1000
G. NILSSON,H. CHRISTENSEN, P. PAGSBERG, AND S. 0. NIELSEN
Transient Electrons in Pulse-Irradiated Crystalline Water and Deuterium Oxide Ice by G. Nilsson,* H. Christensen, A B Atomenergi, Studavik, Nykoping, Sweden
P. Pagsberg, and S. 0. Nielsen Danish AEC Research Establishment, Risd, Roskilde, Denmark
(Received August $4, 1971)
Publication costs assisted by The Swedish Atomic Research Council
Crystalline samples of HzO and DzO ice were irradiated with electron pulses from a Linac and the optical absorption spectrum of the transient electron was recorded at different temperatures from - 5 to -190". At - 10' the position of the absorption peaks in the two matrices are a t 682 (1.82 & 0.01 eV) and 668 nm (1.86 f 0.01 eV), respectively. The peak energy continuously increases as the temperature becomes lower, and a t about -130" constant values of 1.90 and 1.94 eV are reached. The electron decay could be accounted for by a second-order reaction between the electron and a species present in a concentration greater than its own. This second species is very probably the proton, and the proton excess was found to increase as the temperature decreases. The rate constants for the reaction in HzO and DzO ice a t - 10" are (1.95 rt 0.06) X 1011 and (0.42 & 0.04) X 1011M-' see-', respectively, and the activation energies are 4.5 =t1.0 and 6.8 d= 1.0 kcal/mol. The yield of electrons (G) decreases rapidly from 0.95 a t - 5" to 0.05 a t - 50". Our interpretation of the kinetics gives a reasonable explanation to this fact.
Introduction The localized excess electron in the solid aqueous phase was first observed as a long-lived species in irradiated hydroxide-water glasses at 77"K.l#2 In this matrix the electron is characterized by an esr singlet and an optical absorption band in the visible part of the s p e ~ t r u m . The ~ trap is supposed to be a hydroxide anion v a c a n ~ y and , ~ thc degree of structural disorder of the matrix was found to be of great importance for the trapping.6-8 Crystalline ice, on the other hand, would be expected to have a low concentration of defects suitable as electron traps and, since self-trapping as in liquid water would not be feasible due to the long dielectric relaxation time, it was at first believed that the electron could not be localized in this matrix. In spite of this the optical absorption spectrum of a transient electron was observed by pulse radiolysis of crystalline ice at temperatures not far from its melting point.g lo The absorption spectrum of this new species closely resembles that of the solvated electron in liquid water (eaq-), The yield is, however, less and the lifetime is shorter. The lifetime increases with decreasing temperature and the yield decreases At 77"K, at which the electron is stable, the yield ( G ) is only 2 X 10-4.13814 I n the present investigation the study of the localized electron in crystalline ice has been extended to lower temperatures (- 190") and to DzO ice. The temperature shift of the absorption peak has been recorded in both H20 and DzO ice. The temperature variation The Journal of Physical Chemistry, Vol. 76, N o . 7 , 1972
of the yield has also been studied, and the kinetics could be accounted for by a much simpler reaction scheme (with only one type of electrons) than that given (1) D. Schulte-Frohlinde and K . Eiben, Z. Naturforsch. A , 17, 445 (1962). (2) J. Jortner and B . Sharf, J . Chem. Phys., 37, 2506 (1962). (3) See, for example, the review articles by (a) L. Kevan in "Radiation Chemistry of Aqueous Systems," 19th L. Farkas Memorial Symposium, Hebrew University, Jerusalem, Dec 27- 29 1967, G. Stein, Ed., Weizmann Science Press of Israel, Jerusalem, 1968, p 21; (b) D. Schulte-Frohlinde and K. Vacek in "Current Topics in Radiation Research," Vol. 5, M.Ebert and A. Howard, Ed., NorthHolland Publishing Co., Amsterdam, 1969, p 39; (c) J. K . Thomas in "Advances in Radiation Chemistry," Vol. 1, M. Burton and J. L. Magee, Ed., Wiley-Interscience, New York, N.Y . ,1969, p 