- Email: [email protected]
Development of laser-synchronized picosecond pulse radiolysis system
Radiation Physics and Chemistry 60 (2001) 313–318
Development of laser-synchronized picosecond pulse radiolysis system Y. Yoshidaa,*, Y. Mizutania, T. Kozawaa, A. Saekia, S. Sekia, S. Tagawaa, K. Ushidab a
The Institute of Scientific and Industrial Research, Osaka University Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan b The Institute of Physical and Chemical Research (RIKEN) Hirosawa 2-1, Wako-shi, Saitama 351-01, Japan
Abstract A new picosecond pulse radiolysis system for absorption spectroscopy by using a 20 ps single electron pulse from the 28 MeV L-band electron linac and a single femtosecond laser pulse was developed for the research of the ultra-fast phenomena in primary processes of radiation chemistry. The electron pulse was used for the irradiation source and the laser pulse was used for the analyzing light. Both pulses are synchronized within several picoseconds by controlling a common radio frequency. The available analyzing light was from 360 to 1000 nm by using white-light continuum. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Picosecond pulse radiolysis; Electron linac; Femtosecond laser; Synchronization
1. Introduction Primary processes of radiation chemistry are very important to know the whole reaction processes induced by high-energy radiation. Pulse radiolysis technique is one of the most powerful methods to research the primary processes, because very fast reaction can be detected directly. In order to measure the very fast reaction by picosecond pulse radiolysis, the time-resolved emission or absorption spectroscopy is required. The studies on excited states and energy transfer were done by using picosecond pulse radiolysis for emission spectroscopy (Tagawa et al., 1979, 1980, 1982; Katsumura et al., 1980, 1982a, b) with a very fast response light detector such as a streak camera (Kobayashi et al., 1984; Yoshida et al., 1987). In absorption spectroscopy, the streak camera and very fast response detector (Tagawa et al., 1983)
*Corresponding author. Tel.: +81-6-6879-8511; fax: +81-66879-4346. E-mail address: [email protected] (Y. Yoshida).
were also used. However, the experiment had the difficulty to produce very intense analyzing light. The stroboscopic method does not require the very fast light detector. The time resolution is mainly decided by the pulse width of the radiation source and the analyzing light. Several types of picosecond pulse radiolysis system were developed since the first picosecond pulse radiolysis by Hunt et al. (Bronskill et al., 1970) in 1970. Argonne group (Jonah, 1975) developed the picosecond pulse radiolysis by using a single electron pulse from a L-band linac in 1980. Later the so-called twin-linac system (Tabata et al., 1985; Kobayashi et al., 1987; Yoshida et al., 1989) was developed at Tokyo University in 1985. In these systems, the Cherenkov light pulse produced by the high-energy electron was used for the analyzing light. Although the primary processes of radiation chemistry were studied by using pulse radiolysis, they were not good at the infrared spectroscopy, because of the low intensity of the Cherenkov light in this wavelength region. A picosecond pulse radiolysis system (Yoshida and Tagawa, 1992; Yoshida et al., 1993) by using the laser diode instead of the Cherenkov light was developed in 1991. The system enabled the absorption spectroscopy
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from visible to near-infrared region. However, it was difficult to obtain complete spectra, because the laser diode was not tunable light source. Recently, a new picosecond pulse radiolysis system has been developed at Osaka University. The femtosecond laser, which is synchronized with the picosecond electron pulse, is used for the analyzing light. The tunable laser pulse could cover the wide wavelength region from ultra violet to infrared by the non-linear optical effect. In this paper, the new system and its performance were described.
