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Triplet-triplet interactions in organic scintillators
Volume 7, number 2
CHEMICAL PHYSICS LETTERS
TRIPLET-TRIPLET
INTERACTIONS
IN
15 October 1970
ORGANIC
SCINTILLATORS
v
J. B. BIRKS ** CJtemistry
Division.
Argonne
Argonne,
National Laboratory,
Illinois,
USA’
Received 25 August 1970
Data on the prompt and delayed scintillation emission of an organic liquid solution are analysed to obtain the triplet-triplet interaction parametera.
An ionizing particle impinging on a unitary organic scintillator X (e.g., an anthracene crystal or an aromatic liquid) yields molecule in higher excited n-electronic singlet and triplet states, which are populated by direct excitation and/or molecular ion recombination, and which rapidly internally convert to the lowest excited singlet (SIX) and triplet (TlX) states, respectively [l]. The scintillation emission consists of a prompt component PX, which decays exponentially in a few nsec with the SIX fluorescence lifetime fX, and a delayed component DX , which decays non-exponentially in a few psec. The yield (per 100 eV) of PX is G(PX) = G(SIX)
+Px ,
(1)
where G(SlX) is the SIX yield, and Q~x is the Six fluorescence quantum yield. DX is due to the process
G (DX) = @TX G(SlX)l (kXTT/2”TT)@‘FX,
SIX +
(3)
where GfTlX) is the TlX yield; @TX( = k&kx) quantum yield from SIX; km is the rate of SI~-T~~ intersystem crossing; kX( =1/7X) is * Based on work performed under the auspices of the
U.S. Atomic Energy Commission. ** On leave of absence from The Schuster Laboratory, University of Manchester. Manchester 13, UK, to which reprint requests should be addressed.
soy -sax
+ qy 9
(4)
the rate of which is controlled by SIX migration and diffusion [3]. The yield of Py is
G(Py) =
i2) 9 TIX + TIX -S1x+Sox the rate of which is controlled by TIX migration and/or diffusion, and which yields delayed SIX fluorescence and an unexcited molecule SOx [2]. The yield of DX is
[G(TIX)+
the rate of SIX decay; kXTT is the rate parameter of process (2); and kTT is the total rate parameter of TlX quenching by TI~-TI~ interaction. The scintillation emission of a binary organic liquid soluticn scintillator, consisting of an aromatic solvent X, cc._“ining a more fraction c of a fluorescent solute Y, also comprises a prompt component Py and a delayed component DY, each due to fluorescence from Sly, the first excited singlet state of Y. PY is due to energy transfer from SIX to Sly,
G(Slx) (
1
:y;,)
@‘FY>
where uyx (=kyx/RX) is the Stern-Volmer coefficient of S~X-S~X energy transfer; RYE is the rate parameter of process (4); and @PY is the Sly fluorescence quantum yield. Voltz and Laustriat [4],have proposed that Dy in liquid solution scintillators is due to process (2), followed by process (4). An analysis [5] of the DV decay of oxygen-saturated and airsaturated solutions of toluene containing 8 g 1-l PBD in terms of this model gives triplet lifetimes of 110 nsec and 400 nsec, respectively, which extrapolate to a triplet lifetime of 1.3 psec in the absence of oxygen. VoLtz et ai. [5] identify this with the soluenr triplet (TlX} lifetime. This identification is incorrect, since the triplet lifetime of the all@ benzene liquids is only = 10 - 2C nsec [6-81. It is proposed that the triplet lifetime of 1.3 ~1set is that of the solute triplet (Tlu) state, and 293
Volume 7. numaer 2
CHEMICAL
PHYSICS LETTER5
that DY originates from the following sequence of processes: (i) population of TIX by the ionizing particle and by intersystem crossing from Six; (ii) energy transfer from TIX to TlY, (6) TlX + Soy “Sox + TlY 3 which occurs by electron-fxchange interaction as an efficient diffusion-controlfed process in fluid solutions [S]; and (iii) solute triplet-triplet interaction * TlY + T1Y -%Y+%Y leading to delayed SlY fluorescence. model the DY yield is G(Dy) =
CWIX)
-I-
(7) On this
@TX >wX) i ( +oiyyc 1
