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Effect of metal film thickness on surface-atom coupling
Volume 34, number 3
OPTICS COMMUNICATIONS
September 1980
EFFECT OF METAL FILM THICKNESS ON SURFACE-ATOM COUPLING Arnold ADAMS, R.W. RENDELL 1, R.W. GARNETT and P.K. HANSMA 2
Department of Physics and Quantum Institute, University of California, Santa Barbara, California 93106, USA and Horia METIU 2,3
Department of Chemistry and Quantum Institute, University of Cali[ornia, Santa Barbara, California 93106, USA Received 3 June 1980
Metal film thickness is found to be an important parameter in the nonradiative coupling of atoms to metal surlhces. Silver, sodium, and potasium fihns in the thickness range 8-120 nm are evaporated onto a 40 K substrate in ultra high vacuum. At ! ! K, layers of N2 in the thickness range 3-300 nm are condensed onto the metal films. Low ener~ electrons excite the N, 2D--, 4S (o~)electronic transition. The lifetime of o~is measured as a function of nitrogen thickness and metal film thickness. Results are found to be in good agreement with classical theory based on nonradiative coupling of perpendicular dipoles to surface plasmons of the metal surface.
Nonradiative energy transfer from excited atoms or molecules to surface plasmons of a nearby metal surface is a topic of current interest [ 1 - 1 4 ] . Classical theory [ 10-14] presently accounts for most (not all [3] ) of the experimental data but has not been completely tested. This communication presents experimental data showing the effect of the position of the surface plasmon resonance energy relative to an atom's emission energy. The position of the surface plasmon resonance is altered by changing the metal or the metal thickness. Nitrogen films of controlled thickness were prepared in ultrahigh vacuum (typical operating pressure ~ 7 X 10 -8 Pa ~ 5 X 10-10 Torr) on evaporated Ag, Na and K metal films at 11 K of approximately known thickness, as described previously [21. The thickness of the N 2 film deposited on the substrate was determined by I R.W. Rendell now at National Bureau of Standards, Washington, D.C. 20234. 2 A.P. Sloan Fellow. 3 Camille and l lenry Dreyfus Teacher-Scholar.
counting interference fringes from specularly reflected Hg pen lamp light CA= 253.65 run) directed at 15 ° from the substrate normal hlto the monochromator and PMT for a given exposure (pressure X time), of the substrate to N 2 gas. The measured points fell on a straight line with slope 1100 nm/Pa-sec. This thickness versus exposure calibration factor was independent of deposition rate over the range 0.08 to 1.5 nm/s (7 X 10 -5 to 130 X 10 -5 Pa). We used this measured calibration factor at the lower exposure rates to estimate the thickness of our thinner films of N 2 . The thickness of Na and K films was found using the interference fringe method (thin films of Na and K are nearly transparent at ;k = 253.65 nm) [15]. Silver thick, ness estimates were made by visuaUy comparing the Ag film to calibrated Ag films made in another system. The estimates were thought to be reliable to + 4 nm for film thicknesses less than 20 nm. Greater film thicknesses were estimated by assuming a constant deposition rate. The refrigerator was turned on and at 40 K a known thickness of metal (Ag, Na or K) evaporated onto a sapphire target. The evaporated metal surface was then 417
Volume 34, numl~er 3
OPTICS COMMUNICATIONS
heated to 165 K to desorb any residual gases and then cooled to 11 K. Pressures during metal deposition were never above 1 X 10 -5 Pa. On the first run, 3 nm of research ~ a d e N 2 was deposited. Thicker layers were del,',esiccd in subsequent runs after first desorbing the previous N 2 layer by heating to 100 K. The electron gun excited the N 2 with 200 eV electrons (0.6 pA). The ener~w flux over the approxinmtely 0.2 cm 2 excited area was 0,6 mW/cm 2. The decay curves of the N, 2D ~ 4S (e) transition were measured as a function of N 2 matrix thickness after manually turning off the electron ~ m . Turn off times of less than 0.1 s were achieved. Figs. 1--3 show experimental results for the lifetime of ~ as a function of N 2 fihn thickness, type of metal film. and metal film tltickness. The lifetime reported here is measured after approximately one lifetime has already elapsed. This was done so that the initial n o n exponential decay of the N atoms could be neglected [2]. For the first time Na has been examined and for both Ag and K the measurements as a function of metal and N 2 f'dm thickness are more complete. To compute the theoretical lifetime of oe shown in fi~. 1 - 3 we assume that the radiating dipole rno, nent is perpendicular to the surface for four reasons: I) Our calculations of radiation patterns of perpendicular and parallel dipoles coupled strongly to surface plasmons show that ttle intensity of parallel dipoles should be smaller than that of perpendicular dipoles at an obser-
September 1980 I
I
I
I
I
f
40
~'\ . / / . . o ..........., - ~ ,,
io../ #I
/
------,oo~m no {0)
~
/
......