2. System of picosecond pulse radiolysis by using femtosecond laser Fig. 1 shows the block diagram of the new picosecond pulse radiolysis system by using femtosecond laser. The system was mainly composed of a 28 MeV L-band electron linac (Takeda et al., 1985), a Ti–sapphire femtosecond laser system, a control system of RF and triggers, a light detection system and a personal computer. The linac was composed of a thermal electron gun, two 1/12 (108 MHz) sub-harmonic pre-bunchers, a 1/6 (208 MHz) sub-harmonic pre-buncher, a 1.3 GHz prebuncher, a 1.3 GHz buncher and 1.3 GHz acceleration tube. The pulse width and maximum pulse charge are 20 ps and 67 nC, respectively. The repetition rate of the electron pulse was below 60 Hz. The pulse beam was transported to the irradiation room by an achromatic beam optics. A femtosecond Ti–sapphire laser (Spectra Physics : Tunami) is excited by a 12 W Ar ion laser (Spectra
Physics : 2060-12SA). A mode locker (A/O) of the Ti– sapphire laser was driven by 81 MHz from the control system of RF and triggers. A compensation system for the time jitter (lock to clock system) was equipped to keep the stability of the laser. The system could keep the length of the laser cavity to be constant by the feedback system comparing the oscillation frequency of the cw laser with the reference frequency of 81 MHz. The pulse width of the laser observed by using an autocorrelator was about 60 fs under the operation of 81 MHz. A single laser pulse synchronized with the electron pulse was extracted from the continuous laser pulse train of 81 MHz by the pulse selector (Spectra Physics : 39806S) located after the Ti–sapphire laser. The timing of the extraction was decided by the trigger signal from the control system of RF and triggers. The sample contained in the quartz (the pass length is from 0.5 to 2 cm) cell was irradiated by the picosecond electron pulse from the L-band linac. The single femtosecond laser pulse as the analyzing light also was passed through the sample. The time-dependent absorption was obtained by using a stroboscopic method. The time difference between the electron pulse and the laser pulse was changed by varying both the phase of 81 MHz provided to the laser system and the trigger timing provided to the pulse selector. The two phase shifters changed the phase and trigger timing. The shift range of each phase shifter is 0–3 ns and the shift position was moved with a pulse motor controlled by the computer. The wavelength region for absorption spectroscopy was from 750 to 850 nm, which was decided by the tunable region of the Ti–sapphire laser. The time resolution of transient absorption is decided by the width of the electron pulse, the width of the laser
Fig. 1. Picosecond pulse radiolysis system synchronized with a femtosecond Ti : sapphire oscillator.
Y. Yoshida et al. / Radiation Physics and Chemistry 60 (2001) 313–318
pulse, and the timing jitter between the electron pulse and the laser pulse. It is important for the light detection system to measure the correct amount of analyzing light. The detection system was composed of a Si-photodiode (Hamamatsu : S1722-02), and a digital oscilloscope (SONY Tektronix : TDS684B).
3. System of new picosecond pulse radiolysis by using white-light continuum For absorption spectroscopy with wide measurable wavelength region, the white-light continuum was used as the analyzing light. Fig. 2 shows the system by using the Ti–sapphire femtosecond regenerative amplifier (Spectra Physics : TSA-02) excited by a YAG laser (Spectra Physics : GCR-130-12). The white-light continuum was produced by focusing the high intense laser (1 mJ/pulse) into the water cell. The available wavelength is 360–1000 nm. The repetition rate of the system was below 12 Hz which was decided by the YAG laser. The timing of the amplified laser pulse was decided by the timing of the two Pockel cells installed in the regenerated amplifier. The timing triggers for the Pockels cells were provided from the control system of RF and triggers. However, the amplified laser pulse was still synchronized with 81 MHz, because the seed pulse for the regenerative amplifier was provided from the Ti–sapphire laser. To change time difference between an electron pulse and a laser pulse, the optical delay was inserted after the regenerative amplifier instead of the electronic phase
315
shifter, because the regenerative amplifier was very sensitive to the disturbance of the timing change of the seed pulse. The mechanical shutter was installed in front of the water cell for the generation of the white-light continuum. The function of the shutter is described in next section. The monochoromator was used for absorption spectroscopy. The other components for the white-light continuum were almost same as the system mentioned in Section 2.
4. Synchronization and triggering system In the above two systems, it is a very important technique to synchronize the femtosecond laser system with the L-band linac. Fig. 3 shows the control system for the RF synchronization and the triggering for the laser system and the linac. The basic clock was 27 MHz. The multiplied RF of 27 MHz was provided to the subharmonic pre-bunchers ( 4, 8), the pre-buncher ( 12), the buncher ( 12), and acceleration tube ( 12) of the linac. Also, 81 MHz, which was 3 times of 27 MHz was provided to the laser system. Trigger signals for the L-band linac and the laser system were generated by the triple synchronized system as shown in Fig. 3. The first trigger was synchronized between a start trigger from the computer and the 60 Hz. The delay/counter generated the delayed two trigger based on 27 MHz clock. The final two triggers were synchronized with 108 MHz. Therefore, the timing jitter of the trigger system depended on the timing jitter of
Fig. 2. Picosecond pulse radiolysis system synchronized with a femtosecond Ti : sapphire amplifier for white-light continuum generation.