(8) where suffix 5; refers to Tly; u T( =kZT/kT) is the Stern-Volmer coefficient of H ~x-‘I’~~ energy transfer; k T is the rate parameter of process (6); kT( = 1BTT) is the unimolecular rate of TAX decay when c=O; kyzz is the rate parameter of is the total rate parameter of process (7); k Tly quenchingzz y Tly-TlY interaction; and kZ( = l/rZ) is the unimolecvlar rate of Tly decay. The form of the DY decay is the same as on the Voltz-Laustriat model [4], but CZ replaces TT. At high c O> l/oyx. l/uZT) (5) and (8) reduce, respectively, to GH(PY) = C(Slx) *FY , ‘-+@y)
= 4 cuZ G(Tlx) Lh~y ,
so that
(9) Eq. f9) provides a method of evaluating (YZ. Cundall et ai. [13] have observed GfSlx)/G(TLx)= 1.15 for deoxygenated liquid benzene excited by 4 MeV electrons. Fuchs et aL [ll] have observed GH(PY) = 0.85, GP(Dy) = 0.15, for deoxygenated benzene solutions ‘containing 5 g 1-l (x-NPG excited by 0.624 MeV electrons. (The absolute values of Go and GH(DY) appear low, but the relative values should be reliable. ) ~bsti~~on of these values in f9) gives o!Z = 0.464 for cr -NPo. The possible spin-allowed TlY-TlY quenching processes are: 294
1970
SOY+Soy + kvy, *F f” /+ZY , TlY * SOY TlY +TlY - Sly + SOY \ *GY ; ‘SOY *soy TlY ‘TlY - s0y+50y i
(7a)
TlY’ TlY -, TIYfSOY If the rate parameters of processes
W)
l
t;;;“”
kYZZ~ kGZZ and kZZZ,
Ub) UC) fm
(‘i’), (10) and respectively,
(la) kzz =kyZZW+Q,Zy)+kGZZ+%kZZZ. There are nine possible (Tl +Tly) spin states: one singlet, which can Becay by processes (7) or (10); three triplets, which can decay by process (1X); and five quintets which probably dissociate into TlY+ Tly. If the 9 spin states are formed with equal probability in TRY - T1Y encounters in a fluid solution, and if processes (7), (10) and (11) occur irreversibly, then
(13) kGZ2) ’ Substi~ti~g aZ = 0.406, @ZY = I-@FY = 0.22 (where CBFY= 0.78 for a!-NPG in benzene [12]} and (13) in (12), we obtain kZZZ
= 3(kyZZ +
kGzz
=0.036 kyZZ ; kZZZ=3.11
kyzZ .
Alternatively, if we assume ~YZZ >> kGzz, since the latter process involves a much larger energy dissipation (i.e., a much smaller Franek-Condon factor), then we obtain kGzz
(5a) (*a)
15 Octcber
= 0 ; kzzz
= 3.18kyzz.
which is consistent with (13) within the experimental error. It is thus to be expected that
(14) kyZZ = W9) kdiff , where kaf is the rate parameter of diffusioncontrolled Tly-Tly encounters. Groff et ah 1131have obtained an upper limit of f=PXTT ‘XT
&Fx = 0.36
(15)
for crystal anthracene from observations of the
magnetic field dependence of the SIX delayed fluorescence arising from Tlx-TIX interaction, and ta.kinZkGTT (the parameter analogous to kGzz) = il. SillC!t?*F = 0.9 [I], this COITE!spends
to aT(=&fT
7bT]
= 0.40,
in close
agreement with the value of ‘YZ = 0.404 obtained for ol-Nl?O in benzene solution.
Volume ‘i’, number 2
CHEMICAL PHYSICS LETTERS
REFERENCES [l] J. B. Birks, The theory and practice of scintillation counting (Pergamon Press, Oxford. 1964). [2] T. A-King and R. Voltz, Proc. Roy. Sot. A289 (1966) 424. [3j J. B. Birks and J. C. Conte, Proc. Roy. Sot. A303 0.968) 85. [4] R. Voltz and G. Laustriat, J. Phys. 29 (1968) 159. &5]R, Voltz, H. DuPont and G. Laustriat. J. Phys. 29 4968) 297. [S] F.Wilkinson and J.T.Dnbois, J. Chem. Phys. 39 (1963) 377. I?] R, B. Cundall and P. A. Griffiths. Trans. Faraday Sot. 61 (1965) 1968.
15 October 1970
[8] R.B. Cundall and A. J.R. Voss. C&em. Commun. (1969) 116, [S] J. B. Birks. Photophysics of aromatic molecules (Wiley-Interscience. London snd New Pork. 1970). 1101 _ . R. B. Cundall. G,B. Evans. P. .a. Griffiths and J. P.Keene. j. Phys. Che& 72 (l968) 3871. IllI _ _ C. Fuchs. F. Heise!. R. V&z and A. Cache. Infx+rn. Conf. Organic Scintillators and Liquid ScintilIation Counting, San Francisco (l9’iO). to be pubIished. 1121 _ _ A. Greenberg, MI.Furst and Ii. Kallmann, Intern. Symp. Luminescence, The physics and chemistry of scintiltators (Verlna Karl Thiemip;, hIunich. 1966) p.71. . fl3i R. 3. Groff. R. E. Mercifietd and P. Avakfan. Chem. Phya. Letrers 5 (1970) X68.
