HARALD B. STEENAND JOHAN MOAN
3366
On the Mechanisms of Photobleaching of Trapped Electrons in Ethylene Glycol-Water Glass y Harald B. Steen* and Johan Moan' .Vorsk Hydro's Institute for Cancer Research, Montebello, Oslo 3, Norway
(Received J u n e IS, 1972)
Puhlicalwn costs assisted by the Norsk Hydro's Institute for Cancer Research
The mechanisms of excitation and photobleaching of radiation-induced trapped electrons, et-, have been investigated by studying the effect of electron scavengers on the photobleaching of et- in equivolume ethylene glycol-water glass. OD at, , ,A (Le., at 585 nm) was measured during X irradiation and subsequent exposure to monochromatic light of various wavelengths Ab. The photobleaching quantum yield, 4 , decreases by a factor of about 15 between 366 and 546 nm, while above 546 nm it is constant. The presence of NOn- enhances @ significantly. This effect decreases with increasing Ab. et- appears to exhibit a number of excited levels, some of which are below the ionization potential Ei. Thus, the lowest excited level seems to be at about 1.8 eV whereas E , = (2.4 0.2) eV. The results are in accordance with the assumption that the photoactivation of et- occurs via excited states of et- located above E,, whereas the direct transition of et- to the conduction band seem3 to be insignificant. The excited states of et-, both below and above E;, may react with the solvent glass, ?;.e., with ethylene glycol, as well as with solute molecules.
*
Introduction The solvated electron e,- is characterized by a strong optical absorption usually covering the entire visible region of the spectrum. However, the nature of the optical transitions involved is not fully understood. 'Thus, it is not known whether the excitation of e,occurs to bound or quasi-bound excited levels or directly to the state of a free electron, i.e., to a conduction band, or both. The transient nature of e,- makes detailed studies of its optical properties difficult. Hence, considerable effort has been concentrated on electrons trapped in the rigid glasses formed by various polar liquids at low temperature. Except for their stability these trapped electrons et- appear to be essentially similar to A number oE investigations have shown that when glasses containing et- are exposed to visible light, the electrons disappear and, furthermore, that this photobleaching may be accompanied by photoconductivity, indicating that the absorption of light may bring etinto the state of a free, mobile electron e,-.2 Eisele and &van3 found that for y-irradiated 10 M NaOH a t 79 K the wavelength dependence of the photoconductivity matches the absorption spectrum of et-. They concluded that, in contrast to the predictions of niosl current theories, et- has no excited bound states. Hoviever, they were not able to decide whether the formation of e,- occurs directly from the ground state of et- or by ionization of a quasi-bound excited state, i e., excited states above E,. In the present work we have tried to elucidate further the nature of the excited states of et- and the mechanism of the photoactivation. It appears that in the present glass e$- has at least one bound excited The Journal of Physical Chemistry, Vol. 76, No. 23, 1972
level, i.e., an excited level which is below Ei. Furthermore, the results support the hypothesis that the photoinduced transition of et- to e,n- occurs via quasi-bound excited states of et-. The excited states of et-, both below and above Ei, may react Tvith the solvent as well as with solute molecules.
Theoretical Considerations Photobleaching Kinetics. Since it is easily overlooked that the kinetics of the photoinduced decay of et- depend critically on the concentration of et-, [et-], we derive here the exact formula. Consider the case that all et- in a sample have uniform trap depth, i.e., that all occupied traps are identical, and consequently have the same probability, 4 , to disappear upon absorption of a photon of wavelength Ab. The number of et- disappearing per unit time and volume is then
-I4(1 -
10-ODb)
(1)
where I is the quantum intensity of the bleaching light, a the fraction of this light which is absorbed in the sample, 1 the optical path length in the sample, Eb the extinction coefficient at Ab, and t the bleaching time. Integration of eq 1 gives (1) Fellow of The Norwegian Research Council. (2) Several reviews of this subject have been published. See, for example, A. Ekstrom, Radiat. Res. Rev., 2,381 (1970); F. S. Dainton, Ber. Bzmsenges. P h y s . Chem., 75, 608 (1971). (3) I. Eisele and L. Kevan, J . Chem. Phys., 53, 1867 (1970).
