- Email: [email protected]
Resonant Raman scattering study of undoped fullerene thin films
Synthetic Metals, 49-50 (1992) 543-547
543
Resonant Raman scattering study of undoped fullerene thin films D. Dick, R. E. B e i m e r a n d Z. V. V a r d e n y Departments of Electrical Engineering and Physics, University of Utah, Salt Lake City, UT 84112 (USA) D. Moses, V. I. S r d a n o v a n d F. W u d l Institute of Polymers and Organic Solids and Physics Department, University of California, Santa Barbara, CA 93106 (USA)
Abstract Raman spectra of the fullerenes Ceo and C7o have been measured for photon energies from 2.1 to 3.6 eV. Resonant enhancement of the Raman cross section versus wavelength has been measured for the 270, 497, 1424, 1469, and 1568 cm -1 lines, with the diamond 1332 cm -1 line as a reference. A prominent enhancement occurs near 2.4 and 2.5 eV for the C~o 497 cm -1 and 1469 cm -~ lines, respectively. The C7o 1424 cm -1 and 1568 cm -~ lines increase monotonically in intensity between 2.3 and 2.7 eV. The 270 cm -1 line, which has contributions from both C~o and C7o, exhibits resonance enhancement which is qualitatively similar to that of the C7o lines. All of the lines show resonance enhancement at 3.4 and 3.5 eV.
Introduction The r e c e n t p r o d u c t i o n o f g r a m quantities o f the fullerenes C8o a n d C7o h a s s t i m u l a t e d n u m e r o u s studies o f their p h y s i c a l a n d c h e m i c a l p r o p e r t i e s . The e l e c t r o n i c a n d vibrational p r o p e r t i e s o f t h e s e materials have b e e n studied, b o t h in solution a n d in solid forms, b y UV-Vis a b s o r p t i o n , p h o t o l u m i n e s c e n c e , p h o t o m o d u l a t i o n , e l e c t r o a b s o r p t i o n , Raman, a n d infrared s p e c t r o s c o p i e s [ 1 - 5 ]. A n u m b e r o f t h e o r e t i c a l studies o f the p r o p e r t i e s o f t h e s e fullerenes have also b e e n c o m p l e t e d [ 6 - 8 ] . R e s o n a n t R a m a n s c a t t e r i n g (RRS) p r o v i d e s c o m p l e m e n t a r y i n f o r m a t i o n a b o u t the e l e c t r o n i c a n d vibrational p r o p e r t i e s o f materials. The b e h a v i o r o f the R a m a n c r o s s s e c t i o n as the p h o t o n e n e r g i e s a p p r o a c h t h o s e o f the e x c i t e d states o f t h e s y s t e m p r o v i d e s a m e t h o d o f p r o b i n g the e l e c t r o n i c s t r u c t u r e a n d the i n t e r a c t i o n b e t w e e n e l e c t r o n i c a n d vibrational excitations. W e r e p o r t h e r e the r e s o n a n t R a m a n s p e c t r a o f thin films o f C6o a n d C70. W e also d e s c r i b e the d e g r a d a t i o n o f the thin films w h i c h o c c u r r e d after e x p o s u r e to visible laser light.
Elsevier Sequoia
544
Experimental Thin films o f C~0 and C7o were deposited on a sapphire substrate by evaporation and are described elsewhere [2]. The Raman spectra show that the films are predominantly C60 with a small amount ( = 7%) of C7o. The unpolarized spectra were collected in a backscattering geom et ry by a 0.85 m double m o n o c h r o m a t o r equipped with a GaAs PMT and p h o t o n counting electronics. The instrumental resolution was 4 cm -I. The visible and mid-UV lines of an Ar ion laser and an Ar ion laser p u m p e d dye laser were used to provide excitation in the range 2 . 1 - 3 . 5 eV. The laser pow er varied from 40 to 80 m W and was focused with a cylindrical lens to a 40 ~m by 2 mm spot. The sample was translated approximately 100 ~ m between spectra to avoid problems associated with sample degradation. A small industrial diamond was m ount e d adjacent to the sample on a translation stage, and the scattering efficiencies were measured relative to the diamond 1332 cm -1 Raman line.
