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dc onductivity signals observed following pulsed photoionization of solutions of anthracene in dielectric liquids
CHEMICAL
Volume 91, number 1
dc CONDUCTMTY
PHYSKS
15 Oclober 1982
LETTERS
SIGNALS OBSERVED FOLLOWING PULSED PHOTOIONIZATION
OF SOLUTIONS OF ANTHRACENE M DIELECTRIC LIQUIDS*
Myran C. SAUER Jr., Alexander D. TRIFUNAC, Ronald COOPER* and Dan MEISEL Chemstr_v Dwsion.
Argonne
htitiorurl
Laborarory.
Argonne. Xlbnorc 60139.
USA
Rccclrsd 28 June 198’
dc conductnlt) U-I several hydrocarbon hqulds conlammg anthraccne (2 X 10% M) and hghcr concentrations of efficlenr electron scaren.gcrs is measured iollowmg an I8 ns pulse of 238 nm lght. Wnh cycloheuane as Ihe sohcnl, an mlthl conduc11vicys~n.4. the origin oi wiuch IS uncertain. dec;lbs over = 1 PS and is inithUy more than an order of mxgnlude gxxcr than the much longer hvcd irec-Ion ngnal. Strong eiiccrs of the solvent on this uutbl sg.rwl me observed.
2. Experimental
1. lntroducfion The application
of conductivity
techniques
to the
study of gemmate and free ions produced by pulse radlolysls has besn described by Warman and co-workers [l-3] _The measurement of geminate ion decay kinetics using microwave conductivity is well documented UI the case of pulse radiolysis [l-3], but not m the case of pulsed phoroionlzation. The measurement of dc conductwity signals due to free ions produced by pulsed laser photoionization has been described by Beck and Thomas (41 in work where the decay of conductivity due to the free electron produced in cyclohexane solutions of an-
The experimental arrangement is shown in fig. 1. The 148 nm, Wl/z = 18 ns pulse of light from a Lambda Physlk eximer laser (EMG-102) &verged only slightly as it traversed the 4 cm path between the electrodes of the conductivity cell. The conductivity detection circuitry is also shown in fig. 1, and it is simdar to that described by Beck and Thomas [4]. The electrodes in the conductivity cell are 4 cm X 1.2 cm and 0.7 cm apart. The load resistance, R, and input capacitance, C,, of the oscilloscope result in a rise time of ~5 ns. The oscdloscope (Tektronix 7633,7A13 amplifier, 3.5 ns
thracene or pyrene was measured as a function of electron scavenger concentration to measure electron capture rate constants. The dc coqductiviry method has not been used to detect gemmate Ions. In the present work we have observed photolytically produced dc conductivity signals with unusual properties in hydrocarbon solutions. The purpose of this communication is to descllbe the nature of the observations and to examine the problems in interpretalion.
* Worh performed under the ausp~cs oi rh2 Office oi BX.IC Energy Srwxrs: Divbron of Chcmiccll Sarncr. USDOE under Contract Number W-31-109-ENG-38. ’ Present address Department of PhysIcal Chematry. University of Mclbournc. Parkv4lr. Vlctorn 3052. Austraha.
178
1. Schematic diagram of the expenmenta~ apparatus for pholoionlwtion and measurement of conductkty: EL is the excimer laser. L ISa JO cm iocal length lens, S is a sht (11 mm X 3 5 mm).Visa voltage source (on2 to four 300 V batter@. R ISthe load resistance (92 SZ)and Ci IS the mput apacitance oi the osdloxope (20 pF). Fg
0 OW-2614/82/0000-0000/S
02.75 0 1982 North-Holland
Volume 92. number
2
IS October 198,
CHEMICAL PHYSICS LElTERS
rise time) was used directly for data collection via photographs, or as an amplifier for a Blomation 8100 transient recorder (15 11srise time), in which cz.e data handling was computerized. The conductivity apparatus and data recordmg equipment were enclosed in a double-walled copper screen room. This, as well as a grounded holder, which provided some additional shielding for the conductivity cell, was necessary to reduce electronic noise. The conductivity ceU has suprasd windows for transmitting the 248 nm light, and is equipped with groundglass Joints to allow a syringe technique to be used for introducing degassed samples. The photoionlzation of anthracene at 248 nm requires two photons. Because of the high photon flux (“2 X lOI7 photons per pulse) and the low anthracene concentration (4 X 10IJ molecules in the light path) there IS efficient bleaching of the ground state. Therefore, most of the light passes through the sample despite the irutlal absorbance of zO.3 (per cm) at 2 X 104 M anthracene. The conductivity signal due to free ions is observed to be appro.ximately proportional to the light intensity, presumably because the concentration of excited anthracene (which absorbs the “second”, or ionizing, photon) does not vary much over the range of hght intensity used.
