givcii amount of adsorbed alcohols on the mercury electrode. In more nearly perpendicular orientation, the projected area of normal aliphatic alcohols on the mercury electrode is about the same. Thus, the inhibition effect in this orientation is proportional to the amount of adsorbed alcohols on the mercury and not on the alcohol itself. The deviation of the inhibition effect from regularity in the medium concentration range is explained by some perpendicular orientation and some parallel. With increasing alcohol concentration, it is reasonable that perpendicular orientation predominates; therefore, the rate constants in the presence of the different alcohols become super-
imposable on each other a t a given relative concentration of the alcohol. dcknowledgment. We deeply appreciate support of this work by The Robert A. Welch Foundation of Houston, Texas, and by the Office of Naval Research, Contract NONR375( 15). The authors also express their thanks to Dr. Allen J. Bard and Mr. David Jones for their extensive discussion and reading, and t o many others for their helpful discussion. One of the authors (K. K. N.) also thanks Sumitomo Chemical Company of Japan for the leave of absence which permitted him to undertake this work.
Far-Ultraviolet Spectra of Hydrogen and Hydroxyl Radicals from Pulse Radiolysis of Aqueous Solutions. Direct Measurement of the Rate of H
+ H1
by P. Paysberg, H. Christensen,28J. Rabani,2b G. Ni1sson,2aJ. Fenger, and S. 0. Nielsen Danish Alomic Energy Commission Research Establishment. RisB, Denmark, and AktieboEagel Atornenergi, Sludsuik, Nykuping. Sweden (Received August 8 2 , 1 9 6 8 )
Pulse radiolytic absorption transients have been observed in aqueous solutions between 200 and 300 nm using an 1 1 - M e ~Linac and an optical detection system that allowed accurate measurements (a) down to 200 nm and (b) 0.2 psec after the electron pulse With M HCIOh 0.027 M HZ (p(H2) = 35 atm) transients with second-order decay were observed which had amplitudes that decayed monotonically in the region from 200 to 240 nm. Assigning these transients to free H atoms, the molar decadic absorptivities E at 200, 210, and 240 nm of I3 were found to be 900, 560, and 0 cm-', respectively, and %H+H = (1.55 =!= 0.10) x 1OO ' M-1 sec-l from measurements at 200 and 210 nm. The transients could be completely quenched by addition of O2 resulting in a species with the absorption spectrum of HOz. Furthermore, the transient at 210 nm was M ) was added innot affected when IfClOr was left out of the Hz-saturated solution and E20 ( > 2 X stead. The apparent OH transient in 2 X l W 3 M NzO (no Hz) decayed according to second-order kinetics with a calculated rate constant that after correction for the reaction of H withOH was found to be (1.04 rt 0.10) X 1O'O Jl-l sec-' independent of the wavelength used. The calculated e for OH showed, after correction for the absorbance of € 1 2 0 2 , H., and OH-, one broad absorption maximum near 230 nm with E 530 M-l cm-l and one below 200 nm. The measurements at 200 nm had to be corrected for a substantial contribution from OH- to the observed optical absorption. The calculated values of EH and EOH account quantitatively at all waveiengths used for the initial absorption of the transients in M HClOd (no 132) if it is assumed that H30, if formed, decomposes to yield H f H20 after no longer than 0.2 psec. The light absorption of aqueous solutions of H and to some extent of OH at 200 nm is attributed to a red shift of the water absorption continuum beginning at 186 nm, caused by a partial electron transfer from the first excited singlet state of water to a neighboring H or OH free radical in analogy with the optical transition associated with the @ bands in alkali halide crystals.
+
Introduction Free hydrogen atoms in the gas phase do not absorb light in the ground state a t longer wavelengths than 121.57 nm,3 and it has, in general, been assumed that solutions of free hydrogen atoms in water do not absorb 1igh.t in the far-ultraviolet spectral region (>200 nm). It is the purpose of this report to demonstrate that solutions of free hydrogen atoms in water, produced by pulsed radiolysis in solutions saturated with pressur-
ized Hz, do in fact absorb light appreciably a t 200 nm and to suggest the type of optical transition involved, (1) Presented at the Symposium on Photochemistry and Radiation Chemistry at Natick, Mass., Aprii 22-24, 1968. A preliminary communication has appeared: 9. 0. Nielson, P. Pagsberg, J. Rabani. H. Christensen, and G . Nilsson, Chem. Commun., 1523 (1968). (2) (a) Aktiebolaget Atomenergi, Studsvik, Nykoping, Sweden: (b) Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel. (3) G . Herzberg, "Atomic Spectra and Atomic Structure," 2nd revised ed, Dover Publications, New York. N. Y., 1944, p 25. Volume YS, Number 4 April I960
1030
PAGSBERG, CHRISTENSEN, RABANI,NILSSON,FENGER, AND NIELSEN
Assuming that the optical absorption transients (that are measured with an improved pulse radiolysis technique) are caused by free hydrogen atoms, a direct determination of the rate constant 2kH+H is possible. The value found at room temperature is 2kH+H = (1.55 f 0.10) X 1Olo sec-*. This value should be compared with the values in the literatureke based on indirect methods and ranging from 1.2 X 1O1O to 3.2 X 1O1O sec-1" The absorption spectrum above 200 nm of the OH radical in aqueous solution has been determined by pulse radiolysis of 2 X 10-3 M K20 solutions since it can be compared with the O H spectrum reported by Thomas, et a1.l0 It is shown in the present investigation that the reportedlo apparent difference between the OH spectrum at pH 7 and pH 3 can be understood when the light absorption of H and OH- is taken into account. From the pulse radiolytic absorption transients above 210 nm in 2 X low3M N2O 2kOH+OH can be calculated directly with good accuracy. The value H (1.04 f 0.10) X 1O'O M-l sec-l, found is ~ ~ o K + o = which agrees with the value reported by Thomas, et al.,1° using a similar method. Other values of 2kOH+OH in the l i t e r a t ~ r e ~ are~ ~based ~-~~ on indirect methods and range from 0.8 X 1O1O to 1.26 X 1Olo M-' sec-'.
