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Radiative and radiationless triplet exciton decay in benzophenone
Voiunie 17, number 3
RADIATIVE
1 December
CHEMICAL PHYSICS LETTERS
AND RADIATIONLESS
TRIPLET
EXCITON
DECAY
1972
IN BENZOPHENONE”
Mark SHARNOFF Deportment of Physics.
Utu’venity
of Delaware.
1Vewark.
Delaware
19 711, USA
Received 4 August 1972
The EPR of triplet excitons and of shallow triplet traps in crystalline benzophenone is observed by an optical detection technique. The predominantly radiationless mode of de-excitation of the traps is demonstrated and shown to account
for most of the quenching
of electronic
excitation
The lifetimes of triplet excitons in typical organic lattices like anthracene and naphthalene are known to be orders of magnitude shorter than the radiative lifetimes of the molecules of which they are composed [l] . While superradiance [2] has been suggested as a cause of these anomalously short lifetimes, it is found experimentally that the triplet exciton lifetime rises [3,4] and the quantum yield of phosphorescence falls [5] as anthracene and naphthalene are increasingly refined. The experiments thus indicate that radiationless, rather than superradiative, processes limit the lifetimes of free triplet excitons in organic crystals. Several types of radiationless dissipation of the energy of electronic excitation may take place in nearly ideal crystalline lattices. Individual molecules may be deactivated through coupling with vibron or phonon fields. Superradiationless decay, analogous to superradiance, would also be possible for coherently excited
states in a periodic
potential.
Finally,
radi-
ationless deactivation may be initialed by inelastic scattering or trapping of excitons by dislocations and defects,
either intrinsic
Or extrinsic.
A recent
study
has shown that deep substitutional impurity traps present in concentrations as small as 10e8 mole per mole of anthracene host have dramatic effects on the [6]
lifetimes of triplet excitons in anthracene. The present communication concerns the role of very shallow traps in the radiationless de-excitation of triplet excitons in
benzophenone. * &P&ed
by National Science Foundation
Grant GP-10929.
energy in this system.
Highly purified, orthorhombic [7] single crystals of benzophenone were grown from “spectra” grade hexane solutions of repeatedly zone-refined starting materials. Apart from surface pits, the crystals were of good optical quality and gsve sharp extinctions when viewed between crossed polarizers. The phosphorescence of a typical crystal, observed at 4.2% under excitation from the 3 13 1 A line of a high-pressure mercury arc, is shown in fig. 1. The O-O band lies at 4133.5 c 0.5 A and coincides, within the accuracy of measurement, with the O-O band of the singlettriplet absorption [S] .
The triplet state paramagnetic resonance of the IO] at 4.2”K, con-
crystals, detected optically [9, tained contributions
both from free excitons and from trapped excitations. The excitonic EPR signals
[lo] were homogeneously broadened, characterized by a fine structure tensor whose principal axes c@ incided with the axes of the unit cell of the crystal, and had linewidths indicative of considerable motional narrowing. Both positive and negative signals were ob served (fig. 2), indicating [LO, 1 l] that the spinlattice relaxation times of the excitons were short compared to their phosphorescence lifetimes. Saturation studies [IO] gave spin-!attice relaxation times in
the neighborhood of 0. I iusec. The amplitudes [lo] of the optically detected EPR signals indicated that 30 to 45% of the phosphorescence (as viewed broadbanded through Corning CS 4-72 and CS 5-58 filters) was of excitonic The localized
origin.
nature of the trapped excitatiqns 355
Volume 17, number 3
CHEMICAL PHYSICS LE’i-KRS
I December
1372
Fig. 1. Shore-%ave~ength portion of the pho~horescence spectrum of orthorhombic ben~ophenone at 4.2”K, measured photo electrically with a 0.5 m grating spectrometer. Slit widths were set for 3 resoliition of 1 A. Two ndditional ciusters built on the carbonyl stretching progression occur in the complete spectrum.
t
1
7200
7400
l0000
10200 four*
Fig. 2, Opticaily detected EPR sign& from tripIet excitons (single, smooth peaks) and from triplet trapped excitations Wuctured
continua)
in orthothombic
benzophenone
4.2”K.o!xervedwith external magnetic fieid directed
at along
the crystalline b axis. The microwaves were pulsed on and off at a frequency- of 4 kHz. Excitation of the EPR of the trapped excitations aIways produced increases in the intensity of the phosphorescence. The low- and high-field exciton signal peaks
correspond to changes of +1.6% and -1.2%, respectively, of the phosphorescence observed through CS 4-72 and CS 5-58 iikers.