01
I
1
003
01
03
3 I TIMEI,&
IO
30
ICJJ
FIN. 2 Tune depcndenccof the conductwiry sgnal for 2 x I@ M anthrxcne in several rlkanes uturad ulth SF, (0.05 M III cycloheuane. 0 09 M in n-hcwne. 0 07 M in cyclopcntans. 0.08 M m tsooc~~ne) 151. Tune is measured from the bcgmnm~_~ of the hser pulse.
shown). This signal disappeared in a tme of = lo-’ s, as expected from the measured rate constants for reaction of the electron with snthracene or similar solutes [6], after which time the signals were similar to those shown in fig. 2 where the high concentration of SF6 prevented the obxrvatmn of the electron signal. An important observation is that decreasing the SF6 concentration by two orders of m3gmtudc does not
3. Results
decrease the magmtude of the signals shown in fig. 3,
The observed conductivity signals are shown in fig. 2 for 2 X 10m6M anthracene in cyclopentane, cyclohexane, n-hexane and isooctane, all solutions being saturated with SF6. The experimental data points are not shown, but the signal-to-noise ratio is good, and they correspond closely to the curves shown. Experiments were done which showed that changing condi-
nor do the kinetics change significantly. The large changes in the signal amphtude and decay kmetics caused by changing the solvent are also important. For example, the free ion signal is not much different in cyclohexane and n-hexane, but the ?rnt1al” signal E quite small in n-hexane in comparison with cyclohexane. The effects of light intensity, type of solute being
tions such as the focusing and collimation of the light
photoionized, and type of electron scavenger were also
and the anthracene concentration did not alter the course of the conductivity decay in an unexpected manner. The signal was found to be linear with voltage from 300 to 1200 V, as was expected [4]. The decay of the signals shown in fig. 2 at times longer than ~10 PS is consistent with a homogeneous recombination of free ions with initial concentrations in the range of loo7 M. When SF6 was omitted from the solutions, a strong initial signal due to the electron was observed (data not
investigated. The results will be mentioned here to in-
beam
dicate the complexity of the nature of the observed conductivity slgnal, but will not be discussed further in this letter. The effect of intensity on cyclohe.xane, anthracene, SF6 solutions was that over a factor of 10 in l&t intensity the magmtude of free ion signal was proportional to light intensity, but the “mitial” signal was proportional to approximately the 3/2 power of light intensity.
179
Voh~mc
92.
number 1
CHEMICAL
Electron scavengers other than SF, were used. COl, N,O, and perfluoro-whehane gaxe results similar lo SF6, but C,HjBr showed a strong quenching effect on the conductr~ty, proportiona to the concentration of CzH$i. Use of other solutes (diphenyl, pyrene. 9,IO-drphenyb anthracene, 1,5diphenyloxazole) instead of anthracene caused appreciable changes in the decay kinetics of the “mitial” conductwiry signal as well as in the ratio of “initi3I” to free-ion signaL
4.
Diwwsion
A smixtoryesplanation of the nature of the early conductivity signal, Hhich is obsentd to be largest in the case of cyclohexane sob&Ions (fig. 2) will depend on further work. The fact that the slope of the pfot of log(signal) versus lo&me) m cyclohe\ane from 40 ns to =I fls is near to the value of -0.6 which has been recently found [7,8] to hold for geminate ion recombinnion, tempts us to ascribe this “initial” signal to geminate SF;. anthracene’ ton pairs. However, the observatrons that the magmtude of thus “inmal” signaI and Its decay kinetics do not vary apprecmbly as the SF, concentration is changed, and that the magnitude of the “mltial” signal is very dependent on the solvent suggest that the situation is more complex. The involvemem of a precursor state to the geminate ion pair, such
as has been suggested by Wu and Lipsky [9] in the case
ofphotoionization of ThlPD, may be indicated. The question of how a dc conductivity apparatus responds to geminate ions has not been settled; euperimental observations supporting the possibility of detectmg geminate ions by the dc method are lacking. ‘fheoretical investigations on this subJect are currently being undertaken in these laboratories [IO]. Effects of varying the light intensity, the solute being photoionized, and the electron scavenger have been mentioned in section 3. These observations serve
180
15 October 1982
PHYSICS LETTERS
at present mainly to substantiate the complex nature of the “initial” conductivity signal. We are currently attempting to use the microwave conductivity method to ~vestigate the same systems in order to verify the results of the dc method and to determine whether conductivity behavior typical of geminate ions is obtained_
Acknowledgement
The authors thank Drs. K. Schmidt, C. Jonah and J. Miller for valuable discussions, and Mr. R. Clarke and Ms. P_Walsh for their assistance.