Experimental Section Pulse Irradiation. Electron pulses 0.7-1.5 psec long with an average electron energy of 10-11 MeV were obtained from the Riser Linac. The 250-mh beam was not monoenergetic, and attention had to be paid to the fact that the average electron energy and the ratio dose/ (average pulse current X pulse width) varied somewhat with pulse width. The front and the back of the pulse had a rise time (corresponding to a change between 10 and 90% of full beam current) of ea. 0.2 psec. Two irradiation cells were used. One consisted of a Suprasil quartz cylinder (length 2.55 cm, 1.5 em i.d.) with 1.25-mm thick, optically flat end windows. The cell was irradiated in air along its axis at a position where the diameter of the electron beam (measured between half-intensity points) had increased to ca. 2 cm. The dose averaged over the cell with a 1-psec pulse was ca. 3 X 1020 eV/1. The analyzing light beam entered the cell approximately parallel to but in opposite direction to the electron beam and was reflected by a 3 X 4 mm aluminum mirror coated directly on the outer surface of the cell window facing the Linac. The optical path length was 2 x 2.55 = 5.1 em. To prevent light which was reflected by the first cell window in the light path without entering the cell from reaching the photomultiplier, the two cell windows were mounted so that they formed an angle of a few degrees with each other. The other irradiation cell (Figure 1) was used for irradiations under an atmosphere of 35 atm of hyThe Journal of PhUsical Chemistry
E
Figure 1. Pressure cell. Valves and manometer are not shown, (A) Teflon gasket; (B) quartz window; (C) O-ring; (D) Pyrex cell; (E) stainless steel pressure containment.
drogen. It consisted of a heavy-walled Pyrex tube (length 7.1 cm, 1.2 cm i d . ) equipped with 6-mm thick optical windows of Suprasil quartz that also served as the pressure tight ends (with O-ring seal) in an outer cylinder made of 1.5-mm thick stainless steel that contained the 35 atm of hydrogen. The Pyrex body could be filled through a hole in the Pyrex tube and a corresponding hole with side arm in the stainless steel body. The cell was irradiated from the side in a position where the electron beam after passing a quadrupole magnetic field had a cross section that approximately fitted the inner Pyrex body of the cell. The dose averaged over the cell with a 1-psec pulse was ca. 1020eV/1. The analyzing light beam traversed the cell perpendicularly to the electron beam with an optical path length of 7.1 cm. The timing, shape, and current intensity of every single pulse was monitored by photographing the oscillogram of the output of a toroidal coil wound around the window of the Linac. It was proved by a series of dosimeter runs that the dose absorbed in the cells could be monitored to better than 37, in a short series of runs by measuring the area under the pulse in the oscillogram. In some of the runs the dose given by a single pulse was also monitored by a transmission Thomas, J. P h y s . Chem., 67, 2593 (1963). H. A. Schwarz, ibid., 67, 2827 (1963). H. Fricke and J. K. Thomas, R a d i a t . Res., S u p p l . , 4 , 35 (1964). H. A. Schwarz, ibid., 4, 89 (1964). J. P. Sweet and J. K. Thomas, J. Phys. Chem., 6 8 , 1363 (1964). J. Rabani and D.Meyerstein, ibid., 72, 1599 (1968). (10) J. K. Thomas, J. Rabani, M. 9. Matheson, E. J. Hart, and S. Gordon, ibid., 70, 2409 (1966). (11) H. A. Schwarz, ibid., 66, 255 (1962). (12) M. Anbar and P. Neta, Int. J . A p p l . R a d i a t . I s o t o p e s , 18, (4) J. K.
(5) (6) (7) (8) (9)
493 (1967). (13) J. Rabani
and M.
S.
Matheson, J. P h y s . Chem., 70, 761 (1966).
FAR-UVSPECTRAOF H
AND
1031
OH
monitor of Taimuty type.14 In most of the runs reported here the timing of the electron pulse relative to an advanced trigger pulse was accurate to better than 0.1 psec, and the dose was reproducible to better than 4%. Optical Detection. The analyzing light used to monitor the pulse radiolytic optical absorption transients was taken from an Osram xenon lamp XBO 450 with Suprasil quartz bulb. The lamp had a continuous emission spectrum down to below 200 nm. The transmission of the optical system and in particular the brightness of the Xe arc at 200 nm was found to be so low that not enough light reached the photomultiplier cathode at this wavelength (overall effective relative aperture f/9.9) to permit accurate measurements of the rather weak optical transients of H and OH. The normal Xe lamp current of 28 A, supplied by a currentregulated power supply (Lambda, Model LE 104), was, therefore, increased to ca. 180 A ca. 300 psec before the Linac was fired. The current increment of ca. 150 A was delivered from a 0.04 F capacitor bank of electrolytic capacitors charged to 35 V that were discharged through a transistorized switch and adjustable series regulator.15 By the time the Linac was fired the brightness of the Xe arc had increased 25-30 times over the normal level and remained either at this level or changed slowly and linearly with time with a deviation of less than 0.5% (see Figure l ) for a t least 300 psec after the electron pulse. The reproducibility of consecutive light pulses fired at more than 5-sec intervals was excellent and allowed a scale expansion of as much as 25 times to be used in the optical detection channel. Pulsing the lamp in this way with 180-A current pulses of 800 psec duration increased the brightness of the Xe arc 25-30 times not only a t 200 nm (where it is most needed) but also at longer wavelengths over most of the ultraviolet spectral region. Between 5000 and 10,000 light pulses have been obtained from one Xe lamp before the reproducibility of the light pulses deteriorated and the cathode showed marked corrosion. V i a a system of three lenses made of Suprasil quartz and six vacuum-evaporated aluminum mirrors with a reflectivity of more than 82% each at 200 nm the light beam from the Xe lamp passed through the irradiation cell and subsequently through a Zeiss MM 12 double quartz prism monochromator to an EM1 9558Q photomultiplier operated with 5 or 6 dynodes.16 The monochromator and photomultiplier were placed behind a 10-cm lead shield. When the lenses were properly set at 200 nm so that light of this wavelength was focused on the entrance slit of the monochromator the level of scattered light at 200 nm uras so low that it could not be detected. At the maximum slit width of 2.0 mm the band width of the monochromator at 200 nm was 0.95 nm. The anode resistance of the photomultiplier was 1 kQ which gave a signal of 0.4 V at the maximum
