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Picosecond defect formation by optical conversion from self-trapped excitons in alkali halide crystals
Radiat. Phys. Chem. Vol. 34, No. 4, pp. 625-628, 1989 Int. J. Radiat. AppL Instrum., Part C
0146-5724/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc
Printed in Great Britain. All rights reserved
PICOSECOND DEFECT F O R M A T I O N BY OPTICAL CONVERSION FROM SELF-TRAPPED EXCITONS IN ALKALI HALIDE CRYSTALS Y. SUZUKI, H. ABE and M. HIRAI Department of Applied Physics, Faculty of Engineering, Tohoku University, Aramaki, Aoba, Sendai 980, Japan Abstract--The optical conversion from metastable self-trapped excitons at the lowest triplet state (STEL) to F centers in NaCI single crystals has been observed in picosecond range with time-delayed double excitation spectroscopy. The absorption due to STE L's induced by the first excitation with a KrF laser was instantaneously bleached under the second excitation with a mode-locked ruby laser, while the absorption due to F centers increased within the pulse duration (40 ps) of the ruby laser. The excitation power dependence of the secondarily induced F absorption intensity suggests that the conversion from STE L to F center occurs faster than 2 ps with an efficiency less than 8%.
INTRODUCTION
Radiation- or photo-induced defect formation process in alkali halide crystals is one of the best known chemical reactions in solids (Itoh, 1982). Strong electron-phonon coupling in these materials causes drastic rearrangement of the lattice to yield lattice defects (see for example Toyozawa, 1983). Primary defects by ionizing radiation are Frenkel pairs ( F - H pairs) consisting of an F center (a halogen (X) vacancy occupied by an electron), and an H center (an interstitial halogen atom with a molecular form of X~-). The F - H pairs are produced through a relaxation process of the self-trapped excitons (STE's X~-*), which is a halogen molecular ion ( O f ) binding an extra electron (-*). Since displacement of the X f core of the STE into an interstitial site results in the F - H pair creation, this process can be taken as a sort of isomeric reactions in condensed matters. As shown in Fig. 1, a main reaction pathway leading to the F - H pair creation is established as follows. (i) Selftrapping of a hole generated by ionizing radiation (kl), (ii) electron-capturing of the hole to yield a self-trapped exciton at high excited states: STEH (k2), (iii) internal relaxation of the STE H to a precursor state for the F - H formation (k3), (iv) branching of an F - H pair from the STE H at the precursor state (k4). A number of picosecond spectroscopic studies were made on this process to clarify its dynamics (Fujii et al., 1984; Hirai et al., 1987; Ortega, 1979; Provoost and Jacobs, 1981; Suzuki et al., 1979; Williams et al., 1978). However, since picosecond laser pulses or electron pulses were used to trigger the step (i) in the preceding studies, it was difficult to distinguish the rate-determining process of the F - H formation by such conventional picosecond photolyses. All the rate constants from kt to k 4 of the respective steps must contribute the observed growth rate of the final products: F - H pairs. 625
On the other hand, the time-delayed double excitation spectroscopy was intensively applied to the F - H formation process in recent years (Soda and Itoh, 1981; Tanimura and Itoh, 1984; Williams, 1976; Yoshinari et aL, 1978). This method allows us to observe the last step of the reaction (iv) without any interference from the preceding steps (i)-(iii). In this method, as shown in Fig. 1, the metastable STE at the lowest triplet state (STEL), which is an initial state of the photolysis, is accumulated by the first excitation. The lifetime of the STE L is as long as 10-6-10-3s depending on the crystals, because the radiative transition (n lumi. in Fig. 1) from STEL to ground state is spin forbidden. The STEL is pumped to the STEH by the second excitation before it decays, to induce the F - H formation. In the previous studies, nanosecond light pulses from Q-switched laser were used as the second excitation source to measure the excitation spectra (Soda and Itoh, 1981; Tanimura and Itoh, 1984) or the temperature dependence (Yoshinari et aL, 1978) of the secondary F - H formation. In the present study, we substituted it to a picosecond pulse from a mode-locked ruby laser to measure the branching rate constant (k4) directly. The experimental results of NaC1 crystals are primarily presented in this paper. EXPERIMENTAL
The picosecond spectroscopic system of the double laser photolysis will be described in detail elsewhere (Suzuki et al., 1989). A nanosecond u.v. pulse from a K r F laser (5.00 eV, 20 ns) was employed as the first exciting source to generate the STEL'S through a two photon absorption process. A picosecond red pulse from a mode-locked ruby laser (1.78 eV, 40 ps) was used as the second excitation source to re-excite the STE L's to the STE n's. Since the decay time of the STE L in NaC1 at 14 K is 340/~s, the delay time of the
626
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--
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XFig. 1. A schematic energy diagram for a recombination process of electron and hole to yield F - H pairs (F + Hnn) in alkali halide crystals. The STE's represent the self-trapped excitons.