References
[ 11J-M.Wxmsn. The McrowaueAbsorpttonTechnrque for Studying Ions and lornc Processes. In The study of iarr processes and hbrle
species III chemistry and molecular
biology using ionizing radiation. F. Busi (NATO Advanced Study
eds. J.H. B;Luendaleand Institute, Capn, 1981).
to be published. [2] CAM. van den Ende, L. Nyk0s.J.M. Nurnon 2nd A. Hummel. Rsduz. phys. Chem I.5 (1980) 273. [ 3 1P.P. Infelta. M P. De Haas and J.M. Warman, Radixr. Phys. Chem. IO (1977) 353. [4] C. Beck and J.K. Thomas, J. Chem. Phys. 57 (1972) 3649. [51 L.E.W. Horsmawan den Dool and J.M. Warnan. The Solubrbtres, ma Variety of Orgamc Liqwds, of Some Gaseous Compounds Commonly Used as Sawngers in Radotion Chemicat Studres, Int~~nffers~a~ Reactor Inswute Report 134-81-01.
161 J.H. BaxendaIe, J.P. Keene and EJ. Rasburn, J. Chem. Sot. Faraday frans I 70 ( 1974) 7 18. 171 C.A.M. van den Endc, L.H. Luthjens. J.M. Warrnan and A. Hummel, to bc pubhshed. 181
J-M.Wannan. The
Dynamtcs of Electrorrs and Ions in
Ltqutds, in- The study of fast processes and )JbiL spreles tn chemistry zmd molecuku brology using lomzingndntion, eds. J.H. Baxendale and F. Busr (NATO Non-Polar
Advanced Study Institute. Capn, 1981). to be pubhshed
[PI K.-C. Wu and
S. Lipsky. J. Chem. Phys. 66 (1977) [ 101 K. Schmidt, private commumatlon.
5614.
Volume 91, number 1
dc CONDUCTMTY
PHYSKS
15 Oclober 1982
LETTERS
SIGNALS OBSERVED FOLLOWING PULSED PHOTOIONIZATION
OF SOLUTIONS OF ANTHRACENE M DIELECTRIC LIQUIDS*
Myran C. SAUER Jr., Alexander D. TRIFUNAC, Ronald COOPER* and Dan MEISEL Chemstr_v Dwsion.
Argonne
htitiorurl
Laborarory.
Argonne. Xlbnorc 60139.
USA
Rccclrsd 28 June 198’
dc conductnlt) U-I several hydrocarbon hqulds conlammg anthraccne (2 X 10% M) and hghcr concentrations of efficlenr electron scaren.gcrs is measured iollowmg an I8 ns pulse of 238 nm lght. Wnh cycloheuane as Ihe sohcnl, an mlthl conduc11vicys~n.4. the origin oi wiuch IS uncertain. dec;lbs over = 1 PS and is inithUy more than an order of mxgnlude gxxcr than the much longer hvcd irec-Ion ngnal. Strong eiiccrs of the solvent on this uutbl sg.rwl me observed.
2. Experimental
1. lntroducfion The application
of conductivity
techniques
to the
study of gemmate and free ions produced by pulse radlolysls has besn described by Warman and co-workers [l-3] _The measurement of geminate ion decay kinetics using microwave conductivity is well documented UI the case of pulse radiolysis [l-3], but not m the case of pulsed phoroionlzation. The measurement of dc conductwity signals due to free ions produced by pulsed laser photoionization has been described by Beck and Thomas (41 in work where the decay of conductivity due to the free electron produced in cyclohexane solutions of an-
The experimental arrangement is shown in fig. 1. The 148 nm, Wl/z = 18 ns pulse of light from a Lambda Physlk eximer laser (EMG-102) &verged only slightly as it traversed the 4 cm path between the electrodes of the conductivity cell. The conductivity detection circuitry is also shown in fig. 1, and it is simdar to that described by Beck and Thomas [4]. The electrodes in the conductivity cell are 4 cm X 1.2 cm and 0.7 cm apart. The load resistance, R, and input capacitance, C,, of the oscilloscope result in a rise time of ~5 ns. The oscdloscope (Tektronix 7633,7A13 amplifier, 3.5 ns
thracene or pyrene was measured as a function of electron scavenger concentration to measure electron capture rate constants. The dc coqductiviry method has not been used to detect gemmate Ions. In the present work we have observed photolytically produced dc conductivity signals with unusual properties in hydrocarbon solutions. The purpose of this communication is to descllbe the nature of the observations and to examine the problems in interpretalion.