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Fig. 3. Rf and triggering system.
108 MHz. The one trigger was provided to the electron gun to produce a single electron pulse. The other trigger was provided to the pulse selector or regenerative amplifier. The trigger for the YAG laser was also obtained from the laser trigger. Fig. 4 shows a demonstration of timing control between the laser pulse from the femtosecond Ti–sapphire and the Cherenkov light from the electron pulse observed by a streak camera (Hamamatsu photonics : C1370-01). The time position of the Cherenkov light, which corresponds to the middle pulse in the figure, shows the timing of the electron pulse. Each small pulse shows the laser pulses at every 200 ps, which is changed by the phase shifter. At the highest pulse, the laser pulse and the Cherenkov light are overlapped. In the measurement of the optical absorption, the four operation modes for the linac and the laser (modes A–D) were required. In each mode, both the linac and the laser, only the laser, only the linac and nothing were operated, respectively. The signal obtained in the mode A consists of the analyzing light containing the absorption signal, the Cherenkov radiation, the other radiation noise derived from the electron pulse and the RF noise from the linac. The signal in the mode B contains the analyzing light and the RF noise. The signal in the mode C contains the Cherenkov radiation, the other radiation noise and the RF noise. The RF noise was measured in the mode D. Therefore, real optical density were calculated by Optical density ¼ log ½ðb dÞ=ða cÞ;
ð1Þ
where a; b; c and d is signal intensity obtained in the mode A, B, C and D, respectively. The change of the
Fig. 4. Timing of laser and electron pulse by changing RF phase of laser (observed by streak camera).
modes was controlled by switching the trigger signal with the two on/off circuits, which were controlled by the computer. In the white-light continuum system, the analyzing light was controlled by the mechanical shutter instead of the on/off circuit, because the generative amplifier was unstable for the on/off operation by the circuit.
5. Primary processes of radiation chemistry of n-dodecane The geminate ion recombination (Warman, 1982; Yoshida et al., 1991) in n-dodecane was studied by using
Y. Yoshida et al. / Radiation Physics and Chemistry 60 (2001) 313–318
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References
Fig. 5. Time-dependent behavior in the wavelength region from 600 to 900 nm obtained in n-dodecane.
the new system. The most geminate pairs of an electron and an n-dodecane cation radical produced by highenergy electron recombined rapidly due to the long range of the Coulomb potential. Fig. 5 shows the typical time-dependent behavior of transient absorption obtained in the picosecond pulse radiolysis in liquid n-dodecane monitored from 650 to 900 nm (Tagawa et al., 1989). The cation radicals at 850 nm formed within the time resolution of the system. The decay showed a typical geminate decay. The excited states of n-dodecane at 650 nm showed the complicated time-dependent behavior, because the absorption of cation radical was overlapped at 650 nm. The detailed study on the geminate ion recombination will be reported elsewhere (Saeki et al., 2001).
6. Conclusion The new type of picosecond pulse radiolysis for absorption spectroscopy has been developed by using the synchronization technique between the linac and the femtosecond laser. The good signal/noise ratio could be obtained due to the high intense laser as an analyzing light. The available wavelength region was from 360 to 1000 nm by using white-light continuum. That will be extended to wider wavelength region by using the non-linear effect of the laser, such as secondharmonics generation, third-harmonics generation and optical parametric oscillation. The system will be advanced in next femtosecond pulse radiolysis system, which is under construction (Kozawa et al., 1999, 2000).
