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Saturation of the fluorescence of tetracene in anthracene crystal
Volume 8, number 5
SATURATION
CHEMICAL
OF IN
THE
1 March
PHYSICS LETTERS
FLUORESCENCE
ANTHRACENE
OF
197i
TETRACENE
CRYSTAL
M. KAWABE, Facdty
K. MASUDA and S. NAMBA of Engineering Science, Osaka University, Osaka, Ja$zn
Received 1 December
1970
Saturation of the fluorescenc=z of tetracene in anthracene crystal with increasing excitation was ob-served at liquid nitrogen temperature. Samples were excited by a iow energy electron beam (X 20 kev). The reIation between t!e fluorescence intensity and the electron beam density was explained by solving a steads state rate equation. It was concluded that the snturation of the fluorescence is due to the depletion of ihe tetracene molecules of the ground state .
Tetracene molecule in anthracene crystal is known to accept energy from anthracene with high efficiency. Even at concentration of tetracene as low as 10-6 mole per mole of anthracene, the fluorescence of anthracene is strongly quenched and the green tetraaene fluorescence is observed. By using electron beam excitation which is a method of high density excitation, the saturation of the tetracene fluorescence was observed with ’ fncreasing the excitation beam density. Samples were prepared by recrystallization from N-N’ dimethylformamide solution of anthracene and an appropriate amount of tetracene. The scintillation grade anthracene was purified by passing 100 melting zones and the commercial ultra pure grade tetracene was purified by 8 times sublimation and they were used for recrystallization. Very thin crystals (about 0.2 mm thick) were obtained. Iti this experiment three kinds of samples with different tetracene concentrations, 2.7 X10s6, 6 X 10e6 and 1 X 10e4 mole ratio to anthracene, were used, Hereafter.they are: called samples A,B and C. The concentrations of tetracene were m-red by the method of Kreps et al. [l]. Samples were attached to a copper block cooled by liquid nitrogen. The electron beam was incident at 45O on the flat surface (ab plane). .S~ples were excited by a single short pulse of the electron Meam in order to avoid temperature rise &$I radiation damage of the sample. The pulse width ~~0.2 &sec with rice an! fall times
of about 0.05 psec. The oscilloscope traces of the fluorescence were photographed. They had similar pulse shapes to those of the electron beam except for a very weak fluorescence with 2 long decay time which was negligibly small compared with the total fluorescence intensity. Therefore, the fluorescence intensity was approximated by the fluorescence pulse height. Figs. 1 and 2 show the fluorescence intensity versus beam density. *In fig. 1, c) shows the fluorescence of 4300 A of sample B which is due to anthracene molecule. 0 and a show the fluorescence of 5300 A of samples A and B, respectively. The results of 4300 A show no saturation phenomena, while those of 5300 A saturate at the excitation density of 0.4 A/cm2 and 0.8 A/cm2. Since the fluorescence intensity cue to anthracene moiecules increases linearlye with excitation density, the saturation of 5300 A is not due to ionization quenching or radiation damage. The beam density where the fluorescence begins to saturate increases with increasing the tetracene con$entration. Fig. 2 shows the fluorescence of 5300A of sample C which shows no saturation phenomena within these ex+.ation den&ties. The solid lines are cdculat@ v&es obtained by solving the-rate equations of the concentration of anthracene singlet exciton nA -and tetracene *Cited m&&&nT . dn..
Jr
:-.. -‘Q
=-NAG ;z.;
+d+
- ‘+
nA ,
yI
‘.
(1)
Volume 8. number 5
0
0.5
CHEMICAL PHYSICS LETTERS
1.0
2.0
1.5
Bwm Density f A/C&
1 March 1971
I
Fig. I. Light intensity versus excitation density. 0
shows the fluorescencg of 4300 A. @ and * show the fluorescence of 5300 A of samples A and B. respectiveiy. The solid lines show the calculated values of the density of the excited tetracene molecules(rightside scale) versus excitation density.
0
0.5 EIeam
1.0
1s
2.0
Density [n/&2 f
Fig. 2. Ligitt intensity Versus exzitation density. 3 shows the fluorescence of 5300 A of sample C. The solid line represents the calculated vniucs.