0
-.-I
_I I~ )0
0
I___
N z TH;CKNESS
:2 2 ,o, 1
t 200
_.L. 3CK
(nm)
Fig. 2. Lifetime of the c~transition as a function of N2 thickness for 50 nrn (o) and ! 00 nm (o) Na films. Theoretical curves arc given for 100 rim, 50 tam and 10 nm Na fihns.
vation angle of 15 ° for h ~ 200 nm, though the total integrated intensity of the parallel dipoles is larger. 2) The intensity of the perpendicular dipoles at an observation angle of 15 ~' should be proportional to the total integrated int~ asity (to within ~'5%)for h ~< 100 nm, the region of Most interest. 3) The monochrometer is slightly more sensitive (;~ 30%) to photons from penpendicular dipoles. 4) It was considerably simpler to calculate the curves without a complete angular average. Nevertheless, these calculations could be done and might slightly improve tbe agreement with experiment, though
................ T . . . . . . . .
-T
l
i. . . . .
---------
120 nm
1 "
l
GO-
/ .2
/
u,-J
i,i
t~_J
/
F ,~ ? 0 s~
I
_ ~f, ...,
,L ,'
....... ,
TO
¢/
ot~o" O
t~ n m
......
___L_
i ~J2 7}-41'~2KI'JESS
I 2C)0
(nm)
aq
1
3OO
Fig. !. Lifetime of the ~ trans.tion of matrix isolated N atoms as a function of N2 thickness for 8 nm (o) and 60 mn (e) Ag fihns. Theoretical curves are also yNen. to, the lifetune of oe x~hen no surface is present, is 39 s. Uncertainty in lifetime measurements isabvtr, 101,. 41,~
I
I
_.A
l,h~
Aq (O)
v
/
¢:_
L
\\
o
,,/~ ,7
1
K
(o)
. . . . . . .
IO
nm
K
(O)
. . . .
I()
nm
K
I ........
L _ _ _ I _ _ _ L
IOO 200 N2 THICKNESS (nm)
300
I:ig. 3. Lifetime of the o~transition as a function {}I"N2 thickness for 40 nm (o) and 120 nm (e) K films. Theoretical curves are given for 120 nm,40 nm and 10 tun K I'iims. Note the much stronger c(mpling (as indicated by tile short lifetimes out to N2 thicknesses of",- 120 nm) on K relative to Ag or Na.