responsible for the remaining optically detected EPR signals was betrayed by *&heirinhomogeneousIy broad-
ened character [lo]. The fnle structure tensors of these excitations could be @aced in good correspondence, bo*A in their eigenvalues [12] and in the oriesttations of their principal axes [13, 101, with the four
distinctly oriented benzoph&one molecules in the unit cell. It is accordingly probable that the trapped excitations are localized on. individual benzophenone molecules in the v&i&ties of impurities (X-traps f 141) 3.56
. or intrinsic defects. The signals from the trapped excitations were positive wifhout exception, indicating [lo, 1 l] that their spin-lattice reiaxation times were long in comparison to their phosphorescence lifetimes. The trap signals proved to be of two categories. sSignals of the first category were prominent only when ’ the frequency with which the microwaves were pulsed 1on and off was lower than 200 Hz and died away frapidly as the frequency was raised beyond this value *(fig. 3). Trap signals of the second category persisted Tundiminished to modulation frequencies of 16 to ~~-20kHz before falling off in intensity. Since the per’ sisrence of the signals is a measure of the response ,. time of the spin level popuIations (Le., the character& tic time with which the shortest transient dies away), it rnajj be concluded that the lifetime of the traps of the first type is of the order of ~~ = 2.5 msec while the traps of the second type decay with a period of 72 = 25 ysec or less. Since the radiative lifetime of in-
dividuaf.benzophenone molecules at 77°K is 76 = 8 msec [lS] , it is clear that radiative processes account for about half of the decay rate of trapped excitons of the firpt type. The mode of de-excitation of the second type must, however, be overivhelmingly radiationless. In order
to characterize more closely the region of residual emission from traps of the second type, the experiments were repeated with the CS 4-72 f CS S-58 friter combination replaced by a sharp &toff interference filter centered on the exciton O-O band
Volume 17, number 3
CHEMICAL PHYSICS LETTERS
1 December 1972
curve into two simple exponentials worked well, with one component having a lifetime of 12 + 1 msec and the other component having a lifetime of 1.5 msec or less (an upper limit set by the speed of the phot* graphic shutter used to cut off the exciting light). The two components made contributions to the total intensity of the phosphorescence which were equal, within experimental errcr. When the narrow-band interference filter was used to observe the decay, a similar resolution into components was possible, the short-lived component now accounting for about 2/3 of the transmitted intensity. A comparison of these results with the radiative lifetime, ‘o, and with the quantum yield [ 1.51, TQ =
0.80, of the phosphorescence of benzophenone mole-
L
Fig. 3. Behavior of exciton
1
signals and trap signals with respect
to changes in the microwave modulation frequency. The trap signals diminish rapidly in the octave 20 kHz-40 Hllb a-uis Bracketed interval = 200 G.
kHz.
cules isolated in rigid glassy solution substantiates the indications of the EPR results that considerable radiationless deactivation is occurring in thecrystals. The sllort-lived components in the phosphorescence are more important near the excitonic O-O band and are here ascribed mainly to emission from excitons (which, because of trapping, have short lifetimes) and to residual radiation from the shor!-lived trapped excitations. The long-liired components in the phosphorescence are ascribed to emission from the long-lived trapped excitations. The wavelength dependence of the relative intensities of short and long-lived components sugests that the radiative traps lie deeper than the
(4133.5 A) and having a full width at half maximurn
nearly non-radiative ones. This suggestion is borne out
transmission of 9.3 A. The light used to detect the EPR was accordingly restricted principally to quanta lying within ca. 60 c-m- 1 of the energy of the exciton O-O emission. At microwave modulation frequencies above 2 kHz, the modulation depths of the exciton EPR signals observed through the interference fiiter were equal to their previous values. The trap signals of the second category were nearly as intense, relative
by high resolution optical detection studies employing a grating spectrophotometer [ 16, 171. The G-0 band of the short lived traps occurs at 4143 a, while that of the long lived traps lies 1171 at 4164 8”. An estimate of the efficiency with which the nearly radiationless traps quench the crystalline emission may be made with the help of the assumption that, once an excitation has been localized on one of these traps, the probability that the subsequent de-excitation will be radiative is p = 72/70 = 3 X 10S3. When this number is used in conjunction with the quantum yield, 772, of phosphorescence from the short-lived
to the exciton signals, as before. It may be concluded that the energies of trapped excitations of the second type lie within 60 cm-1 of the triplet exciton band. In order to permit assay of the influence which these traps have upon the transformation of energy within the crystal, the quantum yield of phosphorescence was measured and found to be 0.015 t 0.005. The decay of the phosphorescence at zero magnetic field was also studied. The emission, observed through the CS 4-72 and CS 5-58 filter combination, decaied non-exponentially. Attempts to separate the decay
* None of the trap species which we have found in our own crystals is of the type characterized by Chan and Schmidt f 181 or by Maki and Winscom [ 191. These investigators found no excitonic phosphorescence in their crystals, which suggests that its absence there is correlated with the presence of the traps which they studied.