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ruby pulse to the KrF pulse was fixed to be 100#s with an electronic timing circuit. A conventional pump and probe technique with picosecond quasiwhite light was utilized to obtain time-resolved absorption spectra in picosecond range under the second excitation. Ultrapure graded NaCI single crystals were grown in Utah University. A 11 x 16 x 3.5mm block was held in a cryostat to keep its temperature at 14 K.
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Photon energy (eV)
EXPERIMENTAL RESULTS
Figure 2 shows the absorption spectra of NaC1 at 14 K at various times before and after the second excitation with a ruby laser. The time zero corresponds to the time when the peak of the ruby pulse reaches the crystal. The absorption band peaking around 2.0 eV is due to the STE L's and referred to as the STE absorption, while that peaking around 2.8 eV is due to the F center and referred to as the F absorption. The STE absorption at - 58 ps evidences that the STEL is predominantly generated by the first excitation with a K r F laser. The STE absorption decreases instantaneously by the second excitation as seen in the spectra from - 2 5 to 208 ps, while the F absorption increases as compensation for the decrease of the STE absorption. The OD's at 2.00 eV and 2.77 eV at various times are plotted with open circles in the upper and lower frames in Fig. 3. The former stands for the temporal change of the STE absorption, while the latter for that of the F absorption. The solid curves in the upper and lower frames are the response function of the system simulated assuming a stepwise fall and rise, respectively. An essential formula for the simulation with a convolution method is described in Suzuki et al., (1979), where the pulse durations of the exiting and probing lights are taken into consideration. The respective solid curves in Fig. 3 almost follow the open circles. This suggests that the optical conversion from STE L to F center, in other words, the branching
Fig. 2. Time-resolvedabsorption spectra of an NaC1 crystal at 14 K under the second excitation with a mode-locked ruby laser 100 #s after the first excitation with a KrF laser.
from STEH to F - H pair, occurs with a time constant much shorter than the pulse duration (40 ps) of the ruby laser light. Another notable point in Fig. 1 is a considerable relative change of the STE and F absorption intensities between - 5 8 and 208 ps. The spectral change from - 58 to 208 ps implies that most of the STE L's are converted to the F centers by the second excitation. This population change induced by the second excitation is much larger than that expected from the branching ratio in the first excitation process. The small F absorption in the spectra at - 58 ps suggest that the branching ratio from STEH to F center is less than 10%. Two possible reasons for such a high conversion rate in the second excitation can be considered. One is the selective branching to F - H pairs at photorepopulated STEH. Since the STEL is selectively pumped to the second lowest triplet state termed as bzu by the ruby laser light, this state may have an extremely efficient branching path to F - H pairs. The other is the repetitional excitation of STEL'S restored from STEH during the second excitation with the pulse duration of 40 ps. Even though the net conversion efficiency is not so high, the cyclic excitation and relaxation between STEL and STEH results in the
Picosecond defect formation
627
Soda and Itoh, 1981; Suzuki et aL, 1989). 0.3
o
\
(~1 = (])20"2/411(~' A 2 ) ,
0.2 NoCk ot 14K 0.1
o ODot2.0?eV --
"~
o~1~
Response function
I
0
o
v ~ o 0
I
9
I
I
NoCL at ~4K
8 0
0.4
(1)
where 42 is an efficiency for photo-excited electrons to the Franck~Condon state of the Sl state to populate at the relaxed state, and is assumed to be unity as mentioned. A's are constants determined from the following relation for the excitation intensity (I) dependence of the induced absorption (OD):
o O D at 2.77eV
OO (I) = ODMAx { 1 -- exp( -- AI)}.
