14
T h e Study o f E l e c t r o n D e c a y i n PulseIrradiated Gases by a M i c r o w a v e T e c h n i q u e
Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch014
RICHARD W. FESSENDEN and JOHN M. WARMAN Radiation Research Laboratories, Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pa. 15213 A microwave technique for measuring the decay of elec trons in pulse irradiated gases is described. The technique involves the measurement of the change in resonant fre quency of a microwave cavity caused by a change in the complex conductivity within the cavity when electrons are present. Single pulses of 3 Mev. electrons from a Van de Graaff accelerator are used to ionize the gas. Electron densities as low as107cm-3(total dose ~ 0.3 rad at 10 torr) can be measured accurately. In the absence of diffu sion the method can be used to study electron loss by elec tron capture or electron-ion recombination for pressures as low as 1 torr and as high as at least 200 torr. The potential of the technique is illustrated by results obtained with pulse -irradiated air. *Tphe presence in the gas phase of electrons within a microwave cavity is indicated by a shift of the cavity resonance frequency from that in the absence of electrons. This shift, Δ/, is related to the concentration of electrons according to Equation A (2, 3): M. — A f ~ 2TT ' mf
N
* (_*Λ
2
A
c m . sec." . A plot of τ ι Ρ (— 1/Ktt) vs. l / P i is shown i n Figure 4 for air pressures from 14 to 80 torr. The linearity of this plot is i n accord with the expected pseudo-three body nature of electron capture by oxygen as has been found by other workers (4, 5, 8, 10, 12). The three body rate constant derived from the slope of the line in Figure 4 is 0.65 Χ 10" cm. /sec. The relative efficiencies of Oo and N as third bodies in Reaction 4 have been determined to be 0 / N === 20 (8, 12). Using this value and the known composition of the air sample (21.2% 0 ) the three body rate constant for electron capture i n oxygen alone is calculated to be 2.6 Χ 10" cm. /sec. This value is i n good agreement with previous values of 2.1 χ 10" (12) and 2.0 Χ 10" (8) obtained by microwave and drift tube methods respectively. 0
A
30
1
/ 2
θ 2
a
P
G
2
2
2
2
30
6
30
30
In terms of the above discussion the intercept in Figure 4 corresponds to a limiting (infinite pressure) two body rate constant, fc of 4 χ 10" cm. sec." . This value is considerably lower than the minimum values for fci which can be estimated from several other studies [2.5 χ 10" (13), 4 Χ 10" (5), 7 χ 10" (8,10)]. Because of this inconsistency the value of ki determined from the present results should be considered to be i n doubt. Experiments are presently being carried out to clarify this point. 12
b
3
1
11
11
11
Conclusion The advantages of the use of an electron accelerator for production of ionization in studies of electron reactions is evident from the preceding example. Preliminary experiments with NoO and with N 0 i n propane show that electron capture by N 0 occurs in a way which also depends on the total pressure and so is apparently also a three-body process. The success of these experiments at pressures as high as 200 torr illustrates the relevance to radiation chemistry as compared with other methods such as mass spectrometry which only operate at much lower pressures. 2
2
Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch014
14.
FESSENDEN AND WARMAN
0
1 2
229
Electron Decay
3 4 5 ΡΤί (mm)xlO
6
7
2
α
Figure 4. Dependence of the product of the electron half-life and the oxygen pressure (proportional to the reciprocal of the effective two body rate constant, see text) on the reciprocal air pressure The sensitivity of the method is extremely high so that no problems with build-up of products of the radiolysis are likely. Conversely, how ever, problems with impurities will be serious. The radiolytic method allows the possibility of pre-irradiation as a partial cure. This method is being extended to detailed studies of electron capture by N 0 and other compounds. Several other applications of importance to radiation chemistry are immediately apparent such as studies of ion recombination and the dependence of its rate on the nature of the positive ion. 2
Literature Cited (1) Biondi, Μ. Α., Brown, S.C.,Phys. Rev. 75, 1700 (1949). (2) Biondi, Μ. Α., Rev. Sci. Instr. 22, 500 (1951). (3) Bloch, F., Bradbury, Ν. E., Phys. Rev. 38, 689 (1935). (4) Chanin, L. M., Phelps, Α. V., Biondi, M. A., Phys. Rev. 128, 219 (1962).
Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch014
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RADIATION CHEMISTRY II
(5) Hurst, G. S., Bortner, T. E., Phys. Rev. 114, 116 (1959). (6) McDaniel, E. W., "Collision Phenomena in Ionized Gases," p. 121, Wiley, New York, 1964. (7) Mahan, Β.H.,Young, C. E.,J.Chem. Phys. 44, 2192 (1966). (8) Pack, J. L., Phelps, Α. V., J. Chem. Phys. 44, 1879; 45, 4316 (1966). (9) Slater, J.C.,Rev. Mod. Phys. 18, 659 (1946). (10) Stockdale, J. Α., Christophorou, L. G., Hurst, G. S., J. Chem. Phys. 47, 3267 (1967). (11) Van Lint, V. A. J., Parez, J., Trueblood, D. L., Wyatt, M. E., Rev. Sci. Instr. 36, 521 (1965). (12) Van Lint, V. A. J., Wikner, E. G., Trueblood, D. L., Bull. Am. Phys. Soc. 5, 122 (1960). (13) Young, B. G., Johnsen, A. W., Carruthers, J. Α., Can. J. Phys. 41, 625 (1963). RECEIVED January 12, 1968. This work was supported in part by the U. S. Atomic Energy Commission.
Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
