HYDRATED ELECTROK IN FLASH
Dec., 1963
PHOTOLYSIS O F AQCEOUS SOLUTIOXS
2613
FORMATION OF THE HYDRATED ELECTRON IN THE FLASH PHOTOLYSIS OF AQUEOUS SOLUTIONS' BY Max S. MATHESON, W. A . MULAC, AND JOSEPH RABAKI A rgonne Sational Laboratory, Argonne, Illinozs Received M a y 51, 1963 I n flash photolysis experiments with deaerated aqueous solutions of C1-, Br-, I-, OH-, CKS-, or Fe(CPi)04an absorption in the visible Tvas observed. For several ions examined in detail this absorption was found to be a broad band similar to that recently observed in the pulsed radiolysis of aqueous systems and which was attributed to the hydrated electron. Suppression of this band by scavengers for eaQ-confirms its identity. These results, in support of the interpretation of recent work in the steady-state photolysis of I,,-, show that the hydrated electron is an important intermediate in the photochemistry of many aqueous anion solutions.
Experimental
Introduction For a t least a decade the properties2" and the reactions2h,cof the hydrated electron have been discussed. Recently, Hart and Boag3 by the use of spectrographic plates discovered and studied the absorption spectrum of ea,- formed by the action of 1.8 A b . electrons on water and aqueous solutions. They reported a broad band with a maximum at 7000 A. and that this band was diminished by scavengers known to react with eaq-. Keene4 also observed an absorption near 6000 A. under similar conditions and this was suggested by Matheson5as perhaps due to eaa-. Keene,6who used a photocell, also has reported an absolute reaction rate constant for eaa- 02. Further pulsed radiolysis work7in this Laboratory confirmed that the radiation chemical transient species absorbing in the yellow and red is the hydrated electron. The effect of ionic strength on the ferricyanide ion rate constants for the reactions ea,and eaq- HaO+was that predicted by theory. Reaction rates were measured directly by folloowing the change of optical densityowith time a t 5790 A. Similar results obtainecloat 6850 A. indicated the absorptions a t 5790 and 6850 A. belong to the same species. At the high intensities used and in the absence of electron scavenger, it was s u g g e ~ t e dthat ~ . ~ reaction 1 proceeds
+
+
+
eau-
+ eas- +H2 + 2OH-
(1)
with kl e 1 X 1O1OM - l see.-'. Thus, with the spectrum and properties of the solvated electron being well established, we are now able t,o show that eaq- is also formed by the .flashphot*olysisof anions including OH--, C1-, Br-, I-, Fe(CS)e4-, and CSS-. The formation of eaq- by steady ultraviolet irradiation of I.-,C1-, Br-, and OH- has already been ~ u g g e s t e d . ~Our present direct measurement of the optical absorption of eaq- confirms their conclusions. (1) Based on work performed under the auspices of t h e U. 8. Atomic Energy Commission. (2) (a) R. L. Platzman, U. S. Natl. Research Council, Publ. No. 306, 34, 1953; (b) G. Stein, Discussions Faraday Soc.. 12, 227, 289 (1952); (c) E. Hayon and J. Weiss, Proc. U. it-. Intern. Conf. Peaceful Uses At. Energy, and. Geneva, 29, 80 (1958). (3) (a) E. J. Hart and J. W. Boag, J . A m . Chem. SOC.,84, 4090 (1962); (b) J. W.Boag and E. J. Hart, Nature, 197, 45 (1963). (4) Private communication, ref. 5 . ( 5 ) M. S. Matheson, A n n . Rev. P h y s . Chem., is, 77 (l9G2). (6) J. P. Keene, Nature, 197, 47 (1963). (7) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, J . .4m. Chem. Soc., 8 6 , 1375 (1963). (8) L. M. Dorfrnan and I. A Taub, ibid.. E6, 2370(1963). (9) (a) J. Jortner, M.Ottolenghi. a n d G. Stein, J . Phys. Chem., 66, 2029, 2037, 2042 (1962); (Is) M . Ottolenghi, Ph.D. Thesis, The Hebrew Univereity. Israel, 1962; (0) F. S. Dainton and S.A. Sills, iyature, 186, 879 (1960).
