HAL Id: jpa-00220114
https://hal.archives-ouvertes.fr/jpa-00220114
Submitted on 1 Jan 1980HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
PRIMARY PROCESSES IN RADIATION
DAMAGEColor center formation in KBr under
polarized UV-laser irradiation
K. Kan’No, M. Itoh, Y. Nakai
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 1, Tome 41, Juillet 1980, page C6-301
PR/MARY PROCESSES IN RADIATION DAMAGE.
Color center formation in KBr under polarized UV-laser irradiation
K. Kan'no, M. Itoh (*) and Y. Nakai Department of Physics, Kyoto University, Kyoto 606, Japan
Résumé. — Un dichroïsme important suivant la direction [HO] de la bande d'absorption des centres H de KBr
est observé. Ces centres sont produits par excitation à deux photons utilisant la lumière d'un laser à excimer KrF ( / = 248,5 nm), polarisé le long de la direction [110] du cristal. L'anisotropie apparaît également dans l'émis-sion 7i à partir du niveau triplet le plus bas de l'exciton auto-piégé (STE) ; sa polarisation est alors parallèle à la direction [110].
L'origine de cette anisotropie est discutée en tenant compte de l'excitation anisotrope des excitons auto-piégés de leur niveau triplet le plus bas vers des états excités de trou plus élevés. En d'autres termes il faut tenir compte d'un processus à un photon. Certains excitons auto-piégés à l'état excités, orientés parallèlement à [110] relaxent par l'intermédiaire de la production d'une paire F-H, ce qui explique que la bande d'absorption H soit polarisée suivant [110].
Abstract. - - A definite [110]-dichroism is found in the absorption band of H centers produced under two-photon
excitation of KBr by using [110]-polarized KrF-excimer laser light {). = 248.5 nm). Anisotropy is also confirmed in the it emission from the lowest triplet level of the self-trapped exciton (STE), its polarization being parallel to the [110] direction.
Origin of these anisotropy is discussed in terms of the anisotropic excitation of STE from its lowest triplet level to higher hole-excited state. In other words, presence of one-photon process is also important in this case. Some of the excited STE's with the [110]-orientation relax to branch into a channel to F-H pair production, which results in the [lT0]-polarized H absorption.
1. Introduction. — It is well established that
intrin-sic luminescence and formation of the F and H centers in alkali halides derive from relaxation of electron-hole pairs or free excitons into the configuration of a self-trapped hole (VK center) which has captured an electron in a relatively delocalized orbitals; i.e., self-trapped exciton (STE) states. Intrinsic lumines-cence results from radiative annihilation of STE's in the lowest triplet state (71 emission) or in the excited singlet state (a emission). A series of studies with pulsed electron beam by the Sendai group clarified that the channel leading to the defect formation is different from these luminescent levels [1, 2]. '
With the aim to identify which STE level acts as the precursor to F-H pairs, Williams [3] performed clear experiments with a double excitation technique, where an intense light pulse from a ruby laser was timed to stimulate STE's in KC1 into their higher levels after generating a number of STE's in the lowest triplet state by pulsed electron irradiation. The laser irra-diation was found to give rise to enhanced produc-tion of F centers, indicating that the lowest channel to the defect formation starts from the second excited triplet STE level. Further informations were added
recently by Yoshinari et al. [4, 5]. concerning the conversion efficiency from STE's to the F centers in KC1, KBr and NaCl.
In the present paper, we report that such a promo-tion of the lowest STE's to higher states, when sti-mulated by polarized laser light, leads to the forma-tion of aligned H centers. Electron-hole pairs were created in KBr by means of the two-photon band-gap excitation with pulsed ultraviolet laser light, in contrast with the earlier experiments [3, 4, 5] in which pulsed electron beam was used. The excitons relaxed into the lowest STE level are supposed to be stimulated subsequently to a higher excited STE level through the one-photon process with the same laser pulse.
2. Experimental. — The pulsed UV light from a KrF-excimer laser (248.5 nm, ~ 4 MW, 12 ns) or from a N? laser (337.1 nm, ~ 350 kW, 5 ns) was polarized along the [110] axis of KBr by using a Glan prism (transmittance : ~ 28 % at 248.5 nm, ~ 40 % at 337.1 nm), and focused on a crystal surface. Crys-tals obtained from the Crystal Growth Lab. Univ. of Utah were used. Colored portion looks like a filament along a path of the laser beam. Dichroic absorption spectra of the colored portion with the length of 8 mm, its cross-section being ~0.5 x 1.0 mm2, (*) Present address : Institute of Physics, Faculty of Engineering,
Shinshu University, Nagano 380, Japan.
