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An intermolecular interaction of excited states in crystals and liquids
Volume
2. number
AN
CHElMICAL
8
INTERMOLECULAR IN
PHYSICS
LETTERS
INTERACTION CRYSTALS
AND
OF
December
EXCITED
STATES
LIQUIDS
C. R. GOLDSCHMIDT, Y. TOMKIEWICZ and I. B. BERLMAX* Depadmenfs of PlzysicalCJzemistry andPJlysics. Hebrew Uniz*ersfty, Jerusalem. Received
27 September
1968
Israel
3968
evidence is presented on an excimer-excimer interaction in pyrene crystals and SO~Ua high concentration of pyrene. This interaction is used to explain in part why scintillation pulse shapes when excited by cr-particles contain less of an excimer component than when excited by /J-particles. Experimental tions containing
A pulsed nitrogen laser (AVCO-Everett) has been used to study various mechanisms
beam taking
place in a region of high concentration of excited molecules. Pyrene crystals and solutions containing a large solute concentration (greater than 10 g/l of pyrene in toluene) have been excited by a high intensity pulsed source and the results interpreted in a manner similar to that of Jortner and coworkers 111 in their study of an exciton-exciton interaction in anthracene crystaIs. Pyrene is an ideal solute for these investigations because it forms excimers readily, is very soluble in most aromatic solvents, and at the wavelength of the excitation radiation 3371 b its cross section per molecule [Zl LTis about 2 )(120-16 cm2. At the solute concentrations studied and when excitation is by a low intensity source very little monomer emission is expected at room temperature. Furthermore, at high solute concentrations the probability per unit time for excimer formationkDMC ts greater than 108 set-l, the reciprocal of the pulse width, and saturation effects in the formation of excimers when excited with high intensity sources as previously reported [2] is reduced. The techniques used in making the measurements are the same as decribed elsewhere 121. In fig. 1 are shown curves of excimer pulse height as a function of intensity of the excitation radiation. A no&near increase in yield versus intensity is evident. Our results indicate that at high solute concentrations solutr3 molecuies becomes
the density of excited so large that several
*On a- assignment at the University from Sational Laboratory. Argonne, Illinois.’
520
the Argonne USA.
additional factors, not of apparent significance at ion solute concentrations, have to be considered. Since the laser beam contains about 1015 photons/pulse and these photons are focused on a spot smaller than 1 mm2, practically every moIecule on the front surface, in the area iIluminated by the laser beam, becomes excited. In a solution with 80 g/l of pyrene it can be shown
that at this front surface of total excitation the distance obetween excited pyrene molecules is about 16A. -4ttendant this high concentration of excited states is a rise in temperature and a bimolecular interaction. A high temperature is generated from two major sources: One, the fluorescence quantum yield of pyrene excimers [3] is reported to be 0.75, indicating that about 25% of the excitation energy does not contribute to the fluorescence intensity, but probably ends up as heat. Two, every transition leading to fluorescence has vibrational losses Ieading to a generation of heat. The wave number of the excitation radiation is about 29 640 cm’1 and the first moment of the excimer fluorescence of pyrene [4] is about 20500 cm-l. Therefore electronic energy of 9 140 cm-1 or about 1 eV per transition is converted into vibrational energy. Assuming that half of the excitation energy is dissipated, this energy is computed to be about 6 X lo15 MeV/cm 3. If it is assumed that pyrene crystals and toluene each have a specific heat of 0.4 caL gm’l deg’l it can be shown that this
corresponds to 1.2X lo13 MeV cm-3 deg’l. Thus high temperatures can be generated in the small volume in which the radiation is principally absorbed. In fact in solutions containing 80 g/l of pyrene, cavitation bubbles can be heard .
Volume 2, number 8
CHEMICAL PHYSICS LETTERS
20
Excitation
Fig.
1. Curves
of recorded
intensity versus Curve A Curve B Curve C Curve D -
tntensity .