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. Instr. 41, 333. Jonah, C.D., 1975. A wide–time range pulse radiolysis system of picosecond time resolution. Rev. Sci. Instr. 46, 62. Katsumura, Y., Tabata, Y., Tagawa, S., 1982a. The formation process of the excited state of cyclohexane liquid using picosecond pulse radiolysis. Radiat. Phys. Chem. 19, 267. Katsumura, Y., Tagawa, S., Tabata, Y., 1980. Picosecond pulse radiolysis studies on excitation processes in liquid cyclohexane. J. Phys. Chem. 84, 833. Katsumura, Y., Tagawa, S., Tabata, Y., 1982b. Formation processes of solute excited states in various kinds of solvents studied by means of picosecond pulse radiolysis. Radiat. Phys. Chem. 19, 243. Kobayashi, H., Tabata, Y., Ueda, T., Kobayashi, T., 1987. A twin linac pulse radiolysis system (II). Nucl. Instr. and Meth. B 24/25, 1073. Kobayashi, H., Ueda, T., Kobayashi, T., Tagawa, S., Yoshida, Y., Tabata, Y., 1984. Absorption spectroscopy system based on picosecond single electron beams and streak camera. Radiat. Phys. Chem. 23, 4. Kozawa, T., Mizutani, Y., Miki, M., Yamamoto, T., Suemine, S., Yoshida, Y., Tagawa, S., 2000. Development of subpicosecond pulse radiolysis system. Nucl. Instr. and Meth. A 440, 251. Kozawa, T., Mizutani, Y., Yokoyama, K., Okuda, S., Yoshida, Y., Tagawa, S., 1999. Measurement of far-infrared subpicosecond coherent radiation for pulse radiolysis. Nucl. Instr. and Meth. A 429, 471. Saeki, A., Kozawa, T., Yoshida, Y., Tagawa, S., 2001. Study on geminate ion recombination in liquid dodecane using pico- and subpico- second pulse radiolysis. Radiat. Phys. Chem. 60, 319–322. Tabata, Y., Kobayashi, H., Washio, M., Tagawa, S., Yoshida, Y., 1985. Pulse radiolysis with picosecond time resolution. Radiat. Phys. Chem. 26, 473. Tagawa, S., Hayashi, N., Yoshida, Y., Washio, M., Tabata, Y., 1989. Pulse radiolysis studies on liquid alkanes and related polymers. Radiat. Phys. Chem. 34, 503. Tagawa, S., Katsumura, Y., Tabata, Y., 1979. The ultra-fast process of picosecond time-resolved energy transfer in liquid cyclohexane by picosecond single-pulse radiolysis. Chem. Phys. Lett. 64, 258. Tagawa, S., Katsumura, Y., Tabata, Y., 1980. Evidence for two fast formation processes of solute excited states in liquid toluene solutions studied by picosecond single pulse radiolysis. Radiat. Phys. Chem. 15, 287. Tagawa, S., Katsumura, Y., Tabata, Y., 1982. Picosecond pulse radiolysis studies on the formation of excited singlet states of solute molecules in several saturated hydrocarbon and ethanol solvent. Radiat. Phys. Chem. 19, 125. Tagawa, S., Washio, M., Kobayashi, H., Katsumura, Y., Tabata, Y., 1983. Geminate recombination kinetics of solute radical ions. Radiat. Phys. Chem. 21, 45. Takeda, S., Tsumori, K., Kimura, N., Yamamoto, T., Hori, T., Sawai, T., Ohkuma, J., Takamuku, S., Okada, T., Hayashi, K., Kawanishi, M., 1985. High-current single bunch electron linear accelerator. IEEE Trans. Nucl. Sci. NS-32, 3219.
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Warman, J.M., 1982. The dynamics of electrons and ions in non-polar liquids. In: Baxendale, J.H., Busi, F. (Eds.), The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis. Reidel, Dordrecht. Yoshida, Y., Tagawa, S., 1992. Dynamics of geminate ion pairs in liquid alkanes studied by LL-twin picosecond pulse radiolysis. In: Mataga, N., Okada, T., Masuhara, H. (Eds.), Dynamics and Mechanisms of Photoinduced Electron Transfer and Related Phenomena, Vol. 435. Elsevier Science Publisher, Amsterdam. Yoshida, Y., Tagawa, S., Kobayashi, H., Tabata, Y., 1987. Study of geminate ion recombination in a solute-solvent system by using picosecond pulse radiolysis. Radiat. Phys. Chem. 30, 83.
Yoshida, Y., Tagawa, S., Tabata, Y., 1991. Geminate ion recombination in nonpolar liquid. In: Tabata, Y. (Ed.), Pulse Radiolysis, Vol. 343. CRC Press, Boca Raton, FL. Yoshida, Y., Tagawa, S., Washio, M., Kobayashi, H., Tabata, Y., 1989. Picosecond pulse radiolysis on geminate ion recombination and formation of solute excited state in liquid cyclohexane. Radiat. Phys. Chem. 34, 493. Yoshida, Y., Ueda, T., Kobayashi, T., Shibata, H., Tagawa, S., 1993. Studies of geminate ion recombination and formation of excited states in liquid n-dodecane by means of a new picosecond pulse radiolysis system. Nucl. Instr. and Meth. A 327, 41.