C6-302 K. KAN'NO. M. ITOH AND Y. NAKAl
were measured by the use of a similar optical arrange- ment as described in ref. [6].
Emission spectra were measured at a right angle to the exciting laser light by using a Nikon G250 grating monochromator and an HTV R928 photomultiplier. Anisotropy of the intrinsic n (2.27 eV) emission was detected in the direction parallel to the exciting beam from the back surface of the sample with an appro- priate combination of a polarizer, an interference filter, and a plate of plastics for blocking the laser light. Similar experiments for the a (4.38 eV) emis- sion were not made because of insufficient spectral resolution of an interference filter and of proximity of its short life time to the width of the laser pulse. All the experiments reported here were made at 9 K.
3. Results and discussion. - In figure 1 arc shown
dichroic absorption spectra of the H band obtained after repeated irradiation of 3 and 10 shots with the [I 101-polarized K r F laser pulses. The peak energy (3.24 eV) and the half-width (0.45 eV) agree well with the values obtained in the X-rayed KBr by
the polarized laser light, anisotropy would increase with repeated irradiation of laser pulses since the H centers which have been already formed under the preceding irradiation may be aligned by the succes- sive shots. Thus, it is evident that the dichroism observed in the K r F laser coloration arises from a primary formation process. Formation of [li0]- oriented H centers is confirmed also in KC1 and K1. When KBr was colored by irradiating with the [1 101-polarized N 2 laser light, similar [li0]-dichroism was observed in the H band. However. its anisotropy increased .largely as the irradiation was repeated. In this casc, one-photon energy of the N 2 laser light (3.68 eV) can be absorbed by the H centers already formed because the laser light falls on the high energy tail of the H band. Therefore, the dichroism observed under the N, laser irradiation is, at least, partly due to reorientation of H centers.
Anisotropy was also recognized in the intrinsic n
(2.27 eV) luminescence, when excited with polarized UV laser pulses. In figure 2 are shown oscilloscope traces of the [110]- and [li0]-polarized components Bachmann and Kanzig [7]. It is clearly seen that thk
Excited with fl1Ol-polarized KrF laser resulting H band is polarized perpendicular to the
plane of polarization of the laser light; namely, the H centers produced are aligned preferentially along the [ l i ~ ] direction.
Laser 50 P
Fig. 2 . -Oscilloscope traces of the [110]- and [liO]-polarized components of the intrinsic n (2.27 eV) emission in KBr observed at 9 K under excitation with the [I 101-polarized KrF-excimer laser light.
50 45 41) 35 31)
PHOTON ENERGY ( e V ) 25 of the n emission observed under irradiation with Fig. 1. - Dichroic absorpLion spectra of the colored KBr obtalned the [I 101-polarized KrF laser light. It is evident that at 9 K w ~ t h [110]- and [IlO]-polarized light. The coloration was the emission, characterized by a decay time made at 9 K by irradiating with 3 and 10 pulses of the [1 10]-polarizcd 105 pS a t 9 K, is polarized parallel to the exciting KrF-excimer laser light (Aru = 4.99 eV), by means of the two- light vector, namely, along [ l lo]. Similar results were photon excitation process. obtained also in the case of anisotropic excitation with the N 2 laser light. Since the optical dipole of the
n emission is known to be perpendicular to the mole- Since H centers hardly absorb the 5 eV photons cular axis of the STE, these results mean that the from the laser, dichroism observed does not seem to lowest triplet STE's are aligned preferentially along the result from reorientation of H centers by the pola- [ l i ~ ] axis under irradiation with the [I 101-polarized rized laser light. It should be noted that the degree of UV laser light.
COLOR CENTER FORMATION I N KBr U N D E R POLARIZED UV-LASER C6-303
course of migration through the crystal, and/or during relaxation into the lower levels of STE's. Although relaxation mechanism of frec excitons created in KBr by two-photon absorption process with polarized light has not been clarified as yet, their experiment suggests that the anisotropy observed here arises from a certain process subsequent to the initial two- photon process.
In the course of preliminary measurements, inten- sity ratio of the o emission to the n emission was found to increase as the power density of laser pulses was increased. This means that relative population of the lowest triplet STE's and of the excited singlet STE's varies with the intensity of exciting laser light. It suggests the presence of a cascade-like process, namely, t k re-absorption of one-photon energy of the laser light by STE's. Therefore, the present situa- tion is quite similar to the cases of the experiments with the double excitation technique [3, 4, 51, which leads to enhancerncnt of F-H pair production. In fact, the anisotropy in both the H absorption and the n emission was found to enhance when the laser intensity was increased. That is, the cascade process becomes more appreciable at higher power density of the excitation. This is also the reason why relative intensities of the a and n emissions vary with the power of the laser light.