f
(2)
- - Response function 0.3
0,2
0.1
0
o
I - I O0
I
I
I
0
1O0
2OO
Time (ps)
Fig. 3. Temporal change of the optical density at ZOOeV for the STE absorption and at 2.77 eV for the F absorption in NaCI under the second excitation with a ruby laser. The solid curves are calculated response functions of the detecting system, and are explained in the text.
accumulation of F - H pairs to enhance the apparent conversion rate. In order to criticize the two possibilities, the excitation power dependence of the secondarily induced F absorption height was observed. The changes of OD's at 2.77 eV induced 100 ps after the ruby laser excitation are plotted with open circles in Fig. 4 as a function of the excitation intensity. For comparison, the S,*--S~ absorption intensity of cryptocyanine (7.0 p M in propylene glycol) was also measured as a function of the excitation intensity of the ruby laser. OD at 2.29 eV (peak energy of the 5', ,--$1 absorption) at 100 ps is chosen as a representative of the induced S , ~ S ~ absorption intensity, and is plotted with closed circles in Fig. 4. Since the photo-populated S~ state of cryptocyanine in propylene glycol hardly relaxes to the ground state within a pulse duration of the ruby laser light and an inner filter effect of the $1 state for the ruby laser light seems negligible, we employed it as a standard system to give a unitary quantum efficiency for the ruby laser light. It is obvious that the excitation intensity to saturate the F absorption is about two order stronger than that to saturate the S , ~ S I absorption. This result suggests that the apparently high conversion rate from STE L to F center is mainly caused by the multiple excitation process of the restored STEL. Considering the absorption cross section of the F absorption in NaCI ( a ~ = 3 . 4 x 10-~7cm 2) and the S,~S~ absorption in cryptocyanine ( a 2 = 3 . 4 x 10 -16 cm 2) for the ruby laser light (1.78 eV), the net conversion efficiency (q~l) from STE n to F - H pair is given by the following equation (Lachish et aL, 1976; RPC 34/4--N
Here, ODMAx is the absorption intensity in saturation. The best-fit curves of equation (2) to open and closed circles are depicted in Fig. 4 with solid curves. Since the ruby laser light with the maximum intensity is still incapable of saturating the F absorption height, ODMAx of the F absorption is estimated from the depletion rate of the STE absorption at the maximum excitation intensity. The ratio of A] for the F absorption to A2 for the S , ~ S ] absorption is determined to be 8.2 x 10 -3 from the two curves in Fig. 4. Thus, the conversion efficiency ~ is obtained to be about 8% from equation (1) with the given values of ~b, A , A : , ~1 and tr2. Since the efficiencies for the other branching processes from STEn such as conversion to the singlet state and to the ground state are included in 4h (Soda and Itoh, 1981), the net conversion efficiency to F - H pairs would be less than 8%. Since the net conversion efficiency is so small, the apparently high conversion from STE L to F center seen in Figs 2 and 3 implies that the STEL undergoes multiple excitation with a rather high repetition cycle during the ruby laser irradiation. A brief consideration with using a partial sum of power series (Suzuki et al., 1989) suggests that more than 20 times cyclic excitation and relaxation takes place within the pulse duration of the ruby laser light (40 ps). Consequently,
04
S.~S1 in cryptocyanine t I 0
0.3 z
o,n
o2
~_ 0~
0
NaCl
10
100 INTENSITY ( a.u. )
1000
Fig. 4. Excitation intensity dependence of the change of optical density at 2.77 eV (peak energy of the F absorption) in NaCI at 14 K induced 100 ps after the second excitation with a ruby laser (©). (O) represent those at 2.29 eV (peak energy of the S,,,-S~ absorption) in cryptocyanine (7.0 #M in propylene glycol). Solid curves are described in the text.
628
Y. SUZUKIet al.
a period of the single branching process is estimated to be shorter than 2 ps.
