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Excimer emission of diphenyl derivatives in the pulse radiolysis of 3-methylpentane solutions at low temperature
Journal of Photochemistry
and Photobiology,
A:
Chemistry,
4 7 (1989)
319
EXCIMER EMISSION OF DIPHENYL DERIVATIVES THE PULSE RADIOLYSIS OF 3-METHYLPENTANE SOLUTIONS AT LOW TEMPERATURE M. SZADKOWSKA-NICZE,
J. MAYER
The Institute of Applied Radiation WrBblewskiego 15 {Poland) (Received
September
30, 1988;
- 325
319
IN
and J. KROH
Chemistry,
in revised form
Technical December
University,
93-590
L6d2,
16, 1988)
Summary Using the pulse radiolysis method the excimer formation of diphenyl derivatives in low temperature 3-methylpentane was investigated. For diphenyls two emission bands are found in the temperature range 140 - 230 K. The band at 320 nm is due to monomer fluorescence, whereas the second at 380 - 390 nm is as an excimer emission.
1. Introduction Excimers can be produced as a result of photoexcitation [l, 21 and by ion recombination, involving dimer cations, induced by ionizing radiation [3 - ll]. There are no reports on the photoexcitation of an excimer of diphenyl (DF). Excimer emission of DF has been observed in the radiothermoluminescence of alkane glasses [ 5, 61 and in the pulse radiolysis of a viscous solution in isopentane [9]. A new emission band appearing at 365 - 380 nm has been attributed to the DF excimer fluorescence [ 6,9]. In this paper we describe an application of nanosecond pulse radiolysis at low temperature to the study of excimer formation in the viscous systems DF-3-methylpentane (3MP) and 3,3’-dimethyldiphenyl (DMD)-3MP. Owing to the higher solubility of DMD in 3MP compared with that of DF in 3MP a more detailed study of the excimer formation mechanism is presented.
2. Experimental details 3MP (Fluka, pure grade) was chromatographed through a freshly activated silica gel column and stored under argon. DF (POCh-Poland, analytical grade) was recrystallized twice from ethanol. DMD from Aldrich lOlO-6030/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
320
was used as received. The samples were degassed on the vacuum line in rectangular Spectrosil cells by the freeze-pump-thaw method and sealed under vacuum. The home-built styrofoam-copper cryostat, through which cold nitrogen was passed, enabled experiments to be carried out at temperatures down to 90 K. A linear accelerator (ELU-6, made in the U.S.S.R.) delivering 17 ns and 5 ns electron pulses (approximately 50 and 15 Gy respectively) was used for irradiation. More details concerning the detection system and the accelerator can be found elsewhere [ 11,12 1.
3. Results and discussion Pulse radiolysis of a solution of 5 X 10e3 mol dmh3 DF in 3MP at room temperature generates strong fluorescence during the pulse; the spectrum corresponds exactly to that of the emission from the UV-excited singlet of DF [4, 131. The half-life of the emission, r1,2 = 15 f 1 ns (k = 4.6 X lo7 s-l) is somewhat greater than that reported for UV excitation in cyclohexane (7 1,* = 11.04 ns, k = 6.25 X 10’ s-l) [ 141. We attribute this to the simultaneous formation of the excited states during emission due to fast ion recombination. The spectral distribution of the DF emission from 3MP solution at 175 K is shown in Fig. 1. When measured immediately after the 17 ns pulse, the spectrum resembles that at, room temperature except that beyond 350 nm there is more emission than at room temperature. At longer times, e.g. 50 ns after the pulse, the monomer fluorescence in the range 300 - 330 nm is reduced and at longer wavelengths (approximately 380 nm) a new emission band can be seen (inset (d), Fig. 1). Following earlier suggestions [5, 6, 91 we attribute this delayed