Resonant Raman spectra Representative spectra taken at 2.2, 2.4, 2.7, and 3.4 eV are shown in Fig. 1. Bethune et al. [4] have assigned the strong lines at 497 cm -1 and 1469 cm -z to Cs0, the strong 1568 cm - I line to C7o, and the line which appears at 270 cm -1 in our s pe c t r a to a combination of Cso and C7o lines. We have tentatively assigned the line at 1424 cm -1 (which is not quite resolved in their spectra) to C7o. The broad background features between
"O I: 0 O
3.42 eV
>-
2.70 eV
zo II
0
400 800 1 2 0 0 1 6 0 0 2000 RAMAN SHIFT (cm "1)
Fig. 1. R e p r e s e n t a t i v e R a m a n s p e c t r a of the fullerene thin films at v a r i o u s p h o t o n e n e r g i e s normalized relative to the d i a m o n d 1332 c m -z line.
545
2 0 0 cm -1 and 5 0 0 cm -~, and between 6 7 0 cm -~ and 7 7 0 cm -1 are due to scattering from the substrate. Moderate to large variations in the scattering intensities of the Raman lines can be seen in the spectra; the integrated intensities of the lines are plotted versus p h o t o n energy in Fig. 2(a) and 2(b). The C7o 1424 cm -1 and 1568 c m - 1 lines are constant from 2.0 to 2.3 eV and monotonically increasing between 2.3 and 2.7 eV, in rough correspondence with the absorption spectrum of CTo [ 1 ], suggesting that the RRS intensity of these lines may be associated with the absorption feature at 2.7 eV. The 2 7 0 cm -I line s h o w s similar behavior, perhaps caused by enhancement of the C7o c o m p o n e n t of the line. Minor Raman lines at 426, 706, 1098, and 1513 cm -~ were observed to be strongly enhanced in the 3.4 and 3.5 eV spectra; however, no quantitative measurement of the enhancement was made. The 1513 c m - ] line has been
1.5
J a 497 cm-1 I o 1469 cm-1 <:~
LM
o o
1.0
i!05 {.~nr m~
o~
Z
~o~io~~~ # ~ o
m
0.0 2.0
I
,
I
I
I
I
I
, I
I
I
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
PHOTON ENERGY IN eV
(a) 1.0
270
cm-1
I.M ,,-,.
A
¢< _~ 0 . 8
×
1424
cm-1
o
1568
cm-1
ul
e..~:
i,l.i tr ~-P-
o o o o x
0.6
©
0.4
x >(
~4
~j4Xy~
© ~
0.2 Z
zx A~zx~
A
m
0.0 2.0
(b)
2.2
2.4
2.6
PHOTON
2.8
3.0
ENERGY
3.2
3.4
3.6
IN e V
Fig. 2. Integrated Raman intensities of the (a) C60 and Co) C7o lines plotted vs. photon energy. The 2 7 0 cm -1 line is included in (b) since it b e h a v e s similarly to the CTo lines ( s e e text).
546 assigned to C70 [4], but the origins of the other lines have not been determined. The C60 497 cm -~ line exhibits a peak at 2.4 eV, while the 1469 cm -1 line shows a similar feature near 2.5 eV. The absorption spect rum of C80 shows only a broad weak feature, which extends from 1.8 to 2.8 eV; this has been attributed to first-order forbidden transitions. The electroabsorption spectra [3] show a distinct derivative-like feature at 2.4 eV, which provides strong evidence for a forbidden transition at this energy. Resonance effects near forbidden transitions are possible due to the quadrupole c o m p o n e n t of the Raman tensor. Although there is a characteristic reduction factor of (1/137) 2 associated with the quadrupole component, the resonant enhancem ent is often large enough that the RRS intensity of a forbidden line at resonance is larger than that of an allowed line off resonance [9].
