Polarized luminescence from KI: Sn2+

Journalof Luminescence 24/25 (1981) 205—208 North-Holland Poblislsing Company

205

POLARIZED LUMINESCENCE FROM KI:Sn~

0.

Dc Si

Dang’,

0.1.

Rimkin

and P.U.M. Jacobs~

Damartment of Chemistry McGill University Montreal, Quebec CANADA

INTEC200CION 2+

Che enission observed after A band exsitatinn of El nstivatsd with blet structure with necks at 2.21 oW (AT1) and 2,39 oW (Ac 2) [1].

Sn~

has

a dou-

Eoth bands cr1—

ginate from relaxed excited states (EEC) with strong coupling to the Eg lattice modes [C); the dnublet structu”s arises from the electrostatic perturbatinn nn the Cn’~ center due to the presence of a nearby charge—compensatIng cation vacancy [3). Recently, we discussed the Jahn Teller (CT), spin—orbit and cation vacancy perturbations on the EtC of En~ inns in El [1]. However, a complete model for the Nn~~cationvacancy complex, including its orientation with respect to the CT axis and the precise location of the vacancy (nearest neighbor (nn), next nn (nnn), or both) has yet to be given. The nrosent study was undertaken in the hone of elucidating these questions. To date there have been no published measurements of either the asimuthal or temperature dependence of the polarization (P) of the 2t Mo present new emission ~rom EI:Cn~. Rukuda [i,3]has reported the dependence of P on here excitaresults for the for asimuthal dependence, P(o), at 3.5 E and for the tempermture detion wavelength both A~ and ATC omissions of EI:Cn pendence, P(T), for both AT

(so)

1 and ATC omission bands. In addition p(x0~) has boon measured at various excitation wavelengths. These data, together with the temperature dependence of the luminescent decay times given in our previous paper (11 are Interpreted In terms of the model presented in reference 1, and load to a more complete understanding of the nature of the CnC+_cation vacancy complex.

EXPEETh~ETAL The polarismtion measurements were made using a straight line optical arrangement with excitation and detection in the same direction. Excitation was accomplished by passing the light from a 1000 M Xe lamp through a solution filter (CoCa3 + 015024 in 020) to absorb infrared, and then focusing It on the slits of a Cohin Yvon model H—la monochrnmeter. The light emerging from the excitation monochrome— tar was collimated by a second lens, passed through a Clan—Thompson prism polarizer (provided with a dividing circle and vernier which permitted angle measurement of better than ± 0.1 degrees) and then focused on the crystal which was mounted in an Oxford Instruments model CR100 continuous flow He cryostat. The temperature could be controlled to ± 0.1 H and the actual sample temmeratures were monitored using a Lakeshore Cryogenics Ci diode mounted just below the crystai on the copper holder. The emitted light was then collimated by another lens, and analysed using a polaroid sheet polariser (mounted in a dividing circle like the first polarisor). A quarter wave plate was mounted with the analyser, fixed at 350, so that the light would be dapolarised prior to entering the analyzing mcnnchrometer (a Rausch and Lomb model 33—86—20). Finally the light was focused on an EEl 9558QE photomul— tiplier tube and the D.C. ~hotomultiplier output was recorded directly on an X—Y recorder. The X—axis of the recorder was linked (via a battery and potentiometer) to the wavelength of the analyzing monnchrometer.

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2’

Di. Simkin et al. / Polarized luminescence ftom KI:Sn

we would expect the degenerate cases to have nearly temperature independent polarization until relative high I, while the non—degenerate cases should have a pronounced temperature dependence even at low temperatures due to thermallization be— tweon the doublet pair (say Z~ and By) — that is we expect a low temperature limit of nearly 1, with P(T) decreasing with T and approaching 1/3 as kI approaches the energy separation of the two radiative levels. We find p(;) for A 11 to be nearly constant with I up to a 60 K, while for A12 there is a pronounced temperature dependence, P(T) decreasing with T above 01 8 K. Below this temperature 2(T) also falls off rapidly because the emission originates from the trap levels (see Figure 1). We may thus conclude that either: 1) AT1 arises from centers Al, A1’p

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Figure 1. Temperature dependence of the polarization for AT1 ( D ) and AT2 (0). The temperature dependence of the decay time T, which was the same for both emissions [1], is also included for comparison (~).

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