J. Phys. Chem. 1903, 87, 4096-4098
4096
Nanosecond Tlme-Resolved Conductlvlty Studies of Pulsed-Ionized Ice. 3. The Electron as a Probe for Defects in Doped Ice Marlnus Kunst, John M. Warman,' Matthljs P. de Haas, Interunlvers#y Reactor Instffute, Mekelweg 15, Delft, The Netherlands
and Johan 6. Verberne Biophysics Department, Free University, De Boelelaan, Amsterdam, The Netherlands (Received: August 23, 1982; I n Final Form: January 13, 1983)
The conductivity transients resulting from the formation of conduction electrons on pulsed ionization of ice are characterized by an after-pulse height, proportional to the yield and mobility of electrons, and a decay rate reflecting interaction of electrons with extrinsic or intrinsic defects in the medium. By studying the effects of additives on these two parameters we can obtain information about the resulting changes in the nature and/or amount of defects present. Results for the classic ice dopes HF, NH,, and NH4Fare presented and discussed.
Introduction Because of the high mobility of conduction electrons in icel3, this species can provide an extremely sensitive probe for small amounts of impurities or defects in the crystalline lattice. For example, the rate constant for reaction with protons has been determined2to be approximately 10l6M-' s-l and even reactions with neutral entities can occur with rate constants as high as ca. 1014 M-' s-l., Only 1 ppm of a neutral defect capable of trapping the electron could therefore limit the lifetime of the conductivity transient in pulse-ionized ice to approximately 10 ns and positively charged defects could have a much more drastic effect. Dielectric measurements have shown that the concentration of the primary defects in ice can be dramatically influenced by the incorporation of small amounts of HF and NH3.4 In the present paper we show how these dopants, as well as NH4F, can have a pronounced effect on the electron transients. A preliminary, qualitative discussion of the relationship between these effects and the changes which are thought to occur in the concentrations of point defects is given. Experimental Section The time-resolved microwave conductivity technique was used for all measurements in the present Solutions of NH4F and HF were prepared from triply distilled water and were nitrogen bubbled and degassed as for pure water samples. Apart from the final stages of degassing in a Pyrex bulb the HF solutions were stored and handled in PTFE vessels. The NH, solutions were made by adding a known amount of gaseous ammonia to a water sample on a vacuum line after the degassing procedure. Results and Discussion In Figure 1are shown the effects of HF, NH3, and NH4F doping on the electron transients in ice with the pure ice case (for the same dose) being given by the dashed curves. (1)J. B. Verberne, H. Loman, J. M. Warman, M. P. de Haas, A. Hummel, and L. Prinsen, Nature (London),272, 343 (1978). (2) J. M. Warman, M. P. de Haas, and J. B. Verberne, J.Phys. Chem., 84, 1240 (1980). (3) J. M. Warman, in 'The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis", J. H. Baxendale and F. Busi, Ed., Reidel, Boston, 1982, p 433. (4) P. V. Hobbs, "Ice Physics", Clarendon, Oxford, 1974. ( 5 ) M. P. de Haas, Ph.D. Thesis, University of Leiden, The Netherlands. 1977. (6)P. P. Infelta, M. P. de Haas, and J. M. Warman, Radiat. Phys. Chem., 10, 353 (1977). (7) J. M. Warman in ref 3, p 129.
All three compounds are seen to have a pronounced effect indicating a substantial change in the nature and/or concentration of the point defects responsible for the trapping of conduction electrons. In Figure 2 is illustrated the way in which HF and NH, are thought to influence the concentration of intrinsic orientational (Bjerrum) and ionic defects in ice.4 These processes can be represented schematically by the following equations, for HF HF Q HF*d 1 (1)
+
HF
+ H 2 0 zi F . d + H,O+.l
(2)
and for NH3
+d NH, + H 2 0 e NH4+.l + OH-.d NH, e NH3.1
(3) (4)
The rule used for incorporating orientational defects, symbols 1 and d, in eq 1-4 is that if substitution of a chemical species by a water molecule would result in a D or L orientational defect then this is denoted by d or 1 following the chemical symbol of that species. On doping with equal amounts of NH, and HF, corresponding to a solution of NH4F, it is considered that the ionic and orientational defects introduced by the individual components will be almost nullified and a situation resembling more closely pure ice should be rein~tated.~ We will now discuss qualitatively the effects of these dopants on the electron transient in terms of this classical model of their effects when incorporated in the ice lattice. A quantitative interpretation of the data requires a more thorough kinetic analysis which is not yet complete but will be presented in a forthcoming publication. Ammonia. Of the three additives, NH, has the most readily explainable effect on the electron transient since it simply increases the rate of electron decay, Figure 1. This indicates the formation of permanent or "deep" trapping sites over and above those originally present in pure ice. For a lo-, M solution, the electron lifetime approaches a plateau value at low temperatures (< -80 "C)of approximately 20 ns which is an order of magnitude shorter than in pure ice. With increasing temperature the trapping rate increases but with an activation energy which is considerably lower than found for pure ice. As a result the lifetimes in the NH,-doped samples approach those in pure ice at high temperatures as is illustrated by the Arrhenius plots of the trapping rate in Figure 3.