DISCUSSION A generation process of STEL's in NaCI crystals was first time-resolved in picosecond range by Williams et al. (1978). They used the fourth harmonic of a mode-locked Nd : YAG laser for band to band excitation. Although multiple components included in the observed rise curve of the STE absorption made the analysis rather complex, they concluded that the STEL'S in NaC1 are formed from electronhole pairs (X°+ e in Fig. 1) faster than 5 ps. This result is consistent with the present one ( < 2 ps) on the branching step of STEL from STEH, and the both suggest that the relaxation from STEH to STEL occurs extremely fast in this crystal. In other alkali halides, the observed growth times of STEL range from 10 to 200 ps (Fujii et al., 1984; Hirai et al., 1987; Ortega, 1979; Provoost and Jacobs, 1981; Suzuki et al., 1979; Williams et al., 1978). Several authors discussed the non-radiative transition process from STE H to STE L (or F center) (Kabler and Williams, 1978; Soda and Itoh, 1981; Song and Leung, 1979; Suzuki et al., 1979). Suzuki et al. (1979) confirmed that a lower activation barrier between STEH and STEL than a vibrational energy is required to give a transition time of several picoseconds within a framework of Huang and Rhy's (1950) static formula. Most authors took the non-radiative transition from STEH to STE L to occur before thermal equilibrium is achieved at the STEH level. Song and Leung (1979) calculated the transition probabilities during the relaxation at high vibrational levels of the STEn, and obtained transition times of several picoseconds for the transition at the vibrational levels around the crossing point of the potential curves of the STE L and the STE L. Since the energy of vibration coupling to STE's are assumed to be about 10 meV (Song and Leung, 1979; Suzuki et al., 1979), a period of only 4 time vibrations corresponds to 2 ps. The fast transition time observed in the present study may be explained from such a dynamical mechanism of non-radiative transition.
CONCLUSION A spectroscopic system for time-delayed double laser photolysis has been developed to observe picosecond chemical reactions from photo-excited intermediates. The system was successfully applied to the investigation on defect formation process in alkali halide crystals. Among the consecutive reaction steps included in the process, the branching step from STEH's to STEL'S and F - H pairs was selectively observed to occur faster than 2 ps in NaCI crystals even at 14 K. An athermal mechanism of nonradiative transition to predominate the branching step was suggested for NaC1 crystals. Extended studies to the other alkali halides are required. Acknowledgment--The authors thank Mr E. Kitamura and
Mr S. Nakayama for their cooperation to construct the double laser photolysis system. REFERENCES
Fujii K., Kikuchi R., Katagiri S., Tsumori K. and Kawanishi M. (1984) Proc. 4th Int. Conf. Ultrafast Phenomena, Monterey, p. 402. Springer Series in Chemical Physics 38. Springer, Berlin. Hirai M., Suzuki Y., Hattori H., Ehara T. and Kitamura E. (1987) J. Phys. Soc. Jpn 56, 2948. Huang K. and Rhys A. (1950) Proc. R. Soc. A204, 406. Itoh N. (1982) Adv. Phys. 31, 491. Kabler M. N. and Williams R. T. (1978) Phys. Rev. B 18, 1948. Lachish U., Shafferman A. and Stein G. (1976) J. Chem. Phys. 64, 4205. Ortega J. M. (1979) Phys. Rev. B 19, 3232. Provoost J. and Jacobs G. (1981) Phys. Status Solidi b 104, K149. Soda K. and Itoh N. (1981) J. Phys. Soc. Jpn 50, 3988. Song K. S. and Leung C. H. (1979) Solid State Commun. 32, 565. Suzuki Y., Okumura M. and Hirai M. (1979) J. Phys. Soc. Jpn 47, 184. Suzuki Y., Abe H. and Hirai M. (1989) J. Phys. Soc. Jpn (Submitted). Tanimura K. and Itoh N. (1984) J. Phys. Chem. Solids 45, 323. Toyozawa Y. (1983) Semicon. Insul. 5, 175. Williams R. T. (1976) Phys. Rev. Lett. 36, 529. Williams R. T., Bradford J. N. and Faust W. L. (1978) Phys. Rev. B 18, 7083. Yoshinari T., Iwano H. and Hirai M. (1978) J. Phys. Soc. Jpn 45, 936, 1926.