IO
December 1968
(Number
at
Slides)
excitation intensity. laser pulse. for calibration. 10 g/l pyrene in toluene. 80 g/i pyrene in toluene. pyrene crystal.
and in pyrene crystals thermal shock noises can be heard. After a crystal has been exposed to several bursts of the laser beam a discoloration can be observed at the focal spot. Assuming a coefficient of thermal diffusion [5] of 3 X 10-3 cm2/sec it has been calculated that a high temperature remains in the region of excitation for several hundred nsec much longer than the decay time of the pulse. Concomitant with a rise in temperature an increase in the amount of fluorescence in the monomer region is observed. In solutions with a pyrene concentration of 10 g/l or less the decay time is exponential indicating that thermal quenching is the only quenching mechanism. The existence of a sharp break in the fluorescence decay curve in a solution with 80 g/l of pyrene and in pyrene crystals is explained by the assumption of a second mechanism. a bimolecular mechanism. Jortner and co-workers [l J have analyzed their data from an anthracene crystal in terms of an exciton-exciton interaction. Our data can also be analyzed in terms of such a bimolecular process with favorable resuIts. If the rate equation is written as
intensity (in arbitrary units) versus exp(at). A straight line is expected from competing bimolecular and monomoLecular mechanisms with the intercept of the ordinate being equal to &/a If 6 were zero or very smaL1 in comparison to cz, the line would pass through the origin. That the line passing through the experimental points is straight and does not go through the origin is evidence of a bimolecular mechanism. This mechanism in Liquids is assumed to be an excimer-excimer interaction and it is aided by diffusion. To explain the similar data from crystals an excimeric exciton-excimerit exciton interaction must be postuLated.
“lp--
’
I 15
where a and b are, respectively, the rate constant for monomolecular and bimolecular decay, integration of the above equation leads to: +=
exp
(at)(:+&)
Fig. 2 is a plot of the reciprocal
-z.
of the excimer
’i
’
I
not
2
I
Fig. 2. Piot of reciprocal excimer fluorescence intensity versus esp (at}. where a is the reciprocal of the escimer decay time under the esperimental conditions (40 nsec for solution and 54 nsec for crystal\. When the pulse shapes are normalized and the computations are started at the same vdue of time. identical curves are obtained for the 80 g/l solutions and
the crystal.
521
Volume
2. number
8
CHEMlCAL
PHYSICS
Although the possibility of an excimeric exciton in crystals hw.;been discussed.bjr Birks et al. [Cl further stuc!?~? are in progress to investigate this peculiar species and its interactions. These concepts can be us&d to explain succesfully some results obtained by one of the authors (IBB) [71_ Scintillation pulse shapes from a solution of cyclohexatie and about 40 g/l _of 2,5 - diphenyloxazole (PPG) when excitation was by &her a-particles or P-particles were studied and recorded. Since the fluorescence decay time of PPO monomers at this concentration is about 1 nsec and that of the excimer about 14 nsec, the long component of a recorded pulse shape is primarily due to excimer fluorescence. It was observed that when excitation was by means of o-particles the pulse shape contained a much smaller excimer component relative to that when excitation was by @particles. This decrease in excimer intensity when excitation is by oc-particles can now be explained by the two above mechanisms; a temperature rise and an excimer-excimer interaction. That heating takes place along the a-track is we11 known as indicated by the often used l;hrase “thermal spike*. A rise in temperature partially quenches the excimer fluorescence and facilitates a back reaction to produce excited and ground state monomers. However, the rise in temperature and its effects last longer than the decay &me of the excited states and not just lo-11 set as computed by Galanin for anthracene crystals. Finally the high density of excimers give rise to mutual interactions leading to a fur*****
522
_
December
LETTZRS
ther decrease
in excimer
1968
intensity.
We wish to thank Professors A. Weinreb and G. Stein for interesting discussions.
REFERENCES
[1] A. Bergman, M. Levine and J. Jortner, Phys. Rev. Letters 18 (1967) 593. [2] C. R. Goldschmidt, Y. Tomkiewicz and I. B. Berlman. Chcm. Phys. Letters 2 (1968) 569. [3]J. B. Birks. D. J. Dyson and I. H. Munro. Proc. Roy. Sot. (London) A275 (1963) 575. [4] I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules (Academic Press Inc., New York, N. Y. 1965) p. 174. [5] M. D. Galanin. Opt. i Spectroskopiya 4 (1958) 758. [6] J. B. Birks. A. A.Kazzaz and T. A.King, Proc. Roy. Sot. (London) A291 (1966) 556. [7j I. B. Berlman and D. J. Steingraber (unpublished results).