Development of laser-synchronized picosecond pulse radiolysis system Y. Yoshidaa,*, Y. Mizutania, T. Kozawaa, A. Saekia, S. Sekia, S. Tagawaa, K. Ushidab a
The Institute of Scientific and Industrial Research, Osaka University Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan b The Institute of Physical and Chemical Research (RIKEN) Hirosawa 2-1, Wako-shi, Saitama 351-01, Japan
Abstract A new picosecond pulse radiolysis system for absorption spectroscopy by using a 20 ps single electron pulse from the 28 MeV L-band electron linac and a single femtosecond laser pulse was developed for the research of the ultra-fast phenomena in primary processes of radiation chemistry. The electron pulse was used for the irradiation source and the laser pulse was used for the analyzing light. Both pulses are synchronized within several picoseconds by controlling a common radio frequency. The available analyzing light was from 360 to 1000 nm by using white-light continuum. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Picosecond pulse radiolysis; Electron linac; Femtosecond laser; Synchronization
1. Introduction Primary processes of radiation chemistry are very important to know the whole reaction processes induced by high-energy radiation. Pulse radiolysis technique is one of the most powerful methods to research the primary processes, because very fast reaction can be detected directly. In order to measure the very fast reaction by picosecond pulse radiolysis, the time-resolved emission or absorption spectroscopy is required. The studies on excited states and energy transfer were done by using picosecond pulse radiolysis for emission spectroscopy (Tagawa et al., 1979, 1980, 1982; Katsumura et al., 1980, 1982a, b) with a very fast response light detector such as a streak camera (Kobayashi et al., 1984; Yoshida et al., 1987). In absorption spectroscopy, the streak camera and very fast response detector (Tagawa et al., 1983)
*Corresponding author. Tel.: +81-6-6879-8511; fax: +81-66879-4346. E-mail address: [email protected] (Y. Yoshida).
were also used. However, the experiment had the difficulty to produce very intense analyzing light. The stroboscopic method does not require the very fast light detector. The time resolution is mainly decided by the pulse width of the radiation source and the analyzing light. Several types of picosecond pulse radiolysis system were developed since the first picosecond pulse radiolysis by Hunt et al. (Bronskill et al., 1970) in 1970. Argonne group (Jonah, 1975) developed the picosecond pulse radiolysis by using a single electron pulse from a L-band linac in 1980. Later the so-called twin-linac system (Tabata et al., 1985; Kobayashi et al., 1987; Yoshida et al., 1989) was developed at Tokyo University in 1985. In these systems, the Cherenkov light pulse produced by the high-energy electron was used for the analyzing light. Although the primary processes of radiation chemistry were studied by using pulse radiolysis, they were not good at the infrared spectroscopy, because of the low intensity of the Cherenkov light in this wavelength region. A picosecond pulse radiolysis system (Yoshida and Tagawa, 1992; Yoshida et al., 1993) by using the laser diode instead of the Cherenkov light was developed in 1991. The system enabled the absorption spectroscopy
0969-806X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 3 6 8 - 6
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from visible to near-infrared region. However, it was difficult to obtain complete spectra, because the laser diode was not tunable light source. Recently, a new picosecond pulse radiolysis system has been developed at Osaka University. The femtosecond laser, which is synchronized with the picosecond electron pulse, is used for the analyzing light. The tunable laser pulse could cover the wide wavelength region from ultra violet to infrared by the non-linear optical effect. In this paper, the new system and its performance were described.