* Worh performed under the ausp~cs oi rh2 Office oi BX.IC Energy Srwxrs: Divbron of Chcmiccll Sarncr. USDOE under Contract Number W-31-109-ENG-38. ’ Present address Department of PhysIcal Chematry. University of Mclbournc. Parkv4lr. Vlctorn 3052. Austraha.
178
1. Schematic diagram of the expenmenta~ apparatus for pholoionlwtion and measurement of conductkty: EL is the excimer laser. L ISa JO cm iocal length lens, S is a sht (11 mm X 3 5 mm).Visa voltage source (on2 to four 300 V batter@. R ISthe load resistance (92 SZ)and Ci IS the mput apacitance oi the osdloxope (20 pF). Fg
0 OW-2614/82/0000-0000/S
02.75 0 1982 North-Holland
Volume 92. number
2
IS October 198,
CHEMICAL PHYSICS LElTERS
rise time) was used directly for data collection via photographs, or as an amplifier for a Blomation 8100 transient recorder (15 11srise time), in which cz.e data handling was computerized. The conductivity apparatus and data recordmg equipment were enclosed in a double-walled copper screen room. This, as well as a grounded holder, which provided some additional shielding for the conductivity cell, was necessary to reduce electronic noise. The conductivity ceU has suprasd windows for transmitting the 248 nm light, and is equipped with groundglass Joints to allow a syringe technique to be used for introducing degassed samples. The photoionlzation of anthracene at 248 nm requires two photons. Because of the high photon flux (“2 X lOI7 photons per pulse) and the low anthracene concentration (4 X 10IJ molecules in the light path) there IS efficient bleaching of the ground state. Therefore, most of the light passes through the sample despite the irutlal absorbance of zO.3 (per cm) at 2 X 104 M anthracene. The conductivity signal due to free ions is observed to be appro.ximately proportional to the light intensity, presumably because the concentration of excited anthracene (which absorbs the “second”, or ionizing, photon) does not vary much over the range of hght intensity used.
01
I
1
003
01
03
3 I TIMEI,&
IO
30
ICJJ
FIN. 2 Tune depcndenccof the conductwiry sgnal for 2 x I@ M anthrxcne in several rlkanes uturad ulth SF, (0.05 M III cycloheuane. 0 09 M in n-hcwne. 0 07 M in cyclopcntans. 0.08 M m tsooc~~ne) 151. Tune is measured from the bcgmnm~_~ of the hser pulse.
shown). This signal disappeared in a tme of = lo-’ s, as expected from the measured rate constants for reaction of the electron with snthracene or similar solutes [6], after which time the signals were similar to those shown in fig. 2 where the high concentration of SF6 prevented the obxrvatmn of the electron signal. An important observation is that decreasing the SF6 concentration by two orders of m3gmtudc does not
3. Results
decrease the magmtude of the signals shown in fig. 3,
The observed conductivity signals are shown in fig. 2 for 2 X 10m6M anthracene in cyclopentane, cyclohexane, n-hexane and isooctane, all solutions being saturated with SF6. The experimental data points are not shown, but the signal-to-noise ratio is good, and they correspond closely to the curves shown. Experiments were done which showed that changing condi-
nor do the kinetics change significantly. The large changes in the signal amphtude and decay kmetics caused by changing the solvent are also important. For example, the free ion signal is not much different in cyclohexane and n-hexane, but the ?rnt1al” signal E quite small in n-hexane in comparison with cyclohexane. The effects of light intensity, type of solute being
tions such as the focusing and collimation of the light
photoionized, and type of electron scavenger were also
and the anthracene concentration did not alter the course of the conductivity decay in an unexpected manner. The signal was found to be linear with voltage from 300 to 1200 V, as was expected [4]. The decay of the signals shown in fig. 2 at times longer than ~10 PS is consistent with a homogeneous recombination of free ions with initial concentrations in the range of loo7 M. When SF6 was omitted from the solutions, a strong initial signal due to the electron was observed (data not
investigated. The results will be mentioned here to in-
beam
dicate the complexity of the nature of the observed conductivity slgnal, but will not be discussed further in this letter. The effect of intensity on cyclohe.xane, anthracene, SF6 solutions was that over a factor of 10 in l&t intensity the magmtude of free ion signal was proportional to light intensity, but the “mitial” signal was proportional to approximately the 3/2 power of light intensity.