allowed anode current16 and a response time of the optical detection channel of 80 nsec. The electrical signal went directly to a fast, cascaded, emitter follower via a protective transistor limiter that limited the signal to 0.7 V. The emitter follower drove the signal undistorted through 15 m of 7 5 4 coaxial cable to two Tektronix 555 oscilloscopes with type W plug-in units. To protect the vertical amplifiers in the oscilloscopes at high gain from overloading and subsequent slow recovery due to the strong signal from the Cerenkov radiation, a clamping circuit” with a fast electronic switch was placed in the input to the oscilloscopes. A fraction of 1 psec before the electron pulse started, this clamping circuit was adjusted to switch the inputs of the oscilloscopes over to some arbitrary signal that, however, was visible on the oscilloscope screens. The input of the oscilloscopes was again switched back to the transmission line from the photomultiplier, 0.2 psec after the electron pulse and the Cerenkov radiation had faded away. The clamping signal and the fast response of the system is visible in Figure 2. The overall system was shown to be linear in its response to changing light intensity. Dosimetry. The absorbed dose was measured by filling the cells with M K Z e ( CN) 6 dissolved in triply distilled water saturated with NzO and subsequently shaken less than 5 min before irradiation with 5 ml of air per 100 ml of solution in the syringe. The oxygen in the air acted as an H atom scavenger, preventing their reaction with Fe(CN)&-.13 G(Fe(CN)P) was equated with GOH G, = 2.65 2.6 = 5.25,18 and the molar decadic absorptivity E of Fe(CN)&was taken as 1000 at 420nm.13 Dosimeter runs calibrated the electron pulse monitor so that the dose for every electron pulse could be calculated from the monitor reading. A Jena WG 6 light filter that cuts out wavelengths shorter than 300 nm minimized the photochemical oxidation of Fe( CN) 64-. NzO was purified as in ref 13. The variation of dose with the distance from the accelerator in the 2.55-cm two-pass cell was measured with what amounted essentially to a cell with five 5-mm long compartments that could be filled individually one after the other with dosimeter solution while the other compartments contained water. The dose in the rear 5 mm of the cell as seen from the Linac was found to be ca. 40% of the highest dose measured in the front 5 mm of the cell. The dose increased almost linearly through the cell. The relative dose variation within the 7.1-cm pressure cell was not measured but it was estimated to be less marked than the corresponding variation in the 2.55-cm cell.
+
+
(14) 9. I. Taimuty a n d B. 9. Deaver, Jr., Rev. Sei. Instrum., 3 2 , 1098 (1961). T.Hviid and s. o. Nie,sen, in preparation, (IS) J. P. Keene, J. sei. Instrum., 41,493 (1964); Rev. sci. Instrum.,
:;;Eo
~ ~ r ~ ~ ~ ; nG s, Nilsson, en, p, Pagsberg, and i b i a . , in press.
s, o,
Volume YS, Number 4
Nielsen,
April 1969
PAGSBERG, CHRISTENSEN, RABANI,NILSSON,FEKGER, AND NIELSEN
1032
AI
B'
~2 T I.
I -
+
62
I
t
l
I
I
I
plastic bag. Before filling the pressure cell it was flushed carefully with H2, and while still being flushed it was then filled with the deaerated and ilr-saturated solution and rapidly connected to a combined vacuumpressure line that supplied the pressurized HP, and NzO and 02 when used. The cell was taken through several cycles of evacuation to 0.5 atm and pressurization with H2 to 35 atm. The cell had a volume ratio of gas/liquid of ca. 4: 1, and the inner Pyrex body of the cell with quartz windows had room for a 0.3-ml gas bubble without interfering with the light path through the cell (Figure 1). This gas bubble was in good contact with both the liquid and the gas inside the cell while the cell was being shaken. Measurements a t 250 nm of the OH absorption transient in low3Jl HClOl indicated that complete gas equilibration was established after 30 min of gentle shaking. All irradiations were done at 23-25".
Results A3
83 Figure 2. Oscilloscope traces of light intensity us. time: X = 200 nm, slit = 2.0 mm, and response time ca. 80 nsec. One large ordinate division corresponds to 5.37% transmission and one large abscissa division to 20 psec for A traces and 5 Msec for B traces. Tracm with the same number were triggered simultaneously. A2 and R2: light transmission of 7.1 om of 1OW M HClOa 4-0.027 M Hz after irradiation with a 1.2-psec electron pulse that had just faded away 7.7 psec after start of the two oscilloscope traces. A3 and 8 3 : Cerenkov light intensity recorded under conditions identical with those used in A2 and B2 but with no analyzing light. The oscillations before the electron pulse were due to electrical noise pick-up, A1 and B1: light intensity from pulsed Xe lamp (100% transmission) recorded under conditions identical with those used in A2 and B2 but with no electron pulse. The arbitrary signal level to which the oscilloscope inputs were clamped during the duration of the electron pulse is visible in all the traces from 6 to 7.7 psec after sweep start.