103. (4) M.J. Blandamer, L. Shields, and M.C. R . Symons, Nature, 199, 902 (1963), and J . Chem. SOC., 4352 (1964). (5) T. Henriksen, Radiat. Res., 23, 63 (1964). (6) K . Eiben and D . Schulte-Frohlinde, 2.Phys. Chem. (Frankfurt am Main), 4 5 , 2 0 (1965). (7) D. Schulte-Frohlinde in "Radiation Research," Third International Congress of Radiation Research, Cortina d'ilmpezzo, JuneJuly, 1966, G . Silini, Ed., North-Holland Publishing Co., Amsterdam, 1967, p 251. (8) H . Barzynski and D. Schulte-Frohlinde, Z. Naturforsch. A , 22, 2131 (1967). (9) V. N. Shubin, V. I. Zhigunov, V. 1. Zolotarevsky, and P. I. D o h , Nature, 212, 1002 (1966), and Dokl. Akad. Nauk S S S R , 174, 416 (1967). (10) G. Nilsson, H . C. Christensen, J. Fenger, P. Pagsberg, and S. 0. Nielsen, Abstracts, Eight International Free Radical Symposium Novosibirsk USSR, 1967. (11) G. Nilsson, H . C. Christensen, J. Fenger, P. Pagsberg, and S.0 . Nielsen, Aduan. Chem. Ser., No. 8 1 , 7 1 (1968). (12) I. A. T a u b and K . Eihen, J . Chem. Phys., 49, 2499 (1968). (13) K. Eiben and I. A. Taub, Natz~re,216,783 (1967). (14) 0. F. Khodzhaev, E. G. Ershov, and A. K . Pikaev. Tz?. Akad. Nauk S S S R , Ser. Khim., 246 (19fiRl
PULSE-IRRADIATED CRYSTALLINE HzO AND DzO ICE
1001 Electron
by Taub and Eiben.IZ I n addition our interpretation of the kinetics gives a reasonable explanation to the rapid decrease in the yield when the temperature decreases.
Thermocouple
Ice s a m p l e
Experimental Section Large blocks of transparent ice were grown in a quartz tube from triply distilled HzO or DzO (DtO 99.98%), which was degassed by shaking and evacuation. The degassing procedure included saturation of the water three times at a pressure of 1 atm with hydrogen or helium gas that had passed a liquid nitrogen cooled trap. The quartz tube, surrounded by a thermally controlled jacket and filled with the gas to 1 atm pressure, was then placed in a refrigerator, main-. tained a t -20", and the currents in the four heating coils of the jacket were adjusted in such a way that the freezing started a t the water surface, proceeded down the tube axis and stopped about 20 mm from the bottom of the tube. Ice blocks, 70 mm long and 30 mm in diameter, were obtained in about 4 days. Examination of the blocks with polarized light showed that they generally consisted of a few large crystals oriented in different directions. The blocks were stored at -20" for several days, and before each run a 30-mm long sample was cut out from the middle section of a block with a hot platinum wire. The sample was heated to - 5 " , and its end faces were made plane with a smooth warm aluminum plate and polished with a filter paper. The sample was then quickly transferred to the cryostat, cooled to the desired temperature, and irradiated. The cryostat loaded with an ice sample is shown in Figure 1. Cooling is achieved by a pulsed stream of liquid nitrogen, and an evacuated window cylinder and an electrically heated wire (not shown) are used to avoid frosting of the front window. Within the temperature range 0 to -190" the temperature was regulated with a precision of d~0.5"as measured by a small thermocouple in direct contact with the sample surface. This simple method of cryostating also proved useful for growing single crystals of organic compounds a t subambient temperatures. The method for temperature regulation is therefore described elsewhere. The optical detection system is fully described elsewhere16 and comprises the following principal optical and electronic components: an Osram XBO, 450-W high-pressure xenon lamp, a Zeiss MM12 double quartz prism monochromator, a dc-coupled EM1 9558 Q photomultiplier tube, and two Tektronix 555 double beam oscilloscopes equipped with Type W plug-in units. Three of the oscilloscope beams were used to record the time profile of the transient in three different time scales. The fourth beam was used to record the time profile of the electron pulse, as monitored by the current induced in a coil surrounding the electron beam.