2. System of picosecond pulse radiolysis by using femtosecond laser Fig. 1 shows the block diagram of the new picosecond pulse radiolysis system by using femtosecond laser. The system was mainly composed of a 28 MeV L-band electron linac (Takeda et al., 1985), a Ti–sapphire femtosecond laser system, a control system of RF and triggers, a light detection system and a personal computer. The linac was composed of a thermal electron gun, two 1/12 (108 MHz) sub-harmonic pre-bunchers, a 1/6 (208 MHz) sub-harmonic pre-buncher, a 1.3 GHz prebuncher, a 1.3 GHz buncher and 1.3 GHz acceleration tube. The pulse width and maximum pulse charge are 20 ps and 67 nC, respectively. The repetition rate of the electron pulse was below 60 Hz. The pulse beam was transported to the irradiation room by an achromatic beam optics. A femtosecond Ti–sapphire laser (Spectra Physics : Tunami) is excited by a 12 W Ar ion laser (Spectra
Physics : 2060-12SA). A mode locker (A/O) of the Ti– sapphire laser was driven by 81 MHz from the control system of RF and triggers. A compensation system for the time jitter (lock to clock system) was equipped to keep the stability of the laser. The system could keep the length of the laser cavity to be constant by the feedback system comparing the oscillation frequency of the cw laser with the reference frequency of 81 MHz. The pulse width of the laser observed by using an autocorrelator was about 60 fs under the operation of 81 MHz. A single laser pulse synchronized with the electron pulse was extracted from the continuous laser pulse train of 81 MHz by the pulse selector (Spectra Physics : 39806S) located after the Ti–sapphire laser. The timing of the extraction was decided by the trigger signal from the control system of RF and triggers. The sample contained in the quartz (the pass length is from 0.5 to 2 cm) cell was irradiated by the picosecond electron pulse from the L-band linac. The single femtosecond laser pulse as the analyzing light also was passed through the sample. The time-dependent absorption was obtained by using a stroboscopic method. The time difference between the electron pulse and the laser pulse was changed by varying both the phase of 81 MHz provided to the laser system and the trigger timing provided to the pulse selector. The two phase shifters changed the phase and trigger timing. The shift range of each phase shifter is 0–3 ns and the shift position was moved with a pulse motor controlled by the computer. The wavelength region for absorption spectroscopy was from 750 to 850 nm, which was decided by the tunable region of the Ti–sapphire laser. The time resolution of transient absorption is decided by the width of the electron pulse, the width of the laser
Fig. 1. Picosecond pulse radiolysis system synchronized with a femtosecond Ti : sapphire oscillator.
Y. Yoshida et al. / Radiation Physics and Chemistry 60 (2001) 313–318
pulse, and the timing jitter between the electron pulse and the laser pulse. It is important for the light detection system to measure the correct amount of analyzing light. The detection system was composed of a Si-photodiode (Hamamatsu : S1722-02), and a digital oscilloscope (SONY Tektronix : TDS684B).
3. System of new picosecond pulse radiolysis by using white-light continuum For absorption spectroscopy with wide measurable wavelength region, the white-light continuum was used as the analyzing light. Fig. 2 shows the system by using the Ti–sapphire femtosecond regenerative amplifier (Spectra Physics : TSA-02) excited by a YAG laser (Spectra Physics : GCR-130-12). The white-light continuum was produced by focusing the high intense laser (1 mJ/pulse) into the water cell. The available wavelength is 360–1000 nm. The repetition rate of the system was below 12 Hz which was decided by the YAG laser. The timing of the amplified laser pulse was decided by the timing of the two Pockel cells installed in the regenerated amplifier. The timing triggers for the Pockels cells were provided from the control system of RF and triggers. However, the amplified laser pulse was still synchronized with 81 MHz, because the seed pulse for the regenerative amplifier was provided from the Ti–sapphire laser. To change time difference between an electron pulse and a laser pulse, the optical delay was inserted after the regenerative amplifier instead of the electronic phase
315
shifter, because the regenerative amplifier was very sensitive to the disturbance of the timing change of the seed pulse. The mechanical shutter was installed in front of the water cell for the generation of the white-light continuum. The function of the shutter is described in next section. The monochoromator was used for absorption spectroscopy. The other components for the white-light continuum were almost same as the system mentioned in Section 2.
4. Synchronization and triggering system In the above two systems, it is a very important technique to synchronize the femtosecond laser system with the L-band linac. Fig. 3 shows the control system for the RF synchronization and the triggering for the laser system and the linac. The basic clock was 27 MHz. The multiplied RF of 27 MHz was provided to the subharmonic pre-bunchers ( 4, 8), the pre-buncher ( 12), the buncher ( 12), and acceleration tube ( 12) of the linac. Also, 81 MHz, which was 3 times of 27 MHz was provided to the laser system. Trigger signals for the L-band linac and the laser system were generated by the triple synchronized system as shown in Fig. 3. The first trigger was synchronized between a start trigger from the computer and the 60 Hz. The delay/counter generated the delayed two trigger based on 27 MHz clock. The final two triggers were synchronized with 108 MHz. Therefore, the timing jitter of the trigger system depended on the timing jitter of
Fig. 2. Picosecond pulse radiolysis system synchronized with a femtosecond Ti : sapphire amplifier for white-light continuum generation.
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Fig. 3. Rf and triggering system.