0022-3654/83/2087-4096$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 21, 1983 4097
Electron as a Probe for Defects in Doped Ice
-66' C
[NH~I 037(-4)
0
10
20
30
-t
-60' C
[NH,F:
0 T i m e ( ns)
20
IO
-
(-3)
O I6
30
40
Flgure 1. The transient conductivity resulting from pulsed irradiation of frozen aqueous solutions of NH, NH,F, and HF for the concentrations and at the temperatures indicated. The total dose in the pulse was ca. 20 rd except for the NH,F data for which ca. 60 rd was used. The dashed lines are pure Ice data for the same total dose. DOAFNPST-
E
I
NH,
+
\
I
I
\
Ii .\' \
'\ \
.
\.
\ 4
\ . \
. \
..
.
\
. \ \
. \
\
\ \
\
Flgwe 2. Simple, onedimensional lattice representation of the effects on orientational and ionic point defects of incorporation of the dopants HF and NH, in the ice lattice.
I t should be mentioned here that, while a reduction in the electron lifetime was invariably found in NH3-doped samples, the actual value of the lifetime at a given temperature and NH3 content were somewhat irreproducible from sample to sample. The potential defects responsible for electron trapping according to eq 3 and 4 would be the D defect and the ammonium ion. We exclude the latter on the basis of the much lower trapping rate found in NH,F samples for which a considerably higher concentration of NH,+ would be expected than in NH3samples due to the much stronger proton-donating power of HF compared with H,O. This leaves therefore the increased concentration of D defects as the explanation for the increased electron decay rate. As in the case for pure icea we believe the D defect to be to a large extent complexed by vacancies leading to (8)M. P. de Haas, M. Kunst, J. M. Warman, and J. B. Verberne, J. Phys. Chem., this issue.
i
\ !
\ \
'\ '\
Flgure 3. Arrhenius plot of the intrinsic trapping rate of conduction electrons in M NH,doped ice. The dashed line is for pure ice.
dressed vacancies. These are considered to be the actual electron-trapping sites in pure and NH3-doped ice. d+vsdv (5) Hydrogen Fluoride. The electron transient is extremely sensitive to additions of HF, as can be seen by the data in Figure 1. The changes which occur are complex and indicate the creation of two trapping sites on incorporation
4098
The Journal of Physical Chemistty, Vol. 87, No. 21, 1983
____ +
n
1i-7 10-6
lo-:
0 0
.
10-
-W
tE
I -
( 3 s
z
IO c
L a a t
-
o\-
\
r-
n
-
a W w n
7
IO
-
\ \ \
\
61
IOL
4
5
6 , o ~ / T (OK-‘)
Figure 4. Arrhenius plots of the rate of localization of conduction electrons at deep traps in HFdoped ice for the concentrations shown. The dashed line is for pure ice.
of this compound. One of these sites, Ts, appears to result in only temporary or “shallow”trapping of the electron and the second, T D , in permanent or “deep” trapping. Accordingly the data have been preliminary analyzed with a reaction scheme including e-
+TD
-
eD
In Figure 4 is shown an Arrhenius type plot of the values of the deep trapping rate k f l D derived from this analysis. The reader familiar with the effects of H F on the highfrequency conductivity of ice (see for example the data of Von Hippel et al.9 and Camplin et al.lO)will be aware of the similarity in the dependence on H F concentrations of (9) A. R. Von Hippel, R. Mykolajewycz, A. H. Runck, and W. B. Westphal, J. Chem. Phys., 57, 2560 (1972). (10) G. C. Camplin, J. W. Glen, and J. G. Paren, J. Glaciol., 21, 123 (1978).
Kunst et ai.
the form of the temperature dependence of u, and that for kfl,. A fuller analysis of the H F data and comparison with data from dielectric studies will be presented in a future publication. The present opinion of the authors is that the additional deep trapping site in HF-doped ice is the proton formed via (2). The shallow site is possibly the net positive charged HF-d center. As can be seen from eq 1, the concentration of the latter will of course be equal to the concentration of L defects introduced by HF. A slight reduction in the overal deep trapping rate observed at close to the melting point is considered to be due to depression of the dressed vacancy concentration by L defect formation which more than compensates for the increase in trapping by proton formation. Ammonium Fluoride. As mentioned previously, for NH4F-doped ice a situation close to that found in pure ice is to be expected in so far as the intrinsic orientational and ionic defects are concerned. The rate of electron trapping is in fact found to be considerably reduced by the incorporation of this compound in the lattice as is shown by the data in Figure 1. This increase in electron lifetime contrasts with the significant decreases found for both NH3 and H F alone. The effects of the latter compounds are therefore not simply compensated when they are present in equal amounts. To a considerable extent the increase in electron lifetime caused by NH4F can be attributed to a decrease in the electron mobility. This decrease in mobility is apparent as a decrease in the end-of-pulse signal height. The reduction in mobility can be explained by the same shallow trapping by HFed as proposed for the case of H F alone since even on complete compensation of L and D defects the HFad center will remain. Even on taking into account the reduction in mobility due to shallow trapping, it appears that the rate of deep trapping of electrons while in the conduction band state is slightly less in NH4F-doped ice than in pure ice. This could possibly be explained by a slight preferrential incorporation of neutral H F in the lattice compared with NH3. This would result in a net increase in the L defect concentration over that in pure ice which could lead to a suppression of the dressed vacancy concentration. In conclusion it can be said that, a t least on the basis of a qualitative interpretation of the data, no processes or species appear to be required to explain the electron conductivity transients in the doped samples over and above those given by Eq 1-4. Whether or not this will still be the case on closer quantitative comparison with dielectric data remains to be seen. Registry No. H,O, 7732-18-5;HF, 7664-39-3; NH3, 7664-41-7; NHdF, 12125-01-8.
