Capture of the Trapped Electron in Alcohol Glasses
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Capture of the Trapped Electron in Alcohol Glasses A. Namlki, M. Noda, and T. Higashlmura' Research Reactor Institute. Kyoto University, Kyoto, Japan (Received July 23, 1975) Publication costs assisted by the Research Reactor Institute
This is a paper on the scavenging of the presolvated electron. In many matrices, the secondary electron is stabilized in shallow traps a t first, and then it becomes solvated to deepen the potential well. Because the tunneling rate depends strongly on the height of the potential, scavenging by tunneling from the presolvated state possibly overcomes the tunneling from the solvated state even if the lifetime of the presolvated state is very short. Scavengers (benzene, fluorobenzene, chlorobenzene, bromobenzene, iodobenzene, o -chlorotoluene, o-bromotoluene, and o-dichlorobenzene) were added to alcohols (methanol, ethanol, 1-propanol, and 2-propanol) and were cooled rapidly to make clear glasses. These glasses were irradiated by y rays at 77 K with the dose of about 0.2 Mrad. The yield of the solvated electron was estimated from the optical densit y of the wavelength of the peak of the absorption spectrum. The scavenging efficiency is defined as 1/[S]1/2 where
In order to explain the dependence on the electron affinity, one must introduce a factor F (inefficiency factor called by J. Miller) into a usual equation for the tunneling rate. F is expressed as F = $p(E,)AE,, where 0 is the fractional solid angle within which the trapped electron sees the scavenger molecule, and AE, is the width of the resonant vibrational level of the scavenger. p ( E , ) is the probability density of the nuclear configuration of the scavenger molecule at which the difference of the energy between potential curves of the neutral molecule and that of the anion is equal to E,. E , is the energy which is given to the electron upon tunneling and is expressed as E , = Vo - (V-E) - p - , where VOis the energy of the quasi-free electron in the matrix and pis the polarization energy of the matrix. Taking E, = 0.5 eV and using the Morse funcand AE, = eV, 0 = tion for the molecular potential (following Steelhammer and Wentworth), one can obtain the tunneling radius numerically. Calculated values are shown in Table I.
TABLE I: Data for l/[S], ( M - I ) , Experimental Tunneling Radius, a, and Calculated One, a' (in A ) Solvent MeOH EtOH 10 nsec 2.5 psec 7 0-PhCH,CI PhCl o-PhCH,Br PhBr 0-PhC1,
9.0 9.3 11.1 11.7
12.4
12.1
12.2 12.9 13.2 13.4
6.4 12.4 10.7 13.1 15.5
16.4 21.7 23.0 25.0 31.9
[SI1l2is the concentration at which the yield of the solvated electron becomes half of that in pure matrices. When the scavenging efficiency is plotted against the adiabatic electron affinity of the scavenger molecule (see Table I), we obtain the linear relationships for every matrice. For every scavenger, the efficiencies in the l-propanol glasses are larger than those in the ethanol glass. This suggests that the electron tunneling does not occur from the solvated state but from the unsolvated state, because the absorption spectra of the solvated electrons in both matrices are very similar to each other. The solvation time of the trapped electron in 1-propanol is larger than that in ethanol. Therefore, the former electron has larger chance of tunneling than that of the latter electron. If we assume the same depths of the trapped electron levels in both solvents, the difference between the tunneling distances in both matrices is expressed as
where V-E is the energy level of the trapped electron and 7's are the solvation times. From the experimental values of the scavenging efficiencies, one obtains T(1-PrOH) = 175 psec. Similar calculation gives 10 nsec for the solvation time of methanol.
16.6 18.2 18.5 19.1 20.7
10.2 20.3 28.5 21.0 23.4
34.9 41.1 39.7 46.0 52.3
1-PrOH
14 psec 23.3 24.6 24.3 25.5 26.6
12.7 22.7
21.0 23.4 25.9
Discussion S. RICE. That semilogarithmic electron yields vs. scavenger concentration give straight-line plots merely reflects the fact that the electron reaction is pseudo first order.
G. R. FREEMAN. The semilogarithmic relationship between solvated electron concentration and scavenger concentration could be obtained by several mechanisms. One is the tunneling process that you mentioned. Another is the stochastic model of nonhomogeneous kinetics that is often used for charge scavenging in irradiated systems. What you have called tunneling distances might actually be related to the migration distances of the electrons before becoming localized in deep traps. However, this would not affect the other conclusions in your interesting paper. T. HIGASHIMURA. The semilogarithmic relationship can be obtained in the case of capture of the epithermal electron, as you say. What I wanted to emphasize in my talk is the strong correlation between the scavenging efficiency and the solvation time. N. KLASSEN.Your estimate of 10 nsec for the solvation time in MeOH a t 77 K agrees with our pulse radiolysis experiments in which we see very little ir absorption component to the absorption band a t a time of -20 nsec in a pure MeOH glass. Our pulse radiolysis results with ethanol glasses at 77 K show that the stable spectrum of et- is largely reached only after milliseconds which is much slower than the 2.4 Msec reported by Richards and Thomas. The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
R. F. Marzke and W. S. Glaunsinger
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A. K. PIKAEV. ( I ) Have you measured the scavenging effects at different temperatures? (2) Did you make any attempts to observe the spontaneous decay of et,- after y-irradiation in your systems?
solvated in both matrices? At 4 K the relaxation times in both matrices are so long that they should not enter in.
T. HIGASHIMURA. (1)No, we have not obtained scavenging efficiencies a t different temperatures. In cases of solutions different from those which I mentioned, we obtained larger scavenging efficiencies at 4.2 K than a t 77 K. This phenomenon supports the tunneling mechanism. [J.Phys. Ch.em. 76, 3744 (1972) and Int. J . Racliat. Phys. Chern., 6, 393 (1974)]. (2) No, all data are at 5 min after radiolysis.
T. HIGASHIMURA. Yes. The tunneling distance at 4 K, a d ' , and that a t 77 K, a77', satisfy the equation a4' - 077' = (l/fl-X)In ( ~ 4 / ~ 7 7for ) each solvent. Here, 74 must be taken as the time interval between irradiation and measurement. Taking 5 min for 7 4 and values in the table for 777, a4' becomes 26.3 8, larger than a77' in ethanol and 23.9 8, larger than a77' in 1-PrOH. In this simplification, the scavenging efficiency becomes dependent only on the depth of the trapped state, V-E, for the 4 K experiment. If we take the same depth in both EtOH and 1-PrOH, the scavenging efficiency at 4 K becomes the same in both matrices.
L. KEVAN.Would you predict the same Sl/z values for a solute in EtOH and in 1-PrOH at 4 K where the trapped electron is un-
Proton Magnetic Resonance Study of Metal-Ammonia Compounds R. F. Marrke Department of Physics, Arizona State University, Tempe, Arizona 8528 1
W. S. Glaunsinger' Department of Chemistry, Arizona State University, Tempe, Arizona 8528 1 (Received August 4, 1975)
The proton magnetic resonance (lH NMR) spectra of Ca(NH&, Ba(NH&, and Li(NH& have been recorded in the temperature range 20-150 K using a broad-line, symmetrical-bridge spectrometer. In the hexaammines very narrow first-derivative line widths are observed (
