- Email: [email protected]
A re-evaluation of the initial yield of the hydrated electron in the picosecond time range
ARTICLE IN PRESS
Radiation Physics and Chemistry 72 (2005) 169–172 www.elsevier.com/locate/radphyschem
A re-evaluation of the initial yield of the hydrated electron in the picosecond time range Yusa Muroya, Mingzhang Lin, Guozhong Wu, Hokuto Iijima, Koji Yoshii, Toru Ueda, Hisaaki Kudo, Yosuke Katsumura Nuclear Engineering Research Laboratory, Graduate School of Engineering, The University of Tokyo, 2-22 Shirakata-shirane, Tokai-mura, Naka-gun, Ibaraki 319-1188, Japan Received 28 July 2003; accepted 26 February 2004
Abstract The yield of the hydrated electron in the picosecond time range has been re-evaluated with an ultrafast pulse radiolysis system using a laser photocathode RF-gun in combination with a conventional one, and a value of 4.170.2 per 100 eV of absorbed energy at 20 ps was derived. This is consistent with recent experimental results using a time correlation method [Bartels et al., J. Phys. Chem. A 104, 1686-1691 (2000)] and with Monte-Carlo calculations [Muroya et al., Can. J. Chem. 80 1367-1374 (2002)]. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrated electron; Picosecond yield; RF-photocathode; Laser-driven pulse radiolysis system
1. Introduction Water radiolysis is important not only in radiation chemistry but also in radiation biology, nuclear technology, and environmental radiation protection, because an understanding of the radiolysis of aqueous solutions is a key to basic knowledge in these fields (Farhataziz and Rodgers, 1987; Jonah and Rao, 2001; Mozumder and Hatano, 2004). After the discovery of the hydrated electron (e aq) (Boag and Hart, 1963; Keene, 1963), measurements of its yield at early times have been extensively investigated. The first measurement was carried out by a stroboscopic pulse radiolysis system at the University of Toronto (Bronskill et al., 1970), where the temporal behavior in a pulse train (0–350 ps), using an S-band linac, was observed using Cerenkov light as Corresponding author. Tel.: +81 3 5841 6979; fax: +81 3 5841 8624. E-mail address: [email protected] (Y. Katsumura).
an analyzing light. A G-value of 4.170.2 per 100 eV of absorbed energy at 30 ps was reported (Wolff et al., 1973). Almost the same value was reported by a collaborative work between the University of Toronto and the Argonne National Laboratory (ANL) (Hunt et al., 1973). The same stroboscopic method has been developed at Hokkaido University and applied to the measurement of the G-value of the hydrated electron. A value of 4.870.3 at 30 ps was reported (Sumiyoshi and Katayama, 1982; Sumiyoshi et al., 1985). In the 1970s, a single picosecond electron pulse became available at ANL and a Gðe aq Þ of 4.170.1 at 200 ps (Jonah et al., 1973) was reported using a fast vacuum photodiode. Soon after, by combining the single picosecond pulse and a 2701 magnet, a single picosecond pulse stroboscopic system covering the time range of ps to a few ns was constructed at ANL (Jonah, 1975) and a Gðe aq Þ of 4.670.2 at 100 ps (Jonah et al., 1976) was reported. However, the ANL group recently employed the time correlation method and re-evaluated the value at zero
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time to be 4.070.2 (Bartels et al., 2000). In addition, the discrepancy between kinetic and scavenging data was also reconsidered (Pimblott et al., 1996), and a Gðe aq Þ value of 4.8 was chosen as a reasonable value. The picosecond yield of Gðe aq Þ is an important value but the reported values are scattered from 4.0 to 4.8, so no clear consensus exists. Firstly, this is partly due to the difficulty of data analysis obtained by the stroboscopic method, because, in the pulse train, experimental data for later pulses are affected by earlier pulses. Secondly, in order to evaluate the G-value, dose evaluation is inevitable but a reliable dosimetric method has not been established in the ps time range. In the present experiment, to resolve the existing discrepancy in the ps yield of Gðe aq Þ; temporal behavior in the ps time range was obtained by a newly installed ultrafast pulse radiolysis system with a laser photocathode RF-gun at the Nuclear Engineering Research Laboratory, University of Tokyo (Muroya et al., 2001, 2002a). Under the same experimental conditions, the yield in the sub-microsecond time range was also observed by the conventional pulse radiolysis method. Since the value of the hydrated electron yield has an accepted value of about 2.8 at 0.1 ms (Buxton 1972; Shiraishi et al., 1988), a reliable G-value of e aq at ps can be easily determined. Our present data strongly support the recent results of Bartels et al. (2000) at ANL and are consistent with recent Monte-Carlo calculations (Muroya et al., 2002b).