2. number
AN
CHElMICAL
8
INTERMOLECULAR IN
PHYSICS
LETTERS
INTERACTION CRYSTALS
AND
OF
December
EXCITED
STATES
LIQUIDS
C. R. GOLDSCHMIDT, Y. TOMKIEWICZ and I. B. BERLMAX* Depadmenfs of PlzysicalCJzemistry andPJlysics. Hebrew Uniz*ersfty, Jerusalem. Received
27 September
1968
Israel
3968
evidence is presented on an excimer-excimer interaction in pyrene crystals and SO~Ua high concentration of pyrene. This interaction is used to explain in part why scintillation pulse shapes when excited by cr-particles contain less of an excimer component than when excited by /J-particles. Experimental tions containing
A pulsed nitrogen laser (AVCO-Everett) has been used to study various mechanisms
beam taking
place in a region of high concentration of excited molecules. Pyrene crystals and solutions containing a large solute concentration (greater than 10 g/l of pyrene in toluene) have been excited by a high intensity pulsed source and the results interpreted in a manner similar to that of Jortner and coworkers 111 in their study of an exciton-exciton interaction in anthracene crystaIs. Pyrene is an ideal solute for these investigations because it forms excimers readily, is very soluble in most aromatic solvents, and at the wavelength of the excitation radiation 3371 b its cross section per molecule [Zl LTis about 2 )(120-16 cm2. At the solute concentrations studied and when excitation is by a low intensity source very little monomer emission is expected at room temperature. Furthermore, at high solute concentrations the probability per unit time for excimer formationkDMC ts greater than 108 set-l, the reciprocal of the pulse width, and saturation effects in the formation of excimers when excited with high intensity sources as previously reported [2] is reduced. The techniques used in making the measurements are the same as decribed elsewhere 121. In fig. 1 are shown curves of excimer pulse height as a function of intensity of the excitation radiation. A no&near increase in yield versus intensity is evident. Our results indicate that at high solute concentrations solutr3 molecuies becomes
the density of excited so large that several
*On a- assignment at the University from Sational Laboratory. Argonne, Illinois.’
520
the Argonne USA.
additional factors, not of apparent significance at ion solute concentrations, have to be considered. Since the laser beam contains about 1015 photons/pulse and these photons are focused on a spot smaller than 1 mm2, practically every moIecule on the front surface, in the area iIluminated by the laser beam, becomes excited. In a solution with 80 g/l of pyrene it can be shown
that at this front surface of total excitation the distance obetween excited pyrene molecules is about 16A. -4ttendant this high concentration of excited states is a rise in temperature and a bimolecular interaction. A high temperature is generated from two major sources: One, the fluorescence quantum yield of pyrene excimers [3] is reported to be 0.75, indicating that about 25% of the excitation energy does not contribute to the fluorescence intensity, but probably ends up as heat. Two, every transition leading to fluorescence has vibrational losses Ieading to a generation of heat. The wave number of the excitation radiation is about 29 640 cm’1 and the first moment of the excimer fluorescence of pyrene [4] is about 20500 cm-l. Therefore electronic energy of 9 140 cm-1 or about 1 eV per transition is converted into vibrational energy. Assuming that half of the excitation energy is dissipated, this energy is computed to be about 6 X lo15 MeV/cm 3. If it is assumed that pyrene crystals and toluene each have a specific heat of 0.4 caL gm’l deg’l it can be shown that this
corresponds to 1.2X lo13 MeV cm-3 deg’l. Thus high temperatures can be generated in the small volume in which the radiation is principally absorbed. In fact in solutions containing 80 g/l of pyrene, cavitation bubbles can be heard .
Volume 2, number 8
CHEMICAL PHYSICS LETTERS
20
Excitation
Fig.
1. Curves
of recorded
intensity versus Curve A Curve B Curve C Curve D -
tntensity .