Resistance thermometer
;
Dispenser
"/1.'i
window c)ilindcr
Suprasil windows
Figure 1. The cryostat and the light passage. The evacuated window cylinder and an electrically heated wire (not shown) are used to avoid frosting of the front window.
The oscilloscope traces were recorded on Type 3000 Polaroid film, enlarged four times and analyzed. The overall time constant of the detection channel was 80 nsec. The cryostat was irradiated from the mirror end with single electron pulses from a Varian V-7700 linear accelerator equipped with a single-pulse trigger generator.17 The peak current of the pulse was 250 mA, and the average energy was 11 MeV. The pulse length could be varied from 0.25 to 4 psec. The absolute dose was obtained from the recorded absorption trace of the hexacyanoferrate(II1) ions nm = 1000 M-' cm-l)I8 produced by the electron pulse in a polystyrene cell, replacing the ice sample, which was filled with NzO-saturated, 1 mM hexacyanoferrate(I1) solution. The cell had thin polystyrene windows and the same length as the ice sample. For relative dosimetry we utilized the time profiles of the electron pulses as recorded on the oscilloscope screen. The mean dose rate in the half of the sample facing the evacuated window cylinder was 20% less than the mean dose rate in the rear half. This inhomogeneity in dose rate along the sample axis was accounted for. Results and Discussion 1. Spectra. The transient optical absorption spectrum of crystalline ice recorded after a single electron pulse is very similar, both in shape and position, to that caused by the solvated electron in liquid water. On this basis the absorption has been assigned to a localized excess electron eS-, in structure almost identical with (15) T. Dahlgren, T. Gillbro, G. Nilsson, and A. Lund, J . Sci. Instrum., 4, 61 (1971). (16) H. C. Christensen, G. Nilsson, P. Pagsberg, and 8. 0. Nielsen, Rev. Sei. Instrum., 40, 786 (1969). (17) J. Fenger, Nucl. Instrum. Methods, 74,95 (1969). (18) J. Rabani and M. 8. Matheson, J . Phus. Chem., 70,761 (1966).
The Journal of Physical Chemistry, Vol. 76, No. 7 , 1078
1002
G. NILSSON,H. CHRISTENSEN, P. PAGSBERG, AND S. 0. NIELSEN 21 1.8
1
I
I
1
I
-
I
* 1.6E! X
7
5
-I
1.4-
c
I
1.2-
C
c
0,
.-
'6
1
-
.I-
5 OBc 0
I-
C
.-E" 0.6C
X
w
04 a2
1
3
2
4
Energy ( c V ) 300 600
500 600 700 800 WAVELENGTH (nm)
900
Figure 2. Absorption spectra of the electron in HzO ice ( B , extinction coefficient). The temperatures are given in order of decreasing peak heights; dose 6 X 10" eV/g; pulse length 2.6 psec.
eaq-. Q-12, The spectra of the excess electron in HzO ice recorded a t different temperatures in the range -5 to -135" are shown in Figure 2 . All the spectra are broad bands with a single maximum and no structure. The peak height decreases with decreasing temperature, and the shapes of the spectra in general agree with those reported by Shubin, et ~ l . and , ~ by Taub and Eiben. l 2 For electrons in the hydroxide-water glasses a t 77°K the extinction coefficient, e, is known, since the electron concentration can be measured by esr spectroscopy. A "best" value of 2.0 X lo4 M-l cm-' for emax, differing very little from the value for water (Emax =- 1.85 X lo4 M-l cm-1),20has been reported.21 For the transient electron in crystalline ice, on the other hand, the extinction coefficient is unknown. Its value can, however, be estimated if me presuppose a temperature-independent oscillator strength, f. The value of emax as calculated from the optical density is then found to increase very slowly with decreasing temperature below -50", and by taking f = 0.33 the curve could be extrapolated to emax = 2.0 X lo4 M-' cm-1 a t 77°K. We thus obtain a value of the oscillator strength for the electron in H,O ice which is smaller The Journal of Physical Chemistry, Vol. 76, N o . 7 , 1972
Figure 3. Absorption spect'ra of the electron in HzO ice and the absorption spectrum of the solvated electron in liquid water.22 The spectra for the electron in ice are calculated using an oscillator strength of 0.33.
