J. Phys. Chem. 1981, 85, 2021-2026
fering from its unmodified counterpart by a constant factor, Le., invariant with distance of separation. Acknowledgment. We are most grateful to Professor Bruno H. Zimm, Department of Chemistry, University of
2021
California, San Diego, who called to our attention the work of 0. Sinanoglu, and to the William and Mary Beedle Fellowships Fund (awarded to G.P.U.), the UCLA Academic Senate Research Committee, and the Department of Chemistry for financial support.
Electron Trapping in Glassy Normal Alcohols. A Pulse Radiolysis Study at Temperatures Down to 6 K G. V. Buxton,' J. Kroh,' and G. A. Salmon The Universiry of Leeds, C&rklge Radiation Research Centre, CookrMge Hospital, Leeds LS 16 606, United Kingdom (Received: November 10, 1980: In Final Form: February 18, 1981)
End-of-pulse spectra of the trapped electron (e,) have been measured in glassy methanol + 5 mol % water, ethanol, 1-propanol,1-propanol + 5 mol % water, 1-butanol, and 1-pentanol in the temperature range 6-115 K. The spectra depend on the temperature and the alcohol, and it is inferred from the kinetic behavior that they represent the initial distributions of e;. The results are explained in terms of a multitrap (small-polaron) model according to which electrons are trapped by OH groups and by at least two other, shallower traps associated with the alkane moiety. It is concluded that trapping by the shallow traps is more efficient at low temperatures and trapping by the OH groups is more efficient at high temperatures. Spectral changes with time are ascribed to migration of e; from shallow to deep traps.
Introduction The mechanism by which excess electrons, generated by ionizing radiation or photoionization,are trapped in liquids and amorphous and crystalline solids continues to attract attention from experimentalists and theoreticians alike.2 On the experimental front effort is being increasingly directed toward obtaining information about the optical absorption spectrum of the trapped electron, e t , in the very early stages of its existence, since this is most relevant to the actual trapping process. The most recent techniques employed include pulse radiolysis at cryogenic temperatures down to 6 KS5 with submicrosecond time resolution, both pulse radiolysis and flash photolysis at ambient temperature with subnanosecond6and even subpi~osecond~ resolution, and steady-state radiolysis at 4.2* and 1.6 Ka9 An important conclusion arising from such work is that the initial trapping process results in the formation of more than one kind of trapped electron, e.g., in crystalline ice,1° certain aqueous glasses.11J2and liquid6and glassf alcohols. (1)Senior Visiting Fellow from Sept 1978 to March 1979. On leave from Institute of Applied Radiation Chemistry, Technical University of Lodz, Lodz, Poland. (2)see, for example, J. Phys. Chem., Colloq. Weyl, 5th, 84 (1980). (3)(a) G. V. Buxton, K. Kawabata, and G. A. Salmon, Chern. Phys. Lett., 60,48(1978);(b) K. Kawabata, G. V. Buxton, and G. A. Salmon, ibid., 64,487(1979);(c) G.V. Buxton, J. Kroh, and G. A. Salmon, ibid., 68,554(1979);(d) G. V. Buxton and G. A. Salmon, ibid., 73,304(1980). (4)N. V. Klassen and G. G. Teather, J. Phys. Chem., 83,326 (1979). (5)J. W. van Leeuven, L. H. Straver, and H. Nauta, J.Phys. Chem., 83,3008 (1979). (6)(a) G. A. Kenney-Wallace and C. D. Jonah, Chem. Phys. Lett., 39, 596 (1976); (b) ibid., 47,362 (1977). (7)J. M.Wiesenfeld, and E. P. Ippen, Chem. Phys. Lett., 73,47(1980). (8)M. Ogasawara, K.Shimizu, K. Ycahida, J. Kroh, and H. Yoshida, Chem. Phys. Lett., 64,43 (1979). (9)G. Dolivo and L. Kevan, J. Chem. Phys., 70,2599 (1979). (IO) G.V. Buxton, H. A. Gillis, and N. V. Klassen, Can. J.Chem., 55, 2385 (1977). (11)G. V. Burton, H.A. Gillis, and N. V. Klaasen, Can. J. Chern., 54, 367 (1976). (12)T.Q.Nguyen, D. C. Walker, and H. A. Gillis, J.Chern. Phys., 69, 1038 (1978). 0022-3854/81/2085-2021$01.25/0
Subsequent time-dependent spectral shifts have been variously attributed to trap deepening through dipole relaxation9J3or migration from shallower to deeper t r a p ~ . ' ~ J ~ For alcohols it has been suggested16that there are two kinds of trapping sites, one associated with the hydroxyl group and the other with the alkyl group. Alcohols, therefore, are appealing systems in which to investigate electron trapping since the size of the alkyl group can be varied systematically and hence the relative importance of the OH and alkyl moieties in the trapping process determined. In addition, subsequent interconversionbetween different forms of trapped electrons can be followed. In this paper we present results of a pulse radiolysis study of the normal aliphatic alcohols from methanol to pentanol in the temperature range 6-115 K. These show that the initial trapping is dependent on the alcohol and the temperature. The results for 1-propanol glass, which were presented in a preliminary report,3Eshowed that the end-of-pulse spectrum at 6 K is totally different from previously reported spectra obtained by pulse radiolysis at 77 K or steady-state radiolysis a t liquid-helium temperatures.