The experimental procedure has been described elsewhere.I0 We used an optical cell 25 cm. long and 1 cm. in diameter prepared from high purity fused silica in order to have good transmission of shorter wave lengths. The high purity silica photoflash lamp (6 mm. i.d. X 30 cm. long) n'as operated a t 12 kv. and 7.5 pf. and the spectroflash lamp at 15 kv. and 1 pf. Photographic plates used were Eastman Fodak 103-0 (for the 2500-5500-8. region), 103-F (3500-6500 A . ) , and I-N (5500-7500 8 . ) . The unsensitized plates were developed following the manufacturer's recommendations. In Fig. 7. the oscilloscope trace shows the variation of intensity for both lamps as a function of time. Sensitivity of the two phototube circuits were adjusted to give approximately the same signal height for each lamp. In the lower picture the trace of the spectroflash lamp only is seen. I n the upper is shown the trace for the photoflash lamp with a very minor contribution from the spectroflash lamp. Such tracee were taken with all flashes in order to check the reproducibility. All solutions were prepared using triply distilled water" and Analar grade chemicals. The solutions were deaerated by shaking vigorously with pure argon in a syringeI2for 1 min. and repeating with fresh argon four or five times and then transferring the solution to the cell under argon. The concentration of M. 0 2 was less than 5 X
Results Grossweiner and Mathesonlo found that X2- radical ions (where X represents a halide atom) are formed by the flash photolysis of the halides in water. These transients have a main gbsorption band with peaks a t 3850, 3600, and 3500 A. for 12-, Brz-, and C k , respectively, and a common smaller band near 3000 8. We have now extended this work to the longer wave lengths where eaq- absorbs. In Fig. 2 the absorption 1%' KC1 in spectrum obtained by flashing 3 X neutral 0.2 .?If methanol (added to suppress the back reaction of C1 or Clz- with e.,-) is given for the wave length region 3500-7500 A. With a 10-psec. delay between lamps, a broad absorption band with a maximum a t about 6700 8. is observed. The shape of the absorption band in Fig. 2 was corrected for the variation of y with wave length by calibrating the plates with neutral density filters of known light transmittance and using the appropriate number of spectroflashes through the solutioon (no photolysis flash). For the region 35006500A. (103-Fplate) three oflasheswereneeded,and l5for the region of 5500-7500 A. (1-N plate). The experimental error in D., the optical density of the transient in solution, is about jz0.035 O.D. unit for the 103-F plates, and about k0.075 for the 1-N plates. Thus, the curve in Fig. 2 (and in the following similar figures) should be regarded as semiquantitative only. The (10) L. I. Grossweiner and M. 5. Matheson, J. Phya. Chem., 61, 1089 (1967). (11) E. J. Hart, J . A m . Chem. SOC.,73, 68 (1951). (12) C. B. Senvar and E. .J Hart, PTOC U . N . Intern. Conf Peaceful Uses At. E n e i g y , 2nd Geneva, 29, 19 (1958).
AI. S. ~IATHESON, W. A. MULAC,AXD J. RABANI
2614
phere of NzO caused complete disappearance of the band (Fig. 3e). The N20was distilled under vacuum three times before use and finally distilled into the solution under one atmosphere of iV20pressure. These results show that the hydrated electron, eaq-, is formed under these conditions by the photolysis of the C1anion. Under our experimental conditions, the light is absorbed by the CI- and not by the water or the methanol present.13 The photochemical reaction is
I
INTENSITY
I
Vol. 67
-
TIME
Fig. I.-Variation of light output for photaflaah (upper) and spectroflash (lower) lamps ua. time. Both traces are triggered simultaneously, showing an Il-FC. delay between the lamps. 0 60
C L - -1; CI
+ CHsOH
Clz-
3500
5500
4500
I
d
6500
7500
---f
CH,OII
+ €I+ + CI-
(3a)
or
045
030
(2)
but the 0.2 Af methauol reacts with the Cl atomsshJ4or ClZ-loion radicals CI
0 0 2
+ eaq-
0
A161
Fig. 2.4orrected spectrum of en"- obtained in deaerated, aqueous 3 X IO-: A I KCI 0.2 M CHIOH. The left ordinate is for the 103-F plate (350W6500 A,), the right ordinate for the I-N plnte (5500-7500 A,); IlLpsec. delay.