179
Voh~mc
92.
number 1
CHEMICAL
Electron scavengers other than SF, were used. COl, N,O, and perfluoro-whehane gaxe results similar lo SF6, but C,HjBr showed a strong quenching effect on the conductr~ty, proportiona to the concentration of CzH$i. Use of other solutes (diphenyl, pyrene. 9,IO-drphenyb anthracene, 1,5diphenyloxazole) instead of anthracene caused appreciable changes in the decay kinetics of the “mitial” conductwiry signal as well as in the ratio of “initi3I” to free-ion signaL
4.
Diwwsion
A smixtoryesplanation of the nature of the early conductivity signal, Hhich is obsentd to be largest in the case of cyclohexane sob&Ions (fig. 2) will depend on further work. The fact that the slope of the pfot of log(signal) versus lo&me) m cyclohe\ane from 40 ns to =I fls is near to the value of -0.6 which has been recently found [7,8] to hold for geminate ion recombinnion, tempts us to ascribe this “initial” signal to geminate SF;. anthracene’ ton pairs. However, the observatrons that the magmtude of thus “inmal” signaI and Its decay kinetics do not vary apprecmbly as the SF, concentration is changed, and that the magnitude of the “mltial” signal is very dependent on the solvent suggest that the situation is more complex. The involvemem of a precursor state to the geminate ion pair, such
as has been suggested by Wu and Lipsky [9] in the case
ofphotoionization of ThlPD, may be indicated. The question of how a dc conductivity apparatus responds to geminate ions has not been settled; euperimental observations supporting the possibility of detectmg geminate ions by the dc method are lacking. ‘fheoretical investigations on this subJect are currently being undertaken in these laboratories [IO]. Effects of varying the light intensity, the solute being photoionized, and the electron scavenger have been mentioned in section 3. These observations serve
180
15 October 1982
PHYSICS LETTERS
at present mainly to substantiate the complex nature of the “initial” conductivity signal. We are currently attempting to use the microwave conductivity method to ~vestigate the same systems in order to verify the results of the dc method and to determine whether conductivity behavior typical of geminate ions is obtained_
Acknowledgement
The authors thank Drs. K. Schmidt, C. Jonah and J. Miller for valuable discussions, and Mr. R. Clarke and Ms. P_Walsh for their assistance.
References
[ 11J-M.Wxmsn. The McrowaueAbsorpttonTechnrque for Studying Ions and lornc Processes. In The study of iarr processes and hbrle
species III chemistry and molecular
biology using ionizing radiation. F. Busi (NATO Advanced Study
eds. J.H. B;Luendaleand Institute, Capn, 1981).
to be published. [2] CAM. van den Ende, L. Nyk0s.J.M. Nurnon 2nd A. Hummel. Rsduz. phys. Chem I.5 (1980) 273. [ 3 1P.P. Infelta. M P. De Haas and J.M. Warman, Radixr. Phys. Chem. IO (1977) 353. [4] C. Beck and J.K. Thomas, J. Chem. Phys. 57 (1972) 3649. [51 L.E.W. Horsmawan den Dool and J.M. Warnan. The Solubrbtres, ma Variety of Orgamc Liqwds, of Some Gaseous Compounds Commonly Used as Sawngers in Radotion Chemicat Studres, Int~~nffers~a~ Reactor Inswute Report 134-81-01.
161 J.H. BaxendaIe, J.P. Keene and EJ. Rasburn, J. Chem. Sot. Faraday frans I 70 ( 1974) 7 18. 171 C.A.M. van den Endc, L.H. Luthjens. J.M. Warrnan and A. Hummel, to bc pubhshed. 181
J-M.Wannan. The
Dynamtcs of Electrorrs and Ions in
Ltqutds, in- The study of fast processes and )JbiL spreles tn chemistry zmd molecuku brology using lomzingndntion, eds. J.H. Baxendale and F. Busr (NATO Non-Polar
Advanced Study Institute. Capn, 1981). to be pubhshed
[PI K.-C. Wu and
S. Lipsky. J. Chem. Phys. 66 (1977) [ 101 K. Schmidt, private commumatlon.
5614.