Materials and Methods. The general techniques of pulse radiolysis have been treated in detail.1s-20 We used the "syringe" techniquez1 with 100-ml Summit syringes made of Pyrex; they were baked a t 470". The filling and emptying of the two-pass cell under 1 atm of He was remote-controlled.17 The following chemicals were used: triply distilled water; NzO purified by bubbling through NaOH-pyragallol;la HC104, SO%, Merck analytical grade; Ar purified by distillation; and Hz purified carefully by passing it through 2 m of molecular sieve a t - 196' and subsequently through a dust filter. Solutions were stored for up to 30 hr in syringes which, in turn, were kept in a moistened The Journal of Physical Chemistry
The performance of the optical detection channel is shown in Figure 2. At 200 nm and with a response time of 80 nsec, the peak-to-peak noise on the 100% light level corresponded to less than 2% of light transmission. The strong Cerenkov signal was almost completely suppressed by the clamping circuit which also allowed apparently true measurements to begin 0.2 psec after the electron pulse had faded away. As seen from trace B3 of Figure 2, long-lived luminescence from the cell windows and satellite pulses from the photomultiplier were practically absent. This was not true a t the much lower intensities obtained with an unpulsed Xe lamp. CH and . Z ~ H + H . When the 7.1-cm pressure cell was filled with 10-3 M HC104 0.027 M Hz (pHa = 35 atm) and irradiated with 1.2-psec electron pulses, optical absorption transients were observed (Figure 2) from 200 to 240 nm that appeared to be independent of (a) the number (at least 10) of electron pulses previously given to the same solution and (b) the particular stock solution of HC104 used. To calculate the absorbance of the transients they were first corrected for the small slope of the 100% transmission level (see Figure 2, Bl). The optical transmission scale (see Figure 2) was then calibrated from the oscilloscope gain and an oscillogram with gain one showing both the 1 0 0 ~ oand 0% transmission levels. Finally, the absorbance of the transients as a function of time after the
+
(18) L. M. Dorfman and M . 9. Matheson in "Progress in Reaction Kinetics," Vol. 111, G. Porter, Ed., Pergamon Press, Oxford, 1965, Chapter 6. (19) J. W. Boag in "Actions Chimiques et Biologiques des Radiations," M. Haissinsky, Ed., Masson et Cie. Paris, 1963, Series VI, Chapter 1. (20) J. P. Keene in "Pulse Radiolysis," & Ebert, I. J. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., Scadernic Press, lnC., Kew York, N. Y.,1956,p 1. (21) E. J. Hart, 9. Gordon, and J. IC. Thomas, J . P h y S Chem., 6 8 , 1271 (1964).
FAR-UVSPECTRA OF H
AND
1033
OH I
1
I
80
1 1 1 2 2
60
ZkH+H eK.
210.0 200.0 200.0 200.0 200.0 200.0
1
0
::
Wavelength, nm
Expt
70
E C
+
and 2 k H + H in 10-8 ill IIClO~ 0.027 M Hp
Table I:
50
n
L1
M - 1
c111-1
M-1
x sec-1
563 915 918 91 5 971 785
1.51 1.53 1.44 1.99 1.25
Av” 900 f 300
Av 1.55 & 0.100
1.67
“ A t 23-25“, bAverage of values a t 200nm only. OStandard deviation. In the calculation of 2kH+H the corresponding values of CH were used.
‘=. 40 30
20
lo
b
I
I 20
10 sec
Figure 3. Second-order plot of the decay of the pulse radiolytic transient B2 in Figure 2. Absorbance is true decadic absorbance minus decadic “asymptotic” absorbance read 160 psec after the electron pulse. The time is microseconds after the end of the electron pulse.
end of the electron pulse was calculated and corrected by subtracting the “asymptotic” absorbance read at 160 psec after the electron pulse from all the values. The reciprocal of this corrected absorbance was then plotted against time (see Figure 3). The decay of the transients was found to agree well with second-order kinetics a t 200 and 210 nm. At higher wavelengths the transients were too weak to warrant kinetic analysis. In the pressure cell hydrated electrons, eaq-, and OH are rapidly removed by reactions with HsOf and HS, respectively. If we assume (as will be supported in the following) that both reactions result in the formation of H, the observed optical transients may be provisionally assigned to H. The half-times of eaqand OH are 0.03 and 0.5 psec, respectively, under our conditions11*22a-c compared with the ca. 8 psec half-time of the optical transients (see Figure 3). Only a small error in the calculation of the apparent molar decadic absorptivity, EH, and of the rate constant 2kH+H is introduced by assuming that only H is present at the end of the electron pulse. In extrapolating (absorbance)-l to time zero the reactions during the first microsecond of the optical transients are disregarded. Taking G, = 2.6, GOH = 2.65, and GH = 0.55 l8 and applying a 5% correction for recombination during the electron pulse, CH at time zero immediately after the electron pulse is calculated to be 8.43 X M in the experiment treated in Figure 3. This value, together with the absorbance at time zero (obtained from ex-
trapolation along the straight line in Figure 3) and the light path 7.1-cm, give CH = 915 M-’ cm-’ at 200 nm. A value of 2 k H f H = 1.53 X 1O’O M-’ see-l is obtained from the slope of the line by multiplication with 7.1 X 915. The results of other runs on two different days were treated in a similar way and are summarized in Table I. At 210 nm EH has dropped to 563, but practically the same value (1.51 X 1O1O M-I see-l) was found for 2 k H f H . Although the optical transients observed a t wavelengths above 210 nm were not treated kinetically, absorbances at time zero were derived from them by extrapolation and BH was calculated. The results are given in Figure 4.
1 I
1000
I
I
I
I
I
1
1
nrn Figure 4. Absorption spectrum of the pulse radiolytic transient observed in 10-8 M HClOa 0.027 M Hz a t 23-25’ calculated as molar decadic absorptivity of free hydrogen atoms.