cene and tetracene. In the case of sampfes A, B and C, 0.65, 0.34 and 0.64 are used as the &T --‘@Yzk(NT -nT)?zA -F, values of R according to Benz et al. [2]. In order T to get the relation between% and beam density, vT/NT and NAG should be ~ILOWTL TT/‘HT C~UI where NA and NT are the cancer&rations of be calculated assuming “T = 1.3~ 10-g see 131. The scintillation efficiency of anthracene exanthracene and tetracene, respectively, N G cited by an electron beam has been measured is the rate of generation of anthracene sing4 et excLtonwhich is assumed to be proportional to to be 0.057 at - 7OoC [4] and the Lightyield shows small temperature dependence below 200°K [5]. the excitation density, TA and 7T are the life The value of 0.057 was used as conversion eftimes of the anthracene exciton and the tetracene ficiency of electron beam into tight at liquid excited molecules and k[NT-njl~)is the energy nitrogen temperature, The penetration depth of transfer probability. Since the pulse width of the electron beam is long compared with the life an electron beam of 20 keV was measured to be about 5 @XI[6), Using the above values, time of the fluorescence, the system is approxiNAG can be calculated on the assumption that mately in steady state, therefore, NAG is independent of NT. In the case of pure anthracene N-&Gttv = WXO.051, where W is the in(3) put power density. When anthracene is excited by an electron beam of 1 A/cm2, 20 keV, NAG By e&inatiOn of nA in eqs. (I) and (2) On the is 3.8~ 102* crne3 se& and 1;is calculated to above condition, nT is abtained as follows: be 4.91, 2.t7 and 0.13 for samples A,3 and C, nT=;iu;f (x+&.1 -&2+2 (R-l)xc(R+~~2]~'z~, respectively, The solid curves in figs. Z and 2 show eq. (4) as a function of beam density inw stead-of x. Since the absofute emission power where% = NACrT/NT and R = l/kNT rA’ R is of the sample could not be measured, the relaconsidered to be approximately equal to the tive %ntensitywas de&&d such @at the experi~fiuokescence intensity r&o of anthracene to t mental results might fit the calculz$ed curves. tetracene whenn C< NT, because in eq. (1) the The saturation is weU explained by the calcusecond and the third terms of the,right side are lated curves. The saturatiodis due to the first proportion+ to fluorescence intensity of anthra- . term of the right &de of eq. (2). _ 449
:.‘ti&m~e 6, nyber.5 .;-RFFEENCES--
I_
._ :
. :
I C~EMIC&,PHYSiCS’LE’&Eks _’
--
,’
D1s;I.Kreps..M.?~n-8nd B.&GOT?, AnaIl Cheui. -37 (1965j. 586. ,-.[2] K. W.Benz and H. C. Wolf, Z., Naturforsch. 19a ..’ (1964).177. :
I.
_
..
.
._-.
t
’
1 ,March 1971.
.
f3] &.D;GaIanin and Z;A.ChZztmkova, ‘bpt. i spektroskopiya’ 1 (1956) 115. ‘. (4) J.B.Birks, Phys. Rev, 94 (195& 196i. 151 W. F. von Kienzle, .Z. Naturforsch; :9a (1964) 756,‘ [6] M. Kayabe, K.Masuda and_S. Nsmba, J. Appl. Phys ,; ,: : to be published.
-_
SATURATION
CHEMICAL
OF IN
THE
1 March
PHYSICS LETTERS
FLUORESCENCE
ANTHRACENE
OF
197i
TETRACENE
CRYSTAL
M. KAWABE, Facdty
K. MASUDA and S. NAMBA of Engineering Science, Osaka University, Osaka, Ja$zn
Received 1 December
1970
Saturation of the fluorescenc=z of tetracene in anthracene crystal with increasing excitation was ob-served at liquid nitrogen temperature. Samples were excited by a iow energy electron beam (X 20 kev). The reIation between t!e fluorescence intensity and the electron beam density was explained by solving a steads state rate equation. It was concluded that the snturation of the fluorescence is due to the depletion of ihe tetracene molecules of the ground state .
Tetracene molecule in anthracene crystal is known to accept energy from anthracene with high efficiency. Even at concentration of tetracene as low as 10-6 mole per mole of anthracene, the fluorescence of anthracene is strongly quenched and the green tetraaene fluorescence is observed. By using electron beam excitation which is a method of high density excitation, the saturation of the tetracene fluorescence was observed with ’ fncreasing the excitation beam density. Samples were prepared by recrystallization from N-N’ dimethylformamide solution of anthracene and an appropriate amount of tetracene. The scintillation grade anthracene was purified by passing 100 melting zones and the commercial ultra pure grade tetracene was purified by 8 times sublimation and they were used for recrystallization. Very thin crystals (about 0.2 mm thick) were obtained. Iti this experiment three kinds of samples with different tetracene concentrations, 2.7 X10s6, 6 X 10e6 and 1 X 10e4 mole ratio to anthracene, were used, Hereafter.they are: called samples A,B and C. The concentrations of tetracene were m-red by the method of Kreps et al. [l]. Samples were attached to a copper block cooled by liquid nitrogen. The electron beam was incident at 45O on the flat surface (ab plane). .S~ples were excited by a single short pulse of the electron Meam in order to avoid temperature rise &$I radiation damage of the sample. The pulse width ~~0.2 &sec with rice an! fall times
of about 0.05 psec. The oscilloscope traces of the fluorescence were photographed. They had similar pulse shapes to those of the electron beam except for a very weak fluorescence with 2 long decay time which was negligibly small compared with the total fluorescence intensity. Therefore, the fluorescence intensity was approximated by the fluorescence pulse height. Figs. 1 and 2 show the fluorescence intensity versus beam density. *In fig. 1, c) shows the fluorescence of 4300 A of sample B which is due to anthracene molecule. 0 and a show the fluorescence of 5300 A of samples A and B, respectively. The results of 4300 A show no saturation phenomena, while those of 5300 A saturate at the excitation density of 0.4 A/cm2 and 0.8 A/cm2. Since the fluorescence intensity cue to anthracene moiecules increases linearlye with excitation density, the saturation of 5300 A is not due to ionization quenching or radiation damage. The beam density where the fluorescence begins to saturate increases with increasing the tetracene con$entration. Fig. 2 shows the fluorescence of 5300A of sample C which shows no saturation phenomena within these ex+.ation den&ties. The solid lines are cdculat@ v&es obtained by solving the-rate equations of the concentration of anthracene singlet exciton nA -and tetracene *Cited m&&&nT . dn..