0146-5724/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc
Printed in Great Britain. All rights reserved
PICOSECOND DEFECT F O R M A T I O N BY OPTICAL CONVERSION FROM SELF-TRAPPED EXCITONS IN ALKALI HALIDE CRYSTALS Y. SUZUKI, H. ABE and M. HIRAI Department of Applied Physics, Faculty of Engineering, Tohoku University, Aramaki, Aoba, Sendai 980, Japan Abstract--The optical conversion from metastable self-trapped excitons at the lowest triplet state (STEL) to F centers in NaCI single crystals has been observed in picosecond range with time-delayed double excitation spectroscopy. The absorption due to STE L's induced by the first excitation with a KrF laser was instantaneously bleached under the second excitation with a mode-locked ruby laser, while the absorption due to F centers increased within the pulse duration (40 ps) of the ruby laser. The excitation power dependence of the secondarily induced F absorption intensity suggests that the conversion from STE L to F center occurs faster than 2 ps with an efficiency less than 8%.
INTRODUCTION
Radiation- or photo-induced defect formation process in alkali halide crystals is one of the best known chemical reactions in solids (Itoh, 1982). Strong electron-phonon coupling in these materials causes drastic rearrangement of the lattice to yield lattice defects (see for example Toyozawa, 1983). Primary defects by ionizing radiation are Frenkel pairs ( F - H pairs) consisting of an F center (a halogen (X) vacancy occupied by an electron), and an H center (an interstitial halogen atom with a molecular form of X~-). The F - H pairs are produced through a relaxation process of the self-trapped excitons (STE's X~-*), which is a halogen molecular ion ( O f ) binding an extra electron (-*). Since displacement of the X f core of the STE into an interstitial site results in the F - H pair creation, this process can be taken as a sort of isomeric reactions in condensed matters. As shown in Fig. 1, a main reaction pathway leading to the F - H pair creation is established as follows. (i) Selftrapping of a hole generated by ionizing radiation (kl), (ii) electron-capturing of the hole to yield a self-trapped exciton at high excited states: STEH (k2), (iii) internal relaxation of the STE H to a precursor state for the F - H formation (k3), (iv) branching of an F - H pair from the STE H at the precursor state (k4). A number of picosecond spectroscopic studies were made on this process to clarify its dynamics (Fujii et al., 1984; Hirai et al., 1987; Ortega, 1979; Provoost and Jacobs, 1981; Suzuki et al., 1979; Williams et al., 1978). However, since picosecond laser pulses or electron pulses were used to trigger the step (i) in the preceding studies, it was difficult to distinguish the rate-determining process of the F - H formation by such conventional picosecond photolyses. All the rate constants from kt to k 4 of the respective steps must contribute the observed growth rate of the final products: F - H pairs. 625
On the other hand, the time-delayed double excitation spectroscopy was intensively applied to the F - H formation process in recent years (Soda and Itoh, 1981; Tanimura and Itoh, 1984; Williams, 1976; Yoshinari et aL, 1978). This method allows us to observe the last step of the reaction (iv) without any interference from the preceding steps (i)-(iii). In this method, as shown in Fig. 1, the metastable STE at the lowest triplet state (STEL), which is an initial state of the photolysis, is accumulated by the first excitation. The lifetime of the STE L is as long as 10-6-10-3s depending on the crystals, because the radiative transition (n lumi. in Fig. 1) from STEL to ground state is spin forbidden. The STEL is pumped to the STEH by the second excitation before it decays, to induce the F - H formation. In the previous studies, nanosecond light pulses from Q-switched laser were used as the second excitation source to measure the excitation spectra (Soda and Itoh, 1981; Tanimura and Itoh, 1984) or the temperature dependence (Yoshinari et aL, 1978) of the secondary F - H formation. In the present study, we substituted it to a picosecond pulse from a mode-locked ruby laser to measure the branching rate constant (k4) directly. The experimental results of NaC1 crystals are primarily presented in this paper. EXPERIMENTAL
The picosecond spectroscopic system of the double laser photolysis will be described in detail elsewhere (Suzuki et al., 1989). A nanosecond u.v. pulse from a K r F laser (5.00 eV, 20 ns) was employed as the first exciting source to generate the STEL'S through a two photon absorption process. A picosecond red pulse from a mode-locked ruby laser (1.78 eV, 40 ps) was used as the second excitation source to re-excite the STE L's to the STE n's. Since the decay time of the STE L in NaC1 at 14 K is 340/~s, the delay time of the
626
Y. SUZUKIet al. X °+ e
--
0.4
k
X~+e
~
STEH
k3
~
NoCL ot 14K KrF ~ Rubyex.