2. Experimental Time-resolved transient absorption spectroscopy was performed by a pump-and-probe method. Details and performance of the system have been reported elsewhere (Muroya et al., 2001, 2002a). Briefly, an S-band linear accelerator with a laser photocathode RF-gun that is triggered with 265-nm laser pulses prepared by thirdharmonic generation using a femtosecond Ti:Sapphire laser (795 nm), is operated at a repetition rate of 10 Hz. The laser pulse is split by a half mirror and used for two purposes; as an analyzing light for the pump and probe measurement and as an injector for the photocathode RF-gun after the third-harmonic generation. An electron beam with pulse duration of 3 ps (full-width at halfmaximum), charge of 1 nC, and energy of 20 MeV, is produced as an irradiation source. The electron beam and the fs laser are synchronized within 2 ps (root mean square value) and an optical delay stage is employed for changing the time interval. Absorbance was calculated from the Lambert–Beer law expressed as log10(I0/I), where I0 and I represent the intensity of the reference and absorbed light, respectively. The intensity of the two light signals was measured using Si PIN photodiodes (Hamamatsu/S1722-02) and an oscilloscope (Hewlett-
Packard/HP54845). Measurement as a function of time was done by repeating the following three procedures: (i) detection of laser intensities by the photodiodes and the oscilloscope, (ii) absorbance calculation, and (iii) adding optical delay of length dl corresponding to the time difference dt(=dl/c), where c represents the velocity of light. Kinetic measurements for times greater than 10 ns were carried out at 633 nm, the He–Ne laser wavelength. It is essential that both kinetic and pump–probe measurements be done under the same conditions. Therefore, a few filters with pin-holes were employed and special care was taken to keep the light path of the He–Ne laser beam the same as that of the 795-nm laser beam, because the spatial distribution of dose significantly depends on the position. Since the absorption spectra in ps and ns time ranges are assumed to be identical, the absorbances obtained at each wavelength were normalized by the absorption coefficients at 633 and 795 nm, respectively (Jou and Freeman, 1977). Millipore water, bubbled with nitrogen, was used as the target sample. In order to protect the sample from accumulation dose, a flow system was used, in which the sample was supplied from a reservoir at a rate of 3 ml/ min and passed through irradiation cells with optical path lengths of 5 or 18 mm.
3. Results and discussion The temporal behavior of the hydrated electron was measured by using the pump-and-probe method in the picosecond range and one typical data set is shown in Fig. 1, where a 5-mm optical cell was used. The rise time was 9 ps and this can be assumed to be the total time resolution of the present data. According to the fs laser photolysis literature, the solvation time of the hydrated electron is less than 1 ps (Migus et al., 1987; Lu et al., 1989; Alfano et al., 1993). A formula, dtT ¼ dtF þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dt2P þ dt2J ; where dtT, dtF, dtP, and dtJ represent the total time resolution, the time resolution due to the flight difference between the electron beam and the laser light through the sample, the pulse duration, and the time jitter between the laser and the electron pulses, respectively, gives the total time resolution (Muroya et al., 2002a). Taking into account the time resolution of 5 ps due to the flight difference in the 5-mm cell, the pulse duration of the electron beam of 3 ps, and the time jitter of about 2 ps, the observed rise time of 9 ps seems reasonable. Two signals in the ps time region obtained with the 5and 18-mm optical cells are plotted in Fig. 2 after normalization of both signal intensities at 100 ps. Data at less than 9 and 20 ps are excluded because of the total time resolutions of 9 and 20 ps in the experiments using
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Absorbance
0.08 0.06 9ps 0.08
0.04
0.04 0.02
0 0
10
20
100
150 Time /ps
30
40
0 0
50
200
250
300
Fig. 1. Temporal behavior of the hydrated electron in the picosecond time range measured by the ultrafast pulse radiolysis system. A cell with 5-mm optical length and a pulse duration of 3 ps were employed. The growth part of the absorption is shown in the inset.