295
CHEMICAL PHYSICS LETTERS
TRIPLET-TRIPLET
INTERACTIONS
IN
15 October 1970
ORGANIC
SCINTILLATORS
v
J. B. BIRKS ** CJtemistry
Division.
Argonne
Argonne,
National Laboratory,
Illinois,
USA’
Received 25 August 1970
Data on the prompt and delayed scintillation emission of an organic liquid solution are analysed to obtain the triplet-triplet interaction parametera.
An ionizing particle impinging on a unitary organic scintillator X (e.g., an anthracene crystal or an aromatic liquid) yields molecule in higher excited n-electronic singlet and triplet states, which are populated by direct excitation and/or molecular ion recombination, and which rapidly internally convert to the lowest excited singlet (SIX) and triplet (TlX) states, respectively [l]. The scintillation emission consists of a prompt component PX, which decays exponentially in a few nsec with the SIX fluorescence lifetime fX, and a delayed component DX , which decays non-exponentially in a few psec. The yield (per 100 eV) of PX is G(PX) = G(SIX)
+Px ,
(1)
where G(SlX) is the SIX yield, and Q~x is the Six fluorescence quantum yield. DX is due to the process
G (DX) = @TX G(SlX)l (kXTT/2”TT)@‘FX,
SIX +
(3)
where GfTlX) is the TlX yield; @TX( = k&kx) quantum yield from SIX; km is the rate of SI~-T~~ intersystem crossing; kX( =1/7X) is * Based on work performed under the auspices of the
U.S. Atomic Energy Commission. ** On leave of absence from The Schuster Laboratory, University of Manchester. Manchester 13, UK, to which reprint requests should be addressed.
soy -sax
+ qy 9
(4)
the rate of which is controlled by SIX migration and diffusion [3]. The yield of Py is
G(Py) =
i2) 9 TIX + TIX -S1x+Sox the rate of which is controlled by TIX migration and/or diffusion, and which yields delayed SIX fluorescence and an unexcited molecule SOx [2]. The yield of DX is
[G(TIX)+
the rate of SIX decay; kXTT is the rate parameter of process (2); and kTT is the total rate parameter of TlX quenching by TI~-TI~ interaction. The scintillation emission of a binary organic liquid soluticn scintillator, consisting of an aromatic solvent X, cc._“ining a more fraction c of a fluorescent solute Y, also comprises a prompt component Py and a delayed component DY, each due to fluorescence from Sly, the first excited singlet state of Y. PY is due to energy transfer from SIX to Sly,
G(Slx) (
1
:y;,)
@‘FY>
where uyx (=kyx/RX) is the Stern-Volmer coefficient of S~X-S~X energy transfer; RYE is the rate parameter of process (4); and @PY is the Sly fluorescence quantum yield. Voltz and Laustriat [4],have proposed that Dy in liquid solution scintillators is due to process (2), followed by process (4). An analysis [5] of the DV decay of oxygen-saturated and airsaturated solutions of toluene containing 8 g 1-l PBD in terms of this model gives triplet lifetimes of 110 nsec and 400 nsec, respectively, which extrapolate to a triplet lifetime of 1.3 psec in the absence of oxygen. VoLtz et ai. [5] identify this with the soluenr triplet (TlX} lifetime. This identification is incorrect, since the triplet lifetime of the all@ benzene liquids is only = 10 - 2C nsec [6-81. It is proposed that the triplet lifetime of 1.3 ~1set is that of the solute triplet (Tlu) state, and 293
Volume 7. numaer 2
CHEMICAL
PHYSICS LETTER5
that DY originates from the following sequence of processes: (i) population of TIX by the ionizing particle and by intersystem crossing from Six; (ii) energy transfer from TIX to TlY, (6) TlX + Soy “Sox + TlY 3 which occurs by electron-fxchange interaction as an efficient diffusion-controlfed process in fluid solutions [S]; and (iii) solute triplet-triplet interaction * TlY + T1Y -%Y+%Y leading to delayed SlY fluorescence. model the DY yield is G(Dy) =
CWIX)
-I-
(7) On this
@TX >wX) i ( +oiyyc 1