Degradation We have observed degradation of the fuUerene thin films after exposure to laser light while taking the Raman spectra. The Raman spectra at 2.4 eV o f a certain spot on the sample before and after eight 20 min exposures at p h o t o n energies of 2.4 to 2.7 eV at an intensity of 65 W / c m 2 (At laser visible lines) appear as shown in Fig. 3. The intensities of the Raman lines have decreased dramatically, and the spectrum is superimposed on a broad background which increases toward lower energies. Based on the relative change in the 497, 1424, 1469, and 1568 cm -1 lines, it appears that C~0 molecules are more susceptible to damage than C~o. Moving to a nearby spot on the film restores the original spectrum. Photoinduced damage after 20 min, the time needed to acquire a single Raman spectrum, appears to be small. Degradation of fullerenes in solution has been observed [10], with partial or complete fragmentation of the closed-cage structure; however, 5000 ~ 4000 o
DEGRADED
m I- ~ 3000
_===.
e 2000 5= ,~o ,S,~1000
z
I
400
I
800
I
1200
I
1600
RAMAN SHIFT (cm "1)
I
2000
Fig. 3. Raman spectra taken at 2.4 eV before (initial) and after (degraded) 160 rain of exposure to 2.4-2.7 eV fight at 65 W/cm2. The origins for both spectra are identical.
547 s u c h d e g r a d a t i o n w a s r e p o r t e d t o o c c u r o n l y o n e x p o s u r e t o u l t r a v i o l e t light. O x i d a t i o n of t h i n films a f t e r e x p o s u r e t o t h e a t m o s p h e r e [11 l, a n d p h o t o n i n d u c e d o x i d a t i o n o f m n l t i l a y e r films o f C6o a n d s o l i d 02 [12] h a v e a l s o b e e n r e p o r t e d . Kroll e t a l . h a v e p r o p o s e d p o s s i b l e p r o c e s s e s f o r r e a c t i o n o f C~o w i t h 02 i n v o l v i n g v i s i b l e p h o t o n s [12], a n d s u c h a r e a c t i o n m a y e x p l a i n t h e d e g r a d a t i o n w e h a v e o b s e r v e d in o u r t h i n films.
Conclusions W e h a v e m e a s u r e d t h e r e s o n a n c e R a m a n s p e c t r a o f CBo a n d C7o o v e r t h e r a n g e 2 . 1 - 3 . 5 eV. T h e C6o l i n e s s h o w e d a p r o m i n e n t e n h a n c e m e n t n e a r 2 . 4 - 2 . 5 eV, w h i c h c a n b e a t t r i b u t e d t o r e s o n a n t e n h a n c e m e n t f r o m a f o r b i d d e n t r a n s i t i o n a t 2.4 eV. F u r t h e r s t u d y o f t h e 2 . 7 - 3 . 4 eV r e g i o n , w h i c h w a s n o t a c c e s s i b l e w i t h o u r l a s e r s o u r c e s , will h e l p c h a r a c t e r i z e r e s o n a n c e effects in C6o a n d C7o. T h e s t u d y o f s a m p l e s o f p u r e C6o o r C7o will a l l o w t h e o r i g i n s of the Raman lines to be d e t e r m i n e d unambiguously. Finally, we have observed s i g n i f i c a n t d a m a g e t o f u l l e r e n e t h i n films a f t e r e x p o s u r e t o v i s i b l e l a s e r light, a n d r e c o m m e n d t h a t e x p o s u r e o f s a m p l e s t o light b e k e p t t o a m i n i m u m .
Acknowledgements The w o r k at the University of Utah was partially s u p p o r t e d by SDIO/ ONR C o n t r a c t s N 0 0 0 1 4 - 9 0 - C - 0 0 3 5 a n d N 0 0 0 1 4 - 9 0 - J - 1 8 4 1 , a n d b y t h e U t a h L a s e r I n s t i t u t e . T h e w o r k at UCSB w a s s u p p o r t e d b y DMR 8 8 - 2 0 9 3 3 a n d by NSF grant #CHE 89-08323.
