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Thermally stimulated luminescence of irradiated caffeine
538
Journal of Luminescence 31 & 32(1984)538-540 North -1—lolland, Amsterdam
THERMALLY STIMULATED LUMINESCENCE OF IRRADIATED CAFFEINE M. Shah JAHAN Physics Department, Memphis State University, Memphis, Tennessee 38152, USA* D. Wayne COOKE Los Alamos National Laboratory, Los Alamos, New Mexico
87545, USA
We report the first thermally stimulated luminescence measurements in UVand x—irradiated caffeine. An analysis of the glow peaks resulted in determination of trapping parameters which, in conjunction with attendant emission spectra data, allowed us to use an energy-level model to explain the observations. The data and model both tend to support previous results obtained from biological systems. 1. INTRODUCTION Interest in the study of the effect of UV- and x—irradiation on caffeine has resulted from the discovery that radiation-induced repair of chromosomal aberrations in DNA is inhibited by caffeine if it is added immediately after irradiation, prior addition being without effect’. In an effort to gain knowledge regarding radiation-induced charge trapping and deexcitation mechanisms in caffeine, we used the thermally stimulated luminescence (TSL) technique and investigated the system in the temperature interval 77-300 K. 2. MATERIALS AND METHODS Pressed pellets of caffeine powder were used for this investigation. Details of the x-ray and UV sources, and of the TSL apparatus used can be found 2. elsewhere 3. RESULTS Shown in fig. 1 are typical UV- and x-ray-induced glow curves for caffeine resulting from excitation at 77 K and subsequent heating to 300 K. As indicated by the solid line, the x-ray—induced curve exhibits TSL peaks at 102, 128, 158 and 198 K, whereas the UV—induced glow (dashed line) is characterized by peaks at 120 and 208 K. Concomitant emission spectra of each glow peak, *This work was supported by a Foremost-McKesson Foundation, Inc. grant of Research Corporation and partially by the Memphis State University Faculty Research Grant Fund. This support does not necessarily imply endorsement of research conclusions by the University. 0022—2313/84/$03.OO© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
MS. Jaltan, D.W. Cooke / Luminescence of irradiated caffeine
539
whether UV— or x—ray—induced, were similar and consisted of a band with two well-defined maxima at 405 and 480 nm (fig. 2). The relative intensities of these two peaks changed dramatically with temperature; this change results either from a temperature—dependent emission process or from different spectra associated with higher temperature glow peaks. Glow peak parameters such as activation energy (E), frequency factor (s), heating rate (8) and kinetics order (2) were extracted for each glow peak using experimental and theoretical methods, and the ones that best describe the experimental glow peaks are listed in Table 1. Since we were unable to resolve the UV-induced peak at 120 K into its components, and since the 208 K peak was of insufficient intensity, our glow curve analysis was limited to the x-ray-induced TSL peaks.
c8
:2
___________
60
140
220
300
TEMPERATURE (K) FIGURE 1 Typical glow curves of irradiated caf— feine. The solid line corresponds to 5 R) glow the (2.22 x i0 curvex-ray-induced and the dashed line represents the UV—induced (254 nat excitation) one. Irradiations were performed at 77 K and the sample was subsequently heated to 300 K.
_________
300
400
500
600
WAVELENGTH (nat) FIGURE 2 Characteristic emission spectra of the TSL glow peaks of fig. 1. Theinterval letters correspond to the temperature in which the spectra were recorded: A(100—11OK),B(132-138K),C(158—162K). The spectra have been corrected for the nonlinear response of the monochromator grating and PMT.
4. DISCUSSION The experimental results can be explained by the model shown in fig. 3.
In
the temperature regime 100-150 K, thermal energy releases trapped charges allowing them to fall into the first excited singlet ~ or triplet (Ti) state (dashed lines).
