i f f e r e n c e i n shapes o f t h e curves as w e l l as t h e shape o f each i n d i v i d u a l curve.
The r e s u l t s o f our t h r e e s p e c t r a l measurements c o n f i r m t h a t molecules nearest i s - lands do decay most r a p i d l y and t h a t t h i s subset o f molecules has a spectrum s h i f t e d t o s h o r t e r wavelength compared t o t h e spectrum o f t h e weakly coupled molecules between t h e islands. Not o n l y does each i n d i v i d u a l experiment i n d i c a t e a s p e c t r a l s h i f t as i t probes t h e inhomogeneous environment o f t h e i s l a n d f i l m b u t spectra from d i f f e r e n t experiments e x h i b i t s t r i k i n g resemblances. Thus, emission (a) f r o m mole- c u l e s on Ag/Al, (b) from molecules most r e s i s t a n t t o photochemical degradation on Ag/Si02, and (c) from molecules w i t h t h e f a s t e s t f l u o r e s c e n t decay r a t e s on Ag/Si02 a l l show s i m i l a r spectra. These molecules a r e s p a t i a l l y near i s l a n d s and, having very r a p i d surface-induced decay r a t e s , spend l e s s t i m e i n photochemically r e a c t i v e and o t h e r e x c i t e d states. I n contrast, emission (a) from molecules on SiO (b) f r o m molecules l e a s t r e s i s t a n t t o photochemical degradation on Ag/Si02, and (c?' from molecules w i t h l o n g e r f l u o r e s c e n t decay r a t e s on Ag/SiO a l l s h w s i m i l a r spectra but a t a l o n g e r wavelength t h a n those discussed above. ~ 2 e s e molecules a r e f a r from i s l a n d s and, having only weak coupl i n g t o surface-induced modes, have residence times i n photochemically r e a c t i v e and o t h e r e x c i t e d s t a t e s more l i k e t h e f r e e mole- c u l e.
Our experimental methods have a l l w e d us t o probe t h e inhomogeneity of t h e i s l a n d f i l m and have a l s o revealed a dramatic s p e c t r a l s h i f t o f t h e emission f r o m molecules nearest t h e islands. This s h i f t i s induced by t h e electrodynamic c o u p l i n g of t h e e x c i t e d s t a t e s o f t h e adsorbate t o t h e plasma resonances i n t h e f i l m . For t h e most
JOURNAL DE PHYSIQUE
Wavelength (nm)
F i g u r e 3. Temporal e v o l u t i o n o f normalized spectrum o f RuTBP on s i l v e r - i s l a n d f i l m . Times marked on each curve represent t h e temporal window a f t e r pulsed ex- c i t a t i o n d u r i n g which fluorescence i s c o l l e c t e d .
s t r o n g l y coupled molecules, t h e s u r f ace-induced decay
r a t e may be so f a s t t h a t i t can compete s u c c e s s f u l l y w i t h t h e very f a s t i n t e r - and i n t r a m o l e c u l a r pro- cesses which r e l a x t h e e x c i t e d s t a t e o f t h e mole- c u l e b e f o r e i t emits i n a f r e e s t a t e such as on SiO
.
Thus, a molecule strong?y coupled t o t h e plasma reso- nance may emit b e f o r e t h i s r e l a x a t i o n occurs and w i t h a spectrum s h i f t e d compared t o t h e f r e e case. I n addi- t i o n , w h i l e t h e s p e c t r a l shape o f t h e f r e e molecule emission . i s determined by t h e Frank-Condon f a c t o r s connecting t h e ground and e x c i t e d s t a t e s ; when t h e molecule i s s t r o n g l y cou- p l e d t o t h e plasma reso- nance, t h e frequency de- pendence o f t h e r a d i a t i v e y i e l d of s u r f ace-i nduced decay channels w i l l a l s o a f f e c t t h e o v e r a l l r a d i a - t i v e e f f i c i e n c y of each t r a n s i t i o n o f t h e adsorbed molecule. This would a l s o cause s t r o n g l y coup1 ed molecules t o emit w i t h a d i f f e r e n t spectrum t h a n weakly coupled o r f r e e mol ecul es.
References
1. A recent review i s Chang R. K., and Furkak T. E., eds., "Surface Enhanced Raman S c a t t e r i n g " (Plenum Press, New York, 1981).
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&,
245 (1981).3. Weitz D. A., e t al., J. Chem. Phys. 78, 5324 (1983).
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8. Garoff S., e t . al., Chem. Phys. Lett. 93, 283 (1982).
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