than that for the electron in water (f = 0.71).22 This is in agreement with the calculations of Fueki, et aLZa It may also be pointed out that the treatmcnt o f f as temperature independent (as it is for F centers in alkali halides) is a reasonable approximation supported by the form of the theoretical expression for this quan-
tit^.^^ By taking f = 0.33 thc spectra shown in Figure 3 are obtained, which are thus based on a temperatureindependent oscillator strength, and the value of Emax for alkaline ice at 77°K. From this figure, which also shows the eaq- spectrum,22we conclude that the electron absorbs in the same spectral range in ice and water. The absorption peak is, however, shifted to a higher energy in ice and thc half-width of the peak is smaller in the solid phase. All bands shown have a comparatively sharp low energy edge and a tail on the high energy side. (19) B. G. Ershov and A. K . Pikaev, Advan. Chem. Ser., No. 81, 1 (19G8). (20) E. M. Fielden and E. J. Hart, Radiat. Res., 32, 564 (1967). (21) H . Hase and L. Kevan, J . Chem. Phys., 54,908 (1971). (22) E. J. Hart and M. Anbar, "The Hydrated Electron," WileyInterscience, New York, N . Y., 1970, p 40. (23) K. Fueki, D.-F. Peng, and L. Kevan, J. Phys. Chem., 74, 1976 (1970). (24) J. Jortner, S. A. Rice, and E. G. Wilson, "Solutions Metalammoniac. Colleque Weyl, Lille (1963)," G. Lepoutre and M. J. Sienko, Ed., W. A. Benjamin, New Yorlr, N. Y., 1964, p 245.
PULSE-IRRADIATED CRYSTALLINE HzO AND DzO ICE
:!
*
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1
1
1
I
'
I
0.
\.-.-$ o b
3b0
--
-y.....a&-+&
_ . A .
A0
*
40
600 WAVELENOTH I n m )
700
1
800
1
900
Figure 4. Absorption spectrum of the electron (broad peaks), the OD radical (276 nm) and the DO2 radical (250 nm) a t -6" ( x ) and a t -101' ( 0 ) ; dose, 1.2 X lo1*eV/g; pulse length 3.0 psec.
Raising the temperature broadens the band in ice (the half-width increases from 0.52 eV at -50" to 0.70 eV a t -5") and shifts A, to longer wavelengths, which are properties common to the F center, e,,-, and eNHa-. The absorption of irradiated DzO ice is very similar to that of HzO ice. Overall spectra obtained a t -6 and - 101" are shown in Figure 4. The broad absorption band is due to the electron. The absorption bclow 300 nm very much resembles that found by Taub and Eiben a t -98 and - 14" in HzO ice.1z In accordance with these authors assignment of the 280-nm peak to the OH radical and the 230-nm peak to the HOz radical, we assign the 275-nm peak to the OD radical and the 250-nm peak to the DO2 radical. Figure 5 shows the absorption spectrum of the electron in DzO ice a t different temperatures from -6 to -190". As fieen from the figure the peak height at first decreases with decreasing temperature as in HZO ice. Below -.loo", however, it increases again (see also Figure 8). I11 the same way as in HzO ice the band maximum is shifted to higher energy with decreasing temperature. At the same temperature, however, the absorption maximum is at a shorter wavelength in DzO ice than in HBO ice, which is clearly shown in Figure 6 for -5", where the energy difference between the maxima is 0.04 eV. All points in the spectra shown refer to minimum light transmission as read from the oscilloscope traces and the values of EG are corrected for the decay during the electron pulse. No displacement of the absorption peak during the lifetime of the transient was detected in the temperature range -10 to -40'; ;.e., the full trapping energy was developed within 0.25 psec after the electron pulse. 2 . T h e Temperature Shift of the Absorption Peak. The shift of the band maximum with temperature is shown in Figure 7. To avoid arbitrariness in the determination of Amax, the midpoints of a number of hori-
m
e
L! 0 w
I
i
WAVELENGTH (nm 1 Figure 5. Absorption spectra of the electron in DzO ice (e, extinction coefficient). The temperatures are given in order of decreasing peak heights; dose (1.1-1.2) X 10l8eV/g; pulse length 2.7 to 3.0 psec.