Volume 34, number 3
OPTICS COMMUNICATIONS
as discussed below, there are other probable sources of error. The theory takes account of electron range in the N 2 film, effect of the dipole image field on the dipole and the effect of metal film thickness on the surface plasmon resonance as described previously in classical terms [2]. Dielectric constants used in the theory are contained in refs. [ 1 5 - 1 7 ] . The theoretical fits to the data are good showing that classical theory [12 ] seems to be valid for z f> 3 nm for various metal thicknesses and for both free electron-like metals (Na and K) and Ag (which has a strong interband transition near the surface plasmon resonance). The small deviations of experiment from theory may be due to many causes: I) roughness of the metal surface relaxing the momentum conservation requirement on surface plasmon radiation [2] ; 2) imprecise knowledge o" metal dielectric constants (this can be a 30% effect; there is no general agreement on dielectric constants for most metals, probably due to different conditions of preparation); 3) uncertainties in the effective electron range due to production of secondary electrons; and 4) the assumption of perpendicular dipoles discussed above. Fig. 1 shows stronger coupling (shorter lifetime) between N atoms close to the Ag surface (N 2 fihal thickness <~ 50 nm) if the Ag surface is thin. This is caused by a splitting of the surface plasmon resonance into higher energy and lower energy resonances due to interactions of the electron density oscillations on each surface of the thin Ag film [18]. (See fig. 4.) The higher energy mode is at 1 + exp(-Kxd ) times the e lergy of the thick film mode; the lower energy at 1 exp(-Kxd ) where d is the thickness of the film avd Kx = (to/c)[eoe !/(Co + el)] 1/2. It is the lower e,tergy resonance of the thin Ag film which is strongly interacting with o~. A similar, though less pronounced effect is observed on Na (fig. 2, N 2 thickness < 30 ran). K p., . .,.,.~. -,,v~, I. ..... .I,,, n . 3) , .i,' ,,,,. o p p o s i t e ,,,('r,,,,) . , , . . , . . iv.~,, . . . , s,.~fac~" .... p l a s m o n s o n t h i n K f i l m s are less easily e x c i t e d b y o~
than surface plasmons on thick K films. This is because the K surface plasmon resonance (Eqp ~ 2.58 eV) is already near the emission energy of a (Ea ~ 2.37 eV) and so the splitting of the K st~rface plasmon resonance moves the less sharp lower energy resonance to an energy lower than Ea.. The closeness of E a to ESI, for K also gives very strong coupling to the surface (strong coupling out to 120 nm). it is interesting to note that
September 1980
THICK FILM" + ++
+
++
THIN FILM: (o) LOWER ENERGY +++
+++
+++
+++
(b) HIGHER ENERGY
+++
+++
/////////////////////.//; + + +
+ + +
+.++
Fig. 4. The surface plasmon modes on each side of a thin film combine to give a higher energy, and lower energy mode than for a thick film. The distribution of charges is for one wavelength at a given instant.
the coupling of a to thick films of Na is slightly stronger than to thick films of Ag even though the surface plasmon energy of Ag (Esp ~ 3.57 eV) is nearer to E a than is the surface plasmon resonance of Na (Esp 3.71 eV). This is due to the free electron-like nature of Na. The lifetimes for N 2 thicknesses greater than about 50 nm on Ag and Na or 150 nm on K are dominated by the interaction of the dipole with its reflected image field. This gives an oscillatory effect to the lifetimes as a function of N 2 thickness and is more pronounced on the thick metal films than on the thin metal films since the thick metal films are better reflectors - they can more easily form an image of a dipole that is far from the metal surface. Though classical theory fits all of the dat~i at present, there are extensions of the theory that must be made. Metal surface roughness has been shown to be an important parameter in atom-surface coupling [2], and some theory is now being done [19], but it is fair to say that the experiments and theory are not in agreement. Refinements in theory and better characterization of the metal surface roughness are necessary before ' a final judgment can be made. Also, the theory has not been tested for atoms or molecules whose emission energy is nearly identical to the metal's surface plasmon resonance. Even the N atoms coupling to a thick K surface, the strongest coupling demonstrated to date, is small compared to the coupling expected from an atom 41q
voh~Inc 34. number 3
OPTICS COMMUNICATIONS
or molecu'., whose emissicm energy is coincident with ll~e K surface plasmon resonance (factor of ~ 10), Fin:,~iv. the experiments of tlarris et al. [ 1 ], and Rosseili and Brus [3] are attempting to answer the question of whether classical theory is valid within a few atomic radii of the surthce. ~ l e experiments of Harris et al. indicated tha~ lbr a molecule not near the surface plasmon energy of the metal, classical theory describes the molecular quantum efficiency for molecule-surface separations as small as 0.8 nm. Recent calculations [20] ~Lsing a jellium model and random phase approximation ate in general agreement with this conclusion. The experiments of Rossetti and Brus do not agree with classical theory for molecule-surface separations less than 13 rim. This disagreement may be related to surface roughness o~ an impurity layer on the surface. The authors would like to thank Dr. L.E. Brus and Professor C.B. Harris ~o, sending us preprints of unpublished work. This work was supported by the National Science Foundation under grant numbers DMR76-38423 and CHE78-16181. ¢"
¢
References It ] \ . (ampion. A.R. (;allo. C.B. tlarris, II.J. Robota and 1'.\I. '~khiim~rc. ('hem. Phys. I ett. 73 ¢1980) 447.