G-value /molec.(100 eV)-1
6 (b2)
5
G(e-aq)=2.38
(b1) G(e-aq)=2.7
4 3
(a1)
2
(a2)
G(e -aq)=4.1
1 0 10-12
10-11
10-10
10-9 Time /s
10-8
10-7
10-6
Fig. 2. Time dependence of the hydrated electron yield in the picosecond and sub-microsecond time ranges. The time profiles (a1) in the picosecond range and (a2) in the picosecond and subms ranges were taken with the 5- and 18-mm optical cells, respectively. Solid (b1) and dotted (b2) lines are from MonteCarlo calculations reported by Muroya et al. (2002b) and Frongillo et al. (1998), respectively. The G(e aq) value of 2.38 at 300 ns gives a value of 4.170.2 at 20 ps (see text).
the 5- and 18-mm cells, respectively. The reproducibility in the ps time range is satisfactory, irrespective of independent experiments with different optical cells. For the measurements in the sub-microsecond time region, just after the ps measurements with the 18-mm optical cell, a He–Ne laser was introduced keeping the experimental system. Data obtained by kinetic spectroscopy at times shorter than 10 ns have also been eliminated because of the photodiode response time of 10 ns. The G-value of the hydrated electron at 120 ns has been reported to be around 2.8 (Buxton, 1972). At 70
171
and 300 ns, values of 2.93 and 2.67, respectively, have also been determined (Shiraishi et al., 1988) at a dose of 3.8 Gy per pulse. These values are consistent with the evaluation from the product analysis of the scavenging capacity at 108 s1 (Draganic´ and Draganic´, 1973). In addition, it was also reported that the decay of the hydrated electron is dependent on the dose per pulse (Shiraishi et al., 1988). From the absorbance of the hydrated electron obtained in the picosecond time scale, the dose per pulse was estimated to be 12–15 Gy. The decay behavior under the present dose conditions is similar to that reported by Shiraishi et al. (1988), supporting the reliability of the present experimental results. Taking into consideration the present dose per pulse, a value of 2.38 for the yield of the hydrated electron at 300 ns was chosen. Then, this gives a value of 4.170.2 for G(e aq) at 20 ps. The value of 4.1 is identical to early and recently reported values of 4.170.2 (Hunt et al., 1973a; Wolff et al., 1973b; Jonah et al., 1973; Bartels et al., 2000), but is in sharp contrast to the values of 4.6 (Jonah et al., 1976) and 4.8 (Sumiyoshi and Katayama, 1982; Sumiyoshi et al., 1985). It is clear that there is a large discrepancy between the present experimental results as (a1) and (a2) and previous Monte-Carlo simulations of water radiolysis (b2) in Fig. 2 (Frongillo et al., 1998). A re-evaluation of Monte-Carlo calculations (Muroya et al., 2002b), indicated as (b1) in Fig. 2, predicted a lower value for G(e aq) in the picosecond time range. A G-value of 4.4–4.5 at 1 ps was obtained after re-examination of certain adjustable parameters that intervene in the physical and physicochemical stages, such as the thermalization distance of subexcitation electrons, the recombination cross section of the electrons with their water parent cations prior to thermalization, and the branching ratios of the different competing mechanisms in the fragmentation decay of excited water molecules. In view of the small decay from 10 to 20 ps, the calculated value of G(e aq) is in good agreement with the present experimentally determined value. From the temporal behavior in the sub-microsecond region, both calculations are equivalent. It is clear in Fig. 2 that the experimental data after 100 ns are deviating from the calculations. This is due to the homogeneous reactions of the hydrated electron with itself, OH and other chemical species dependent on the pulse dose.
Acknowledgements We thank Prof. M. Uesaka for his encouragement during the experiment. We also thank Prof. J.-P. JayGerin (University of Sherbrooke) for valuable comments from a theoretical aspect.