long wavelength emission to DF excimers. Similar results were obtained for DMD solutions in 3MP in the concentration range (5 - 20) X 10e3 mol dm- 3. The emission spectra of lo-* mol dmm3DMD in 3MP at various temperatures are shown in Fig. 2. At room temperature the fluorescence of the solution peaks at 320 nm as observed for DF. Using a 5 ns pulse (approximately 15 Gy) the half-life of the DMD fluorescence’(5 X 10d3 mol dmM3) is estimated to be 15.7 * 1 ns (k = 4.4 X lo7 s-l). Lowering of the temperature to 180 K has no influence on the spectral distribution of the light emitted during the pulse. The tail at h > 360 nm seems to be mainly due to Cerenkov emission_ At longer times, i.e. 40 ns and 100 ns after the beginning of the pulse, the monomer fluorescence is significantly reduced. However, a new emission band is observed with X,,, at 390 nm. The apparent half-lives of the emissions (lo-’ mol dmm3 DMD in 3MP at 180 K) measured at 320 nm and 390 nm are equal to approximately 17.4 ns and 55.7 ns respectively. The decay of the 390 nm fluorescence is very strongly temperature dependent. The half-lives of this emission vary
321
h, nm
Fig. 1. Emission spectra from pulse-irradiated (17 ns pulse) 5 X 10M3 mol dmd3 DF in 3MP at 175 K. The intensities of fluorescence were measured during the pulse at the maximum of the intensity-time profiles as shown in the insets (a) and 50 ns after the beginning of the pulse (0). Insets: oscilloscope traces of emissions: (a) 320 nm at 298 K (50 mV per division, 50 ns per division); (b) 380 nm at 298 K (10 mV per division, 50 ns 380 nm per division); (c) 320 nm at 175 K (50 mV per division, 50 ns per division); erenkov at 175 K (10 mV per division, 50 ns per division); the spike corresponds to radiation.
from 43.5 ns to 126.5 ns in the temperature range 200 - 140 K. The apparent activation energy calculated from these data is equal to 4.5 kJ mol-l. At the lowest temperature, i.e. 100 K the spectrum of light emitted during the pulse resembles that at room temperature. The influence of temperature on the intensity of the emission from 10V2 mol dm- 3 DMD in 3MP measured at 320 nm and 390 nm is shown in Fig. 3. The fluorescence intensities are not corrected for the variation in quantum efficiency of the DMD fluorescence with temperature. The relative intensity of the DMD fluorescence at 320 nm measured during the pulse (Fig. 3(a)) gradually decreases with a ‘lowering of temperature to 100 K. This result can be explained as being due to the slowing down of DMD ion recombination with decreasing temperature as a result of the viscosity effect. The relative intensity of emission at 390 nm measured just after the pulse at the maximum of the signal first increases as the temperature decreases, passes through a maximum at approximately 190 K and then decreases. In the temperature range 170 - 230 K, where the emission at 390 nm can be seen, a plateau with small distortions in intensity is observed
322
300
400 A,
500
nm
Fig. 2. Emission spectra from pulse-irradiated (17 ns pulse) 10e2 mol drne3 DMD in 3MP at various temperatures: A, 298 K; 0, 180 K; 0, 100 K. The intensities of fluorescence were measured during the pulse at the maximum of the intensity-time profiles as shown in the insets. At 180 K the fluorescence intensities were also measured at 40 ns (a) and 100 ns (0) after the beginning of the pulse. Insets: oscilloscope traces of emissions: (a) 320 nm at 298 K (50 mV per division, 50 ns per division); (b) 390 nm at 298 K (10 mV per division, 50 ns per division); (c) 320 nm at 180 K (full line) and at 100 K (broken line) (50 mV per division, 50 ns per division); (d) 390 nm at 180 K (full line) and at 100 K (broken line) (10 mV per division, 50 ns per division).