357
Volume 17, number
3
CHEMICAL PHYSICS LETTERS
traps alone (the intensities of the optically detected EPR signals, averaged over crystalline o~entation, indicate that this yield is approximately one third of the rapid component of the phosphorescence, or about ‘17%of the total yield of 0.015), the fraction of excitation quanta which are eventually transferred to these traps becomes Q/P = 0.8.
The short-lived traps thus account for nearly all of the radiationless degradation of the electronic excitation of the crystal. Superradiationless de-excitation of tripIet exciton states would accordingly seem not to be important in benzophenone. The concentration of the sh&ow trapping sites can be roughly estimated from the correlation times [lo] of the triplet exciton motion, whose average value is re = 1.1 X 10N1” set, and from the pSospllorescence lifetime, 7 = v. r. = 6 mscc, of individual benzc” phenone molecules. It can be inferred [ 171 that at 4.2’K 8 shallow trap, once occupied by an excitation, does not release it agdin to the exciton band of the crystal. By implication, the triplet exciton lifetime wouid be ~~ = (1 -a2jP> r = 1.2 msec. During its Iifetime the exciton will sample N = T~/T, = I. I X IO7 lattice sites, only one of which need be a shallow trap. Our experiments thus demonstrate the effectiveness of very small concentrations of shallow, radiationlessiy deactivated traps in Iimiting the lifetimes of triplet excitons in the orthorhombic phase of benzophenone.
3%
1 December
1972
References
[II P. Avakian 121D.P. Craig
and R.E. Merritield, Mol. Cryst. 5 (1968) 37. and L.A. Dissado, J. Chem. Phys 48 (1968)
516. [31 Y. Lupien and D.F. Williams, Mol. Cryst. 5 (1968) 1. [41 K.W. Benz. 2. Naturforsch. 23a (1969) 298. 18a (1963) is1 A. P&stl and XC. Wolf, 2. Naturforseh. 724. I61 R.P. Groff, R.E. MerrifIeld, P. Avakian and Y. Tomkiewicz. Phys. Rev. Letters 25 (1970) 105. 12 (1967) I71 E.B. Vu1 and G.&f. Lobanova, ~rist~o~afiya 411 [Soviet Phys Cryst. 12 (1967) 3551. S. Dym, R.M. Hochstrasser and hl. Schafer, J. Chem. Phys. 48 (1968) 646. M. Sharnoff, J, Chem. Phys. 46 (1967) 3263. hf. Sharnoff, Symp. Faraday Sot. 3 (1969) 137. Y. Ghan and M. Sharnoff, J. Luminescence 3 (1970) 155. R.F. Clements, Y. Ghan and M. Sharnoff, unpublished measurements. r131 hf. Sharcoff, J. Chem. Phys. 5 1 (1969) 45 1. [I41 H. port and H.C. Wolf, in: The triplet state, ed A.B. ZahIan (Cambridge Univ. Press, London, 1967) p. 393. 1151 E.H. Gilmore, G.R. Gibson and D.S. hIcClure, I. Chem. Phys 23 (1955) 399; 20 (1952) 829. 116f M. Sharnoff and E.B. lturbe, Phys. Rev. Letters 27 (1971) 576. [ 171 hf. Sharnoff and E.B. fturbe, J. Luminescence, to be published, [ 181 I.-Y. Chan and J. Schmidt, Syrnp. Faraday Sot. 3 (1969) 156. f 191 Cf. Winscom and A.H. hfaki, Cbem. Phys. Letters 12 (1971) 264.
