Radiation Chemistry-II

18

Ionic Processes i n

γ-Irradiated

3 - M P Glass

I. KOSA SOMOGYI and J. BALOG

Downloaded by UNIV OF ROCHESTER on June 11, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch018

Central Research Institute for Physics, P.O.B. 49, Budapest 114, Hungary Studies of the electrical conductivity as a function of tem perature on gamma-irradiated 3-methylpentane glass are reported. During irradiation steady-state currents of 1.7 X 10-8 and 5.7X10-10amp. have been measured at 290° and 77°K., respectively. The temperature dependence of the conductivity curves during warming reveals three popula tions of charge carriers which can be freed from traps to the conduction band at 77°, 85°-185° and 200°K. The two thermoluminescence intensity peaks observed at 100° and 163°K. confirm charge migration in these temperature ranges, but no intensity peak appears at 77°K. Τ η recent years the investigation of ionic processes has become one of the main efforts in radiation chemistry. The existence of the "blue electrons" predicted by Platzman's theory (17) has been confirmed in different irradiated materials, and their properties and reactions have been extensively studied. This progress in our knowledge about the radiation-induced ionic processes arises mainly from the use of pulse radiolysis,flashphotolysis, and matrix isolation techniques. Despite the fact that one of the most characteristic properties of hydrated and solvated electrons is their electric charge, surprisingly few investigations have covered the conductivity of the current induced by the migration of these charges. This applies particularly to simple organic solids, including organic glasses, though the latter are extensively used for the optical identification of ions formed by irradiation. This is probably caused partly by the fact that no satisfactory theory is available for interpreting the data obtainable from current measure ments during and after irradiation, partly by the experimental difficulties involved in using sensitive electrometers and high resistance measuring cells required for the measurements. The electric current induced in organic liquids by irradiation was studied by Freeman (7,8,9), Hummel and Allen (12, 13), and Gibaud 291 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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(11), who measured steady-state current at low applied field. Under these conditions most of the charge carriers recombine before reaching the electrodes, and the measured "free electron" yields are much lower than those obtained by other methods.


Conductivity measurements on irradiated solids (2, 4, 5) revealed the similarity of the mechanisms of photo- and radiation-induced con ductivity and indicated the importance of the role of traps and recombi nation centers which may exist at positions of disorder in the liquid or solid structure. Radiation-produced impurities can also act as traps. Only a few conductivity measurements on organic glasses have been reported. Albrecht et al. (14, 15) studied the photoconductance of 3-methylpentane ( 3 - M P ) while Viseall and Willard (18) determined the conductivity of the same compound after y-irradiation. In an earlier paper (16) we reported the observations on the conductivity of irradiated 2-methyltetrahydrofuran. The presence of trapped electrons and ions that can be freed by illumination or thermally can be inferred from the current increase caused by their movements. The temperature depend ence of the conductivity in these glasses suggested the occurrence of structural changes and indicated the existence of traps with different depths. This paper reports measurements of the electrical conductivity and the thermoluminescence of γ-irradiated 3-MP. Experimental 3-MP was purified by stirring with concentrated H S 0 , followed by several passages through 1.50 meters of freshly activated silica gel, then fractional distillation from P 0 . The end product was stored in evacuated ampoules provided with breakseals. Diphenyl was distilled at 10" torr, repeatedly recrystallized from high purity ethyl alcohol, and sublimated in high vacuum. A lead borosilicate cell of about 5 cc. capacity was used. The leads from the two plane-parallel 1-sq. cm. Pt electrodes were coated with glass. The electrode spacing was 1 mm. The dimensions of the cell are shown in Figure 1. Long leadouts were used to reduce as much as possible the conductivity caused by vapor condensation on the wires since the protective effect of the hydrophobic silicone coating proved insufficient at the temperatures used. The background current of the evacuated empty cell was measured as less than 10" to 10" amp. and that of the unirradiated sample as less than 10" amp. at 5 Χ 10 volts/cm. F r o m about 200°K. up the blank current through the empty cell gradually increased, i n some cases up to 10" amp. at 500 volts. The measured values were corrected for the blank currents. To eliminate the probable changes i n resistivity owing to irradiation, the cell was heated to 300°C. i n an oven after each measuring cycle (blank with empty cell, unirradiated sample; irradiated sample, blank with irradiated empty cell). 2