420
September 1980
[2] A. Adams, R.~ Rendell, W.P. West, H.P. Broida, P.K. Hansma and ti. Metiu, Phys. Rev. B, (as. pted). [3] R. Rossett" ~r./L.E. Brus, J. Chem. Phys., (accepted). [~1 L.E. Brus, J. Chem. Phys., (submitted). i5] W.H. Weber and C.F. Eagen, Optics Lett. 4 (1979) 236. I6] I. Pockrand, J.D. Swalen, R. Santo, A. Brillante and M.R. Philpott, J. Chem. Phys. 69 (1978) 4001. [7] T. Lopex-Rios and G. Vuye, Surf. Sci. 81 (1979) 529. [8] K.H. Drexhage, Progress in Optics XII, ed. E. Wolf tNorthHolland, Amsterdam, 1974) p. 165. [9] H. Kurczewska and H. Bassler, J. Lumin 15 (1977) 261. [10] H. Morawitz and M.R. Philpott, Phys. Rev. BI0 (1974) 4963. [11] H. Kuhn, J. Chem. Phys. 53 (1970) 101. [ 12] R.R. Chance, A. Prock and R. Silbey, Advances in Chemical Physics, Vol. XXXVII, eds. I. Prigogine and S.A. Rice (Wiley, New York, 1978) p. 1. [13] S. Efrima and tl. Metiu, Chem. Phys, Lett. 60 (1978) 59; J. Chem. Phys. 70 (1979) 1602, 1939, 2297. [14] S. Mukhopadhyay and S. Lundqvist, Phys. Scr. 17 (1978) 69; T. Maniv and H. Metiu, J. Chem. Phys. 72 (1980) 1996. [15] J. Monin and G.A. Boutry, Phys. Rcv. B9 (1974) 1309. [16] H.J. ltageman, W. Gudat and C. Kunz, Opti,'al constants from the far infrared to the X-ray region: Mg, AI, Cu, Ag. Au, Bi, C and A1203, Dautsdles Electronen-Synchrotron DESY Report SR-74[7 (May, 1974). [ 17] American Institute of Physics Itandbook, 2nd Ed., ed. D.E. (;ray (McGraw-lliil, New York, 1963) p. 6-34. [18] !I. Raethcr, Physics of thin fihns (Academic Press, New York, 1977)9,p. 145. [191 P.K. Aravind and II. Metiu, Chem. Phys. Lett. 74 (1980) 301. [20] (;. Korzenicwski. T. Maniv and ll. Mctiu, ('hem. Phys. kerr. 73 (1980) 212.
OPTICS COMMUNICATIONS
September 1980
EFFECT OF METAL FILM THICKNESS ON SURFACE-ATOM COUPLING Arnold ADAMS, R.W. RENDELL 1, R.W. GARNETT and P.K. HANSMA 2
Department of Physics and Quantum Institute, University of California, Santa Barbara, California 93106, USA and Horia METIU 2,3
Department of Chemistry and Quantum Institute, University of Cali[ornia, Santa Barbara, California 93106, USA Received 3 June 1980
Metal film thickness is found to be an important parameter in the nonradiative coupling of atoms to metal surlhces. Silver, sodium, and potasium fihns in the thickness range 8-120 nm are evaporated onto a 40 K substrate in ultra high vacuum. At ! ! K, layers of N2 in the thickness range 3-300 nm are condensed onto the metal films. Low ener~ electrons excite the N, 2D--, 4S (o~)electronic transition. The lifetime of o~is measured as a function of nitrogen thickness and metal film thickness. Results are found to be in good agreement with classical theory based on nonradiative coupling of perpendicular dipoles to surface plasmons of the metal surface.