Experimental Section Methanol and ethanol (BDH Aristar) and 1-propanol (BDH Analar) were used as received. 1-Butanol (BDH Laboratory Reagent) and 1-pentanol (Hopkin and Williams Analar) were refluxed over sodium borohydride (10 g L-l) for 24 h and fractionally distilled, and the middle fraction comprising 75% of the total was collected. Glassy samples were made by injecting argon-purged alcohol into (13)L.Kevan, J. Chem. Phys., 56,838 (1972). (14)J. H.Baxendale and P. H. G. Sharpe, Chem. Phys. Lett., 39,401 (1976). (15)R. L. Bush and K. Funabashi, J. Chem. SOC.,Faraday Trans. 2, 73,174 (1977). (16)T . Shida, S. Iwata, and T. Watanabe, J. Phys. Chern., 76,3683 (1972).
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 14, 1981
a hollow rectangular stainless-steel block which had been cooled by immersion in liquid nitrogen. After the filled block was recooled to 77 K, three demountable sides were knocked off and the sample was rapidly transferred to a CF 500 cryostat (Oxford Instruments) at 90-100 K. The dimensions of the samples were generally 5 X 7 X 24 mm, the optical path being 5 mm and the radiation path 7 mm, but in some cases the optical path was 7.5 or 10 mm. The sample itself was in direct contact with the heat-transfer gas, helium, in the cryostat on three sides including those exposed to the radiation and analyzing light beams. The temperature of the sample was measured with an Au + 0.03% Fe-chrome1 thermocouple with the reference junction immersed in liquid Nz. The thermocouple was calibrated in liquid N2 and liquid He and was assumed to give a linear response at intermediate temperatures. The experimental temperature range was restricted to 2 6 K because, when the cryostat contained liquid He in the light path, the radiation pulse induced erratic changes in light transmission by the liquid. The electron pulse, from a 3-MeV Van de Graaff accelerator, was 0.6 ps long and provided a dose of -6 krd in the samples. Dosimetry was carried out by using the absorption at 650 nm due to trapped electrons in 12 M LiC1/DzO glass at 77 K for which Gt = 2.92 X lo4 (ref 17 and 18) and the density is 1.32 g mL-l." The dose absorbed in the alcohol glasses was calculated on the assumption that the absorbed dose per unit volume is proportional to the electron density of the medium. The densities of the alcohol glasses at 77 K were measured dilatometrically and found to be 0.97 g mL-' for ethanol and 1-propanol and 0.96 mL-' for methanol + 5% HzO, 1-butanol, and 1-pentanol. The analyzing light source was a 150-W Xe arc lamp providing flashes of 5-ms duration and was filtered to exclude wavelengths below 340 nm to prevent yellow coloration of the samples which seems to arise from photolysis of radiation products. The photodetectors used were an EM1 9781 photomultiplier for X C 600 nm, an EG & G SHS-100 silicon photodiode for 600 < X C 1100 nm, and a Barnes Engineering A-100 In-As photodiode for X 2 1100 nm. Signals from the photodetectors were fed via a Tektronix R 7912 transient digitizer to a Data General Nova 2/10 computer for storage and processing. Tests showed that the glassy alcohol samples could be pulse irradiated repeatedly without any apparent effect on the spectral shape, yield, or rate of decay of e;, and results were reproducible from one sample to another. Between successive electron pulses the trapped electrons either decayed spontaneously or were bleached away by the analyzing light. No photobleaching was detected up to 100 ps after the pulse, the longest time at which measurements were made. At 6 K a transient radiolytic species is generated in the He gas which absorbs at 1100 nm,19 consequently absorption measurements at this wavelength have been excluded from the spectra at 6 K reported below.
Results In all of the glasses strong absorption bands in the solvents made measurements of the spectra of e; impos(17)H. A. Gillis, G. G. Teather, and G. V. Buxton, Can. J.Chem., 56, 1889 (1978). (18) Gc is the product of G, the radiation chemical yield in unita of molecules per 100 eV, and e, the molar decadic extinction coefficient in unita of Mzl cm-'. (19) G. V. Buxton, K. Kawabata, and G. A. Salmon, to be submitted for publication.
Buxton et al.