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References Alfano, J.C., Walhout, P.K., Kimura, Y., Barbara, P.F., 1993. Ultrafast transient-absorption spectroscopy of the aqueous solvated electron. J. Chem. Phys. 98, 5996–5998. Bartels, D.M., Cook, A.R., Mudaliar, M., Jonah, C.D., 2000. Spur decay of the solvated electron in picosecond radiolysis measured with time-correlated absorption spectroscopy. J. Phys. Chem. A 104, 1686–1691. Boag, J.W., Hart, E.J., 1963. Absorption spectra of ‘hydrated’ electron. Nature 197, 45–47. Bronskill, M.J., Taylor, W.B., Wolff, R.K., Hunt, J.W., 1970. Design and performance of a pulse radiolysis system capable of picosecond time resolution. Rev. Sci. Instrum. 41, 333–340. Buxton, G.V., 1972. Nanosecond pulse radiolysis of aqueous solutions containing proton and hydroxyl radical scavengers. Proc. R. Soc. London A 328, 9–21. Draganic´, Z.D., Draganic´, I.G., 1973. Studies on the formation of primary yields of hydroxyl radical and hydrated electron in the g-radiolysis of water. J. Phys. Chem. 77, 765–772. Farhataziz, L., Rodgers, M.A.J. (Eds.), 1987, Radiation Chemistry. Principles and Applications. CVH Publishers, New York. Frongillo, Y., Goulet, T., Fraser, M.-J., Cobut, V., Patau, J.P., Jay-Gerin, J.-P., 1998. Monte Carlo simulation of fast electron and proton tracks in liquid water. II. Nonhomogeneous chemistry. Radiat. Phys. Chem. 51, 245–254. Hunt, J.W., Wolff, R.K., Bronskill, M.J., Jonah, C.D., Hart, E.J., Matheson, M.S., 1973. Radiolytic yields of hydrated electrons at 30 to 1000 picoseconds after energy absorption. J. Phys. Chem. 77, 425–426. Jonah, C.D., 1975. A wide-time range pulse radiolysis system of picosecond time resolution. Rev. Sci. Instrum. 46, 62–66. Jonah, C.D., Rao, B.S.M. (Eds.), 2001, Radiation Chemistry. Present Status and Future Trends, Studies in Physics and Theoretical Chemistry 87. Elsevier Science, Amsterdam. Jonah, C.D., Hart, E.J., Matheson, M.S., 1973. Yields and decay of the hydrated electron at times greater than 200 pisoseconds. J. Phys. Chem. 77, 1838–1843. Jonah, C.D., Matheson, M.S., Miller, J.R., Hart, E.J., 1976. Yield and decay of the hydrated electron from 100 ps to 3 ns. J. Phys. Chem. 80, 1267–1270. Jou, F.-Y., Freeman, G.R., 1977. Shapes of optical spectra of solvated electrons. Effect of pressure. J. Phys. Chem. 81, 909–915.
Keene, J.P., 1963. Optical absorptions in irradiated water. Nature 197, 47–48. Lu, H., Long, F.H., Bowman, R.M., Eisenthal, K.B., 1989. Femtosecond studies of electron-cation geminate recombination in water. J. Phys. Chem. 93, 27–28. Migus, A., Gauduel, Y., Martin, J.L., Antonetti, A., 1987. Excess electrons in liquid water: first evidence of a prehydrated state with femtosecond lifetime. Phys. Rev. Lett. 58, 1559–1562. Mozumder, A., Hatano, Y. (Eds.), 2004, Charged Particle and Photon Interactions with Matter. Chemical, Physicochemical, and Biological Consequences with Applications. Marcel Dekker, New York. Muroya, Y., Watanabe, T., Wu, G., Li, X., Kobayashi, T., Sugahara, J., Ueda, T., Yoshii, K., Uesaka, M., Katsumura, Y., 2001. Design and development of a sub-picosecond pulse radiolysis system. Radiat. Phys. Chem. 60, 307–312. Muroya, Y., Lin, M., Watanabe, T., Wu, G., Kobayashi, T., Yoshii, K., Ueda, T., Uesaka, M., Katsumura, Y., 2002a. Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser. Nucl. Instrum. Methods Phys. Res. A 489, 554–562. Muroya, Y., Meesungnoen, J., Jay-Gerin, J.-P., Filali-Mouhim, A., Goulet, T., Katsumura, Y., Mankhetkorn, S., 2002b. Radiolysis of liquid water: an attempt to reconcile Monte-Carlo calculations with new experimental hydrated electron yield data at early times. Can. J. Chem. 80, 1367–1374. Pimblott, S.M., LaVerne, J.A., Bartels, D.M., Jonah, C.D., 1996. Reconciliation of transient absorption and chemically scavenged yields of the hydrated electron in radiolysis. J. Phys. Chem. 100, 9412–9415. Shiraishi, H., Katsumura, Y., Hiroishi, D., Ishigure, K., Washio, M., 1988. Pulse-radiolysis study on the yield of hydrated electron at elevated temperatures. J. Phys. Chem. 92, 3011–3017. Sumiyoshi, T., Katayama, M., 1982. The yield of hydrated electrons at 30 picoseconds. Chem. Lett. 1887–1890. Sumiyoshi, T., Tsugaru, K., Yamada, T., Katayama, M., 1985. Yields of solvated electrons at 30 picoseconds in water and alcohols. Bull. Chem. Soc. Jpn. 58, 3073–3075. Wolff, R.K., Bronskill, M.J., Aldrich, J.E., Hunt, J.W., 1973. Picosecond pulse radiolysis. IV. Yield of the solvated electron at 30 picoseconds. J. Phys. Chem. 77, 1350–1355.