Nonradiative energy transfer from excited atoms or molecules to surface plasmons of a nearby metal surface is a topic of current interest [ 1 - 1 4 ] . Classical theory [ 10-14] presently accounts for most (not all [3] ) of the experimental data but has not been completely tested. This communication presents experimental data showing the effect of the position of the surface plasmon resonance energy relative to an atom's emission energy. The position of the surface plasmon resonance is altered by changing the metal or the metal thickness. Nitrogen films of controlled thickness were prepared in ultrahigh vacuum (typical operating pressure ~ 7 X 10 -8 Pa ~ 5 X 10-10 Torr) on evaporated Ag, Na and K metal films at 11 K of approximately known thickness, as described previously [21. The thickness of the N 2 film deposited on the substrate was determined by I R.W. Rendell now at National Bureau of Standards, Washington, D.C. 20234. 2 A.P. Sloan Fellow. 3 Camille and l lenry Dreyfus Teacher-Scholar.
counting interference fringes from specularly reflected Hg pen lamp light CA= 253.65 run) directed at 15 ° from the substrate normal hlto the monochromator and PMT for a given exposure (pressure X time), of the substrate to N 2 gas. The measured points fell on a straight line with slope 1100 nm/Pa-sec. This thickness versus exposure calibration factor was independent of deposition rate over the range 0.08 to 1.5 nm/s (7 X 10 -5 to 130 X 10 -5 Pa). We used this measured calibration factor at the lower exposure rates to estimate the thickness of our thinner films of N 2 . The thickness of Na and K films was found using the interference fringe method (thin films of Na and K are nearly transparent at ;k = 253.65 nm) [15]. Silver thick, ness estimates were made by visuaUy comparing the Ag film to calibrated Ag films made in another system. The estimates were thought to be reliable to + 4 nm for film thicknesses less than 20 nm. Greater film thicknesses were estimated by assuming a constant deposition rate. The refrigerator was turned on and at 40 K a known thickness of metal (Ag, Na or K) evaporated onto a sapphire target. The evaporated metal surface was then 417
Volume 34, numl~er 3
OPTICS COMMUNICATIONS
heated to 165 K to desorb any residual gases and then cooled to 11 K. Pressures during metal deposition were never above 1 X 10 -5 Pa. On the first run, 3 nm of research ~ a d e N 2 was deposited. Thicker layers were del,',esiccd in subsequent runs after first desorbing the previous N 2 layer by heating to 100 K. The electron gun excited the N 2 with 200 eV electrons (0.6 pA). The ener~w flux over the approxinmtely 0.2 cm 2 excited area was 0,6 mW/cm 2. The decay curves of the N, 2D ~ 4S (e) transition were measured as a function of N 2 matrix thickness after manually turning off the electron ~ m . Turn off times of less than 0.1 s were achieved. Figs. 1--3 show experimental results for the lifetime of ~ as a function of N 2 fihn thickness, type of metal film. and metal film tltickness. The lifetime reported here is measured after approximately one lifetime has already elapsed. This was done so that the initial n o n exponential decay of the N atoms could be neglected [2]. For the first time Na has been examined and for both Ag and K the measurements as a function of metal and N 2 f'dm thickness are more complete. To compute the theoretical lifetime of oe shown in fi~. 1 - 3 we assume that the radiating dipole rno, nent is perpendicular to the surface for four reasons: I) Our calculations of radiation patterns of perpendicular and parallel dipoles coupled strongly to surface plasmons show that ttle intensity of parallel dipoles should be smaller than that of perpendicular dipoles at an obser-
September 1980 I
I
I
I
I
f
40
~'\ . / / . . o ..........., - ~ ,,
io../ #I
/
------,oo~m no {0)
~
/
......