zontal lines cutting an absorption curve were connected, and the intersection between this line and the curve was taken as the maximum point. The errors indicated are estimated ones. The figure includes results of Gottschall and Hart for ea,- in liquid HZOlz5 of Brown, et aLlZ6and Schindewolf, et u Z . , ~ ~for ea,- in liquid DzO, and of Taub and Eiben for the trapped electron in HzO ice.lZ As seen from the figure, the shift of the band maximum is nowhere a linear function of the temperature, as it is for e,,- in liquid water25 and as it was reported by Taub and Eiben to be also for ice.12 Instead the energy reaches a constant value at about - 130". The positions of the band maxima a t the plateau are 1.90 eV for HzO ice and 1.94 eV for D.0 ice. At -10" the values are 1.82 f 0.01 and 1.86 f 0.01 eV, respectively. The energy difference is 0.04 eV throughout the whole temperature range, which can be compared with a differerice of 0.05 eV for the liquids2' and 0.12 eV for the 10 M alkaline-water glasses at 77°K.6 At 0" the curve for the liquid phase fits smoothly to the curve (25) W. C. Gottschall and E. J. Hart, J . Phys. Chem., 71, 2102 (1967). (26) D. M. Brown, F. S. Dainton, J. P. Keene, and D. C. Walker, Proc. Chem. Soc., 266 (1964). (27) U. Schindewolf and R. Olinger, Der. Runsenoes. Phvs. Chem., 72, 1066 (1968).
The Journal of Physical Chemistry, Vol. 76, No. 7, 1078
1004
G. NILSSON,H. CHRISTENSEN, P. PAGSBERG, AND S. 0. NIELSEN
I
I
I
I
1
I
I
I
I
I
3 TEMPERATURE (OC)
Figure 8. Temperature variation of the product of the extinction coefficient and G for the electron in HzO ( X ) and DzO ice (a).
0'
1
200
300
I
I
I
I
400 500 600 700 WAVELENGTH (nm1
I
I
800 900
Figure 6. Absorption spectra of the electron in HzO ( 0 )and DaO ice ( X ) at -5'; dose 6 X 10'7 eV/g; pulse length 2.7 psec.
22.0 I
19-
+
'
1
t
-
IS5 2 I?-
15
-
-200 I4 -200
\ -150 -150
-100 -100
-50 -50
0 0
r50 r50
. .1 1w w
temperatures. A similar increase would probably have been found also for H2O ice if the measurements had been extended to still lower temperatures. This is corroborated by the fact that below -100" the decay becomes faster in HzO ice. The slightly higher value of emaxGfor D20 ice is in accordance with the behavior of eaq- in the liquid phases where there is an increase in both G (11%)and emsx (28%) when passing from light to heavy water.29 On the basis of emax from Figure 3 the temperature variation of G for HzOice has been calculated. The results are shown in Table I. The large variation of G with temperature is remarkable since the yield in liquid water is almost independent of the temperature between +4 and +90°.26
TEMPERATURE I'C)
Figure 7. Temperature shift of the maximum of the absorption band of the electron in 1 1 2 0 (%) and DzO ice (f). The figure includes results of Gottschall and Hart for eaq- in liquid HZOa5 (@), of Brown, et aZ.,Z6(+) and of Schindewolt, et aZ.,27 H )(I for eaq- in liquid DzO and of Taub and Eiben12 (+) for electrons in HzO ice.
Table I: The Temperature Variation of G for Localized Excess Electrons in HzO Ice
Temp, "C
-5
for the solid phase. The absence of a discontinuity a t that point is a strong indication that the trapping mechanism is essentially the same in the two phases. It may also be pointed out that the temperature shift of ,A, as pictured in Figure 7 resembles very much the temperature shift of the optical absorption band of the F center. For both, A, reaches a limiting value a t low temperatures.28 3. Yield and Kinetics. The product cmaxG(Figure 8) decreases with decreasing temperature, at first very fast, between -50 and - 100" considerably slower, and finally it increases in D20 ice at temperatures below -100". The increase is due to a population of shortlived electrons which are not observed at the higher The Journal of Physical Chemistry, Vol. 76, No. 7, 1979
- 10 - 31
- 50 - 96 - 135 a
rmaxG, M -1 om-' e8-/100 eV ( X 109
12.21 8.66 2.40 0.91 0.30 0.11
emsx, M - 1 em-'
(x
104)
1.29 1.53 1.83 1.90 1.94" 1.96"
G, ee-/lOO eV
0.95 0.57 0.13 0.05 0.02 0.01
Extrapolated values.