Radiation Physics and Chemistry 72 (2005) 169–172 www.elsevier.com/locate/radphyschem
A re-evaluation of the initial yield of the hydrated electron in the picosecond time range Yusa Muroya, Mingzhang Lin, Guozhong Wu, Hokuto Iijima, Koji Yoshii, Toru Ueda, Hisaaki Kudo, Yosuke Katsumura Nuclear Engineering Research Laboratory, Graduate School of Engineering, The University of Tokyo, 2-22 Shirakata-shirane, Tokai-mura, Naka-gun, Ibaraki 319-1188, Japan Received 28 July 2003; accepted 26 February 2004
Abstract The yield of the hydrated electron in the picosecond time range has been re-evaluated with an ultrafast pulse radiolysis system using a laser photocathode RF-gun in combination with a conventional one, and a value of 4.170.2 per 100 eV of absorbed energy at 20 ps was derived. This is consistent with recent experimental results using a time correlation method [Bartels et al., J. Phys. Chem. A 104, 1686-1691 (2000)] and with Monte-Carlo calculations [Muroya et al., Can. J. Chem. 80 1367-1374 (2002)]. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrated electron; Picosecond yield; RF-photocathode; Laser-driven pulse radiolysis system
1. Introduction Water radiolysis is important not only in radiation chemistry but also in radiation biology, nuclear technology, and environmental radiation protection, because an understanding of the radiolysis of aqueous solutions is a key to basic knowledge in these fields (Farhataziz and Rodgers, 1987; Jonah and Rao, 2001; Mozumder and Hatano, 2004). After the discovery of the hydrated electron (e aq) (Boag and Hart, 1963; Keene, 1963), measurements of its yield at early times have been extensively investigated. The first measurement was carried out by a stroboscopic pulse radiolysis system at the University of Toronto (Bronskill et al., 1970), where the temporal behavior in a pulse train (0–350 ps), using an S-band linac, was observed using Cerenkov light as Corresponding author. Tel.: +81 3 5841 6979; fax: +81 3 5841 8624. E-mail address: [email protected] (Y. Katsumura).
an analyzing light. A G-value of 4.170.2 per 100 eV of absorbed energy at 30 ps was reported (Wolff et al., 1973). Almost the same value was reported by a collaborative work between the University of Toronto and the Argonne National Laboratory (ANL) (Hunt et al., 1973). The same stroboscopic method has been developed at Hokkaido University and applied to the measurement of the G-value of the hydrated electron. A value of 4.870.3 at 30 ps was reported (Sumiyoshi and Katayama, 1982; Sumiyoshi et al., 1985). In the 1970s, a single picosecond electron pulse became available at ANL and a Gðe aq Þ of 4.170.1 at 200 ps (Jonah et al., 1973) was reported using a fast vacuum photodiode. Soon after, by combining the single picosecond pulse and a 2701 magnet, a single picosecond pulse stroboscopic system covering the time range of ps to a few ns was constructed at ANL (Jonah, 1975) and a Gðe aq Þ of 4.670.2 at 100 ps (Jonah et al., 1976) was reported. However, the ANL group recently employed the time correlation method and re-evaluated the value at zero
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.09.011
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time to be 4.070.2 (Bartels et al., 2000). In addition, the discrepancy between kinetic and scavenging data was also reconsidered (Pimblott et al., 1996), and a Gðe aq Þ value of 4.8 was chosen as a reasonable value. The picosecond yield of Gðe aq Þ is an important value but the reported values are scattered from 4.0 to 4.8, so no clear consensus exists. Firstly, this is partly due to the difficulty of data analysis obtained by the stroboscopic method, because, in the pulse train, experimental data for later pulses are affected by earlier pulses. Secondly, in order to evaluate the G-value, dose evaluation is inevitable but a reliable dosimetric method has not been established in the ps time range. In the present experiment, to resolve the existing discrepancy in the ps yield of Gðe aq Þ; temporal behavior in the ps time range was obtained by a newly installed ultrafast pulse radiolysis system with a laser photocathode RF-gun at the Nuclear Engineering Research Laboratory, University of Tokyo (Muroya et al., 2001, 2002a). Under the same experimental conditions, the yield in the sub-microsecond time range was also observed by the conventional pulse radiolysis method. Since the value of the hydrated electron yield has an accepted value of about 2.8 at 0.1 ms (Buxton 1972; Shiraishi et al., 1988), a reliable G-value of e aq at ps can be easily determined. Our present data strongly support the recent results of Bartels et al. (2000) at ANL and are consistent with recent Monte-Carlo calculations (Muroya et al., 2002b).