Jr
:-.. -‘Q
=-NAG ;z.;
+d+
- ‘+
nA ,
yI
‘.
(1)
Volume 8. number 5
0
0.5
CHEMICAL PHYSICS LETTERS
1.0
2.0
1.5
Bwm Density f A/C&
1 March 1971
I
Fig. I. Light intensity versus excitation density. 0
shows the fluorescencg of 4300 A. @ and * show the fluorescence of 5300 A of samples A and B. respectiveiy. The solid lines show the calculated values of the density of the excited tetracene molecules(rightside scale) versus excitation density.
0
0.5 EIeam
1.0
1s
2.0
Density [n/&2 f
Fig. 2. Ligitt intensity Versus exzitation density. 3 shows the fluorescence of 5300 A of sample C. The solid line represents the calculated vniucs.
cene and tetracene. In the case of sampfes A, B and C, 0.65, 0.34 and 0.64 are used as the &T --‘@Yzk(NT -nT)?zA -F, values of R according to Benz et al. [2]. In order T to get the relation between% and beam density, vT/NT and NAG should be ~ILOWTL TT/‘HT C~UI where NA and NT are the cancer&rations of be calculated assuming “T = 1.3~ 10-g see 131. The scintillation efficiency of anthracene exanthracene and tetracene, respectively, N G cited by an electron beam has been measured is the rate of generation of anthracene sing4 et excLtonwhich is assumed to be proportional to to be 0.057 at - 7OoC [4] and the Lightyield shows small temperature dependence below 200°K [5]. the excitation density, TA and 7T are the life The value of 0.057 was used as conversion eftimes of the anthracene exciton and the tetracene ficiency of electron beam into tight at liquid excited molecules and k[NT-njl~)is the energy nitrogen temperature, The penetration depth of transfer probability. Since the pulse width of the electron beam is long compared with the life an electron beam of 20 keV was measured to be about 5 @XI[6), Using the above values, time of the fluorescence, the system is approxiNAG can be calculated on the assumption that mately in steady state, therefore, NAG is independent of NT. In the case of pure anthracene N-&Gttv = WXO.051, where W is the in(3) put power density. When anthracene is excited by an electron beam of 1 A/cm2, 20 keV, NAG By e&inatiOn of nA in eqs. (I) and (2) On the is 3.8~ 102* crne3 se& and 1;is calculated to above condition, nT is abtained as follows: be 4.91, 2.t7 and 0.13 for samples A,3 and C, nT=;iu;f (x+&.1 -&2+2 (R-l)xc(R+~~2]~'z~, respectively, The solid curves in figs. Z and 2 show eq. (4) as a function of beam density inw stead-of x. Since the absofute emission power where% = NACrT/NT and R = l/kNT rA’ R is of the sample could not be measured, the relaconsidered to be approximately equal to the tive %ntensitywas de&&d such @at the experi~fiuokescence intensity r&o of anthracene to t mental results might fit the calculz$ed curves. tetracene whenn C< NT, because in eq. (1) the The saturation is weU explained by the calcusecond and the third terms of the,right side are lated curves. The saturatiodis due to the first proportion+ to fluorescence intensity of anthra- . term of the right &de of eq. (2). _ 449
:.‘ti&m~e 6, nyber.5 .;-RFFEENCES--
I_
._ :
. :
I C~EMIC&,PHYSiCS’LE’&Eks _’
--
,’
D1s;I.Kreps..M.?~n-8nd B.&GOT?, AnaIl Cheui. -37 (1965j. 586. ,-.[2] K. W.Benz and H. C. Wolf, Z., Naturforsch. 19a ..’ (1964).177. :
I.
_
..
.
._-.
t
’
1 ,March 1971.
.
f3] &.D;GaIanin and Z;A.ChZztmkova, ‘bpt. i spektroskopiya’ 1 (1956) 115. ‘. (4) J.B.Birks, Phys. Rev, 94 (195& 196i. 151 W. F. von Kienzle, .Z. Naturforsch; :9a (1964) 756,‘ [6] M. Kayabe, K.Masuda and_S. Nsmba, J. Appl. Phys ,; ,: : to be published.
-_