(8) where suffix 5; refers to Tly; u T( =kZT/kT) is the Stern-Volmer coefficient of H ~x-‘I’~~ energy transfer; k T is the rate parameter of process (6); kT( = 1BTT) is the unimolecular rate of TAX decay when c=O; kyzz is the rate parameter of is the total rate parameter of process (7); k Tly quenchingzz y Tly-TlY interaction; and kZ( = l/rZ) is the unimolecvlar rate of Tly decay. The form of the DY decay is the same as on the Voltz-Laustriat model [4], but CZ replaces TT. At high c O> l/oyx. l/uZT) (5) and (8) reduce, respectively, to GH(PY) = C(Slx) *FY , ‘-+@y)
= 4 cuZ G(Tlx) Lh~y ,
so that
(9) Eq. f9) provides a method of evaluating (YZ. Cundall et ai. [13] have observed GfSlx)/G(TLx)= 1.15 for deoxygenated liquid benzene excited by 4 MeV electrons. Fuchs et aL [ll] have observed GH(PY) = 0.85, GP(Dy) = 0.15, for deoxygenated benzene solutions ‘containing 5 g 1-l (x-NPG excited by 0.624 MeV electrons. (The absolute values of Go and GH(DY) appear low, but the relative values should be reliable. ) ~bsti~~on of these values in f9) gives o!Z = 0.464 for cr -NPo. The possible spin-allowed TlY-TlY quenching processes are: 294
1970
SOY+Soy + kvy, *F f” /+ZY , TlY * SOY TlY +TlY - Sly + SOY \ *GY ; ‘SOY *soy TlY ‘TlY - s0y+50y i
(7a)
TlY’ TlY -, TIYfSOY If the rate parameters of processes
W)
l
t;;;“”
kYZZ~ kGZZ and kZZZ,
Ub) UC) fm
(‘i’), (10) and respectively,
(la) kzz =kyZZW+Q,Zy)+kGZZ+%kZZZ. There are nine possible (Tl +Tly) spin states: one singlet, which can Becay by processes (7) or (10); three triplets, which can decay by process (1X); and five quintets which probably dissociate into TlY+ Tly. If the 9 spin states are formed with equal probability in TRY - T1Y encounters in a fluid solution, and if processes (7), (10) and (11) occur irreversibly, then
(13) kGZ2) ’ Substi~ti~g aZ = 0.406, @ZY = I-@FY = 0.22 (where CBFY= 0.78 for a!-NPG in benzene [12]} and (13) in (12), we obtain kZZZ
= 3(kyZZ +
kGzz
=0.036 kyZZ ; kZZZ=3.11
kyzZ .
Alternatively, if we assume ~YZZ >> kGzz, since the latter process involves a much larger energy dissipation (i.e., a much smaller Franek-Condon factor), then we obtain kGzz
(5a) (*a)
15 Octcber
= 0 ; kzzz
= 3.18kyzz.
which is consistent with (13) within the experimental error. It is thus to be expected that
(14) kyZZ = W9) kdiff , where kaf is the rate parameter of diffusioncontrolled Tly-Tly encounters. Groff et ah 1131have obtained an upper limit of f=PXTT ‘XT
&Fx = 0.36
(15)
for crystal anthracene from observations of the
magnetic field dependence of the SIX delayed fluorescence arising from Tlx-TIX interaction, and ta.kinZkGTT (the parameter analogous to kGzz) = il. SillC!t?*F = 0.9 [I], this COITE!spends
to aT(=&fT
7bT]
= 0.40,
in close
agreement with the value of ‘YZ = 0.404 obtained for ol-Nl?O in benzene solution.
Volume ‘i’, number 2
CHEMICAL PHYSICS LETTERS
REFERENCES [l] J. B. Birks, The theory and practice of scintillation counting (Pergamon Press, Oxford. 1964). [2] T. A-King and R. Voltz, Proc. Roy. Sot. A289 (1966) 424. [3j J. B. Birks and J. C. Conte, Proc. Roy. Sot. A303 0.968) 85. [4] R. Voltz and G. Laustriat, J. Phys. 29 (1968) 159. &5]R, Voltz, H. DuPont and G. Laustriat. J. Phys. 29 4968) 297. [S] F.Wilkinson and J.T.Dnbois, J. Chem. Phys. 39 (1963) 377. I?] R, B. Cundall and P. A. Griffiths. Trans. Faraday Sot. 61 (1965) 1968.
15 October 1970
[8] R.B. Cundall and A. J.R. Voss. C&em. Commun. (1969) 116, [S] J. B. Birks. Photophysics of aromatic molecules (Wiley-Interscience. London snd New Pork. 1970). 1101 _ . R. B. Cundall. G,B. Evans. P. .a. Griffiths and J. P.Keene. j. Phys. Che& 72 (l968) 3871. IllI _ _ C. Fuchs. F. Heise!. R. V&z and A. Cache. Infx+rn. Conf. Organic Scintillators and Liquid ScintilIation Counting, San Francisco (l9’iO). to be pubIished. 1121 _ _ A. Greenberg, MI.Furst and Ii. Kallmann, Intern. Symp. Luminescence, The physics and chemistry of scintiltators (Verlna Karl Thiemip;, hIunich. 1966) p.71. . fl3i R. 3. Groff. R. E. Mercifietd and P. Avakfan. Chem. Phya. Letrers 5 (1970) X68.
295