+
(22) (a) J. P. Keene, Radiat. Res., 2 2 , 1 (1964); (b) L. L f . Dorfman and I. A. Taub, J. Amer. Chem. Soc., 8 5 , 237 (1963);( c ) 9. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, (bid.,
85, 1375 (1963). Volume 79, Number 4
April 1989
PAGSBERG, CHRISTENSEN, RABANI,NILSSON, FENGER,AND NIELSEN
1034
In agreement with the assumption made that the optical transients are due to H, it was found that the optical transient observed at 210 nm was unaffected iM HC104. N20, which by substitution of N2O for was difficult to add in known concentrations to the pressure cell, was present in a concentration estimated as M > c(NzO) > 2 X loea M . Good measurements at 200 nm of the pulse radiolytic transients in the presence of high concentrations of K 2 0 are difficult because of the N20 light absorption. The absorption spectrum of OH is needed to obtain the absorption spectrum of H, independently, from the measurements in the two-pass cell (no Hz). The following subsection describes the measurements on OH. EOH and ~ ~ o H + o H . When the two-pass cell was filled with 2 X M N20 and irradiated with 1.1-psec electron pulses, optical transients with half-times less than 10 psec were observed from 200 nm to well above 300 nm. The oscillograms of the transients were treated in the same way as described above for the transients in the pressure cell in order to calculate absorbance as a function of time. The decay of the absorbance followed second-order kinetics to a good approximation at all wavelengths investigated as shown by second-order plots of (absorbance)-l us. time (see Figure 5). Kinetic data were collected from solutions that were irradiated only once. Doubling the N2O
concentration did not appear to affect the pulse radiolytic transients. Estimates were made of the error in the second-order rate constant produced by the nonhomogeneity of the radiation field. Computation showed that the error in the second-order rate constant calculated from the second-order plot deviated less than 2% from the true second-order rate constant. Similar results have recently been reported by S a ~ e and r ~ by ~ B~ag.~~ The pulse radiolytic transients in N2O solutions have been assigned to OH r a d i ~ a l s . ~ O ~To ~ 5calculate ~~~ the molar decadic absorptivity of free OH, EOH, and the rate constant 2 k 0 ~ + we ~ ~must , first introduce a c o r rection for the reaction of the primary yield of H atoms ( GH = 0.55) with the large excess of OH radicals present at time zero immediately after the electron G O H = 5.25). Noting that pulse ( G ( 0 H ) = G, 2 k O H f O H is of the order of 1.0 X 1Olo M-l sec-’ 0~10-13 and that the value of JCH+OH may be taken as 1.2 X 1Olo M-l ~ e c - l ,this ~ correction was introduced by defining an effective molar decadic absorptivity e’ for the sum of H and OH ( H and OH maintain approximately the same ratio H/OH throughout the decay of the transient because 2 k O H + O H rn ~ H + o H ) . E’ is related to the absorbance calculated from the transients as described above by eq 1 and to EH, EOH, and the yield of H202 after total decay of the transient, G(H202), by eq 2.
+
absorbance I
/
1
50
0
a
40
0
‘=.
=
=
60
n
+ Ge + G H ) ~ ’
+
5.l(c~
+
(GOH
COH)E’
@,)€OH
(1)
+ GHEH
- (G(Hz02) - G H ~ o ~ ) E H(2) ~o~ G(H202) - G H ~ o ~
70
8c
(GOH
=
30 20
+
~ . ~ ( G o HGe)2(Goz
+ + G H ) - ~ (3) Ge
Equation 3 is easily derived by statistical reasoning under the assumption that H and OH are kinetically equivalent. This assumption is also necessary for eq 1-3 to be exact. For H and OH to be kinetically equivalent 2kH+H would have to equal 2kOH+OH = ~ H + O H . However, due to the large excess of OH over H the value of 2kH+H is not critical. From the readings of the electron pulse monitor that was calibrated with the ferrocyanide dosimeter the value of the sum COH CH at time zero was calculated after correcting for a small recombination (ca. 10%) during the 1.1-psec electron pulse. COH CH was found to be ca. 3 X 10-5 M in most of the experiments. e’ was then calculated from eq 1 using the absorbance at time zero as obtained from extrapolation along the best-fitting straight line in the second-order plot (see
+
+
10
I 10
0 psec
Figure 5. Second-order plot of the decay of the pulse radiolytic transient in 2 X 10-3 M NtO at 200 nm. Absorbance is true decadic absorbance minus decadic “asymptotic” absorbance read 160 bsec after the 1.1-psec electron pulse. The time is microseconds after the end of the electron pulse. Lower curve: uncorrected for OH- light absorption; upper curve: correoted for OHlight absorption. The Journal of Physical Chemistry
(23) M. 0. Sauer, Jr., ANL-7327. >fay 1967. (24) J. W. Boag, T r a n s . Faraday Soc., 64, 677 (1968). (25) J. K. Thomas, ibdd., 61, 702 (1965). (26) D. M. Brown, F. S. Dainton. and D. 0. Walker in “Pulse Radiolysis,” M. Ebert. J. P. Keene, A. J . Swallow, and J. H . Baxendale, Ed., Academic Press, Inc., New York, N. Y.,1065, p 221.
FAR-UVSPECTRA OF H
X'avelength, nin
AND
1035
OH
e', M - 1
c111-1
200 .o 200 .o 200.0 210.0
695b 4330 463d 387d)
210 .0 220 .o 230.0 240.0 250 .O
411 456 470 432
509d 447 523 543 494
0.91 1.20 1.10 0.98
0.91 463
250.0
0.97
413
Ak6 1.04 f 0.04' a A t 23-25'. Uncorrected for OH- light absorption. Corrected for OH- light absorption. In 2 X M NnO 4 X M HClOc. 8 Excluding value uncorrected for OH- light absorption. 1 Standard deviation. 0 Corrected for the reaction H 4-OH.