2. Experimental Time-resolved transient absorption spectroscopy was performed by a pump-and-probe method. Details and performance of the system have been reported elsewhere (Muroya et al., 2001, 2002a). Briefly, an S-band linear accelerator with a laser photocathode RF-gun that is triggered with 265-nm laser pulses prepared by thirdharmonic generation using a femtosecond Ti:Sapphire laser (795 nm), is operated at a repetition rate of 10 Hz. The laser pulse is split by a half mirror and used for two purposes; as an analyzing light for the pump and probe measurement and as an injector for the photocathode RF-gun after the third-harmonic generation. An electron beam with pulse duration of 3 ps (full-width at halfmaximum), charge of 1 nC, and energy of 20 MeV, is produced as an irradiation source. The electron beam and the fs laser are synchronized within 2 ps (root mean square value) and an optical delay stage is employed for changing the time interval. Absorbance was calculated from the Lambert–Beer law expressed as log10(I0/I), where I0 and I represent the intensity of the reference and absorbed light, respectively. The intensity of the two light signals was measured using Si PIN photodiodes (Hamamatsu/S1722-02) and an oscilloscope (Hewlett-
Packard/HP54845). Measurement as a function of time was done by repeating the following three procedures: (i) detection of laser intensities by the photodiodes and the oscilloscope, (ii) absorbance calculation, and (iii) adding optical delay of length dl corresponding to the time difference dt(=dl/c), where c represents the velocity of light. Kinetic measurements for times greater than 10 ns were carried out at 633 nm, the He–Ne laser wavelength. It is essential that both kinetic and pump–probe measurements be done under the same conditions. Therefore, a few filters with pin-holes were employed and special care was taken to keep the light path of the He–Ne laser beam the same as that of the 795-nm laser beam, because the spatial distribution of dose significantly depends on the position. Since the absorption spectra in ps and ns time ranges are assumed to be identical, the absorbances obtained at each wavelength were normalized by the absorption coefficients at 633 and 795 nm, respectively (Jou and Freeman, 1977). Millipore water, bubbled with nitrogen, was used as the target sample. In order to protect the sample from accumulation dose, a flow system was used, in which the sample was supplied from a reservoir at a rate of 3 ml/ min and passed through irradiation cells with optical path lengths of 5 or 18 mm.
3. Results and discussion The temporal behavior of the hydrated electron was measured by using the pump-and-probe method in the picosecond range and one typical data set is shown in Fig. 1, where a 5-mm optical cell was used. The rise time was 9 ps and this can be assumed to be the total time resolution of the present data. According to the fs laser photolysis literature, the solvation time of the hydrated electron is less than 1 ps (Migus et al., 1987; Lu et al., 1989; Alfano et al., 1993). A formula, dtT ¼ dtF þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dt2P þ dt2J ; where dtT, dtF, dtP, and dtJ represent the total time resolution, the time resolution due to the flight difference between the electron beam and the laser light through the sample, the pulse duration, and the time jitter between the laser and the electron pulses, respectively, gives the total time resolution (Muroya et al., 2002a). Taking into account the time resolution of 5 ps due to the flight difference in the 5-mm cell, the pulse duration of the electron beam of 3 ps, and the time jitter of about 2 ps, the observed rise time of 9 ps seems reasonable. Two signals in the ps time region obtained with the 5and 18-mm optical cells are plotted in Fig. 2 after normalization of both signal intensities at 100 ps. Data at less than 9 and 20 ps are excluded because of the total time resolutions of 9 and 20 ps in the experiments using
ARTICLE IN PRESS Y. Muroya et al. / Radiation Physics and Chemistry 72 (2005) 169–172 0.1
Absorbance
0.08 0.06 9ps 0.08
0.04
0.04 0.02
0 0
10
20
100
150 Time /ps
30
40
0 0
50
200
250
300
Fig. 1. Temporal behavior of the hydrated electron in the picosecond time range measured by the ultrafast pulse radiolysis system. A cell with 5-mm optical length and a pulse duration of 3 ps were employed. The growth part of the absorption is shown in the inset.