+
AIAI.
Fig. 3.-Corrected transient spectra from deaerated, aqueona 3 X 10-aM KCI 0.2 M CH.OH: (a) IO-rse,:.delay; (b) 31)wo.delay; (c) R&w~edelay; e. (d) IO-pscc. dclry, with 0.02 atm. air added; (e) IO-wc. delay, 1 xtm. N20.
+
D. values (Fig. 2) calculatcd from the 103-F and 1-N plates were brought into coincidence by using different scales. The difference bctween the I).-valnes in the two platcs, althongli within the limits of the experimental error, may he in part due to changes in eaq- concentration produced in the two experiments. Figure 3a shows the absorption after a delay of 10 psec. In Fig. :3b is Seen the decreased absorption after 30 psec, and after 80 psec. (Fig. 3c) the optical density is zero within experimental error. In Fig. 3d the solution was saturated with a mixture of 1:50 of air-argon, and this caused a partial suppression of the absorption band. On the other hand, saturation with an atmos-
+ CH30H -+ CH,OH + H + + 2C1-
(ah)
so that the absorption bandlo of CL- is not seen. The spectrum after 10-mec. delay is similar to that obtained by Hart and Boag' in the pulsed radiolysis of water. This indicates the identity of the flash photolysis and pulsed radiolysis produced species. Since in pulsed radiolysis it was unequivocally shown3-* that the solvated electron is reponsible for the absorption in the yellow to near-infrared, the similarity supports reaction 2. Moreover, the effect of a scavenger specific for esq-, ie., KzO,ls is consistent with this interpretation. To make sure that this effect is not due to small traces of oxygen which may be present in the N20 the experiments of Fig. 2d were carried out. There it is shown that in the presence of Ozhigher in concentration than expected as an impurity in the KzO experiments, the band in the red still exists. This shows that both Nz0 and Ozreact with the species which has the absorption band in the rcd, consistent with the identification as due to c."-. To check this point, furthcr cxperimcnts with 3 X Af CI- (KCl or HCI) in the absence of methanol were carried out. The results are represented in Fig. 4. Thc spectrum a t 2500-5500 A. was taken with a 103-0 plate (1 flash). No y corrections were made for this plate. The region between 3500 and 6500 A. was investigated with a 103-F plate as before. In Fig. 4c the parts taken with the 103-F and 103-0 plates were brought into coincidence by multiplying the 103-0 results by 0.6. In Fig. 4a for 3 X A4 neutral KCI, the absorption bands of both eaq- and CIZ- are seen. There is probably some overlapping especially in the violet. The addition of 4 X N H$O, to 3 X A4 KCI causes the disappearance of the ea"- band but does not eliminate the Clz- band (Fig. 4b). I n Fig. 4c the effect of hydrogen ion concentration on thc two bands is investigakd undcr conditions where only H+ and CI- are initially present (3 X lo-' M HCI). The results show the CI,- band, consistent with the previous resultsL0(at pH 6 and 0.1 M Cl-). Again no absorption of eay- is seen as expected, since H+ is known to be a scavenger'," for electrons. Thus the addition of H+ (13) I . L. Weeks. 0. M. A. C. Meahurn. and 8. Gordon. Rodidion RZ~., 19,559 (1988). (141 L. M. Thasrd and M.Burton, 3. Pbya. Chem., 6T. 59 (19081. (16) (a) I>. Ilaint"" ant1 11. H. I'elemon. /'roc. no". SO