+
lency of H and OH, 2kOH+OH may be calculated with good accuracy from the slope of the second-order plot by multiplication with the corresponding value of e' and with the length of the light path, 5.1 cm. The resulting values of 2kOH+OH are given in Table 11. If kH+oI-I is approximately equal to 2 k o H + o H as assumed above, the present treatment using E' is better than the kinetic treatment where the presence of H is completely neglected. Such a treatment leads to a value of ~ ~ O H + Othat H is overestimated by ca. 10%. The values found for 2koH+OH in Table I1 in the region from 210 to 250 nm all fall between 0.91 X 1Olo and 1.20 X 10'O M-' sec-'. The value found at 200 nm, however, using the same method of making a second-order plot (Figure 5, lower curve) and calculating E' and 2kOH+OH, leads to 2ko~+oH = 1.74 X lOlo (Table 11). We believe that this is caused by the neglect of OH- ions formed by reaction 4. OH- absorbs light at 200 nm with EOH- = 1000.27 The absorptivity is almost an order of magnitude smaller at 210
NtO Figure 5). e' and EOH as calculated from eq 2 and 3 substit'uting the values of G,, GOH, and GH already ~ ref 27, are used, CH from Figure 3, and E H ~ Ofrom given in Table 11. In applying the correction for light absorption by H atoms we anticipate the final conclusion (see Discussion) that all results obtained are consistent with an assignment of the spectrum in Figure 4 to H atoms. Some values of EOH are plotted in Figure 6. Owing to the approximate kinetic equiva-
+ eaq-
-
Nz
+ OH- + OH
(4)
nm, and the contribution of OH- to the pulse radiolytic optical transients can be neglected at this wavelength and above. At 200 nm, on the other hand, light absorption by OH- makes an appreciable contribution to the observed optical transients as shown by the difference between the upper and lower curves in Figure 5. OH- formed by reaction 4 and possibly by other reactions (making the initial yield OOH- > 0) in the neutral solution decays by reacting with an equivalent amount of Ha0+ with a rate constant k(OHHaO+) = 1.4 x loll M-l sec-1,28and it is easy to calculate COH- at any given time 1 (in seconds) after the end of the electron pulse from eq 5
+
*0°
600 700 c
'E 500
I
( c o H - ) - ~ = (c'oH-)-'
(liters/mole) (5)
U
-'a 400
c
t ._
-. 300 L.)
200 100
"ZOO
+ 1.4 X 10l1X t
250 nm
Figure 6. *Sbaorptionspectrum of the pulse radiolytic transient observed in 2 X M N20 calculated as molar decadic absorptivity of free OH radicals after correction for the light absorption of HaOz, 13, and OH-.
300
where C'OH- is the concentration of OH- immediately after the electron pulse. As there is extensive recombination of OH- and H30+ during the 1.1-psec electron pulse, C'OH- does not depend very much on the value chosen for GOH-. For the run shown in Figure 5 cooHwas calculated to be ca. 1.0 X M. When the upper second-order plot in Figure 5 that is corrected for the OH- light absorption is used to determine C' and 2kOH+OH, a value of 2k0H+OII = 1.20 X 1O'O 1M-' sec-' is found, in good agreement with the values determined a t the other wavelengths. It is, furthermore, in agreement with the suggestion that OH- makes an appreciable contribution to the pulse (27) Landolt-Bornstein Tables, 6th ed. "Atomic and Molecular Physics," Vol. I "Molecules 11," part 3, Springer-Verlag. Berlin, 1951,p 231. (28) M. Eigen and L.DoMayer, 2.Efektrochem., 59, 986 (1955). Volume 73, Number 4
April 1969
1036
PAGSBERG, CHRISTENSEN, RABANI, NILSSON,FENGER, AND NIELSEN
radiolytic transients a t 200 nm in 2 X M K 2 0that the pulse radiolytic transients in 2 X M NzO 4X M HC104 was found to give “normal” values of 2kOH+OH at 210 and 200 nm (0.95 X 1O1O and 1.16 X lolo, respectively) without any correction for OH- light absorption (Table 11). In the acidified solution OHdisappears in a pseudo-first-order reaction with HaO+ with a decay time of less than 0.2 psec. The average of all the values of 2kOH+OH in Table I1 that either have been corrected for OH-- light absorption or do not need this correction is 1.04 X 1Olo with a standard deviation of 0.04 X lolo. Owing to the small systematic errors introduced by the data treatment, we give 2 k O H + O H as (1.04 f 0.10) X 1O’O M-l sec-l. Pulse Radiolytic Transients in lop3M HC1O4. When the two-pass cell was filled with 10-3 M HC104 and irradiated with 1.2-psec electron pulses, optical transients were observed from 200 nm to well above 300 nm. As will be further described in a future report, the initial absorbance of these transients could, after a small correction for recombination during the electron pulse, be quantitatively accounted for by the initial yields already usedls of OH, H, and cap- (that was assumed immediately to be transformed into H) supplemented with O H I O z = 0.7 l* and the absorption spectra of H , OH, and H20z given in Figures 4 and 6 and in ref 27, respectively. Quenching of the “H-Transient” by 02. The pressure cell was filled with 10-3 M HC10, 0.027 M Hz ca. lou3 fif 02 (it was difficult to add 02 in known concentration to the pressure cell) and irradiated with a 1.2-psec electron pulse. A radiation induced absorbance was observed 0.2 psec after the end of the electron pulse in the region investigated from 200 to 250 am. In this wavelength region the absorbance did not appear to decrease during the first 100 psec after the pulse which indicates that the “H-transient” that is observed in the absence of 02 between 200 and 240 nm (Figure 4) is completely quenched by 0 2 . The species resulting from the quenching has an absorption spectrum given in Figure 7 which is based on the absorbance measured 0.2 psec after the electron pulse and on the assumption that the G value of the resulting species is equal to GOH G e GH = G(H0z). A comparison of Figure 7 with the reportedz9spectrum of HOz leads to the conclusion that the “H-transient” is quenched by oxygen to give HO, radicals and no other uv-absorbing species. The small discrepancy between the H 0 2 spectrum in Figure 7 and the reportedz9 HO, spectrum in the region above 210 rim where they are both known should not be stressed because our experiments with oxygen added are not as accurate as the other experiments reported in this paper. Thus for unknown reasons not investigated further the absorbance after the pulse in the presence of 0 2 increased very slightly with time at some of the wavelengths investigated.