The kinetic behavior of the electron in the temperature range -5 to - 50" is characterized by increasing half-life with decreasing temperature (about ten times) and a longer half-life in DzO ice than in H20 ice (about (28) J. J. Markham, Solid State Phys., Suppl. 8, 1 (1966). (29) E. M. Fielden and E. J. Hart, Radiat. Res., 33,426 (1968).
PULSE-IRRADLATED CRYSTALLINE H20AND D20 ICE two times). The kinetics can neither be fitted to a first- nor to a simpler second-order plot, and it is not a composite first-order decay, for if the optical density is divided by the dose the resulting curves are not superimposable. The decay and the yield are furthermore not affected by the total dme given to the sample. Most samples were therefore pulsed repeatedly. At present there is no general agreement regarding the reaction order of the electron in crystalline ice. Taub and Ei;benl2 characterized the kinetics as predominantly simple second order above - 14"and as first order in the temperature range -40 to -70". To account for the change in rate law with temperature they postulate an equilibrium between immobile and mobile electrons and a reaction scheme where the immobile electrons react with mobile protons, giving rise to a second-order decay, and the mobile electrons react with OH and other radiation-produced species and give rise to a pseudo-first-order decay. Since there is a temperature-dependent competition between the two decay modes, the overall decay changes from mainly second order a t - 14" to mainly first order a t -70". On the other hand, Pernikova, et U Z . , ~ O studied the initial decay rate of the electron after the pulse and concluded that the kinetics could be accounted for by the parallel occurrence of a true monomolecular process and a sum of bimolecular reactions. To gain some information about the kinetics from our data, different kinetic models were fitted to sets of decay curves obtained by varying the pulse length only. The fits were performed by a computer, and each experimental point (optical density) was weighted according to the relative standard deviation assigned to it. The dose inhomogeneity along the sample axis was taken into account by dividing the sample into two halves having a dose rate ratio of 1 :0.8. The dose rate was also corrected, as it changed a little with the pulse length. The goodness of the fit as given by the standard deviation i [L'A2/(n- s)]~", where A is the difference between the experimental and computed values for each point, n the number of points, and s the number of parameters, was then used to select the best model for the electron decay. Examination of our data showed that two decay curves for the electron have the characteristic feature of crossing each other if the decays start a t different electron concentrations at the end of the electron pulse. This behavior is most simply explained by general second-order kinetics, the electron reacting with a species present in a concentration greater than its own. The reaction partner is very probably the proton and one plausible explanation to the electron deficiency may be that some electrons form dimers as they do in the hydroxide-water glassess1 and probably also in ~ a t e r . ~ This ~ - ~would ~ show up as an electron deficiency since the dimers, absorbing in the near-infrared part of the s;pectrum, are not defected by us. How-
1005
a31
0.25 d
5 0.20 'f 'z1
1. I
d
. 0
0
I :
I
I
5
10
b
I
I
15 Time ( p s )
x
B
I
1
20
25
Figure 9. The fit of experimental data to the general second-order decay at - 10' for excess electrons in HzO ice: unfilled symbols, experimental points ; filled symbols, computed points. Pulse lengths: 1.79 (A+); 0.77 (0,0 ) ;0.68 ( 0 , m) and 0.42 psec (V, V).
ever, when testing a model with dimers, it was found that the rate constant for the reaction of the dimers with protons was almost zero, and the standard deviation of the rate constant was very large. The precision of the fit would thus not be less if the rate constant for the dimer reaction was put equal to zero. This was indeed the fact and the remaining equations de,-/dt = PG,,- - ke,-H+
(1)
dH+/dt = PGH+- kea-H+
(2)
where P is the dose rate, G the yield, k (p sec-') the rate constant, and e,- and H+ are the optical densities of, respectively, localized electrons and protons, describe the kinetics very well, as can be seen from Figure 9. The computed curves cross each other and G,,-/GH+, which is
