COMPARISON OF
THE
REACTIVITIES OF DRYAND MOBILEELECTRONS
data support the hypothesifi that the HT arc produced iriitinlly with a nonuniform spatial distribution reflccting the nonuniform deposition of radiation energy but ns they move slowly from their initial trapping sites the distribution gradually becomes more sptttially uniform. Thr dose region from 0 t o 1.2 hlrads in nhich (7‘1T~)’/’ inrrenscs reflects the cohange from spatial notiuniformity to spatial uniformity. At doses between 1.2 and 2.6 Rlrads (12-2.5 hr of irradiation) the total number of HT is still increasing but by these times the spatial distribution is uniform so that (TIT2)”’dewcases sharply. At doses above 2.5 llrads, there is no further increase in the number of HT (Figure 3), their spatial distribution remains uniform, and no more change in (T1T2)*/’occurs. If this hypothesis is valid, it indicates that relaxation time data not only reflect steady-state spatial distribu-
1941
tions but may also give useful insights into changes in these distributions with certain variables such as radiation dose and time.
Aclcnozuledyment. Partial financial support for this research was provided by United States Atomic Energy Commission, contract No. AT( 11-1)-1057-8, which furnished chemical supplies and provided partial salary for one of the investigators. National Aeronautics and Space Administration Grant NGR-17-002-050 provided partial salary for one of the investigators and a technical assistant. U. S. Air Force Contract F1962869-C-0009 furnished funds for part of the epr apparatus. Thanks are also expressed to Professor Larry Kevan, Wayne State University, Detroit, Mich. for valuable discussions and t o Gisela Dreschhoff for her assistance in performing the dta analyses.
A Comparison of the Reactivities of Dry and Mobile Electrons by Harald B. Steen,* Olav Kaalhus,l and Magne Kongshaug’ Norsk Hydro’s Institute for Cancer Research, Montebello, Oslo 8,Norway
(Received December 28,1970)
Publication costs assiated by the Norah Hydro’s Institute for Cancer Research
The scavenging efficiencies of various electron scavengers with regard to mobile electrons (em-) have been determined from the yield of the X-ray-induced trapped electrons (et-) in an ethylene glycol-water glass at 77’K, by measuring the rate of increase of the optical density of the glass at 585 nm during X-irradiation. The scavenger concentrations needed to halve G(et-) were found to be 0.047 M for HzOz, 0.035 M for NaNOs, 0.020 M for CC13COOH,0.26 M for ClCH&OOH, and 0.41 M for HCI. G(et-) was not noticeably affected by the presence of 1 M NaC1, indicatingthat ionic strength was not important. The apparent reaction kinetics were not homogeneous. One reason for this seems to be the scavenging of the precursor of e,,,-, Le., the dry electrons (e-). Comparing the scavenging efficiencies of e,- with those previously measured for e- under similar conditions, we conclude that the reactivities of these entities are generally different,
Introduction One of the basic assumptions in the new model for the radiolysis of aqueous solutions recently put forward by Hamill2is that electrons may react with solutes prior t o hydration, that is, while they presumably are still moving with thermal-or higher-energies as unsolvated particles. According to this model these “dry electrons” (e-) should generally react with molecules which are efficient scavengers of hydrated electrons (e,,-) with the important exception of hydrogen ions (H30nq +) towards which they should be essentially Unreactive. This hypothesis has now been experimentally supported by Wolff, et u Z . , ~ who were able to perform pulse radiolysis of aqueous solutions with a time resolution of about 20 psec. They found that various electron scavengers reacted noticeably with the pre-
cursor of eaq-, Le., presumably with e-, when present in concentrations above 0.1 M . I n agreement with Hamill’s hypothesis HSOaq+was found to be essentially unreactive. Additional experimental support has been reported by Steenj4who measured the yield of X-rayinduced phosphorescence of indole in ethylene glycolwater glass at 77°K as a function of the concentration of various electron scavengers, the reasoning being that approximately 90% of this phosphorescence appears to be due to geminate recombination between indole cat(1) Fellow of the Norwegian Cancer Society. (2) (a) W. H. Hamill, J . Chem. Phys., 49, 2446 (1968); (b) W. H.
Hamill, J. P h y s . Chem., 73, 1341 (1969). (3) R . K . Wolff, M. J. Bronskill, and J. W. Hunt, J . Chem. Phys., 53, 4211 (1970). (4) H. B . Steen, J . P h y s . Chem., 74, 4059 (1970).
T h e Journal of Physical Chemistry, Vol. 76, N o . 13. 1071
1942 ions and e-,6 The scavenging efficiencies of the various scavengers with regard to e- obtained in this way are in good agreement with those found by Wolff, et ul., when compared on a relative scalc. The question arises whether thesc “dry electrons” are the same, ie., with respect to reactivity, as the “mobile electrons” (e,],-) previously observed in various glassy solutions by several authors.6-+ Hamil12b suggests that this is indeed the case. I