In KBr, it is more reasonable to suppose that the K r F and N, laser light can induce the hole transitions, rather than the electron transitions, in the STE's, since one-photon energies of the laser light are both far above those of the electron transitions [9]. There- fore, the V,-like transition o f holcs from the bonding a,-orbital to the antibonding a,-orbital, leaving the electron in the lowest Isa,-orbital,
3C: (a,, IsaJ + 3Z,+ (og, lsag) ,
may be responsible for the cascade process. In this process, the [I 101-polarized UV laser light is not effective to stimulate the STE's in the [li0]-orientation among six equivalent V,-like configurations, so that the [llO]-oriented STE's will be major part of sur- viving ones in the lowest triplet state. This leads to
the anisotropic n luminescence with the [I 101-polari- zation. It is consistent with the results obtained in NaCl by Williams [3], in which the triplet-state lumi- nescence became anisotropic after the holes of STE's were excited into the a,-hole orbitals with the polariz- ed 347 nm laser light.
Some of the STE's thus excited relax to the singlet state to enhance the a emission, and some branch into a precursor state from which additional F-H pairs are produced. Results obtained in the present study sug- gest that aligned H centers are produced as a result of anisotropic excitation of the hole transition in STE's with polarized light. In this connection, it is interest- ing to recall that alignment of H centers has been observed also by Reuter et a!. [lo] in the experiments where U 2 centers were converted to H centers by bleaching with polarized UV light. Thus, it is evident that there is a channel of defect formation where the resulting H centers have been distributed in a preferred orientation which is correlated with the anisotropy in an excitation process.
In the present experiments, the [li0]-oriented H centers are supposed to be formed from the [110]- oriented STE's which are excited preferentially by absorbing the [I 101-polarized UV laser light. This suggests that the presence of a rotational motion is necessary in the excited STE's on the pathway of F-H pair formation. Recently, Kabler and Williams [ l l ] proposed the mechanism that the initial motion of halogen ions to form an H center may include a rota- tion of the axis of the original STE. The results we obtained will be another example of optically induced reorientation of the STE in its higher hole-excited state, which leads to the production of F-H pairs.
Acknowledgement. - The authors would like to
express their thanks to Professor Y. Toyozawa for his kind advice on the cascade-like process under laser light irradiation. They are also indebted to Dr. K. Nakamura for his contribution to constructing laser apparatus and for valuable everyday discussion. The present study was partially supported by a Grant- in-Aid for Scientific Research from the Ministry of Education in Japan.
DISCUSSION
Question. - P. D. TOWNSEND. Reply. - KEN-ICHI KAN'NO.
The production efficiency of stable defects in KI I agree with your views. In the present case, F-H seems much higher than normal. This suggests that pairs are supposed to be produced non-coaxially via
reorientation to stable F-H pairs is taking place the rotational motion of halogen ions in the higher during the higher state excitation. excited STE's. This non-coaxial configuration prevents the F-H pairs from their mutual annihilation, leading to stable defects.
References
[ l ] HIRAI, M., K O N I , ~ , Y., YOSHIUARI, T. and UETA, M . , J . P11ys. [2] KOUDO, Y., HIRN, M and UEIA, M., .I. Phys. Soc Jpn 33
C6-304 K . KAN'NO, M. ITOH A N D Y. NAKAI
[3] WILLIAMS, R. T., Phys. Rev. Lett. 36 (1976) 529.
(41 YOSHINARI, T., IWANO, H. and HIRIU, M., J. Phys. Soc. Jpn 45 (1978) 936.
[5] YOSHINARI, T., IWANO, H. and HIRAI, M., J. Phys. Soc. Jpn 45
(1978) 1926.
[6] I~orr, M , KAN'NO, K. and NAKAI, Y , Solid State Commun. 26
(1978) 929.
[7] BACHMANX, K. and KANZIG, W., Phys. Kondens. Muter 7
(1968) 284.
[8] KAMEJIMA, T., FUKUDA, A and SHIONOYA, S , J Phys. Soc. Jpn 27 (1969) 1549.
[9] WILLIAMS, R T. and KABLER, M. N., Phys. Rev. B 9 (1974) 1897.
[lo] REUTER, G., SCHWAN, L. and SPAETH, J. M., Phys Status Solidi B 53 (1972) K 29 ;
REUTER, G. and SPAETH, J. M., Abstract of International Conference on Color Centers in Ionic Crystals, Sendai,
1974, D 38.