I
0
\
I
I
800
LOO
I
1200
I
I
I
2000
1600
h/nm
Figure 1. Effect of temperature on the endof-pulse spectra of e< In (- - -) glassy methanol (CH30H 5 % H20)and (-) glassy ethanol. The data shown as 0 and 0 are steady-state data at 1.6 K taken from ref 9 and normalized to the pulse data at 600 nm.
+
n - C3H70H
-
8 -t
s1 2 -
I '. 01
LOO
I
I
800
1
I
1200 h/nm
I
I
1600
1
I
2000
1
Flgure 2. Effect of temperature on the endof-pulse spectra of q-in glassy l-propanol. The data shown as 0 are steady-state data at 4.2 K taken from ref 24.
sible in the regions 1550 I X 5 1800 nm and X > 2100 nm. Under certain conditions a small amount of radiation-induced light emission was observed at wavelengths between 400 and 600 nm. End-of-pulse spectra and subsequent changes of absorption with time were corrected for this effect. The uncertainty in the Gt values is less than *lo%. Figures 1-4 show the effect of temperature on the endof-pulse spectra in methanol + 5 mol % H20, ethanol, 1-propanol, 1-butanol, and 1-pentanol, respectively. Water was added to the methanol system because the neat alcohol does not form a glass. This system was not sufficiently transparent in the infared for measurements to be made
The Journal of Physical Chemistty, Vol. 85, No. 14, 1981 2023
Electron Trapping in Glassy Normal Alcohols I
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2
I
I
CH30D
I
I
I
1
800
I
I
I
1200
I
coo
2000
1600
800
1200 h/nm
h/nm
Flgure 3. Effect of temperature on the end-of-pulse spectra of 81- in glassy 1-butanol. The data shown as 0 are steady-state data at 4.2 K taken from ref 25. I
I
1
I
I
I
I
I
I
5 m o l YO D20
6t
6K
LOO
+
1600
2000
Flgure 5. Effect of temperature on the end-of-pulse spectra of e; in glassy methanol (CH30D 5 mol % D,O).
+
> 1 I
I
n-C3H70H
+
5 mol % H20
n - CSH11OH
L -
I
3 3'
Y
4
0 ,
1
4 0
4
LO0
800
1200 h/nm
2000
1600
Flgure 4. Effect of temperature on the endof-pulse spectra of e; in glassy 1-pentanol. The data shown as 0 are steady-state data at 4.2 K taken from ref 25.
a t X > 1500 nm, so spectra were also measured in the corresponding deuterated system, CH,OD 5 mol % DzO, and are shown in Figure 5. Similar data for these methanol glasses were reported by Perkey and SmalleyZ0while this work was in progress. To assess the possible influence of the water on these spectra, we also made measurements in a glass comprising 1-propanol + 5 mol % HzO, and the spectra are presented in Figure 6. Comparison of these data with those in Figure 2, and also comparison of the relevant data in Table I, show that this small fraction of water has no effect on either the end-of-pulse spectra or the decay of e,. From these results we conclude that data
+
~~~
~
(20) L. M. Perkey and J. F. Smalley, J. Phys. Chem., 83,2959 (1979).
0 LOO
800
1200
1600
2000
h/nm
Figure 8. Effect of temperature on the end-of-pulse spectra of e; in glassy 1-propanol -t 5 mol % H,O.
for methanol + 5 mol % water pertain to electrons trapped in methanol. A t 6 K the end-of-pulse spectra are of three types: (a) a band with, X at 700 nm in methanol; (b) a band with A- at 1500-1600 nm in ethanol; and (c) a band which rises monotonically to the limit of observation (2100 nm) in the three higher alcohols. With increasing temperature the bands in methanol and ethanol both show progressive blue shifts. In the higher alcohols, on the other hand, there fmt develops a broad maximum beyond 1500 nm which shows some blue shift, and then a second band appears in the region 400-1000 nm. At a given temperature, the development of this band is strongest in 1-propanol and weakest in 1-pentanol. At each temperature, the decay of e; was followed a t 500, 1000, 1500, and 2000 nm, except where the signals
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Buxton et al.
The Journal of Physical Chemistty, Vol. 85, No. 14, 1981
1
-,r-ri
TABLE I: Effect of Temperature and Wavelength on the Decay of e+- in Glassy Normal Alcohols4
I
-I
-
T/K
h/nm
500
1000
1500
6 40
CH,OH + 5 mol % H,O 3b 18 0 35
6 37
CH,OD + 5 mol % D,O 4c,d 24d oc, d 41
2000
6 45 76
C,H,OH 12c 12 5d 14d 35d 36d
6 33 51 76 95 115
l-C,H,OH 36 29 22 28 22 32 3 29 4 40 31 69
37 34 36 50 50 83
33 42 44 59 56 78
6 33 50 75 95 115
l-C,H,OH 28 23 30 25 19 24 13 28 31 34 45
25 26 29 32 39 55
19 23 27 36 32 58
6 45 76 115
l-C,H,,OH 14 21 16 30 12 31 23 48
22 19 29 64
1 5e 16e 2Te 40e
6 33 51 76 95
1-C,H,OH 32 26 16 4 0
+
l
o
l
12 lgd 62d
1
. '.
l
\
\
-L.
