COMMUNICATIONS TO THE EDITOR
1175 Subnanosecond Observations of the Solvated Electron
Table I: Ammonia Synthesis from NI and Hs over the FePc-Na Complex Filma N H I formed in
N2,cm
H2,cm
T, OC
10 10 10 10
30 30
110
30
30 30 10
20 hr, m l
STP
0.26 0.92 3.64 4.60
170 240 260 240
1.80
5 The complex film was heated at 200” for 20 hr prior to use and a mixture gas was circulated with a constant rate of 12.2 ml/min.
perature range between 25 and 240”. Hydrogen was also adsorbed to a considerable extent over these complex films. A mixture of Nz and Hz (the total pressure was less than 60 cm) with various relative molar ratio was circulated over the Fe phthalocyaninesodium film in the temperature range between 25 and 260’ for 20 hr. The volume of the closed circulating system was about 140 ml and ammonia produced was collected as in the case of graphite-potassium complex. An appreciable amount of ammonia was obtained at higher temperatures above 110’ as shown in Table I. The catalytic activity of the complex film for the ammonia formation did not change appreciably in more than ten runs. No ammonia and other products were obtained when hydrogen was admitted to the Fe phthalocyanine complex film a t above 280’ in the absence of nitrogen gas. Ammonia was also formed to some extent above 170’ over the Mo, Ti, and Co phthalocyanine eda complex films with sodium or potassium in the Hz-NZ system. A negligible amount of ammonia, on the other hand, was formed over the Cu, Pt, Ni, Zn, and metal-free phthalocyanine eda complexes with sodium when a mixture of Nz and Hz was introduced onto these complex films above 200°, even though they are markedly active for the hydrogen activation reactions. A considerable amount of ammonia was sorbed and was decomposed into a mixture of NZand Hz over Fe phthalocyanine eda complex film above 120’. The activity of the complex films was decreased considerably in the presence of oxygen, but it was almost completely restored by evacuation and hydrogen treatment a t 100’ for a long time.8
Sir: The minimum time resolution obtained with conventional pulse-radiolysis systems has been about 2 X 10-0 sec ( 2 nsec) Recently a new pulse-radiolysis system was described with a theoretical time resolution of about 20 X 10-l2 sec (20 psec).2 The growth and decay of the solvated electron in acidic aqueous solutions (eaq-) have now been detected with this new system. The “stroboscopic” pulse-radiolysis technique2 utilizes, as analyzing light, the Cerenkov light flashes produced in air by the fine-structure pulses of a 48MeV linear accelerator (linac) operating a t a frequency of 2.86 X lo9 Hz, A train of 100 analyzing light flashes is produced by the 100 equispaced fine-structure pulses contained in a single 35-nsec linac output pulse. A simple optical delay varies the phasing between the fine-structure electron pulses and the analyzing light flashes. These light flashes are thus synchronized t o “stop the action” of short-lived absorbing species in a manner similar to that of a stroboscope. Slow alterations of the optical delay allow the growth and decay of a short-lived absorbing species to be followed in the time interval between fine-structure electron pulses (0.35 nsec) , using a conventional, [‘slow” photomultiplier detection system. The theoretical resolving time of this technique was estimated to be about 20 psec.2 This limitation was due to the different velocities in the sample of the analyzing light flashes and the fine-structure electron pulses and also to the estimated width of the fine-structure electron pulses. Some modifications have been made to the stroboscopic pulse-radiolysis system originally describedS2 A conical (axicon) lens was incorporated in the optical
(3) It has been recently found in our laboratory that ammonia was formed siowly at room temperature from a mixture of Nt (10 cm) and HI (45 cm) over the iron phthalocyanine-Na eda complex when the complex was deposited over the activated carbon to have a large surface area.
DEPARTMENT OF CHEMISTRY THEUNIVERSITY OF TOKYO HONGO, BUNKYO-KU, TOKYO, JAPAN
Figure 1. The formation and decay of e., in acidic aqueous solutions as observed by the stroboscopic technique. The MIZUOSUDO periodicity in the patterns corresponds to a change of 10.5 om in MASARUICHIKAWA the optical path of the analyzing light. ThiA distance is equivalent MITSUYUKI SOMA to the 350-psec spacing of the fine-structure pulses.