2

4

5

2

12

13

12

10

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

3

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Ionic Processes

293


Fi

Figure 1. Conductivity cell with sidearm for 3-MP. Dimensions in mm. The purified 3 - M P was distilled into the cell from the sidearm (Figure 1) which was subsequently sealed off, and the system was evacuated by the usual pumping-thawing-freezing technique to a pres sure of 10" torr. The samples were irradiated in a C o gamma source at a dose rate of 0.9 χ 10 e.v./ml. min. The electrical conductivity of the samples was measured either during or after irradiation. In the latter case the conductivity (or the optical) measurements were begun about 1 minute after termination of the irradiation. The samples were protected from the incidence of light when transported from irradiation to measurement. The circuit consisted of a stabilized voltage supply, K F K I type P-13-1RK, and the conductivity cell that was connected with the elec trometer in series. The lower limit of the S E A type 6 - A T C C - 5 electrome ter sensitivity was 10" amp. The variations i n the output current were displayed on a Graphispot, type G R 4 V A D recorder. The sample was warmed by two different methods, both essentially spontaneous. The sample was either exposed to the surrounding air or left to warm slowly inside of the metal block that was cooled to liquid nitrogen temperature before the measurement. The sample temperature was measured by recording the thermo e.m.f. vs. time of an iron constantan thermocouple put in the middle of the opened sample. The temperature was measured to a ± 5 ° accuracy. During the connection of the samples to the electrometer charge loss to grounding was unavoidable. Thus, the current peaks appearing at 77 °K. had to be ignored in interpreting the measured data. 6

6 0

17

13


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For optical measurements, either a Unicam Sp-700 recording spec trophotometer with low temperature cell holder, or spectrofluorimeter displaying the whole emission spectrum with 1-second repetition fre quency was used. Results The temperature dependence of the current in a nonirradiated 3 - M P sample measured at an applied field of 3 Χ 10 volts/cm. is shown in Figure 2. Before the increase in slope at about 200 °K. in some cases a "negative" (opposite to the direction of the field) current peak is observ able. A definite, broad peak that, however, does not exceed 5 χ 10" amp. also appears in the temperature range 90°-140°K.


4

12

Temperature (°K.) 76.9

90.9

Ill.l

200

142.8

Β

A Λ

ft

-10

A

13 I0

Figure 2.

3

Current vs. sample temperature before irradiation (A) and after irradiation (B) with a dose of 1.6 X 10 e.v./gram 18

After irradiation, the current vs. temperature curve changes consid erably (Figure 2 ) . The current surges accompanying the connection of the cell to the measuring circuit indicate the availability of free charges at 77°K. The decay of currents at 77°K. was studied and described by Viseall et al. (18).


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Ionic Processes

The main difference in the temperature dependence after irradiation from that before irradiation is the current peak i n the range from 85° to 185°K. with a maximum at 110°K. After this peak the current decreases to its blank value and rises again at about 200 °K. as for the nonirradiated sample. A t higher doses the position of this peak shifts to somewhat higher temperatures and is preceded by a "negative" peak of increasing magnitude as the dose increases.


-ι

i

_J

90

110

I

I

130 150 Temperature (°K.)

ί !

!

170

Γ ί

L _

190

Figure 3. Radiothermoluminescence curves as a function of tem perature To observe the possible effects of molecular movements and struc tural changes on the resistivity of the sample, thermoluminescence measurements were performed which are known to be very sensitive to these changes (1, 3 ) . Figure 3 shows the thermoluminescence curve for 3 - M P containing 1 0 " M diphenyl. T w o peaks appear—one at about 100°K., the other at 163°K. The spectral distribution of the emitted light (Figure 4) that remains unchanged during the heating period and the decay of the emission show that diphenyl phosphorescence takes place. 4

O n bleaching the irradiated sample with the light from a tungsten lamp, the second current peak between 85° and 180 °K. becomes appre ciably lower (Figure 5) but does not disappear completely. Steady-state current values obtained during irradiation i n the source are summarized in Table I. Separate measurements show that these values


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contain a contribution from the ionization of the surrounding air which, however, does not induce any appreciable error. The steady-state current was recorded during the spontaneous warming of the sample while in the gamma source. The value of the current changes smoothly from that measured at 77 °K. to that obtained at room temperature; thus it does not follow the behavior pattern of the conductivity after irradiation.

Figure 4.

Luminescence spectrum of gamma-irradiated 3-MP ghss containing 10~ M diphenyl 4

There were no detectable changes in the absorption spectra of 3 - M P after an absorbed dose of 3 χ 10 e.v./gram. A sample with a dose of 4 X 10 e.v./gram showed two small peaks, at 322 and 555 m/x, but the peak in the near infrared (10) was not detected in our samples even at this high dose. 18

20

Discussion The thermoluminescence curve suggests that two different kinds of charges are migrating while the sample is left to warm, provided the light emission is caused mainly by the recombination of charged particles. Earlier conductivity data (6) obtained on irradiated frozen hydrocarbons as well as the results of E S R and optical measurements indicate that the first entities freed from traps in the temperature range 77°—115° K . with small volume expansion are electrons, while the second peak above 200 °K. is caused by ions needing more space for their migration.