Those in S~undergo a radiative transition to ground state, S~,
yielding 405 nat radiation. If, on the other hand, they are initially in state T 1, they may ultimately decay to S~,yielding a 480 nm emission. The lifetime
MS. Jalta,t, D. Ii’. (‘ook~/ Luotineace,tce oj irradiated caJjei,ts’
540
of this state is greater than
because the deexcitation represents a spin-
forbidden transition. In either process, the deexcitation obeys first-order kinetics. The energy separation, SE, between S~and T 1 is 480 meV, which is large compared to the thermal activation energies of the 102, 128 and 158 K glow peaks (90, 126 and 165 meV respectively). This value of AE makes it unlikely that charges residing in T1 can undergo intersystem crossing and reach ~l by any thermally activated process.
With increased temperature, however,
the triplet state is quenched, possibly by paramagnetic impurities such as oxygen or, alternatively, it forms excited triplet pairs resulting in an excited singlet state. Either mechanism would reduce the probability of a T1 -e radiative transition, thereby decreasing the intensity of the 480 nm emission (fig. 2). The decay process follows kinetics orders 1.3 and 1.8 for the 158 K and 198 K glow peaks, respectively, due to multiple retrapping of charges. •1
TABLE 1. Summary of glow peak parameters that describe the experimental TSL glow peaks in x-irradiated caffeine. Glow Temp. Peak (mev) E 102
90
128 158
126 165
198
450
5_i (sec )
8 (K.sec
—
2 S
—
TSL TRAPS A = 405 nm 0 eV
eV LiE = 480 meV T 2 8 1’ .5 eV
1.18x10
0.042
1
0
4.73x102
0.058 0 050
1 1 3
0.040
1.8
FIGURE 3 Energy-level diagram depicting the TSL process in irradiated caffeine. I.C. corresponds to intersystem crossing.
lxi 8 7.59x10
5. CONCLUSIONS The previously adopted3 two-state energy-level model explains the results of thermally stimulated processes of radiation—induced charges in caffeine. REFERENCES 1) B. A. Kihlman, Caffeine and Chromosomes
(Elsevier, Amsterdam, 1977).
2) W. Cooke, J. Rhodes, R. Santi and C. Alexander, Jr., J. Chem. Phys. 73 (1980) 3573. 3) 0. W. Cooke, S. L. Fortner and M. S. Jahan, J. Lumin. 26 (1982) 319.
Journal of Luminescence 31 & 32(1984)538-540 North -1—lolland, Amsterdam
THERMALLY STIMULATED LUMINESCENCE OF IRRADIATED CAFFEINE M. Shah JAHAN Physics Department, Memphis State University, Memphis, Tennessee 38152, USA* D. Wayne COOKE Los Alamos National Laboratory, Los Alamos, New Mexico
87545, USA
We report the first thermally stimulated luminescence measurements in UVand x—irradiated caffeine. An analysis of the glow peaks resulted in determination of trapping parameters which, in conjunction with attendant emission spectra data, allowed us to use an energy-level model to explain the observations. The data and model both tend to support previous results obtained from biological systems. 1. INTRODUCTION Interest in the study of the effect of UV- and x—irradiation on caffeine has resulted from the discovery that radiation-induced repair of chromosomal aberrations in DNA is inhibited by caffeine if it is added immediately after irradiation, prior addition being without effect’. In an effort to gain knowledge regarding radiation-induced charge trapping and deexcitation mechanisms in caffeine, we used the thermally stimulated luminescence (TSL) technique and investigated the system in the temperature interval 77-300 K. 2. MATERIALS AND METHODS Pressed pellets of caffeine powder were used for this investigation. Details of the x-ray and UV sources, and of the TSL apparatus used can be found 2. elsewhere 3. RESULTS Shown in fig. 1 are typical UV- and x-ray-induced glow curves for caffeine resulting from excitation at 77 K and subsequent heating to 300 K. As indicated by the solid line, the x-ray—induced curve exhibits TSL peaks at 102, 128, 158 and 198 K, whereas the UV—induced glow (dashed line) is characterized by peaks at 120 and 208 K. Concomitant emission spectra of each glow peak, *This work was supported by a Foremost-McKesson Foundation, Inc. grant of Research Corporation and partially by the Memphis State University Faculty Research Grant Fund. This support does not necessarily imply endorsement of research conclusions by the University. 0022—2313/84/$03.OO© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
MS. Jaltan, D.W. Cooke / Luminescence of irradiated caffeine
539
whether UV— or x—ray—induced, were similar and consisted of a band with two well-defined maxima at 405 and 480 nm (fig. 2). The relative intensities of these two peaks changed dramatically with temperature; this change results either from a temperature—dependent emission process or from different spectra associated with higher temperature glow peaks. Glow peak parameters such as activation energy (E), frequency factor (s), heating rate (8) and kinetics order (2) were extracted for each glow peak using experimental and theoretical methods, and the ones that best describe the experimental glow peaks are listed in Table 1. Since we were unable to resolve the UV-induced peak at 120 K into its components, and since the 208 K peak was of insufficient intensity, our glow curve analysis was limited to the x-ray-induced TSL peaks.