for the 320 nm fluorescence. A similar temperature dependence (as shown in Fig. 3(a)) has been found for pyrene and naphthalene [lo, 111. The decrease in solute fluorescence at 320 nm during the pulse with decreasing temperature is accompanied by delayed emission after the pulse, This effect is due to the relatively slow in particular at low temperature. solute ion recombination which occurs after the pulse in a viscous, low temperature medium. The influence of temperature on the relative intensity of delayed luminescence measured 100 ns after the beginning of the pulse is shown in Fig. 3(b). There is an increase in the monomer fluorescence intensity as the temperature decreases to approximately 230 K, followed by a minimum in the temperature range 160 - 180 K and another maximum at 145 K. The intensity of delayed fluorescence at 390 nm reaches a maximum in the temperature range (160 - 180 K) where the intensity of delayed emission at 320 nm falls to a minimum. This type of dependence of the intensities of luminescence at 320 nm and 390 nm is only observed in the more concentrated solutions of DMD in 3MP (low2 and 2 X 10e2 mol dmp3).
323
4
3 0.5 c 2
I7 La
at 390
at
nm
LO
3-
30
2-
20
10
Fig. 3. The influence of temperature on the relative intensities of emission from lo-’ mol dmp3 DMD in 3MP. IT and I 29s denote the intensities of luminescence at temperature T and 298 K respectively. (a) 0, 320 nm; 0, 390 nm; the intensities were measured during the pulse as described in Figs. 1 and 2. (b) q, 320 nm; I, 390 nm; the intensities were measured 100 ns after the beginning of the pulse.
The dependence of the ratio of the emission intensities at 390 nm and 320 nm 1390/1320(both measured 100 ns after the beginning of the pulse) on DMD concentration at different temperatures is shown in Table 1. As is expected the ratio 1390/1320increases with DMD concentration. Our results indicate that the luminescence at 390 nm may be attributed to excimer emission. Brocklehurst and Russell [3] observed excimer fluorescence from naphthalene solutions and showed that it resulted from the neutralization of dimer cations. This type of mechanism is a common phenomenon [5,6,8]. DF does not form dimer cations 1151, as was recently confirmed by timeresolved fluorescence-detected magnetic resonance [ 16 ] . Excimer excited states are also formed by the usual pathway, i.e. the association of excited molecules (lM*) with unexcited molecules (‘M) in liquids
324 TABLE
1
The influence
of DMD
concentration
and temperature
on the Z390/Z320 ratio
Temperature
/DMDl (mol dmP3)
150K
160K
170 K
5 x 10-3 10-2
0.43 1.30
0.64 1.89
0.87 4.22
2 x 10-2
3.33
4.51
5.74
l&J*+
‘M
lM**
--
(1)
Excited states system are formed controlled process 2DF+ + 2DF\
-
of DF mainly
derivatives in a pulse-irradiated 3MP viscous via ion recombination [ 111 by a diffusion-
iDF* + ‘DF /
(2)
In the temperature range 230 - 300 K ion recombination leads to monomer fluorescence only (Fig. 3(b)). Under certain conditions (viscosity) in the range 140 - 230 K DMD excimer formation can be observed. The half-life of monomer fluorescence in the above temperature range depends on the DMD concentration. At 170 K the half-lives ~i,~ of the DMD fluorescence are equal to 19.2 ns, 15.2 ns and 13.8 ns for 5 X 1O-3 mol dm-‘, lo-’ mol dme3 and 2 X lo-’ mol dmd3 respectively. These results seem to support the (2b) channel of excimer formation. (Following the suggestion of one referee we photoexcited a lo-’ mol drnM3 solution of DMD in 3MP at 175 K. Excimer emission at X,,, = 390 nm was observed. This result seems to support the mechanism proposed by us at least in the case of DMD.) At the lowest temperatures (less than 140 K) monomer fluorescence can again be detected because in the highly viscous liquid the (2b) process is so slow that monomer emission takes place before the molecules make contact. Presumably electron transfer takes place at sufficiently large distances so that recombination of ions only gives monomer fluorescence. Our results seem to disregard the (2a) channel of DF excimer formation suggested by Baxendale and Wardman [9].