108 MHz. The one trigger was provided to the electron gun to produce a single electron pulse. The other trigger was provided to the pulse selector or regenerative amplifier. The trigger for the YAG laser was also obtained from the laser trigger. Fig. 4 shows a demonstration of timing control between the laser pulse from the femtosecond Ti–sapphire and the Cherenkov light from the electron pulse observed by a streak camera (Hamamatsu photonics : C1370-01). The time position of the Cherenkov light, which corresponds to the middle pulse in the figure, shows the timing of the electron pulse. Each small pulse shows the laser pulses at every 200 ps, which is changed by the phase shifter. At the highest pulse, the laser pulse and the Cherenkov light are overlapped. In the measurement of the optical absorption, the four operation modes for the linac and the laser (modes A–D) were required. In each mode, both the linac and the laser, only the laser, only the linac and nothing were operated, respectively. The signal obtained in the mode A consists of the analyzing light containing the absorption signal, the Cherenkov radiation, the other radiation noise derived from the electron pulse and the RF noise from the linac. The signal in the mode B contains the analyzing light and the RF noise. The signal in the mode C contains the Cherenkov radiation, the other radiation noise and the RF noise. The RF noise was measured in the mode D. Therefore, real optical density were calculated by Optical density ¼ log ½ðb dÞ=ða cÞ;
ð1Þ
where a; b; c and d is signal intensity obtained in the mode A, B, C and D, respectively. The change of the
Fig. 4. Timing of laser and electron pulse by changing RF phase of laser (observed by streak camera).
modes was controlled by switching the trigger signal with the two on/off circuits, which were controlled by the computer. In the white-light continuum system, the analyzing light was controlled by the mechanical shutter instead of the on/off circuit, because the generative amplifier was unstable for the on/off operation by the circuit.
5. Primary processes of radiation chemistry of n-dodecane The geminate ion recombination (Warman, 1982; Yoshida et al., 1991) in n-dodecane was studied by using
Y. Yoshida et al. / Radiation Physics and Chemistry 60 (2001) 313–318
317
References
Fig. 5. Time-dependent behavior in the wavelength region from 600 to 900 nm obtained in n-dodecane.
the new system. The most geminate pairs of an electron and an n-dodecane cation radical produced by highenergy electron recombined rapidly due to the long range of the Coulomb potential. Fig. 5 shows the typical time-dependent behavior of transient absorption obtained in the picosecond pulse radiolysis in liquid n-dodecane monitored from 650 to 900 nm (Tagawa et al., 1989). The cation radicals at 850 nm formed within the time resolution of the system. The decay showed a typical geminate decay. The excited states of n-dodecane at 650 nm showed the complicated time-dependent behavior, because the absorption of cation radical was overlapped at 650 nm. The detailed study on the geminate ion recombination will be reported elsewhere (Saeki et al., 2001).
6. Conclusion The new type of picosecond pulse radiolysis for absorption spectroscopy has been developed by using the synchronization technique between the linac and the femtosecond laser. The good signal/noise ratio could be obtained due to the high intense laser as an analyzing light. The available wavelength region was from 360 to 1000 nm by using white-light continuum. That will be extended to wider wavelength region by using the non-linear effect of the laser, such as secondharmonics generation, third-harmonics generation and optical parametric oscillation. The system will be advanced in next femtosecond pulse radiolysis system, which is under construction (Kozawa et al., 1999, 2000).