0.3
kz
O
0.2
!
Q
0.1
I st E X
20Bps
"°'e'" e " * * % • io •
2 nd EX ,.'."
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I
0
F + Ho°
0.1
XFig. 1. A schematic energy diagram for a recombination process of electron and hole to yield F - H pairs (F + Hnn) in alkali halide crystals. The STE's represent the self-trapped excitons.
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ruby pulse to the KrF pulse was fixed to be 100#s with an electronic timing circuit. A conventional pump and probe technique with picosecond quasiwhite light was utilized to obtain time-resolved absorption spectra in picosecond range under the second excitation. Ultrapure graded NaCI single crystals were grown in Utah University. A 11 x 16 x 3.5mm block was held in a cryostat to keep its temperature at 14 K.
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--58ps
I
I
2
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i
I
2.6
2.8
3
Photon energy (eV)
EXPERIMENTAL RESULTS
Figure 2 shows the absorption spectra of NaC1 at 14 K at various times before and after the second excitation with a ruby laser. The time zero corresponds to the time when the peak of the ruby pulse reaches the crystal. The absorption band peaking around 2.0 eV is due to the STE L's and referred to as the STE absorption, while that peaking around 2.8 eV is due to the F center and referred to as the F absorption. The STE absorption at - 58 ps evidences that the STEL is predominantly generated by the first excitation with a K r F laser. The STE absorption decreases instantaneously by the second excitation as seen in the spectra from - 2 5 to 208 ps, while the F absorption increases as compensation for the decrease of the STE absorption. The OD's at 2.00 eV and 2.77 eV at various times are plotted with open circles in the upper and lower frames in Fig. 3. The former stands for the temporal change of the STE absorption, while the latter for that of the F absorption. The solid curves in the upper and lower frames are the response function of the system simulated assuming a stepwise fall and rise, respectively. An essential formula for the simulation with a convolution method is described in Suzuki et al., (1979), where the pulse durations of the exiting and probing lights are taken into consideration. The respective solid curves in Fig. 3 almost follow the open circles. This suggests that the optical conversion from STE L to F center, in other words, the branching
Fig. 2. Time-resolvedabsorption spectra of an NaC1 crystal at 14 K under the second excitation with a mode-locked ruby laser 100 #s after the first excitation with a KrF laser.
from STEH to F - H pair, occurs with a time constant much shorter than the pulse duration (40 ps) of the ruby laser light. Another notable point in Fig. 1 is a considerable relative change of the STE and F absorption intensities between - 5 8 and 208 ps. The spectral change from - 58 to 208 ps implies that most of the STE L's are converted to the F centers by the second excitation. This population change induced by the second excitation is much larger than that expected from the branching ratio in the first excitation process. The small F absorption in the spectra at - 58 ps suggest that the branching ratio from STEH to F center is less than 10%. Two possible reasons for such a high conversion rate in the second excitation can be considered. One is the selective branching to F - H pairs at photorepopulated STEH. Since the STEL is selectively pumped to the second lowest triplet state termed as bzu by the ruby laser light, this state may have an extremely efficient branching path to F - H pairs. The other is the repetitional excitation of STEL'S restored from STEH during the second excitation with the pulse duration of 40 ps. Even though the net conversion efficiency is not so high, the cyclic excitation and relaxation between STEL and STEH results in the
Picosecond defect formation
627
Soda and Itoh, 1981; Suzuki et aL, 1989). 0.3
o
\
(~1 = (])20"2/411(~' A 2 ) ,
0.2 NoCk ot 14K 0.1
o ODot2.0?eV --
"~
o~1~
Response function
I
0
o
v ~ o 0
I
9
I
I
NoCL at ~4K
8 0
0.4
(1)
where 42 is an efficiency for photo-excited electrons to the Franck~Condon state of the Sl state to populate at the relaxed state, and is assumed to be unity as mentioned. A's are constants determined from the following relation for the excitation intensity (I) dependence of the induced absorption (OD):
o O D at 2.77eV
OO (I) = ODMAx { 1 -- exp( -- AI)}.