RADIATIVE
1 December
CHEMICAL PHYSICS LETTERS
AND RADIATIONLESS
TRIPLET
EXCITON
DECAY
1972
IN BENZOPHENONE”
Mark SHARNOFF Deportment of Physics.
Utu’venity
of Delaware.
1Vewark.
Delaware
19 711, USA
Received 4 August 1972
The EPR of triplet excitons and of shallow triplet traps in crystalline benzophenone is observed by an optical detection technique. The predominantly radiationless mode of de-excitation of the traps is demonstrated and shown to account
for most of the quenching
of electronic
excitation
The lifetimes of triplet excitons in typical organic lattices like anthracene and naphthalene are known to be orders of magnitude shorter than the radiative lifetimes of the molecules of which they are composed [l] . While superradiance [2] has been suggested as a cause of these anomalously short lifetimes, it is found experimentally that the triplet exciton lifetime rises [3,4] and the quantum yield of phosphorescence falls [5] as anthracene and naphthalene are increasingly refined. The experiments thus indicate that radiationless, rather than superradiative, processes limit the lifetimes of free triplet excitons in organic crystals. Several types of radiationless dissipation of the energy of electronic excitation may take place in nearly ideal crystalline lattices. Individual molecules may be deactivated through coupling with vibron or phonon fields. Superradiationless decay, analogous to superradiance, would also be possible for coherently excited
states in a periodic
potential.
Finally,
radi-
ationless deactivation may be initialed by inelastic scattering or trapping of excitons by dislocations and defects,
either intrinsic
Or extrinsic.
A recent
study
has shown that deep substitutional impurity traps present in concentrations as small as 10e8 mole per mole of anthracene host have dramatic effects on the [6]
lifetimes of triplet excitons in anthracene. The present communication concerns the role of very shallow traps in the radiationless de-excitation of triplet excitons in
benzophenone. * &P&ed
by National Science Foundation
Grant GP-10929.
energy in this system.
Highly purified, orthorhombic [7] single crystals of benzophenone were grown from “spectra” grade hexane solutions of repeatedly zone-refined starting materials. Apart from surface pits, the crystals were of good optical quality and gsve sharp extinctions when viewed between crossed polarizers. The phosphorescence of a typical crystal, observed at 4.2% under excitation from the 3 13 1 A line of a high-pressure mercury arc, is shown in fig. 1. The O-O band lies at 4133.5 c 0.5 A and coincides, within the accuracy of measurement, with the O-O band of the singlettriplet absorption [S] .
The triplet state paramagnetic resonance of the IO] at 4.2”K, con-
crystals, detected optically [9, tained contributions
both from free excitons and from trapped excitations. The excitonic EPR signals
[lo] were homogeneously broadened, characterized by a fine structure tensor whose principal axes c@ incided with the axes of the unit cell of the crystal, and had linewidths indicative of considerable motional narrowing. Both positive and negative signals were ob served (fig. 2), indicating [LO, 1 l] that the spinlattice relaxation times of the excitons were short compared to their phosphorescence lifetimes. Saturation studies [IO] gave spin-!attice relaxation times in
the neighborhood of 0. I iusec. The amplitudes [lo] of the optically detected EPR signals indicated that 30 to 45% of the phosphorescence (as viewed broadbanded through Corning CS 4-72 and CS 5-58 filters) was of excitonic The localized
origin.
nature of the trapped excitatiqns 355
Volume 17, number 3
CHEMICAL PHYSICS LE’i-KRS
I December
1372
Fig. 1. Shore-%ave~ength portion of the pho~horescence spectrum of orthorhombic ben~ophenone at 4.2”K, measured photo electrically with a 0.5 m grating spectrometer. Slit widths were set for 3 resoliition of 1 A. Two ndditional ciusters built on the carbonyl stretching progression occur in the complete spectrum.
t
1
7200
7400
l0000
10200 four*
Fig. 2, Opticaily detected EPR sign& from tripIet excitons (single, smooth peaks) and from triplet trapped excitations Wuctured
continua)
in orthothombic
benzophenone
4.2”K.o!xervedwith external magnetic fieid directed
at along
the crystalline b axis. The microwaves were pulsed on and off at a frequency- of 4 kHz. Excitation of the EPR of the trapped excitations aIways produced increases in the intensity of the phosphorescence. The low- and high-field exciton signal peaks
correspond to changes of +1.6% and -1.2%, respectively, of the phosphorescence observed through CS 4-72 and CS 5-58 iikers.