IO
December 1968
(Number
at
Slides)
excitation intensity. laser pulse. for calibration. 10 g/l pyrene in toluene. 80 g/i pyrene in toluene. pyrene crystal.
and in pyrene crystals thermal shock noises can be heard. After a crystal has been exposed to several bursts of the laser beam a discoloration can be observed at the focal spot. Assuming a coefficient of thermal diffusion [5] of 3 X 10-3 cm2/sec it has been calculated that a high temperature remains in the region of excitation for several hundred nsec much longer than the decay time of the pulse. Concomitant with a rise in temperature an increase in the amount of fluorescence in the monomer region is observed. In solutions with a pyrene concentration of 10 g/l or less the decay time is exponential indicating that thermal quenching is the only quenching mechanism. The existence of a sharp break in the fluorescence decay curve in a solution with 80 g/l of pyrene and in pyrene crystals is explained by the assumption of a second mechanism. a bimolecular mechanism. Jortner and co-workers [l J have analyzed their data from an anthracene crystal in terms of an exciton-exciton interaction. Our data can also be analyzed in terms of such a bimolecular process with favorable resuIts. If the rate equation is written as
intensity (in arbitrary units) versus exp(at). A straight line is expected from competing bimolecular and monomoLecular mechanisms with the intercept of the ordinate being equal to &/a If 6 were zero or very smaL1 in comparison to cz, the line would pass through the origin. That the line passing through the experimental points is straight and does not go through the origin is evidence of a bimolecular mechanism. This mechanism in Liquids is assumed to be an excimer-excimer interaction and it is aided by diffusion. To explain the similar data from crystals an excimeric exciton-excimerit exciton interaction must be postuLated.
“lp--
’
I 15
where a and b are, respectively, the rate constant for monomolecular and bimolecular decay, integration of the above equation leads to: +=
exp
(at)(:+&)
Fig. 2 is a plot of the reciprocal
-z.
of the excimer
’i
’
I
not
2
I
Fig. 2. Piot of reciprocal excimer fluorescence intensity versus esp (at}. where a is the reciprocal of the escimer decay time under the esperimental conditions (40 nsec for solution and 54 nsec for crystal\. When the pulse shapes are normalized and the computations are started at the same vdue of time. identical curves are obtained for the 80 g/l solutions and
the crystal.
521
Volume
2. number
8
CHEMlCAL
PHYSICS
Although the possibility of an excimeric exciton in crystals hw.;been discussed.bjr Birks et al. [Cl further stuc!?~? are in progress to investigate this peculiar species and its interactions. These concepts can be us&d to explain succesfully some results obtained by one of the authors (IBB) [71_ Scintillation pulse shapes from a solution of cyclohexatie and about 40 g/l _of 2,5 - diphenyloxazole (PPG) when excitation was by &her a-particles or P-particles were studied and recorded. Since the fluorescence decay time of PPO monomers at this concentration is about 1 nsec and that of the excimer about 14 nsec, the long component of a recorded pulse shape is primarily due to excimer fluorescence. It was observed that when excitation was by means of o-particles the pulse shape contained a much smaller excimer component relative to that when excitation was by @particles. This decrease in excimer intensity when excitation is by oc-particles can now be explained by the two above mechanisms; a temperature rise and an excimer-excimer interaction. That heating takes place along the a-track is we11 known as indicated by the often used l;hrase “thermal spike*. A rise in temperature partially quenches the excimer fluorescence and facilitates a back reaction to produce excited and ground state monomers. However, the rise in temperature and its effects last longer than the decay &me of the excited states and not just lo-11 set as computed by Galanin for anthracene crystals. Finally the high density of excimers give rise to mutual interactions leading to a fur*****
522
_
December
LETTZRS
ther decrease
in excimer
1968
intensity.
We wish to thank Professors A. Weinreb and G. Stein for interesting discussions.
REFERENCES
[1] A. Bergman, M. Levine and J. Jortner, Phys. Rev. Letters 18 (1967) 593. [2] C. R. Goldschmidt, Y. Tomkiewicz and I. B. Berlman. Chcm. Phys. Letters 2 (1968) 569. [3]J. B. Birks. D. J. Dyson and I. H. Munro. Proc. Roy. Sot. (London) A275 (1963) 575. [4] I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules (Academic Press Inc., New York, N. Y. 1965) p. 174. [5] M. D. Galanin. Opt. i Spectroskopiya 4 (1958) 758. [6] J. B. Birks. A. A.Kazzaz and T. A.King, Proc. Roy. Sot. (London) A291 (1966) 556. [7j I. B. Berlman and D. J. Steingraber (unpublished results).