TAKAHARU ONISHI KENZITAMARU
RECBIVED NOVEMBER 7,1968
(1) J. W. Hunt and J. K . Thomas, Radiation Res., 32, 149 (1967). (2) M . J. Bronskill and J. W. Hunt, J. P h y s . Chern., 72, 3762 (1968). Volume 79,Number 4
April 1969
COMMUNICATIONS TO THE EDITOR
1176 system to improve focusing of the Cerenkov analyzing light. A signal-averaging computer (Fabritek Model 1062) was used to improve the signal-to-noise ratio of the absorption signal. The most significant improvements, however, were made to the Toronto linac, from which both higher currents and higher energies can now be obtained. The observations reported in this note were made with a 43-meV, 1.5-A beam pulse, 35 nsec in length, focused to a mean diameter of about 5 mm a t the sample position. The absorbed dose per finestructure pulse was measured to be about 170 rads, which produces an absorption signal of 2% (at 575 nm) for
this absorption corresponds to the_ time interval between fine-structure electron pulses, ie., to 350 psec. The apparent lifetime of this absorbing species could be altered by changing the concentration of perchloric acid to 1.5 M as shown in Figure l b . The decays appear to be exponential; the apparent rate constants are compared in Table I to the values obtained by conventional pulse-radiolysis techniques. These rate constants are calculated from the slopes of the straight lines obtained by plotting log (absorbance) against time.
Table I
Concn of HCIOa. Ai
0.75 1.5
Av lifetime (rs7).psec
kea,-
+ HsO+. M-1 sec-1
61 =k 10 29 =k 5
( 2 . 2 =k 0.4) X 1Olo (2.3 =k 0.4) X 10’”
Published vnlues
(2.06 & 0.08) X 1Ol0 a (2.26 =t0.21) X 10IOQ (2.36 0.24) X lolob
L. 11. Dorfman a n d I. A. Taub, J . Amer. Chem. Xoc., 85, 1375 (1963). *S. Gordon, E. J. Hart, Pvl. S. Matheson, J. Rabani, and J. K. Thomas, ibid., 85, 2370 (1963). 0
2.0
$ 9,
$
1.0
4
4
0
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I
I
observed rise-time (10-90%) for the formation of the absorption signal observed in this system is less than 35 psec. Since the minimum resolving time for a 2 em cell length is 20 pseq2 it can be shown that the fine-structure electron pulse is less than 20 psec long.4 This length is very close to the theroretically predicted values. The formation of eaq- is not observed and probably occurs in times shorter t,han the resolving time of the system. No evidence for nonhomogeneous short-lived “spur” type reactions has been observed to date.
Acknowledgments. We are indebted to Professor K. G. McNeill, Mr. E. W. Horrigan, and the operating team of the University of Toronto Linear Accelerator for their efforts in improving the linac beam. We also wish to thank Mr. Vince Terpstra of Ahearn and Soper, Toronto, for the loan of a demonstration Fabritek instrument computer. This work was supported by Grant No. A 4525 of the National Research Council of Canada and by the National Cancer Institute of Canada. M. J. B. is indebted to the National Cancer Institute of Canada for a Research Fellowship and R. K. W. is indebted to the Medical Research Council of Canada for a Studentship. (3) J. P . Keene, Radiation Res., 22, 1 (1964).
system is the convolution of t h e inherent rise time of the 2-cm cell (20 psec) with the identical distributions of the linac flne-structure and 6erenkov light pulses. Assuming a Gaussian distribution for the linac Ane-structure pulse, we calculate t h a t 5 0 % of the high-energy electrons must occur within a burst 15 psec in length. The calculated pulse length would be even shorter if the solvation time for the subexcitation electron cloud, thought t o be 10 to 20 psec, had been considered. (4) The 35-psec rise time of the
Figure 2 shows the absorption spectrum of this species. Observations were limited to the region from 400 t o 650 nm by the optical filters and photomultiplier tube used in the detection system. The known spectrum for e,,-, obtained in microsecond times,3 is shown for comparison, normalized a t 525 nm. We conclude that the observed absorption is due to the solvated electron. The normal bimolecular rate HaOf seems to describe constant for the reaction e,, the kinetics observed st subnanosecond times. The
+
The Journal of Physical Chemistry
DEPARTMENT OF MEDICAL BIOPHYSICS UNIVERSITY OF TORONTO AND ONTARIO CANCER INSTITUTE TORONTO 5, ONTARIO,CANADA RECEIVED JANUARY 7, 1969
M. J. BRONSKILL R. K. WOLFF J. W. HUNT