responsible for the remaining optically detected EPR signals was betrayed by *&heirinhomogeneousIy broad-
ened character [lo]. The fnle structure tensors of these excitations could be @aced in good correspondence, bo*A in their eigenvalues [12] and in the oriesttations of their principal axes [13, 101, with the four
distinctly oriented benzoph&one molecules in the unit cell. It is accordingly probable that the trapped excitations are localized on. individual benzophenone molecules in the v&i&ties of impurities (X-traps f 141) 3.56
. or intrinsic defects. The signals from the trapped excitations were positive wifhout exception, indicating [lo, 1 l] that their spin-lattice reiaxation times were long in comparison to their phosphorescence lifetimes. The trap signals proved to be of two categories. sSignals of the first category were prominent only when ’ the frequency with which the microwaves were pulsed 1on and off was lower than 200 Hz and died away frapidly as the frequency was raised beyond this value *(fig. 3). Trap signals of the second category persisted Tundiminished to modulation frequencies of 16 to ~~-20kHz before falling off in intensity. Since the per’ sisrence of the signals is a measure of the response ,. time of the spin level popuIations (Le., the character& tic time with which the shortest transient dies away), it rnajj be concluded that the lifetime of the traps of the first type is of the order of ~~ = 2.5 msec while the traps of the second type decay with a period of 72 = 25 ysec or less. Since the radiative lifetime of in-
dividuaf.benzophenone molecules at 77°K is 76 = 8 msec [lS] , it is clear that radiative processes account for about half of the decay rate of trapped excitons of the firpt type. The mode of de-excitation of the second type must, however, be overivhelmingly radiationless. In order
to characterize more closely the region of residual emission from traps of the second type, the experiments were repeated with the CS 4-72 f CS S-58 friter combination replaced by a sharp &toff interference filter centered on the exciton O-O band
Volume 17, number 3
CHEMICAL PHYSICS LETTERS
1 December 1972
curve into two simple exponentials worked well, with one component having a lifetime of 12 + 1 msec and the other component having a lifetime of 1.5 msec or less (an upper limit set by the speed of the phot* graphic shutter used to cut off the exciting light). The two components made contributions to the total intensity of the phosphorescence which were equal, within experimental errcr. When the narrow-band interference filter was used to observe the decay, a similar resolution into components was possible, the short-lived component now accounting for about 2/3 of the transmitted intensity. A comparison of these results with the radiative lifetime, ‘o, and with the quantum yield [ 1.51, TQ =
0.80, of the phosphorescence of benzophenone mole-
L
Fig. 3. Behavior of exciton
1
signals and trap signals with respect
to changes in the microwave modulation frequency. The trap signals diminish rapidly in the octave 20 kHz-40 Hllb a-uis Bracketed interval = 200 G.
kHz.
cules isolated in rigid glassy solution substantiates the indications of the EPR results that considerable radiationless deactivation is occurring in thecrystals. The sllort-lived components in the phosphorescence are more important near the excitonic O-O band and are here ascribed mainly to emission from excitons (which, because of trapping, have short lifetimes) and to residual radiation from the shor!-lived trapped excitations. The long-liired components in the phosphorescence are ascribed to emission from the long-lived trapped excitations. The wavelength dependence of the relative intensities of short and long-lived components sugests that the radiative traps lie deeper than the
(4133.5 A) and having a full width at half maximurn
nearly non-radiative ones. This suggestion is borne out
transmission of 9.3 A. The light used to detect the EPR was accordingly restricted principally to quanta lying within ca. 60 c-m- 1 of the energy of the exciton O-O emission. At microwave modulation frequencies above 2 kHz, the modulation depths of the exciton EPR signals observed through the interference fiiter were equal to their previous values. The trap signals of the second category were nearly as intense, relative
by high resolution optical detection studies employing a grating spectrophotometer [ 16, 171. The G-0 band of the short lived traps occurs at 4143 a, while that of the long lived traps lies 1171 at 4164 8”. An estimate of the efficiency with which the nearly radiationless traps quench the crystalline emission may be made with the help of the assumption that, once an excitation has been localized on one of these traps, the probability that the subsequent de-excitation will be radiative is p = 72/70 = 3 X 10S3. When this number is used in conjunction with the quantum yield, 772, of phosphorescence from the short-lived
to the exciton signals, as before. It may be concluded that the energies of trapped excitations of the second type lie within 60 cm-1 of the triplet exciton band. In order to permit assay of the influence which these traps have upon the transformation of energy within the crystal, the quantum yield of phosphorescence was measured and found to be 0.015 t 0.005. The decay of the phosphorescence at zero magnetic field was also studied. The emission, observed through the CS 4-72 and CS 5-58 filter combination, decaied non-exponentially. Attempts to separate the decay
* None of the trap species which we have found in our own crystals is of the type characterized by Chan and Schmidt f 181 or by Maki and Winscom [ 191. These investigators found no excitonic phosphorescence in their crystals, which suggests that its absence there is correlated with the presence of the traps which they studied.
