Time-resolved study of resonant secondary emission in NaNO2 crystal

Journal of Luminescence 31 & 32(1984)561-563 North-Holland, Amsterdam

561

TIME—RESOLVED STUDY OF RESONANT SECONDARY EMISSION IN NaNO

2 CRYSTAL

Hiroshi MURATA, Toshihiro KOBAYASHI and Risc KATO Department of Physics, Kyoto University, Kyoto, 606, Japan Behaviors of Raman—like and luminescence—like components of the resonant secondary emission in NaNO2 have been investigated by using a tunable dye laser. Nano—second time resolved measurements of the secondary emission have shown that the time response of the Raman—like component is faster than that of the luminescence—like component but is slower than that of the exciting light. This is likely due to hot luminescence character of the Raman—like component. Relaxation processes of the resonantly excited state are discussed based on the polariton picture. 1. INTRODUCTION It is known that NaNO2 crystal shows Raman-like and luminescence—like components (abbr. as R— and L—components, hereafter) in its secondary emission when excited resonantly 1A~-*1B~ in the region nearR-component the zero—phonon line v~) of with the sin)~l The shifts in (parallel the glet absorption ( change in the exciting photon energy ~, while the L—component appears at the same position as the ordinary luminescence. In the present work, the relaxation processes of resonantly excited state in WaND 2 has been studied by the measurement of nano—second time—resolved spectra of both components and by the measurement of intensity ratio of the components in changing ~

and temperature.

2. EXPERIMENTAL Highly purified NaNO2 crystals were grown by using zone—refined material. Absorption and luminescence lines induced by NO~impurity near

line were

reduced remarkably in 2theTime—resolved zone—refined crystal. Inhomogeneous broadening of thea measurements were carried out by using line was also reduced. N 2—dye laser system and a boxcar integrator with time resolution of —2ns.

3. RESULTS AND DISCUSSION Figure 1 shows typical time—integrated spectra in the region of 2v2 line of the secondary emission at 2K under the excitation around v00. As seen in the figure, R—component shows resonant enhancement at and becomes weak as exceeds

~

On the other hand.

nent as ~i exceeds

[—component appears separately from R—compo—

and grows up rapidly.

0022—231 3/84/$03 .00 © Elsevier Science Publishers BY. (North-Holland Physics Publishing Division)

In addition to R— and [—corn—

ii. .lii,rata et

562

cii.

/ Resonant

s’eondare emission in 7sa,\O~eristai

2V

180 15N0~ ponents, isotopic lines Li, U. end ~y (due to N0 and N0170, respectively) ap— pear strongly when coin— cides with the respective pa—

2-~ine ~-Lumi.- ~ b(aa)b ike ~o ~ excited ‘° ~ ~ ft ~ Raman-hke

R

~

38464~i~®

1 ~

38474

1

50

sitions of isotopic absorption

/

lines.2 Figure 2 shows

3847

response of

i iiji~

z 0 38480 U

,.

3845

time

nents under the excitation at c.~o +5cm~ (3848.2A). Re-

~

(1)

the

R— and L—compo—

3850A

~ ~

100

sponse

of

R—component

is

faster than that of L-component, but slower than that of a)

38477 a)

4102

exciting light.

38500 ~D—~----—38510

This behavior

t

is likely due to hot lumines—

4110 4120 WAVELENGTH (4)

cence character of R—compo— nent. Then, the relaxation processes of resonantly ex—

Fig.l Time—integrated spectra of resonant secondary emission. Inserted figure shows . absorption spectrum of line and positions of excitation for reference.

cited

state

based on the polariton

picture

(shown in Fig.3, scheinatical— ly).

________________________________

will be explained

Under

the

excitation

slightly above a 00, the upper

laser

ex at3848.2A b(aa)6 laser —---Reman-like Lomi —like

5

/~

p

branch created.

polariton

will

be

It decays directly

by emitting a2—phonons and gives rise to k—component. In

.~

li

addi ti on to the direct decay, the polariton relaxes to the

P.

~

I:..

-P

Z

lower branch with relaxation -

F-

time i Then it decays by emitting a2—phonons giving

Z

1 —5

II

0

Ill 5 10 15 TIME (nsec)

rise to L—component. 20

If we

25

assume

that the rate of the

direct decay Fig.2 Time responses of Raman— and luminescence—like components. Solid curve fitted to Raman—like component is obtained by the convolution of time response of laser and exp[—(l/T0+l/T)t] with T07ns and T12n5.

is l/T~, where

1B T0(~7ns) is the life time of 1 state, the intensity ratio of L— to R—components is given

H. Murata et al.

_

/ Resonant secondary

emission in NaNO

7 crystal

563

~?lB1s~

.~

~SOrPUOn~~

Fig.3

Schematic diagram of polariton picture for the optical processes which give rise to Raman—like and luminescence—like components.

by IL/IR~rTO/T, and the decay of P—component will be described approximately by exp[—(l/T0+l/r)t]

(see Fig.2).

From the observed intensity ratio

‘L’’R’ a

typical relaxation time i is estimated to be —l2nsec at 2K under the excitation at 3848.2A (v1~o00+5cm~)for a polished sample. The relaxation time depends sensitively on the surface condition of the sample;

it is much shorter (t3ns)

for a rough surface and relatively longer (~-l7ns) for cleaved surface. shows strong dependence on

and becomes shorter as

where the absorption due to phonon side band starts to increase. indicates that the relaxation time i

It also

exceeds v00+l5cm~ This fact

of the polariton is made shorter by

acoustic phonon scattering. Preliminary measurements of the temperature variation of R— and L—components show that the intensity ratio IL/Ip is less sensitive to temperature below 10K, but it increases steeply above 10K.

This indicate that the upper

branch

polariton relaxes rapidly through the interaction with phonons above 10K.

REFERENCES 1) M.Hangyo, H.Yamanaka and R.Kato : J. Phy. Soc. Jpn. 52 (1983) 1064. 2) T.Kobayashi and R.Kato : U. Phy. Soc. Jpn. 53 (1984) 2157.