5 mol % H,O 34 31 33 35 36 40 43 51 41 59
L
0
were too weak or noisy. The data are collected in Table I, and illustrative examples at 6 and 75 K are given in Figure 7 and 8. A t the lower temperatures the rate of change of absorption due to e; after the pulse occurred on a time scale which was long compared with the pulse length, indicating that no significant change in the spectra takes place during the pulse. At the higher temperatures it is more likely that some spectral changes will have occurred during the pulse, but comparison of our data with literature data for ethanolz1and l-propanolZ obtained with shorter pulses indicates that these changes are relatively minor. The data in Table I show systematic trends in the spectral changes with time. As the temperature is raised the decay at 500 nm is decelerated and eventually is replaced by a growth, while at the longer wavelengths the decay is accelerated. In each of the three higher alcohols the whole band decays almost uniformly at the lowest temperatures at similar rates and growth occurs only at relatively high temperatures. This behavior contrasts with that for methanol and ethanol; in methanol scarcely any change in absorption was observed at 500 nm at any tem(21)N. V. Klassen, H. A. Gillis, and G. G. Teather, J. Chem. Phys., 62,2959 (1975). (22)J. H.Baxendale and P. H. G. Sharpe,Int. J. Radiat. Phys. Chem., 8,621 (1976).
80
60
LO t/ps
Figure 7. Decay of e, after a 0.6-ps pulse at 6 K measured at (-) 500 and (- - -) 1500 nm in glassy alcohols: (1) methanol 5 mol % water; (2) ethanol; (3) 1-propanol; (4) l-butanol. The curves have been drawn to pass through the middle of the noise of the original data.
+
34 37 45 57 56
a The numbers are the percentage decay measured over 8 5 ps except where stated. Italicized figures indicate growth. Possible very small growth over 4 ps. 600 nm. Over 4 5 ps. e Over 14 ps, Estimated uncertainty is * 3 in each case.
20
1
I
I
I
I
I
I
T
I
1
1
I LO t /)Is
I
I 60
I
I
1
10 A/A,
0.8
06
0
20
.80
Flgure 8. Decay or growth of e; after a 0.6-pspulse at 75 K measured at (-) 500 and (- - -) 1500 nrn in glassy alcohols: (2) ethanol; (3) 1-propanol: (4) 1-butanol. The curves have been drawn to pass through the middle of the noise of the original data.
perature, while in ethanol growth was observed at this wavelength at 45 K and above.
Discussion Our results at 6 K for CH30H/H20and CH30D/D20 glasses are in agreement with the data of Perkey and Smalley.20 There are no pulse radiolysis data at 6 K for the other alcohols. Comparison of the end-of-pulse spectrum for the trapped electron in ethanol glass at 6 K with steady-state data at 1.6 K9 (Figure 1)shows that, although the position of ,A, is the same, the steady-state spectrum is broader on the high-energy side. Clearly some changes in the initial absorption spectrum must have occurred before the mea-
Electron Trapping in Glassy Normal Alcohols
surements a t 1.6 K could be made. Unfortunately no quantitative optical data are available for comparison. However, for an ethanol glass which was y-irradiated at 4.2 K, the reportedz3value of Ge at 1500 nm is 2 X lo4, as compared with 4.6 X lo4 in Figure 1, whereas at 640 nm the values are 1 X lo4 and 1.36 X lo4, respectively. Thus it seems reasonable to conclude that the major difference between our data and the 1.6 K data is that a population of trapped electrons which absorb in the region of ,A, is not stable in the latter case. This is illustrated in Figure 1, where Dolivo and Kevan'sg data have been normalized to our data at 600 nm. There is no evidence that the electrons which have been lost from the infrared have become more deeply trapped and therefore absorb at shorter wavelengths. The practice of normalizing spectra a t their maxima to demonstrate relaxation of the trapped electron can be misleading. This is because the apparent blue shift so obtained may really be due to selective decay of the longer wavelength part of the spectrum. Analogous, but quantitative, comparisons are made in Figures 2-4 between the end-of-pulse spectra at 6 K and the steady-state spectra at 4.2 K which have been reported very recently for l-propan01,2~and 1-butanol and l-pentanoLZ5 On the basis of their initial shapes and the changes which occur on selective photobleaching and/or thermal annealing to 77 K, the 4.2 K spectra have been interpretedZ4vz5as being due to two distinct kinds of trapped electrons associated with the hydroxyl (visibleabsorbing e;) and alkyl (infrared-absorbing e
