Formation of F centers and STE's in RbBr, RbI and KBr in picosecond range

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Formation of F centers and STE’s in RbBr, RbI and KBr in picosecond range

M. Hirai, Y. Suzuki, M. Okumura

To cite this version:

M. Hirai, Y. Suzuki, M. Okumura. Formation of F centers and STE’s in RbBr, RbI and KBr in picosecond range. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-305-C6-307.

�10.1051/jphyscol:1980677�. �jpa-00220115�

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JOURNAL

DE

^PHYSIQUE

Colloque C6, supplément au n" 7, Tome 41, Juillet 1980, page C6-305

Formation of F centers and STE's in RbBr, Rbl and KBr in picosecond range

M. Hirai, Y. Suzuki and M. Okumura

Department of Physics, Tohoku University, Sendai, Japan

Résumé. — Les temps de formation des centres F(tF) et des excitons auto-picgés dans l'état 3£„+(TSTE) dans les cristaux de RbBr ont été mesurés sous-excitation par absorption à 2 photons du second harmonique d'un laser à rubis. rF = 27 ± 8 ps à 300 K et 30 ± 8 ps à 4 K. TSTE n'est pas observé explicitement, mais apparaît être envi- ron 30 ps. X, à 300 K et tSTE à 4 K pour Rbl sont égaux à 30 ± 10 ps. TF dans KBr à 300 K est égal à 20 ± 10 ps.

De plus, les temps de croissance (Tt) de l'émission a ont été observés en utilisant un accélérateur linéaire.

T, = 10 ± 10 ps, 5 ± 4 ps, 10 ± 10 ps, 60 ± 10 ps respectivement pour KBr, Kl, RbBr et Rbl. Il est probable que la différence entre Ti et tF OU XSTE soit due à la durée de tunneling entre l'état 2p et l'état ls du STE.

Abstract. — Formation times of F centers (tF) and self-trapped excitons at 3Z„ (TSTE) in RbBr crystals have been measured under the excitation by two photon absorption of second harmonics from a ruby laser. TF is 27 ± 8 ps at RT and 30 + 8 ps at LHeT. TSTE is not explicitly observed, but appears to be around 30 ps. tF at RT and TSTE at LHeT in Rbl are both 30 ± 10 ps. TF in KBr at RT is 20 ± 10 ps. In addition, the rise times (T,) of the a emis- sion have been observed by using a linear accelerator. xt in KBr, KI, RbBr and Rbl are 10 ± 10 ps, 5 ± 4 ps, 10 ± 10 ps, and 70 + 10 ps at LHeT, respectively. The difference between t[ and TF or tSTE is tentatively attributed to the time required for the tunneling from the 2p state to the Is state of the STE.

Observation of the formation time of F centers and STE's by ultrashort laser pulse brings us a useful information on the relaxation of excitons and also on the formation mechanism of color centers in alkali halide crystals. Williams and his co-workers [1] were first to observe the formation time, T

F

, of 9 + 5 ps of F centers in KC1 crystals. Suzuki et al. [2] reported 11 + 9 ps for F center formation in KI at RT and

TsTE

= 210 + 15 ps for STE formation near LHeT.

In both KC1 and KI crystals laser illumination creates free electrons and holes. Williams et al. suggested t

^F

may originate from the time to capture a free electron by a hole i.e., by the V

^k

center. The present paper is concerned mainly with the relaxation time of 2p free excitons in RbBr crystals, and t

F

and z

STh

in several crystals will be discussed by the tunneling probability between a higher excited state and

3

27

u+

(<r

u

, lstr^) state in STE.

In figure 1 is shown a schematic diagram of an optical system to observe the rise time of absorption by using a mode locked ruby laser and an echelon as a time resolving unit. Since the 2p exciton band in RbBr is located around 7.20 eV [3], two photon absorption process (7.14 eV) of the second harmonics (broken line) of the ruby laser creates mainly hot excitons at the 2p state. The fundamental light (1.79 eV, solid line with arrow) is used to monitor the growth of the F band peaking at 1.80 eV at RT and at 1.85 eV near LHeT, and also of the absorption

MODE LOCKED RUBY LASER (1GW,40ps,694nm) SINGLE PULSE

SHUTTER

ECHELON

Fig. 1. —A schematic diagram of an optical system to observe the rise time of absorption in picosecond range by using a mode locked ruby laser and an echelon as a time resolving device.

band (Trip, band) due to the STE appearing near 1.8 eV.

Closed and open circles in figure 2 show the growth of the normalized optical density (OD(w)/OD

AV

) at

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980677

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C6-306 M. HIRAI, Y. SUZUKI AND M. OKUMURA

1 . . 1 - -. 1 -. I .-

-200 0 2 M ) 400ps

T I M E

Fig. 2. - The growth of the absorption at 1.79 eV In RbBr at RT (closed circles) and near LHeT (opcn circles) excited by two photon absorption process of second harmonics from a mode locked ruby laser.

1.79 eV in RbBr at RT and LHeT, respectively. By fitting a following equation [I] along circles

the growth time 7,'s are estimated to be 27

f

8 ps and 30

f

8 ps at R T and LHeT, respectively, with

7 , 4

72, where

^{7 2}

is the decay time of the absorption.

The derivation of eq. (1) is described in reference [I].

In figure 3 spectral distribution of OD(n)/ODA, at about 100 ps after the excitation are shown by closed and open circles for RT and LHeT, respectively.