Acknowledgments We wish to thank the accelerator staff for maintaining This work was supported by contract 01.19.02.06.
the machine.
325
References
6 7 8 9 10 11 12 13 14 15 16
J. B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, London, 1970. J. B. Birks, Rep. Prog. Phys.. 38 (1975) 903 - 974. B. Brocklehurst and R. D. Russel, Trans. Faraday Sot., 65 (1969) 2159. C. Deniau, A. Deroulede, F. Kieffer and J. Rigaut, J. Lumin., 3 (1971) 325. B. Brocklehurst, D. C. Bull and M. Evans, J. Chem. Sot., Faraday Trans. 2, 71 (1975) 543. M. Al-Jarrah, B. Brocklehurst and M. Evans, J. Chem. Sot., Faraday Trans. 2, 72 (1976) 1921. J, Kroh, J. Mayer and M. Szadkowska, Faraday Discuss. Chem. Sot., 63 (1977) 91. J. Mayer, M. Szadkowska-Nicze and I. Zuchowicz, Radiaf. Phys. Chem., 21 (1983) 401. J. H. Baxendale and P. Wardman, Int. J. Radiat. Phys. Chem., 3 (1971) 377. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Proc. 6th "Tihany" Symp. on Radiation Chemistry, Balatonszdplak, 1986, p_ 387. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Radiat. Phys. Chem., 32 (1988) 519. J. Kroh, J. Mayer, A. Plonka and M. Szadkowska-Nicze, Radiat. Phys. Chem., in press. S. Karolczak, K. Hodyr, R. Eubis and J. Kroh, J. Radioanal. Nucl. Chem., IO1 (1986) 177. N. Nakamizo and Y. Kanda, Spectrochim. Acta, 19 (1963) 1235. I. B. Berlman and 0. J. Steingraber, J. Chem. Phys., 43 (1965) 2140. A. Kira, S. Arai and M. Imamura, J. Phys. Chem., 76 (1972) 1119. M. F. Desrosiers and A. D. Trifunac, Chem. Phys. Lett., 121 (1985) 382.
and Photobiology,
A:
Chemistry,
4 7 (1989)
319
EXCIMER EMISSION OF DIPHENYL DERIVATIVES THE PULSE RADIOLYSIS OF 3-METHYLPENTANE SOLUTIONS AT LOW TEMPERATURE M. SZADKOWSKA-NICZE,
J. MAYER
The Institute of Applied Radiation WrBblewskiego 15 {Poland) (Received
September
30, 1988;
- 325
319
IN
and J. KROH
Chemistry,
in revised form
Technical December
University,
93-590
L6d2,
16, 1988)
Summary Using the pulse radiolysis method the excimer formation of diphenyl derivatives in low temperature 3-methylpentane was investigated. For diphenyls two emission bands are found in the temperature range 140 - 230 K. The band at 320 nm is due to monomer fluorescence, whereas the second at 380 - 390 nm is as an excimer emission.
1. Introduction Excimers can be produced as a result of photoexcitation [l, 21 and by ion recombination, involving dimer cations, induced by ionizing radiation [3 - ll]. There are no reports on the photoexcitation of an excimer of diphenyl (DF). Excimer emission of DF has been observed in the radiothermoluminescence of alkane glasses [ 5, 61 and in the pulse radiolysis of a viscous solution in isopentane [9]. A new emission band appearing at 365 - 380 nm has been attributed to the DF excimer fluorescence [ 6,9]. In this paper we describe an application of nanosecond pulse radiolysis at low temperature to the study of excimer formation in the viscous systems DF-3-methylpentane (3MP) and 3,3’-dimethyldiphenyl (DMD)-3MP. Owing to the higher solubility of DMD in 3MP compared with that of DF in 3MP a more detailed study of the excimer formation mechanism is presented.