0
-.-I
_I I~ )0
0
I___
N z TH;CKNESS
:2 2 ,o, 1
t 200
_.L. 3CK
(nm)
Fig. 2. Lifetime of the c~transition as a function of N2 thickness for 50 nrn (o) and ! 00 nm (o) Na films. Theoretical curves arc given for 100 rim, 50 tam and 10 nm Na fihns.
vation angle of 15 ° for h ~ 200 nm, though the total integrated intensity of the parallel dipoles is larger. 2) The intensity of the perpendicular dipoles at an observation angle of 15 ~' should be proportional to the total integrated int~ asity (to within ~'5%)for h ~< 100 nm, the region of Most interest. 3) The monochrometer is slightly more sensitive (;~ 30%) to photons from penpendicular dipoles. 4) It was considerably simpler to calculate the curves without a complete angular average. Nevertheless, these calculations could be done and might slightly improve tbe agreement with experiment, though
................ T . . . . . . . .
-T
l
i. . . . .
---------
120 nm
1 "
l
GO-
/ .2
/
u,-J
i,i
t~_J
/
F ,~ ? 0 s~
I
_ ~f, ...,
,L ,'
....... ,
TO
¢/
ot~o" O
t~ n m
......
___L_
i ~J2 7}-41'~2KI'JESS
I 2C)0
(nm)
aq
1
3OO
Fig. !. Lifetime of the ~ trans.tion of matrix isolated N atoms as a function of N2 thickness for 8 nm (o) and 60 mn (e) Ag fihns. Theoretical curves are also yNen. to, the lifetune of oe x~hen no surface is present, is 39 s. Uncertainty in lifetime measurements isabvtr, 101,. 41,~
I
I
_.A
l,h~
Aq (O)
v
/
¢:_
L
\\
o
,,/~ ,7
1
K
(o)
. . . . . . .
IO
nm
K
(O)
. . . .
I()
nm
K
I ........
L _ _ _ I _ _ _ L
IOO 200 N2 THICKNESS (nm)
300
I:ig. 3. Lifetime of the o~transition as a function {}I"N2 thickness for 40 nm (o) and 120 nm (e) K films. Theoretical curves are given for 120 nm,40 nm and 10 tun K I'iims. Note the much stronger c(mpling (as indicated by tile short lifetimes out to N2 thicknesses of",- 120 nm) on K relative to Ag or Na.
Volume 34, number 3
OPTICS COMMUNICATIONS
as discussed below, there are other probable sources of error. The theory takes account of electron range in the N 2 film, effect of the dipole image field on the dipole and the effect of metal film thickness on the surface plasmon resonance as described previously in classical terms [2]. Dielectric constants used in the theory are contained in refs. [ 1 5 - 1 7 ] . The theoretical fits to the data are good showing that classical theory [12 ] seems to be valid for z f> 3 nm for various metal thicknesses and for both free electron-like metals (Na and K) and Ag (which has a strong interband transition near the surface plasmon resonance). The small deviations of experiment from theory may be due to many causes: I) roughness of the metal surface relaxing the momentum conservation requirement on surface plasmon radiation [2] ; 2) imprecise knowledge o" metal dielectric constants (this can be a 30% effect; there is no general agreement on dielectric constants for most metals, probably due to different conditions of preparation); 3) uncertainties in the effective electron range due to production of secondary electrons; and 4) the assumption of perpendicular dipoles discussed above. Fig. 1 shows stronger coupling (shorter lifetime) between N atoms close to the Ag surface (N 2 fihal thickness <~ 50 nm) if the Ag surface is thin. This is caused by a splitting of the surface plasmon resonance into higher energy and lower energy resonances due to interactions of the electron density oscillations on each surface of the thin Ag film [18]. (See fig. 4.) The higher energy mode is at 1 + exp(-Kxd ) times the e lergy of the thick film mode; the lower energy at 1 exp(-Kxd ) where d is the thickness of the film avd Kx = (to/c)[eoe !/(Co + el)] 1/2. It is the lower e,tergy resonance of the thin Ag film which is strongly interacting with o~. A similar, though less pronounced effect is observed on Na (fig. 2, N 2 thickness < 30 ran). K p., . .,.,.~. -,,v~, I. ..... .I,,, n . 3) , .i,' ,,,,. o p p o s i t e ,,,('r,,,,) . , , . . , . . iv.~,, . . . , s,.~fac~" .... p l a s m o n s o n t h i n K f i l m s are less easily e x c i t e d b y o~