Since the solid curve presents the conventional F band at RT, and since the chain curve presents a composition of the F band (broken curve) and the Trip. band (dotted curve) near LHeT, absorption shown by circles originate from the F band at RT,

RbBr

AT loops F AT LHeT

-. .

W \,-,

>

F ...

I

* ^L

^-

^I.

^I^-^. ^. ^' ^. ^- ^-

^.I

1.1 1.6 1.8 2 0 2 2 24

PHOTON ENERGY ( eV )

Flg. 3. - Absorption spectra of RbBr at

-

100 ps after excita- tlon by a ruby laser at RT (closed circles) and near LHeT (open circles). Solid and broken curves show the conventional F band at RT and a t LHcT, respectively. Dotted curve shows the absorption due to the STE. Cham curve is a compos~tion of the broken and dotted curve

and mainly from the F band with partial contribution of the Trip. band near LHeT. Therefore,

7,

obtained in figure 2 arises from the formation of the F band, i.e., stands for

7,

at RT and LHeT, and partially for rST, a t LHeT.

7,

and

T,,

in RbI at RT and LHeT were estimated to be 30 + 10 ps for both cases.

^7,

and r 2 in KBr at RT were 20

f

10 ps and 800

f

200 ps, respecti- vely, as listed on table I, in which

7,

and

T,,,

in crystals investigated earlier are also included.

Leung and Song [4] introduced the breathing mode relaxation around STE to lower the energy barrier for the tunneling process between 3 ~ l ( o , , 2pau) and 'Z;(ou, lsoJ, and estimated the probability W ( T )

⁼

lit, for the tunneling by using the equation introduced by Huang and Rhys [5], with the LO phonon energy hwLo as the phonon energy in the equation. However, since we are taking into the

Table I.

-

The formation time oj'the F center, and the STE,

7,

and

T,.,., ;

the rise and decay times oj'the a emis- sion,

7,

and 72, in alkali haliak crystals.

NaCl NaBr KC1 KBr KI RbBr RbI

e + h e + h e + h 2 p. ex. e + h 2 p. ex. e + h

71

(0, LHeT) (ps)

^-

-

^-

₁₀ ₅ ₁₀ ₍₇₀₎

7,(a, LHeT) (ns)

^- - -

3.4 1.6 3.3 3.7

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FORMATION O F F CENTERS A N D STE's IN RbBr, RbI A N D KBr C6-307

consideration the breathing mode around the STE, it will be much plausible to employ a local phonon with

ho rather than the LO phonon. If a suitable

h can be found and introduced into the equation,

^7,

at 5 K can be made close to r, and

T,,,.

ho's found in such a way are listed on table I. They are smaller than ho,,, but close to

ho,

which is the local phonon energy influencing the F centers at the ground state [6].

Thus the important point is the phonon influencing the STE. Above estimation suggests that the local phonon around the STE rather than the LO phonon is quite effective to explain z, and z,,, by r,, although we could not find direct evidence for 3C,+ as the initial and responsible state for the tunneling. Thus, the breathing type relaxation introduced by Leung and Song with modification of Toyozawa's model [7], along with the introduction of the local phonon by the present work, appears to help the understanding of the mechanism of F + H center formation as far as formation time is concerned.

In addition to the absorption measurement, the rise times of the a emission in KBr, KI, RbBr and RbI at LHeT were observed by using pulsed electron beam (pulse duration of

24

- 56 ps, energy of

35

MeV, current of 2 A) a t Nuclear Engineering Research Lab., Univ. of Tokyo. Figure

4a

shows the pulse shape of the beam with - 56 ps duration. Figure

46

presents the change of the

a

emission intensity in KBr with the rise time (7,) and the decay time (z2) of 10 + ^{10 ps}

and

3.4 f

0.1 ns, respectively. rl's and r,'s of other crystals are listed on table I. Since the pulse duration of the electron beam is rather wide

(-

56 ps), rl's obtained here are not accurate enough. However, 7,'s are comparable to or shorter than

t~

or r,, in most crystals and correspond to the relaxation time of the electron from the conduction band to

Fig. 4. - (a) Pulse shape of the Cerenkov light from water.

(b) Vanation of the a emission Intensity of KBr after excitation by pulsed electron beam (pulse d u r a t ~ o n of

-

56 ps, energy of 35 MeV.

current of

-

^{2 A).}

To clarify the correlation between z, and

7,

or

z,,,,

we need further consideration.

DISCUSSION

Question.

^-

N. KRISTIANPOLLER. Reply.

-

M. HIRAI.

Have you observed in the picosecond or nanosecond No, I did not.

range also the formation of F+ and I centres near LHeT

?

References

[I] WILI.IAMS, R. T., BRADFORD, J. N. and F A I : ~ ~ , W L., Phys [4] LEIJIUG, C. H. and SONG, K. S., Phys. Rev. B 18 (1978) 922 Rev. B 18 (1978) 7038. [5] HAUNG, K. and RHYS, A., Proc R Soc. London, Ser. A 204 [2] SUZUKI, Y. and HIRAI, M., J. Phys. Soc. Japan 43 (1977) 1679. (1950) 406.

[3] FRO~ILICH, D. and STAGINNUS, B., Ph.ys Rev. Lerr. 19 (1967) [6] DAWSOK, R. K. and P(X)LEY, D., Phys. StatusSol1di35 (1969)95.

496. [7] TOYOZAWA, Y., J. Phys. Soc. Japan 44 (1978) 482.