2. Experimental details 3MP (Fluka, pure grade) was chromatographed through a freshly activated silica gel column and stored under argon. DF (POCh-Poland, analytical grade) was recrystallized twice from ethanol. DMD from Aldrich lOlO-6030/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
320
was used as received. The samples were degassed on the vacuum line in rectangular Spectrosil cells by the freeze-pump-thaw method and sealed under vacuum. The home-built styrofoam-copper cryostat, through which cold nitrogen was passed, enabled experiments to be carried out at temperatures down to 90 K. A linear accelerator (ELU-6, made in the U.S.S.R.) delivering 17 ns and 5 ns electron pulses (approximately 50 and 15 Gy respectively) was used for irradiation. More details concerning the detection system and the accelerator can be found elsewhere [ 11,12 1.
3. Results and discussion Pulse radiolysis of a solution of 5 X 10e3 mol dmh3 DF in 3MP at room temperature generates strong fluorescence during the pulse; the spectrum corresponds exactly to that of the emission from the UV-excited singlet of DF [4, 131. The half-life of the emission, r1,2 = 15 f 1 ns (k = 4.6 X lo7 s-l) is somewhat greater than that reported for UV excitation in cyclohexane (7 1,* = 11.04 ns, k = 6.25 X 10’ s-l) [ 141. We attribute this to the simultaneous formation of the excited states during emission due to fast ion recombination. The spectral distribution of the DF emission from 3MP solution at 175 K is shown in Fig. 1. When measured immediately after the 17 ns pulse, the spectrum resembles that at, room temperature except that beyond 350 nm there is more emission than at room temperature. At longer times, e.g. 50 ns after the pulse, the monomer fluorescence in the range 300 - 330 nm is reduced and at longer wavelengths (approximately 380 nm) a new emission band can be seen (inset (d), Fig. 1). Following earlier suggestions [5, 6, 91 we attribute this delayed long wavelength emission to DF excimers. Similar results were obtained for DMD solutions in 3MP in the concentration range (5 - 20) X 10e3 mol dm- 3. The emission spectra of lo-* mol dmm3DMD in 3MP at various temperatures are shown in Fig. 2. At room temperature the fluorescence of the solution peaks at 320 nm as observed for DF. Using a 5 ns pulse (approximately 15 Gy) the half-life of the DMD fluorescence’(5 X 10d3 mol dmM3) is estimated to be 15.7 * 1 ns (k = 4.4 X lo7 s-l). Lowering of the temperature to 180 K has no influence on the spectral distribution of the light emitted during the pulse. The tail at h > 360 nm seems to be mainly due to Cerenkov emission_ At longer times, i.e. 40 ns and 100 ns after the beginning of the pulse, the monomer fluorescence is significantly reduced. However, a new emission band is observed with X,,, at 390 nm. The apparent half-lives of the emissions (lo-’ mol dmm3 DMD in 3MP at 180 K) measured at 320 nm and 390 nm are equal to approximately 17.4 ns and 55.7 ns respectively. The decay of the 390 nm fluorescence is very strongly temperature dependent. The half-lives of this emission vary
321
h, nm
Fig. 1. Emission spectra from pulse-irradiated (17 ns pulse) 5 X 10M3 mol dmd3 DF in 3MP at 175 K. The intensities of fluorescence were measured during the pulse at the maximum of the intensity-time profiles as shown in the insets (a) and 50 ns after the beginning of the pulse (0). Insets: oscilloscope traces of emissions: (a) 320 nm at 298 K (50 mV per division, 50 ns per division); (b) 380 nm at 298 K (10 mV per division, 50 ns 380 nm per division); (c) 320 nm at 175 K (50 mV per division, 50 ns per division); erenkov at 175 K (10 mV per division, 50 ns per division); the spike corresponds to radiation.