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Ionic Processes

Figure 5. Current vs. temperature curves for gamma-irradi ated sample; dose = 10 e.v./ml. (Curve A). Curve B, taken under the same conditions plus 10 minutes bleaching at 77°K. with the unfiltered light of a 100 watt tungsten lamp 18

Table I.

Steady-State Currents in Irradiated 3-MP at Different Temperatures α

Temperature, °K.

Current, amp.

290

1.74 Χ 10" 1.68 Χ 10"

8 8

320

2.00 Χ 10"

77

5.72 Χ 10" 5.72 X 1 0 ' 5.95 Χ 1 0

8

10

10 1 0

" Dose rate = 0.9 Χ 10 e.v./ml. min. Voltage = 300 volts. The values of the current at the same temperature were measured on the same sample that was cooled down sev eral times to 77°K. after warming to room temperature. 17



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There seems to be good agreement between the above picture and the temperature profile of the current. Accordingly, the first row of trapped electrons seems to be liberated at 85°K., ignoring, of course the electrons causing current surges at 77°K. The current caused by the movements of the released electrons has a maximum at about 115°K., then decreases to the value of the dark current to which it becomes equal at 185 °K. This means that the freed electrons can be raised to the conduction band by rearranging the glass structure needing only a very small activation energy. The summing of the charges under the electron peak permits one to evaluate the "virtual" electron yield G(e) if the absorbed dose is known. The "virtual" value may be substantially different from the true radiation-chemical yield since a major fraction of the free electrons is lost by recombination during their migration and thus does not con tribute to the current. The electron peak of a sample irradiated at 77 °K. by a dose of 1.6 to 10® e.v./gram was caused by the passage of 6.5 Χ 10" coulomb charges—i.e., the virtual G(e) = 2.5 X 10~ electrons/100 e.v. Thus, the number of charges so small that 1 in 10 of the molecules in the monolayer adjacent to the electrodes would be able to produce them if they had any charge. Conductivity measurements on solids after irradiation do not seem to yield sufficient information for evaluating the radiation chemical elec tron yield since the number of the radiation-produced charge carriers cannot be determined by this method alone. The conductivity data are, however, useful for identifying these charge carriers, and they permit determination of the temperature range at which the charge carriers react and the rate constants of their reactions. The temperature behavior of the conductivity can be related to the structural changes expected to occur. Investigation must be continued for a better understanding of the mechanism of the radiation-induced electronic movements.

9

5

4

Literature Cited (1) Bagdassarian, Kh. S., Milutinskaya, R. I., Kovalev, Yu. V., High Energy Chemistry (in Russian) 1, 127 (1967). (2) Bube, R. H., "Photoconductivity of Solids," p. 120, Wiley, New York, 1960. (3) Buben, N. Ya., Goldanskii, V. I., Dokl. Akad. Nauk. USSR 162, 370 (1965). (4) Frankevich, E. L., Tal'roze, V. L., Proc. Symp. Radiation Chem. 2nd, AN USSR Moscow, 1962, p. 651. (5) Frankevich, E. L., Tal'roze, V. L., Solid State Phys. 3, 180 (1961). (6) Frankevich, E. L., Uspehi Chim. 35, 1161 (1966). (7) Freeman, G. R.,J.Chem. Phys. 39, 988 (1963). (8) Ibid., p. 1580.


18. soMOGYi AND

BALOG

Ionic Processes

Freeman, G. R., Fayadh, J. M., J. Chem. Phys. 43, 86 (1965). Gallivan, J. B., Hamill, W. H., J. Chem. Phys. 44, 2378 (1966). Gibaud, R., J. Chim. Phys. 64, 521 (1967). Hummel, Α., Allen, A. O.,J.Chem. Phys. 44, 3426 (1966). Hummel, Α., Allen, A. O., Watson, F. H., J. Chem. Phys. 44, 3431 (1966). (14) Johnson, G. E., Albrecht, A.C.,J. Chem. Phys. 44, 3162 (1966). (15) Ibid., p. 3179. (16) Kosa Somogyi, I., "The Chemistry of Ionization and Excitation," p. 116, Taylor & Francis, London, 1967. (17) Platzman, R. L., U. S. Natl. Res. Council Publ. 305, 34 (1953). (18) Wiseall, B., Willard, J. E., J. Chem. Phys. 46, 4387 (1967). RECEIVED January 16, 1968. Downloaded by UNIV OF ROCHESTER on June 11, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch018

(9) (10) (11) (12) (13)

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Radiation Chemistry-II

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