than surface plasmons on thick K films. This is because the K surface plasmon resonance (Eqp ~ 2.58 eV) is already near the emission energy of a (Ea ~ 2.37 eV) and so the splitting of the K st~rface plasmon resonance moves the less sharp lower energy resonance to an energy lower than Ea.. The closeness of E a to ESI, for K also gives very strong coupling to the surface (strong coupling out to 120 nm). it is interesting to note that
September 1980
THICK FILM" + ++
+
++
THIN FILM: (o) LOWER ENERGY +++
+++
+++
+++
(b) HIGHER ENERGY
+++
+++
/////////////////////.//; + + +
+ + +
+.++
Fig. 4. The surface plasmon modes on each side of a thin film combine to give a higher energy, and lower energy mode than for a thick film. The distribution of charges is for one wavelength at a given instant.
the coupling of a to thick films of Na is slightly stronger than to thick films of Ag even though the surface plasmon energy of Ag (Esp ~ 3.57 eV) is nearer to E a than is the surface plasmon resonance of Na (Esp 3.71 eV). This is due to the free electron-like nature of Na. The lifetimes for N 2 thicknesses greater than about 50 nm on Ag and Na or 150 nm on K are dominated by the interaction of the dipole with its reflected image field. This gives an oscillatory effect to the lifetimes as a function of N 2 thickness and is more pronounced on the thick metal films than on the thin metal films since the thick metal films are better reflectors - they can more easily form an image of a dipole that is far from the metal surface. Though classical theory fits all of the dat~i at present, there are extensions of the theory that must be made. Metal surface roughness has been shown to be an important parameter in atom-surface coupling [2], and some theory is now being done [19], but it is fair to say that the experiments and theory are not in agreement. Refinements in theory and better characterization of the metal surface roughness are necessary before ' a final judgment can be made. Also, the theory has not been tested for atoms or molecules whose emission energy is nearly identical to the metal's surface plasmon resonance. Even the N atoms coupling to a thick K surface, the strongest coupling demonstrated to date, is small compared to the coupling expected from an atom 41q
voh~Inc 34. number 3
OPTICS COMMUNICATIONS
or molecu'., whose emissicm energy is coincident with ll~e K surface plasmon resonance (factor of ~ 10), Fin:,~iv. the experiments of tlarris et al. [ 1 ], and Rosseili and Brus [3] are attempting to answer the question of whether classical theory is valid within a few atomic radii of the surthce. ~ l e experiments of Harris et al. indicated tha~ lbr a molecule not near the surface plasmon energy of the metal, classical theory describes the molecular quantum efficiency for molecule-surface separations as small as 0.8 nm. Recent calculations [20] ~Lsing a jellium model and random phase approximation ate in general agreement with this conclusion. The experiments of Rossetti and Brus do not agree with classical theory for molecule-surface separations less than 13 rim. This disagreement may be related to surface roughness o~ an impurity layer on the surface. The authors would like to thank Dr. L.E. Brus and Professor C.B. Harris ~o, sending us preprints of unpublished work. This work was supported by the National Science Foundation under grant numbers DMR76-38423 and CHE78-16181. ¢"
¢
References It ] \ . (ampion. A.R. (;allo. C.B. tlarris, II.J. Robota and 1'.\I. '~khiim~rc. ('hem. Phys. I ett. 73 ¢1980) 447.
420
September 1980
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