c8
:2
___________
60
140
220
300
TEMPERATURE (K) FIGURE 1 Typical glow curves of irradiated caf— feine. The solid line corresponds to 5 R) glow the (2.22 x i0 curvex-ray-induced and the dashed line represents the UV—induced (254 nat excitation) one. Irradiations were performed at 77 K and the sample was subsequently heated to 300 K.
_________
300
400
500
600
WAVELENGTH (nat) FIGURE 2 Characteristic emission spectra of the TSL glow peaks of fig. 1. Theinterval letters correspond to the temperature in which the spectra were recorded: A(100—11OK),B(132-138K),C(158—162K). The spectra have been corrected for the nonlinear response of the monochromator grating and PMT.
4. DISCUSSION The experimental results can be explained by the model shown in fig. 3.
In
the temperature regime 100-150 K, thermal energy releases trapped charges allowing them to fall into the first excited singlet ~ or triplet (Ti) state (dashed lines).
Those in S~undergo a radiative transition to ground state, S~,
yielding 405 nat radiation. If, on the other hand, they are initially in state T 1, they may ultimately decay to S~,yielding a 480 nm emission. The lifetime
MS. Jalta,t, D. Ii’. (‘ook~/ Luotineace,tce oj irradiated caJjei,ts’
540
of this state is greater than
because the deexcitation represents a spin-
forbidden transition. In either process, the deexcitation obeys first-order kinetics. The energy separation, SE, between S~and T 1 is 480 meV, which is large compared to the thermal activation energies of the 102, 128 and 158 K glow peaks (90, 126 and 165 meV respectively). This value of AE makes it unlikely that charges residing in T1 can undergo intersystem crossing and reach ~l by any thermally activated process.
With increased temperature, however,
the triplet state is quenched, possibly by paramagnetic impurities such as oxygen or, alternatively, it forms excited triplet pairs resulting in an excited singlet state. Either mechanism would reduce the probability of a T1 -e radiative transition, thereby decreasing the intensity of the 480 nm emission (fig. 2). The decay process follows kinetics orders 1.3 and 1.8 for the 158 K and 198 K glow peaks, respectively, due to multiple retrapping of charges. •1
TABLE 1. Summary of glow peak parameters that describe the experimental TSL glow peaks in x-irradiated caffeine. Glow Temp. Peak (mev) E 102
90
128 158
126 165
198
450
5_i (sec )
8 (K.sec
—
2 S
—
TSL TRAPS A = 405 nm 0 eV
eV LiE = 480 meV T 2 8 1’ .5 eV
1.18x10
0.042
1
0
4.73x102
0.058 0 050
1 1 3
0.040
1.8
FIGURE 3 Energy-level diagram depicting the TSL process in irradiated caffeine. I.C. corresponds to intersystem crossing.
lxi 8 7.59x10
5. CONCLUSIONS The previously adopted3 two-state energy-level model explains the results of thermally stimulated processes of radiation—induced charges in caffeine. REFERENCES 1) B. A. Kihlman, Caffeine and Chromosomes
(Elsevier, Amsterdam, 1977).
2) W. Cooke, J. Rhodes, R. Santi and C. Alexander, Jr., J. Chem. Phys. 73 (1980) 3573. 3) 0. W. Cooke, S. L. Fortner and M. S. Jahan, J. Lumin. 26 (1982) 319.