References 1 J. P. Hare, H. W. Kroto and R. Taylor, Chem. Phys. Lett., 177 (1991) 394. 2 X. Wei, Z. V. Vardeny, D. Moses, V. I. Srdanov and F. Wudl, Synth. Met., 49-50 (1992) 549 3 S. Jeglinski, Z. Y. Vardeny, D. Moses, V. I. Srdanov and F. Wudl, Synth. Met., 4P-50 (1992) 557. 4 D. Bethune, G. Meijer, W. C. Tang and H. J. Rosen, Chem. Phys. Lett., 174 (1990) 219. 5 W. Kratschmer, K. Fostiropoulos and D. R. Huffman, Chem. Phys. Lett., 170 (1990) 167. 6 R. C. Haddon, L. E. Brus and K. Raghavachari, Chem. Phys. Lett., 125 (1986) 459. 7 F. Negri, G. Orlandi and G. Zerbetto, Chem. Phys. Lett., 144 (1988) 31. 8 D. E. Weeks and W. G. Harter, Chem. Phys. Lett., 144 (1988) 366. 9 R. M. Martin and L. M. Falicov, in M. Cardona (ed.), Light Scattering in Solids, Springer, New York, 1975, Ch. 3, p. 103. 10 R. Taylor, J. P. Parsons, A. G. Avent, S. P. Rannard, T. J. Dennis, J. P. Hare, H. W. Kroto and D. R. M. Walton, Nature (London), 351 (1991) 277. 11 W. M. Tong, D. A. A. Ohlberg, H. K. You, R. S. Williams, S. J. Anz, M. M. Alvarez, R. L. Whetten, Y. Rubin and F. N. Diederich, J. Phys. Chem., 95 (1991) 4709. 12 G. H. Kroll, P. J. Benning, Y. Chen, T. R. Ohno, J. H. Weaver, .L.P.F. Chibante and R. E. Smalley, Chem. Phys. Lett., 181 (1991) 112.
543
Resonant Raman scattering study of undoped fullerene thin films D. Dick, R. E. B e i m e r a n d Z. V. V a r d e n y Departments of Electrical Engineering and Physics, University of Utah, Salt Lake City, UT 84112 (USA) D. Moses, V. I. S r d a n o v a n d F. W u d l Institute of Polymers and Organic Solids and Physics Department, University of California, Santa Barbara, CA 93106 (USA)
Abstract Raman spectra of the fullerenes Ceo and C7o have been measured for photon energies from 2.1 to 3.6 eV. Resonant enhancement of the Raman cross section versus wavelength has been measured for the 270, 497, 1424, 1469, and 1568 cm -1 lines, with the diamond 1332 cm -1 line as a reference. A prominent enhancement occurs near 2.4 and 2.5 eV for the C~o 497 cm -1 and 1469 cm -~ lines, respectively. The C7o 1424 cm -1 and 1568 cm -~ lines increase monotonically in intensity between 2.3 and 2.7 eV. The 270 cm -1 line, which has contributions from both C~o and C7o, exhibits resonance enhancement which is qualitatively similar to that of the C7o lines. All of the lines show resonance enhancement at 3.4 and 3.5 eV.