+
+
+ +
The Journal 01Physical Chamislry
+
t
nm Figure 7 . Absorption spectrum of HOZcalculated approximately (see text) from the initial absorbance of the pulse radiolytic M transients produced by 1.2-psec electron pulses in: 0 , HClOa 2.6 X 10-4 M 02 in the two-pass cell; A, 10-8 M HClOa 0.027 M HI ca. 10-8 M 02 in the pressure cell.
+ +
+
02-quenching experiments were also performed in the two-pass cell filled with 10-3 M HC104 2.6 X M 0 2 (air saturation). Again, it was found that the “H-transient” appeared to have been very nearly completely quenched when absorbance measurements were started a few tenths of 1 psec after the electron pulse with the formation of HOn as the only product. The spectrum of HO2 found that together with the absorbance of OH accounts for the data after making a rough estimate of recombination during the electron pulse is given in Figure 7. The spectra of HOZfound with the two-pass cell and with the pressure cell are not significantly different. An estimate of the rate of quenching of the “H-transient” by 0 2 was provided by pulse radiolysis of lod3M M 0 2 in the two-pass cell. The HC104 2.6 X initial absorbance of the transients observed at 240 nm indicated a degree of scavenging of H atoms of ca. 25%. Likewise, the initial absorbance at 200 nm was found to be consistent with a degree of quenching of the species responsible for the “H-transient” of somewhere between 20 and 40%. The experiments are difficult at this wavelength and are not very accurate because G X E values of only 0.3( GH Ge)( n m - E H O ~ ~ O O nm) e 300 are involved. Taking the degree of quenching observed 0.2 psec after the end of the electron pulse (Le., ca. 0.3 psec after the pulse current, had decreased to half of
+
+
+
(29) C. Czapski and b. M. Durfman, J . Phys. Chem., 68, 1169 (1964).
FAR-UVSPECTRAOF H
AND
1037
OH
its maximal value) as 25% and noting that the width of the electron pulse was 1.2 psec, we may calculate k ( H 02) and the secoridTorder rate constant k of quenching of the “H-transient” by 0 2 .
+
k(€I
+
02)
wk % 0.25C2.6 X
=! 1010 M-1
X (0.3
+ 0.6) X 10-6]-1 (6)
see-1
Discussion The value of e0HZ60 = 405 X-l cm-l found in the present investigation at 260 nm (see Figure 6) agrees well with OH^^^ = 410 M-’ em-l reported by RabaniS3O In addition, our value of 2kOH+OH = (1.04 0.10) X 1010 M-1 sec-’ based on measurements between 200 and 250 nm agrees well with the value of 2kOHfOH = 1.05 X 1Olo M-’ see-I reported from pulse radiolysis of M N2O at 260 nrn by Thomas, et al.1° They used eOHZ60 = 370 M-’ see-’ 25 to calculate 2kOH+OI-Ifrom the slope of the second-order plot of the pulse radiolytic transient apparently without correcting for the presence of H. As seen above this procedure leads to an ca. 10% overestimate of 2kOH+OH which, however, in their case was compensated by the use of a value of EOH that is ca. 10% lower than ours. Thus the good agreement between our value of 2kOH+OH and that of Thomas, et aZ.,10 may serve as a check on the pulse-radiolysis techniques employed. The main advantage of the type of pressure cell used in the present investigation is that it can be filled with pure Hz so that the first single-pulse electron irradiation gives meaningful results. It was found and verified by calculation that up to 10 electron pulses could be given to the same solution of M HC104 0.027 M Hz without producing significant changes in the optical transients observed. With the dose per pulse employed of lozoeV/l. or more, it is a good approximation to assume that the transients have decayed to a constant level 160 psec after the electron pulse. Thus the measured absorbance of the transients at 1 = 160 psec can be used to correct the absorbances of the same transient measured at shorter times with the extrapolated absorbance at
+
+
t = m .