from 43.5 ns to 126.5 ns in the temperature range 200 - 140 K. The apparent activation energy calculated from these data is equal to 4.5 kJ mol-l. At the lowest temperature, i.e. 100 K the spectrum of light emitted during the pulse resembles that at room temperature. The influence of temperature on the intensity of the emission from 10V2 mol dm- 3 DMD in 3MP measured at 320 nm and 390 nm is shown in Fig. 3. The fluorescence intensities are not corrected for the variation in quantum efficiency of the DMD fluorescence with temperature. The relative intensity of the DMD fluorescence at 320 nm measured during the pulse (Fig. 3(a)) gradually decreases with a ‘lowering of temperature to 100 K. This result can be explained as being due to the slowing down of DMD ion recombination with decreasing temperature as a result of the viscosity effect. The relative intensity of emission at 390 nm measured just after the pulse at the maximum of the signal first increases as the temperature decreases, passes through a maximum at approximately 190 K and then decreases. In the temperature range 170 - 230 K, where the emission at 390 nm can be seen, a plateau with small distortions in intensity is observed
322
300
400 A,
500
nm
Fig. 2. Emission spectra from pulse-irradiated (17 ns pulse) 10e2 mol drne3 DMD in 3MP at various temperatures: A, 298 K; 0, 180 K; 0, 100 K. The intensities of fluorescence were measured during the pulse at the maximum of the intensity-time profiles as shown in the insets. At 180 K the fluorescence intensities were also measured at 40 ns (a) and 100 ns (0) after the beginning of the pulse. Insets: oscilloscope traces of emissions: (a) 320 nm at 298 K (50 mV per division, 50 ns per division); (b) 390 nm at 298 K (10 mV per division, 50 ns per division); (c) 320 nm at 180 K (full line) and at 100 K (broken line) (50 mV per division, 50 ns per division); (d) 390 nm at 180 K (full line) and at 100 K (broken line) (10 mV per division, 50 ns per division).
for the 320 nm fluorescence. A similar temperature dependence (as shown in Fig. 3(a)) has been found for pyrene and naphthalene [lo, 111. The decrease in solute fluorescence at 320 nm during the pulse with decreasing temperature is accompanied by delayed emission after the pulse, This effect is due to the relatively slow in particular at low temperature. solute ion recombination which occurs after the pulse in a viscous, low temperature medium. The influence of temperature on the relative intensity of delayed luminescence measured 100 ns after the beginning of the pulse is shown in Fig. 3(b). There is an increase in the monomer fluorescence intensity as the temperature decreases to approximately 230 K, followed by a minimum in the temperature range 160 - 180 K and another maximum at 145 K. The intensity of delayed fluorescence at 390 nm reaches a maximum in the temperature range (160 - 180 K) where the intensity of delayed emission at 320 nm falls to a minimum. This type of dependence of the intensities of luminescence at 320 nm and 390 nm is only observed in the more concentrated solutions of DMD in 3MP (low2 and 2 X 10e2 mol dmp3).
323
4
3 0.5 c 2
I7 La
at 390
at
nm
LO
3-
30
2-
20
10
Fig. 3. The influence of temperature on the relative intensities of emission from lo-’ mol dmp3 DMD in 3MP. IT and I 29s denote the intensities of luminescence at temperature T and 298 K respectively. (a) 0, 320 nm; 0, 390 nm; the intensities were measured during the pulse as described in Figs. 1 and 2. (b) q, 320 nm; I, 390 nm; the intensities were measured 100 ns after the beginning of the pulse.