G-value /molec.(100 eV)-1
6 (b2)
5
G(e-aq)=2.38
(b1) G(e-aq)=2.7
4 3
(a1)
2
(a2)
G(e -aq)=4.1
1 0 10-12
10-11
10-10
10-9 Time /s
10-8
10-7
10-6
Fig. 2. Time dependence of the hydrated electron yield in the picosecond and sub-microsecond time ranges. The time profiles (a1) in the picosecond range and (a2) in the picosecond and subms ranges were taken with the 5- and 18-mm optical cells, respectively. Solid (b1) and dotted (b2) lines are from MonteCarlo calculations reported by Muroya et al. (2002b) and Frongillo et al. (1998), respectively. The G(e aq) value of 2.38 at 300 ns gives a value of 4.170.2 at 20 ps (see text).
the 5- and 18-mm cells, respectively. The reproducibility in the ps time range is satisfactory, irrespective of independent experiments with different optical cells. For the measurements in the sub-microsecond time region, just after the ps measurements with the 18-mm optical cell, a He–Ne laser was introduced keeping the experimental system. Data obtained by kinetic spectroscopy at times shorter than 10 ns have also been eliminated because of the photodiode response time of 10 ns. The G-value of the hydrated electron at 120 ns has been reported to be around 2.8 (Buxton, 1972). At 70
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and 300 ns, values of 2.93 and 2.67, respectively, have also been determined (Shiraishi et al., 1988) at a dose of 3.8 Gy per pulse. These values are consistent with the evaluation from the product analysis of the scavenging capacity at 108 s1 (Draganic´ and Draganic´, 1973). In addition, it was also reported that the decay of the hydrated electron is dependent on the dose per pulse (Shiraishi et al., 1988). From the absorbance of the hydrated electron obtained in the picosecond time scale, the dose per pulse was estimated to be 12–15 Gy. The decay behavior under the present dose conditions is similar to that reported by Shiraishi et al. (1988), supporting the reliability of the present experimental results. Taking into consideration the present dose per pulse, a value of 2.38 for the yield of the hydrated electron at 300 ns was chosen. Then, this gives a value of 4.170.2 for G(e aq) at 20 ps. The value of 4.1 is identical to early and recently reported values of 4.170.2 (Hunt et al., 1973a; Wolff et al., 1973b; Jonah et al., 1973; Bartels et al., 2000), but is in sharp contrast to the values of 4.6 (Jonah et al., 1976) and 4.8 (Sumiyoshi and Katayama, 1982; Sumiyoshi et al., 1985). It is clear that there is a large discrepancy between the present experimental results as (a1) and (a2) and previous Monte-Carlo simulations of water radiolysis (b2) in Fig. 2 (Frongillo et al., 1998). A re-evaluation of Monte-Carlo calculations (Muroya et al., 2002b), indicated as (b1) in Fig. 2, predicted a lower value for G(e aq) in the picosecond time range. A G-value of 4.4–4.5 at 1 ps was obtained after re-examination of certain adjustable parameters that intervene in the physical and physicochemical stages, such as the thermalization distance of subexcitation electrons, the recombination cross section of the electrons with their water parent cations prior to thermalization, and the branching ratios of the different competing mechanisms in the fragmentation decay of excited water molecules. In view of the small decay from 10 to 20 ps, the calculated value of G(e aq) is in good agreement with the present experimentally determined value. From the temporal behavior in the sub-microsecond region, both calculations are equivalent. It is clear in Fig. 2 that the experimental data after 100 ns are deviating from the calculations. This is due to the homogeneous reactions of the hydrated electron with itself, OH and other chemical species dependent on the pulse dose.
Acknowledgements We thank Prof. M. Uesaka for his encouragement during the experiment. We also thank Prof. J.-P. JayGerin (University of Sherbrooke) for valuable comments from a theoretical aspect.
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