Introduction The r e c e n t p r o d u c t i o n o f g r a m quantities o f the fullerenes C8o a n d C7o h a s s t i m u l a t e d n u m e r o u s studies o f their p h y s i c a l a n d c h e m i c a l p r o p e r t i e s . The e l e c t r o n i c a n d vibrational p r o p e r t i e s o f t h e s e materials have b e e n studied, b o t h in solution a n d in solid forms, b y UV-Vis a b s o r p t i o n , p h o t o l u m i n e s c e n c e , p h o t o m o d u l a t i o n , e l e c t r o a b s o r p t i o n , Raman, a n d infrared s p e c t r o s c o p i e s [ 1 - 5 ]. A n u m b e r o f t h e o r e t i c a l studies o f the p r o p e r t i e s o f t h e s e fullerenes have also b e e n c o m p l e t e d [ 6 - 8 ] . R e s o n a n t R a m a n s c a t t e r i n g (RRS) p r o v i d e s c o m p l e m e n t a r y i n f o r m a t i o n a b o u t the e l e c t r o n i c a n d vibrational p r o p e r t i e s o f materials. The b e h a v i o r o f the R a m a n c r o s s s e c t i o n as the p h o t o n e n e r g i e s a p p r o a c h t h o s e o f the e x c i t e d states o f t h e s y s t e m p r o v i d e s a m e t h o d o f p r o b i n g the e l e c t r o n i c s t r u c t u r e a n d the i n t e r a c t i o n b e t w e e n e l e c t r o n i c a n d vibrational excitations. W e r e p o r t h e r e the r e s o n a n t R a m a n s p e c t r a o f thin films o f C6o a n d C70. W e also d e s c r i b e the d e g r a d a t i o n o f the thin films w h i c h o c c u r r e d after e x p o s u r e to visible laser light.
Elsevier Sequoia
544
Experimental Thin films o f C~0 and C7o were deposited on a sapphire substrate by evaporation and are described elsewhere [2]. The Raman spectra show that the films are predominantly C60 with a small amount ( = 7%) of C7o. The unpolarized spectra were collected in a backscattering geom et ry by a 0.85 m double m o n o c h r o m a t o r equipped with a GaAs PMT and p h o t o n counting electronics. The instrumental resolution was 4 cm -I. The visible and mid-UV lines of an Ar ion laser and an Ar ion laser p u m p e d dye laser were used to provide excitation in the range 2 . 1 - 3 . 5 eV. The laser pow er varied from 40 to 80 m W and was focused with a cylindrical lens to a 40 ~m by 2 mm spot. The sample was translated approximately 100 ~ m between spectra to avoid problems associated with sample degradation. A small industrial diamond was m ount e d adjacent to the sample on a translation stage, and the scattering efficiencies were measured relative to the diamond 1332 cm -1 Raman line.
Resonant Raman spectra Representative spectra taken at 2.2, 2.4, 2.7, and 3.4 eV are shown in Fig. 1. Bethune et al. [4] have assigned the strong lines at 497 cm -1 and 1469 cm -z to Cs0, the strong 1568 cm - I line to C7o, and the line which appears at 270 cm -1 in our s pe c t r a to a combination of Cso and C7o lines. We have tentatively assigned the line at 1424 cm -1 (which is not quite resolved in their spectra) to C7o. The broad background features between
"O I: 0 O
3.42 eV
>-
2.70 eV
zo II
0
400 800 1 2 0 0 1 6 0 0 2000 RAMAN SHIFT (cm "1)
Fig. 1. R e p r e s e n t a t i v e R a m a n s p e c t r a of the fullerene thin films at v a r i o u s p h o t o n e n e r g i e s normalized relative to the d i a m o n d 1332 c m -z line.
545
2 0 0 cm -1 and 5 0 0 cm -~, and between 6 7 0 cm -~ and 7 7 0 cm -1 are due to scattering from the substrate. Moderate to large variations in the scattering intensities of the Raman lines can be seen in the spectra; the integrated intensities of the lines are plotted versus p h o t o n energy in Fig. 2(a) and 2(b). The C7o 1424 cm -1 and 1568 c m - 1 lines are constant from 2.0 to 2.3 eV and monotonically increasing between 2.3 and 2.7 eV, in rough correspondence with the absorption spectrum of CTo [ 1 ], suggesting that the RRS intensity of these lines may be associated with the absorption feature at 2.7 eV. The 2 7 0 cm -I line s h o w s similar behavior, perhaps caused by enhancement of the C7o c o m p o n e n t of the line. Minor Raman lines at 426, 706, 1098, and 1513 cm -~ were observed to be strongly enhanced in the 3.4 and 3.5 eV spectra; however, no quantitative measurement of the enhancement was made. The 1513 c m - ] line has been
1.5
J a 497 cm-1 I o 1469 cm-1 <:~
LM
o o
1.0
i!05 {.~nr m~
o~
Z
~o~io~~~ # ~ o
m
0.0 2.0
I
,
I
I
I
I
I
, I
I
I
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
PHOTON ENERGY IN eV
(a) 1.0
270
cm-1
I.M ,,-,.