357
Volume 17, number
3
CHEMICAL PHYSICS LETTERS
traps alone (the intensities of the optically detected EPR signals, averaged over crystalline o~entation, indicate that this yield is approximately one third of the rapid component of the phosphorescence, or about ‘17%of the total yield of 0.015), the fraction of excitation quanta which are eventually transferred to these traps becomes Q/P = 0.8.
The short-lived traps thus account for nearly all of the radiationless degradation of the electronic excitation of the crystal. Superradiationless de-excitation of tripIet exciton states would accordingly seem not to be important in benzophenone. The concentration of the sh&ow trapping sites can be roughly estimated from the correlation times [lo] of the triplet exciton motion, whose average value is re = 1.1 X 10N1” set, and from the pSospllorescence lifetime, 7 = v. r. = 6 mscc, of individual benzc” phenone molecules. It can be inferred [ 171 that at 4.2’K 8 shallow trap, once occupied by an excitation, does not release it agdin to the exciton band of the crystal. By implication, the triplet exciton lifetime wouid be ~~ = (1 -a2jP> r = 1.2 msec. During its Iifetime the exciton will sample N = T~/T, = I. I X IO7 lattice sites, only one of which need be a shallow trap. Our experiments thus demonstrate the effectiveness of very small concentrations of shallow, radiationlessiy deactivated traps in Iimiting the lifetimes of triplet excitons in the orthorhombic phase of benzophenone.
3%
1 December
1972
References
[II P. Avakian 121D.P. Craig
and R.E. Merritield, Mol. Cryst. 5 (1968) 37. and L.A. Dissado, J. Chem. Phys 48 (1968)
516. [31 Y. Lupien and D.F. Williams, Mol. Cryst. 5 (1968) 1. [41 K.W. Benz. 2. Naturforsch. 23a (1969) 298. 18a (1963) is1 A. P&stl and XC. Wolf, 2. Naturforseh. 724. I61 R.P. Groff, R.E. MerrifIeld, P. Avakian and Y. Tomkiewicz. Phys. Rev. Letters 25 (1970) 105. 12 (1967) I71 E.B. Vu1 and G.&f. Lobanova, ~rist~o~afiya 411 [Soviet Phys Cryst. 12 (1967) 3551. S. Dym, R.M. Hochstrasser and hl. Schafer, J. Chem. Phys. 48 (1968) 646. M. Sharnoff, J, Chem. Phys. 46 (1967) 3263. hf. Sharnoff, Symp. Faraday Sot. 3 (1969) 137. Y. Ghan and M. Sharnoff, J. Luminescence 3 (1970) 155. R.F. Clements, Y. Ghan and M. Sharnoff, unpublished measurements. r131 hf. Sharcoff, J. Chem. Phys. 5 1 (1969) 45 1. [I41 H. port and H.C. Wolf, in: The triplet state, ed A.B. ZahIan (Cambridge Univ. Press, London, 1967) p. 393. 1151 E.H. Gilmore, G.R. Gibson and D.S. hIcClure, I. Chem. Phys 23 (1955) 399; 20 (1952) 829. 116f M. Sharnoff and E.B. lturbe, Phys. Rev. Letters 27 (1971) 576. [ 171 hf. Sharnoff and E.B. fturbe, J. Luminescence, to be published, [ 181 I.-Y. Chan and J. Schmidt, Syrnp. Faraday Sot. 3 (1969) 156. f 191 Cf. Winscom and A.H. hfaki, Cbem. Phys. Letters 12 (1971) 264.