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. Instr. 41, 333. Jonah, C.D., 1975. A wide–time range pulse radiolysis system of picosecond time resolution. Rev. Sci. Instr. 46, 62. Katsumura, Y., Tabata, Y., Tagawa, S., 1982a. The formation process of the excited state of cyclohexane liquid using picosecond pulse radiolysis. Radiat. Phys. Chem. 19, 267. Katsumura, Y., Tagawa, S., Tabata, Y., 1980. Picosecond pulse radiolysis studies on excitation processes in liquid cyclohexane. J. Phys. Chem. 84, 833. Katsumura, Y., Tagawa, S., Tabata, Y., 1982b. Formation processes of solute excited states in various kinds of solvents studied by means of picosecond pulse radiolysis. Radiat. Phys. Chem. 19, 243. Kobayashi, H., Tabata, Y., Ueda, T., Kobayashi, T., 1987. A twin linac pulse radiolysis system (II). Nucl. Instr. and Meth. B 24/25, 1073. Kobayashi, H., Ueda, T., Kobayashi, T., Tagawa, S., Yoshida, Y., Tabata, Y., 1984. Absorption spectroscopy system based on picosecond single electron beams and streak camera. Radiat. Phys. Chem. 23, 4. Kozawa, T., Mizutani, Y., Miki, M., Yamamoto, T., Suemine, S., Yoshida, Y., Tagawa, S., 2000. Development of subpicosecond pulse radiolysis system. Nucl. Instr. and Meth. A 440, 251. Kozawa, T., Mizutani, Y., Yokoyama, K., Okuda, S., Yoshida, Y., Tagawa, S., 1999. Measurement of far-infrared subpicosecond coherent radiation for pulse radiolysis. Nucl. Instr. and Meth. A 429, 471. Saeki, A., Kozawa, T., Yoshida, Y., Tagawa, S., 2001. Study on geminate ion recombination in liquid dodecane using pico- and subpico- second pulse radiolysis. Radiat. Phys. Chem. 60, 319–322. Tabata, Y., Kobayashi, H., Washio, M., Tagawa, S., Yoshida, Y., 1985. Pulse radiolysis with picosecond time resolution. Radiat. Phys. Chem. 26, 473. Tagawa, S., Hayashi, N., Yoshida, Y., Washio, M., Tabata, Y., 1989. Pulse radiolysis studies on liquid alkanes and related polymers. Radiat. Phys. Chem. 34, 503. Tagawa, S., Katsumura, Y., Tabata, Y., 1979. The ultra-fast process of picosecond time-resolved energy transfer in liquid cyclohexane by picosecond single-pulse radiolysis. Chem. Phys. Lett. 64, 258. Tagawa, S., Katsumura, Y., Tabata, Y., 1980. Evidence for two fast formation processes of solute excited states in liquid toluene solutions studied by picosecond single pulse radiolysis. Radiat. Phys. Chem. 15, 287. Tagawa, S., Katsumura, Y., Tabata, Y., 1982. Picosecond pulse radiolysis studies on the formation of excited singlet states of solute molecules in several saturated hydrocarbon and ethanol solvent. Radiat. Phys. Chem. 19, 125. Tagawa, S., Washio, M., Kobayashi, H., Katsumura, Y., Tabata, Y., 1983. Geminate recombination kinetics of solute radical ions. Radiat. Phys. Chem. 21, 45. Takeda, S., Tsumori, K., Kimura, N., Yamamoto, T., Hori, T., Sawai, T., Ohkuma, J., Takamuku, S., Okada, T., Hayashi, K., Kawanishi, M., 1985. High-current single bunch electron linear accelerator. IEEE Trans. Nucl. Sci. NS-32, 3219.
318
Y. Yoshida et al. / Radiation Physics and Chemistry 60 (2001) 313–318
Warman, J.M., 1982. The dynamics of electrons and ions in non-polar liquids. In: Baxendale, J.H., Busi, F. (Eds.), The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis. Reidel, Dordrecht. Yoshida, Y., Tagawa, S., 1992. Dynamics of geminate ion pairs in liquid alkanes studied by LL-twin picosecond pulse radiolysis. In: Mataga, N., Okada, T., Masuhara, H. (Eds.), Dynamics and Mechanisms of Photoinduced Electron Transfer and Related Phenomena, Vol. 435. Elsevier Science Publisher, Amsterdam. Yoshida, Y., Tagawa, S., Kobayashi, H., Tabata, Y., 1987. Study of geminate ion recombination in a solute-solvent system by using picosecond pulse radiolysis. Radiat. Phys. Chem. 30, 83.
Yoshida, Y., Tagawa, S., Tabata, Y., 1991. Geminate ion recombination in nonpolar liquid. In: Tabata, Y. (Ed.), Pulse Radiolysis, Vol. 343. CRC Press, Boca Raton, FL. Yoshida, Y., Tagawa, S., Washio, M., Kobayashi, H., Tabata, Y., 1989. Picosecond pulse radiolysis on geminate ion recombination and formation of solute excited state in liquid cyclohexane. Radiat. Phys. Chem. 34, 493. Yoshida, Y., Ueda, T., Kobayashi, T., Shibata, H., Tagawa, S., 1993. Studies of geminate ion recombination and formation of excited states in liquid n-dodecane by means of a new picosecond pulse radiolysis system. Nucl. Instr. and Meth. A 327, 41.