A
¢< _~ 0 . 8
×
1424
cm-1
o
1568
cm-1
ul
e..~:
i,l.i tr ~-P-
o o o o x
0.6
©
0.4
x >(
~4
~j4Xy~
© ~
0.2 Z
zx A~zx~
A
m
0.0 2.0
(b)
2.2
2.4
2.6
PHOTON
2.8
3.0
ENERGY
3.2
3.4
3.6
IN e V
Fig. 2. Integrated Raman intensities of the (a) C60 and Co) C7o lines plotted vs. photon energy. The 2 7 0 cm -1 line is included in (b) since it b e h a v e s similarly to the CTo lines ( s e e text).
546 assigned to C70 [4], but the origins of the other lines have not been determined. The C60 497 cm -~ line exhibits a peak at 2.4 eV, while the 1469 cm -1 line shows a similar feature near 2.5 eV. The absorption spect rum of C80 shows only a broad weak feature, which extends from 1.8 to 2.8 eV; this has been attributed to first-order forbidden transitions. The electroabsorption spectra [3] show a distinct derivative-like feature at 2.4 eV, which provides strong evidence for a forbidden transition at this energy. Resonance effects near forbidden transitions are possible due to the quadrupole c o m p o n e n t of the Raman tensor. Although there is a characteristic reduction factor of (1/137) 2 associated with the quadrupole component, the resonant enhancem ent is often large enough that the RRS intensity of a forbidden line at resonance is larger than that of an allowed line off resonance [9].
Degradation We have observed degradation of the fuUerene thin films after exposure to laser light while taking the Raman spectra. The Raman spectra at 2.4 eV o f a certain spot on the sample before and after eight 20 min exposures at p h o t o n energies of 2.4 to 2.7 eV at an intensity of 65 W / c m 2 (At laser visible lines) appear as shown in Fig. 3. The intensities of the Raman lines have decreased dramatically, and the spectrum is superimposed on a broad background which increases toward lower energies. Based on the relative change in the 497, 1424, 1469, and 1568 cm -1 lines, it appears that C~0 molecules are more susceptible to damage than C~o. Moving to a nearby spot on the film restores the original spectrum. Photoinduced damage after 20 min, the time needed to acquire a single Raman spectrum, appears to be small. Degradation of fullerenes in solution has been observed [10], with partial or complete fragmentation of the closed-cage structure; however, 5000 ~ 4000 o
DEGRADED
m I- ~ 3000
_===.
e 2000 5= ,~o ,S,~1000
z
I
400
I
800
I
1200
I
1600
RAMAN SHIFT (cm "1)
I
2000
Fig. 3. Raman spectra taken at 2.4 eV before (initial) and after (degraded) 160 rain of exposure to 2.4-2.7 eV fight at 65 W/cm2. The origins for both spectra are identical.
547 s u c h d e g r a d a t i o n w a s r e p o r t e d t o o c c u r o n l y o n e x p o s u r e t o u l t r a v i o l e t light. O x i d a t i o n of t h i n films a f t e r e x p o s u r e t o t h e a t m o s p h e r e [11 l, a n d p h o t o n i n d u c e d o x i d a t i o n o f m n l t i l a y e r films o f C6o a n d s o l i d 02 [12] h a v e a l s o b e e n r e p o r t e d . Kroll e t a l . h a v e p r o p o s e d p o s s i b l e p r o c e s s e s f o r r e a c t i o n o f C~o w i t h 02 i n v o l v i n g v i s i b l e p h o t o n s [12], a n d s u c h a r e a c t i o n m a y e x p l a i n t h e d e g r a d a t i o n w e h a v e o b s e r v e d in o u r t h i n films.