Assignment of the “H-Transient.” The pulse radiolytic transient that has been referred to above as the “H-transient” is observed in “pure” form by irradiating the pressure cell filled with M HC104 0.027 M Hz. It has a spectrum shown in Figure 4 where its molar decadic absorptivity has been calculated on the assumption that the transient is due to free H atoms. The same decay kinetics observed at both 200 and 210 nm shows that it is the same species which is absorbing at both wavelengths. We now actually propose to assign the “H-transient” to the presence of dissolved free H atoms. The reason for this assign-
+
ment is that the transient in every respect tested behaves as an optical absorption transient due to free H atoms. Thus, (a) the “pure” transient decays following second-order kinetics (Figure 3 ) . (b) The value calculated for 2kH+H = (1.55 f 0.10) X 1Olo M-‘ see-l under the assumption that the “H-transient” is due to H falls within the range of values of 2kH+H found by indirect r n e t l i ~ d s . ~ - (e) ~ The “H-transient” as ob0.027 M Hz at 210 nm is served in 10-3 M HC104 independent of whether H is formed by reactions 7 or 8
+
Hz e,,
+ OH-H + HaO+
-
+ HzO
H
+ HzO
(7)
(8)
thus also providing evidence that H30, if formed as an intermediate in reaction 8, has a lifetime that does not exceed a few tenths of 1 psec. (d) The initial absorbance of the pulse radiolytic transient in M HC104 can be accounted for a t all wavelengths investigated by the commonly accepted initial radiolytic yields18 and by the absorption spectra of H and OH (Figures 4 and 6) and by that of Hz02. This indicates M HClOl as that the “H-transient” is observed in well (further supported by the analysis of the decay kinetics as will be shown in a future report) with a M HClOl strength relative to that observed in 0.027 M Hz that is proportional to the concentration of free H atoms. (e) The “H-transient” appears to be completely quenchable by O2 with a calculated rate constant k(H 02) M 10‘0 M-’ sec-l assuming the “H-transient” to be due to €1. The only product of the quenching reaction with 0 2 appears to be H02. It should also be noted that the observed secondorder decay of the “H-transient” in conjunction with the calculated, nearly diffusion-controlled second-order rate constant 2kH+H = 1.6 X 1010 M-’ sec-l (assuming the “H-transient” to be due to H with G(I-I) = GOH G, GH = 5.8) make it difficult to assign the “H-transient” to a radiation produced species with G < 0.6. Such an assignment would mean that the species in question would have a second-order decay constant OH-) = 1.4 X 10” M-l sec-l. larger than k(H+ The “H-transient” must, therefore, either be due to free dissolved H atoms or to some other radiationproduced species with G > 0.6 and a lifetime that under our conditions is several microseconds. It has so far not been necessary to postulate the existence of such a species in the radiation chemistry of water, and we prefer to assign the “H-transient” to H atoms. Nature of the Far-Ultraviolet Spectrum of H . The absorption spectrum of H given in Figure 4 is surprising in view of the fact that free hydrogen atoms in the gas phase do not absorb light in the ground state a t longer wavelengths than 121.57 nm.3 A solvent red shift of the absorption spectrum of H of more than
+
+
+
+
+
(30) 5 . Rabani in “Radiation Chemistry of Aqueous Systems,” G . Stein, Ed., Interscience, New York, N. Y . , 1968, p 229.
Volume 79,Number 4 April 1060
1038
PAGSBERG, CHRISTEKSEN, RABANI,NILSSON,FENGER, AND NIELSEN
70 nm (30,000 cm-I) is hardly conceivable. We tentatively interpret, therefore, the absorption spectrum of aqueous solutions of H (Figure 4) as a comparatively small red shift of the first water absorption continuum beginning at 186 nm.31 The water molecules contributing to the red shift must be those in direct contact with the dissolved H atoms. In view of evidence from esr measurements of essentially free H atoms in aqueous phases (see e.g., ref 32a), this red shift must be due to a perturbation of the corresponding excited state only of €120 leaving the ground state essentially unperturbed. Whereas a dissolved H atom does not interact with more than thermal energies with any HzO solvent molecule in its ground state, it can interact much more strongly with a neighboring, electronically excited water molecule H20* via a partial, negative charge transfer from HzO* to H. The energetic conditions for this charge transfer to take place from the first excited (continuous) singlet state of H20(ionization potential 5 12.6 - 6.7 = 5.9 eV32b),associated with the first absorption continuum from 186 to 145 nm, to H (electron affinity 0.77 eV89 is comparable to the conditions for charge transfer in many well-known charge-transfer ~ o m p l e x e s . ~Due ~ to the compensating coulomb attraction the distance over which the charge transfer is to occur is critical for the extent of the charge transfer and hence for the perturbation (red shift) of the water continuum band beginning at 186 nm. A qualitative explanation along this line may be offered for the apparent absence of the solvent perturbation band in aqueous solutions of HOz (Figure 7). The absorption spectrum of aqueous solutions of OH (Figure 6) appears, on the other hand, to show the beginning of a solvent perturbation band a t 200 nm. It may be tentatively interpreted along similar lines as the ab-
The Journal of Physical Chemistry
sorption spectrum of H shown in Figure 4. Our spectrum of OH (Figure 6) differs somewhat from that of Thomas, et a1.,l0 below 220 nm. These authors, however, stated certain reservations concerning their measured spectra below 220 nm, and we believe our OH spectrum to be the more correct. A semiquantitative treatment of the far-ultraviolet absorption spectra of aqueous solutions of H and of OH is under way and will be presented in a future report. The optical transition proposed above to account for these spectra is analogous to the transition believed to be responsible for the fl bands in alkali halides.86
Acknowledgments. We thank the staff of the Accelerator Division, RisZ, for careful Linac operation, K. B. Hansen, E. Mose Christiansen, and K. E. Neisig of the Electronics Department for help with circuitry, K. S@eHgjberg and Tove Rosendahl Hansen for computer calculations, and Farukh Said and C.-G. BlixenFinecke for aid in the experimental work. The special help of Dr. E. J. Hart, Argonne National Laboratory, with pressure cell irradiations is gratefully acknowledged. (31) G. Herzberg, “Electronic Spectra and Electronic Structure of Polyatomic Molecules,” I). Van Nostrand C o . , Princeton, N. J.. 1966, p 585. (32) (a) P. W. Atkins and M. 0. R . Symons, “The Structure of Inorganic Radicals,” Elsevier Publishing Co., Amsterdam, 1967, p 84. (b) T h e ionization potential of water in the gas phase is 12.6 eV. The quantum assoqiated with tho 186-nm absorption edge is 6.7 eV. (33) J. D. Weisner and B. H. Armstrong, Proc. Phys. Soc., 83, 31 (1964). (34) L. J. Oosterhoff in “Modern Quantum Chemistry,” 0. Sinanoglu, Ed., Academic Press, Inc., New York, N. Y., 1965, Part I, p 137. (35) J. J. Markham, Solid Stale Phys., S u p p l . , 8. 1 (1956).