f
(2)
- - Response function 0.3
0,2
0.1
0
o
I - I O0
I
I
I
0
1O0
2OO
Time (ps)
Fig. 3. Temporal change of the optical density at ZOOeV for the STE absorption and at 2.77 eV for the F absorption in NaCI under the second excitation with a ruby laser. The solid curves are calculated response functions of the detecting system, and are explained in the text.
accumulation of F - H pairs to enhance the apparent conversion rate. In order to criticize the two possibilities, the excitation power dependence of the secondarily induced F absorption height was observed. The changes of OD's at 2.77 eV induced 100 ps after the ruby laser excitation are plotted with open circles in Fig. 4 as a function of the excitation intensity. For comparison, the S,*--S~ absorption intensity of cryptocyanine (7.0 p M in propylene glycol) was also measured as a function of the excitation intensity of the ruby laser. OD at 2.29 eV (peak energy of the 5', ,--$1 absorption) at 100 ps is chosen as a representative of the induced S , ~ S ~ absorption intensity, and is plotted with closed circles in Fig. 4. Since the photo-populated S~ state of cryptocyanine in propylene glycol hardly relaxes to the ground state within a pulse duration of the ruby laser light and an inner filter effect of the $1 state for the ruby laser light seems negligible, we employed it as a standard system to give a unitary quantum efficiency for the ruby laser light. It is obvious that the excitation intensity to saturate the F absorption is about two order stronger than that to saturate the S , ~ S I absorption. This result suggests that the apparently high conversion rate from STE L to F center is mainly caused by the multiple excitation process of the restored STEL. Considering the absorption cross section of the F absorption in NaCI ( a ~ = 3 . 4 x 10-~7cm 2) and the S,~S~ absorption in cryptocyanine ( a 2 = 3 . 4 x 10 -16 cm 2) for the ruby laser light (1.78 eV), the net conversion efficiency (q~l) from STE n to F - H pair is given by the following equation (Lachish et aL, 1976; RPC 34/4--N
Here, ODMAx is the absorption intensity in saturation. The best-fit curves of equation (2) to open and closed circles are depicted in Fig. 4 with solid curves. Since the ruby laser light with the maximum intensity is still incapable of saturating the F absorption height, ODMAx of the F absorption is estimated from the depletion rate of the STE absorption at the maximum excitation intensity. The ratio of A] for the F absorption to A2 for the S , ~ S ] absorption is determined to be 8.2 x 10 -3 from the two curves in Fig. 4. Thus, the conversion efficiency ~ is obtained to be about 8% from equation (1) with the given values of ~b, A , A : , ~1 and tr2. Since the efficiencies for the other branching processes from STEn such as conversion to the singlet state and to the ground state are included in 4h (Soda and Itoh, 1981), the net conversion efficiency to F - H pairs would be less than 8%. Since the net conversion efficiency is so small, the apparently high conversion from STE L to F center seen in Figs 2 and 3 implies that the STEL undergoes multiple excitation with a rather high repetition cycle during the ruby laser irradiation. A brief consideration with using a partial sum of power series (Suzuki et al., 1989) suggests that more than 20 times cyclic excitation and relaxation takes place within the pulse duration of the ruby laser light (40 ps). Consequently,
04
S.~S1 in cryptocyanine t I 0
0.3 z
o,n
o2
~_ 0~
0
NaCl
10
100 INTENSITY ( a.u. )
1000
Fig. 4. Excitation intensity dependence of the change of optical density at 2.77 eV (peak energy of the F absorption) in NaCI at 14 K induced 100 ps after the second excitation with a ruby laser (©). (O) represent those at 2.29 eV (peak energy of the S,,,-S~ absorption) in cryptocyanine (7.0 #M in propylene glycol). Solid curves are described in the text.
628
Y. SUZUKIet al.
a period of the single branching process is estimated to be shorter than 2 ps.