.PHQTQBLEACHJN;G OF TRAPPED ELECTRONS
where ODbo is ODI,a t t = 0. The decay of ODb, as calculated from eq 2 , is shown in Figure 1. It can be seen that it is reasonably exponentiaI only when a smalI fraction of the incident light i s absorbed, say for ODb < 0.2. For large values 1 approach a linear time of ODb the decay ~ 4 obviously dependence (see eq 1). Thus, for ODb > 1 this is a good approximation. For intermediate values of ODb, which is perhaps the region which is most commonly utilized, the decay kinetics have no simple analytical _arm at all, Pholoactiuation Models. We shall consider two mechanisms for the photoactivation of et- which are both shown schematically in Figure 2 . For both cases we assume that all et- have the same trap depth and consequently the same ionization potential, i . e . , the same energy difference between e,- and the ground state of et-. The upper diagram of this figure represents what we shall call "the direct activation" model, namely, the hypothesis that by absorption of a photon et- is transfcrred direclly to a free mobile electron e,--, i.e., a direct transition from the ground state to the conduction band. e,, - may be retrapped in competition with reaction with the molecules of the solvent glass itself and with a scavenging solute S. We assume that the origiiial formation of et-, e.g., by ionizing radiation, occurs via the formation of e,-. The following symbols are used: [SI = scavenger concentration, G(et-) = yield of et--, qi = photobleaching quantum yield, 1S]112 = the value of [SI necessary to halve G(et-),
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q- ground slate
Figure 2. Schematic representation of the two models of photoactivation considered here. The upper diagram represents the assumption that by absorption of a photon et- is transformed directly to a free electron e,- whereas in the lower model it is assumed that em- is formed by ionization of an excited state of et-.
[SI, = the value of [SInecessary to double 4, Ict = probability per second that e,- will become trapped, k , = probability per second and per unit concentration of S that e,- will react with S, IC, = probability per second that e,- will react with t h r solvent glass including recombination with positive holes. Subscript 0 denotes that S = 0. We assume that the reactivity of e,- is independent of its mode of formation, i.e., that k,, ICr, and k , are independent of whether e,- results from X-ray-induced ionization or from photoactivation of et- and independent of the energy of the photon which cause3 the photoactivation. Physically this is t o say that electrons released either by ionization or photoactivation are essentially unreactive until they reach the state em- which is conceivably the thermalized free electron. The experimental support for this assumption is discussed below. Assuming that IC, is independent of [SI,one obtains the expression for homogeneous scavenging kinetics Go(et-)/G-(et-)
" I[ 0.05
Oo2
t
0W5i0
+ k,ESl/(kt ikr)
(3)
Measurements of the scavenging kinetics of e,indicate that although eq 3 is not valid for large [SJ, it is a reasonable approximation as long as @o(et-)/ G(eh-) is relatively small, say 5 2 . Accepting this, we have
L3 0.10 0
aoi
= 1
+ + ks[S])/(kt + + ks[Sl) 40 k / ( h+ kr)
[SI,,,= (kt 4 1
2
3
4
5
6
1
8
9
Bleaching time (relative unit4
Figure 1. OD decay kinetics during photobleaching with constant light intensity as calculated from eq 2.
= (kr
=
kr)/ks
(4)
kr
(5) (6)
(4) H. B. Steen, 0. Kaalhus, and M. Kongshaug, J . Phys. Chem., 75, 1941 (1971).
The J O U Tof ~Physical Chemistrv, TOE.7 6 , N o . 29, 1072
HARALD B. STEENAND JOHANMOAN
3368 and hence
Putting
c$/&
=
2 in eq 7 , we find
The present experiments show that for all Ab cn0