The dependence of the ratio of the emission intensities at 390 nm and 320 nm 1390/1320(both measured 100 ns after the beginning of the pulse) on DMD concentration at different temperatures is shown in Table 1. As is expected the ratio 1390/1320increases with DMD concentration. Our results indicate that the luminescence at 390 nm may be attributed to excimer emission. Brocklehurst and Russell [3] observed excimer fluorescence from naphthalene solutions and showed that it resulted from the neutralization of dimer cations. This type of mechanism is a common phenomenon [5,6,8]. DF does not form dimer cations 1151, as was recently confirmed by timeresolved fluorescence-detected magnetic resonance [ 16 ] . Excimer excited states are also formed by the usual pathway, i.e. the association of excited molecules (lM*) with unexcited molecules (‘M) in liquids
324 TABLE
1
The influence
of DMD
concentration
and temperature
on the Z390/Z320 ratio
Temperature
/DMDl (mol dmP3)
150K
160K
170 K
5 x 10-3 10-2
0.43 1.30
0.64 1.89
0.87 4.22
2 x 10-2
3.33
4.51
5.74
l&J*+
‘M
lM**
--
(1)
Excited states system are formed controlled process 2DF+ + 2DF\
-
of DF mainly
derivatives in a pulse-irradiated 3MP viscous via ion recombination [ 111 by a diffusion-
iDF* + ‘DF /
(2)
In the temperature range 230 - 300 K ion recombination leads to monomer fluorescence only (Fig. 3(b)). Under certain conditions (viscosity) in the range 140 - 230 K DMD excimer formation can be observed. The half-life of monomer fluorescence in the above temperature range depends on the DMD concentration. At 170 K the half-lives ~i,~ of the DMD fluorescence are equal to 19.2 ns, 15.2 ns and 13.8 ns for 5 X 1O-3 mol dm-‘, lo-’ mol dme3 and 2 X lo-’ mol dmd3 respectively. These results seem to support the (2b) channel of excimer formation. (Following the suggestion of one referee we photoexcited a lo-’ mol drnM3 solution of DMD in 3MP at 175 K. Excimer emission at X,,, = 390 nm was observed. This result seems to support the mechanism proposed by us at least in the case of DMD.) At the lowest temperatures (less than 140 K) monomer fluorescence can again be detected because in the highly viscous liquid the (2b) process is so slow that monomer emission takes place before the molecules make contact. Presumably electron transfer takes place at sufficiently large distances so that recombination of ions only gives monomer fluorescence. Our results seem to disregard the (2a) channel of DF excimer formation suggested by Baxendale and Wardman [9].
Acknowledgments We wish to thank the accelerator staff for maintaining This work was supported by contract 01.19.02.06.
the machine.
325
References
6 7 8 9 10 11 12 13 14 15 16
J. B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, London, 1970. J. B. Birks, Rep. Prog. Phys.. 38 (1975) 903 - 974. B. Brocklehurst and R. D. Russel, Trans. Faraday Sot., 65 (1969) 2159. C. Deniau, A. Deroulede, F. Kieffer and J. Rigaut, J. Lumin., 3 (1971) 325. B. Brocklehurst, D. C. Bull and M. Evans, J. Chem. Sot., Faraday Trans. 2, 71 (1975) 543. M. Al-Jarrah, B. Brocklehurst and M. Evans, J. Chem. Sot., Faraday Trans. 2, 72 (1976) 1921. J, Kroh, J. Mayer and M. Szadkowska, Faraday Discuss. Chem. Sot., 63 (1977) 91. J. Mayer, M. Szadkowska-Nicze and I. Zuchowicz, Radiaf. Phys. Chem., 21 (1983) 401. J. H. Baxendale and P. Wardman, Int. J. Radiat. Phys. Chem., 3 (1971) 377. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Proc. 6th "Tihany" Symp. on Radiation Chemistry, Balatonszdplak, 1986, p_ 387. J. Mayer, M. Szadkowska-Nicze and J. Kroh, Radiat. Phys. Chem., 32 (1988) 519. J. Kroh, J. Mayer, A. Plonka and M. Szadkowska-Nicze, Radiat. Phys. Chem., in press. S. Karolczak, K. Hodyr, R. Eubis and J. Kroh, J. Radioanal. Nucl. Chem., IO1 (1986) 177. N. Nakamizo and Y. Kanda, Spectrochim. Acta, 19 (1963) 1235. I. B. Berlman and 0. J. Steingraber, J. Chem. Phys., 43 (1965) 2140. A. Kira, S. Arai and M. Imamura, J. Phys. Chem., 76 (1972) 1119. M. F. Desrosiers and A. D. Trifunac, Chem. Phys. Lett., 121 (1985) 382.