Conclusions W e h a v e m e a s u r e d t h e r e s o n a n c e R a m a n s p e c t r a o f CBo a n d C7o o v e r t h e r a n g e 2 . 1 - 3 . 5 eV. T h e C6o l i n e s s h o w e d a p r o m i n e n t e n h a n c e m e n t n e a r 2 . 4 - 2 . 5 eV, w h i c h c a n b e a t t r i b u t e d t o r e s o n a n t e n h a n c e m e n t f r o m a f o r b i d d e n t r a n s i t i o n a t 2.4 eV. F u r t h e r s t u d y o f t h e 2 . 7 - 3 . 4 eV r e g i o n , w h i c h w a s n o t a c c e s s i b l e w i t h o u r l a s e r s o u r c e s , will h e l p c h a r a c t e r i z e r e s o n a n c e effects in C6o a n d C7o. T h e s t u d y o f s a m p l e s o f p u r e C6o o r C7o will a l l o w t h e o r i g i n s of the Raman lines to be d e t e r m i n e d unambiguously. Finally, we have observed s i g n i f i c a n t d a m a g e t o f u l l e r e n e t h i n films a f t e r e x p o s u r e t o v i s i b l e l a s e r light, a n d r e c o m m e n d t h a t e x p o s u r e o f s a m p l e s t o light b e k e p t t o a m i n i m u m .
Acknowledgements The w o r k at the University of Utah was partially s u p p o r t e d by SDIO/ ONR C o n t r a c t s N 0 0 0 1 4 - 9 0 - C - 0 0 3 5 a n d N 0 0 0 1 4 - 9 0 - J - 1 8 4 1 , a n d b y t h e U t a h L a s e r I n s t i t u t e . T h e w o r k at UCSB w a s s u p p o r t e d b y DMR 8 8 - 2 0 9 3 3 a n d by NSF grant #CHE 89-08323.
References 1 J. P. Hare, H. W. Kroto and R. Taylor, Chem. Phys. Lett., 177 (1991) 394. 2 X. Wei, Z. V. Vardeny, D. Moses, V. I. Srdanov and F. Wudl, Synth. Met., 49-50 (1992) 549 3 S. Jeglinski, Z. Y. Vardeny, D. Moses, V. I. Srdanov and F. Wudl, Synth. Met., 4P-50 (1992) 557. 4 D. Bethune, G. Meijer, W. C. Tang and H. J. Rosen, Chem. Phys. Lett., 174 (1990) 219. 5 W. Kratschmer, K. Fostiropoulos and D. R. Huffman, Chem. Phys. Lett., 170 (1990) 167. 6 R. C. Haddon, L. E. Brus and K. Raghavachari, Chem. Phys. Lett., 125 (1986) 459. 7 F. Negri, G. Orlandi and G. Zerbetto, Chem. Phys. Lett., 144 (1988) 31. 8 D. E. Weeks and W. G. Harter, Chem. Phys. Lett., 144 (1988) 366. 9 R. M. Martin and L. M. Falicov, in M. Cardona (ed.), Light Scattering in Solids, Springer, New York, 1975, Ch. 3, p. 103. 10 R. Taylor, J. P. Parsons, A. G. Avent, S. P. Rannard, T. J. Dennis, J. P. Hare, H. W. Kroto and D. R. M. Walton, Nature (London), 351 (1991) 277. 11 W. M. Tong, D. A. A. Ohlberg, H. K. You, R. S. Williams, S. J. Anz, M. M. Alvarez, R. L. Whetten, Y. Rubin and F. N. Diederich, J. Phys. Chem., 95 (1991) 4709. 12 G. H. Kroll, P. J. Benning, Y. Chen, T. R. Ohno, J. H. Weaver, .L.P.F. Chibante and R. E. Smalley, Chem. Phys. Lett., 181 (1991) 112.