DISCUSSION A generation process of STEL's in NaCI crystals was first time-resolved in picosecond range by Williams et al. (1978). They used the fourth harmonic of a mode-locked Nd : YAG laser for band to band excitation. Although multiple components included in the observed rise curve of the STE absorption made the analysis rather complex, they concluded that the STEL'S in NaC1 are formed from electronhole pairs (X°+ e in Fig. 1) faster than 5 ps. This result is consistent with the present one ( < 2 ps) on the branching step of STEL from STEH, and the both suggest that the relaxation from STEH to STEL occurs extremely fast in this crystal. In other alkali halides, the observed growth times of STEL range from 10 to 200 ps (Fujii et al., 1984; Hirai et al., 1987; Ortega, 1979; Provoost and Jacobs, 1981; Suzuki et al., 1979; Williams et al., 1978). Several authors discussed the non-radiative transition process from STE H to STE L (or F center) (Kabler and Williams, 1978; Soda and Itoh, 1981; Song and Leung, 1979; Suzuki et al., 1979). Suzuki et al. (1979) confirmed that a lower activation barrier between STEH and STEL than a vibrational energy is required to give a transition time of several picoseconds within a framework of Huang and Rhy's (1950) static formula. Most authors took the non-radiative transition from STEH to STE L to occur before thermal equilibrium is achieved at the STEH level. Song and Leung (1979) calculated the transition probabilities during the relaxation at high vibrational levels of the STEn, and obtained transition times of several picoseconds for the transition at the vibrational levels around the crossing point of the potential curves of the STE L and the STE L. Since the energy of vibration coupling to STE's are assumed to be about 10 meV (Song and Leung, 1979; Suzuki et al., 1979), a period of only 4 time vibrations corresponds to 2 ps. The fast transition time observed in the present study may be explained from such a dynamical mechanism of non-radiative transition.
CONCLUSION A spectroscopic system for time-delayed double laser photolysis has been developed to observe picosecond chemical reactions from photo-excited intermediates. The system was successfully applied to the investigation on defect formation process in alkali halide crystals. Among the consecutive reaction steps included in the process, the branching step from STEH's to STEL'S and F - H pairs was selectively observed to occur faster than 2 ps in NaCI crystals even at 14 K. An athermal mechanism of nonradiative transition to predominate the branching step was suggested for NaC1 crystals. Extended studies to the other alkali halides are required. Acknowledgment--The authors thank Mr E. Kitamura and
Mr S. Nakayama for their cooperation to construct the double laser photolysis system. REFERENCES
Fujii K., Kikuchi R., Katagiri S., Tsumori K. and Kawanishi M. (1984) Proc. 4th Int. Conf. Ultrafast Phenomena, Monterey, p. 402. Springer Series in Chemical Physics 38. Springer, Berlin. Hirai M., Suzuki Y., Hattori H., Ehara T. and Kitamura E. (1987) J. Phys. Soc. Jpn 56, 2948. Huang K. and Rhys A. (1950) Proc. R. Soc. A204, 406. Itoh N. (1982) Adv. Phys. 31, 491. Kabler M. N. and Williams R. T. (1978) Phys. Rev. B 18, 1948. Lachish U., Shafferman A. and Stein G. (1976) J. Chem. Phys. 64, 4205. Ortega J. M. (1979) Phys. Rev. B 19, 3232. Provoost J. and Jacobs G. (1981) Phys. Status Solidi b 104, K149. Soda K. and Itoh N. (1981) J. Phys. Soc. Jpn 50, 3988. Song K. S. and Leung C. H. (1979) Solid State Commun. 32, 565. Suzuki Y., Okumura M. and Hirai M. (1979) J. Phys. Soc. Jpn 47, 184. Suzuki Y., Abe H. and Hirai M. (1989) J. Phys. Soc. Jpn (Submitted). Tanimura K. and Itoh N. (1984) J. Phys. Chem. Solids 45, 323. Toyozawa Y. (1983) Semicon. Insul. 5, 175. Williams R. T. (1976) Phys. Rev. Lett. 36, 529. Williams R. T., Bradford J. N. and Faust W. L. (1978) Phys. Rev. B 18, 7083. Yoshinari T., Iwano H. and Hirai M. (1978) J. Phys. Soc. Jpn 45, 936, 1926.