Cathodo-luminescence of rare-gas solid solutions

Solid State Communications

Vol. 12, PP. 1—4, 1973.

Pergamon Press

Printed in Great Britain

CATHODO-LUMINESCENCE OF RARE-GAS SOLID SOLUTIONS A. G. Belov, I. Ya. Fugol’ and E. V. Savchenko Physico-Technical Institute of Low Temperatures, Academy of Sciences of the Ukrainian SSR, Kharkov 86, U.S.S.R. (Received 10 October 1972 by

E. A.

Kaner)

Vacuum-ultraviolet luminescence of rare-gas solid solutions of Xe—Ar, Xe—Kr, and Kr—Ar has been measured using excitation by slow electrons. The concentration effect has been studied on the spectra of mixtures at T = 5°K.It is shown that luminescence centers of three tynes atomic, molecular and heteroatomic are formed in the solutions. the bands corresponding to the molecular and heteroatomic centers appear at radiative decay of molecules which are formed in a crystal during excitation. The bands of atomic centers display a fine structure which is due to asymmetric distortion of the surrounding lattice. IdentifIcation of the observed bands has been proposed. —

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EMISSION spectra from pure crystals of heavy rare gases in the vacuum-ultraviolet region consist of wide structureless bands which are due to radiative decay of localized excitons.1 20f the rare gases studied solid neon is an exception. As our measurements have shown, its luminescence spectrum contains a narrow atomic band consisting of five components3 The present paper presents new data on spectra and the fine structure of cathodoluminescence bands of Xe—Ar, Xe—Kr, and Kr—Ar solid solutions at 5°Kin the vacuum-ultraviolet region. For low impurity concentrations the heavy luminescence with a multicomponent structure. In have been observed. addition, rare-gas solutions new bands have of displayed heteroatomic an atomic-type molecules The specimens were prepared by gas condensation onto a cooled substrate. The luminescence was excited caused no structure by electrons distortions of the in energy the ssmples. 500 eV which The spectra were registered on a vacuum monochromator VMR-2 with photoelectric recording. The sensitivity of the recording scheme was 10—i 7W. Spectral resolution was better than o.5A. Changes in the spectra were investigated as the impurity concentration

increased from 0.001 to 15%. A more detailed description of the apparatus and methodology of the measurements is given in the work of Fugol’ et al.4 Energy, eV 6

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2

CAThODO-LIJMINESCENCE OF RARE-GAS SOLID SOLUTIONS

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1498 andof1480 The band widths cm’at consists two A. separate bands with are the 270 maxima and 700 cm~,respectively. Shortwave regions of the bands display a complex structure. For the l480A band we could resolve six components shown in Fig. 3. The band maximum is close to the resonance line of atomic xenon,X = l470A(3P,—1S0). By analogy

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Vol. 12, No. 1

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with3P, this atomic line transition the l480Aofband is the identified S~atomic Xe in argon as the crystal. The observed band splitting is most probably due to the symmetry lowering of the luminescence center, caused by deformation of the surrounding lattice. The location of the band whose maximum is at 1498A corresponds to the 3P 2~S0transition of Xe forbidden in gas spectra. Since an excited atom of Xe in solid solutions brings about an asymmetric deformation3Pof the surrounding lattice, the forbiddenness of the 2 —‘So transition may be lifted in a crystal. 3P The complex l498A-band is probably the atomic 2 S~transition of Xe in the argon crystal. A shift of the band relative to the transition in a free 5 makes 300 cm~towards lower frequencies. Xe atom The luminescence of Kr—Ar solid solution also shows the emission which should be considered as an atomic type. This is the A band shown in Fig. 4(a) which appears at low impurity contents only. The band maximum is located at 11 25A being rather close to the singlet ‘F,—1 S 0 transition of a free Kr atom (1 l65A). The 1l25A-band in the Kr—Ar solid solution seems to correspond to the ‘F1~’S0transition of Kr in the argon lattice. Because of the low intensity we could not resolve the band structure. The singlet transition of krypton in argon has a noticeable blue shift if compared with the gaseous phase (~v= 3010 cm~).Triplet atomic transitions of Kr are not seen seperately since they fall in the vicinity of a molecular band of Ar. —‘

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FIG. 2. Luminescence spectrum of Xe—Kr solid solution. (a) spectrum of Kr; (b) spectrum of the solution at a Xe concentration of 0.01 per cent; (c) ~t Xe concentration 0.1 per cent; (d) at Xe concentration 5 per cent. Figures 1 and 2 show the emission of Xe—Kr and Xe—Ar solid solutions for various concentrations of Xe. Figure 4 illustrates cathode luminescence of a Kr—Ar system. The curves in Figs. 1(a) and 2(a) correspond to spectra from the purest samples of argon and krypton which we were able to obtain after a special chemical and cryogenic purification. The broad structureless bands L in the spectra are due to radiative annihilation of localized excitons and represent the intrinsic luminescence of the host lattice. The bands L1 correspond to molecular type R2 centers. By analogy with the gas spectra the bands L2 can be classified as recombination luminescence of an R center. (The maximum of the band L2 in Ki is located at 2300 A, and therefore not shown Fig. 2(a).)When an impurity is added, the intrinsic luminescence (of the band L) decays very rapidly because of an efficient energy transfer to impurity centers. In our spectra of impurity luminescence we observe at least three different types of centers: atomic (A), molecular (M) and heteroatomic (H). in

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As the impurity concentration increases, the atomic bands disappear and the emission of molecular impurity centers (the bands M in Figs. 1, 2 and 4) increases. The centers of molecluar impurity luminescence appear even at impurity contents of the

Vol. 12, No. 1

CATHODO-LUMINESCENCE OF RARE-GAS SOLID SOLUTIONS

the emission lines of the basic material and impurity, in the luminescence spectra of Kr—Ar and Xe—Kr

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systems is due to formation of heteroatomic molecules in the solid phase of (Kr—Ar) and (Xe—Kr) types.

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An interesting feature has been found in luminescence spectra of solid solutions whose components correspond to neighbor elements in the periodic system: Kr—Ar and Xe—Kr. Spectra of these solutions display new molecular type bands which can be ascribed neither to the emission of the basic material nor to that of impurity. Maxima of these bands denoted by the letter H (see Figs. 2 and 4) are between the molecular bands of the pure components of the solution. Positions in the spectrum as well as intensities of the H bands depend on the impurity content. We believe that the appearance of the bands which are between

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As the content of Kr increases, the emission spectra of the Kr—Ar system show an increase in intensity of the band in the vicinity of X = l48oA, denoted as the K-band. It consists of seven components, viz. 1400,1423,1450,1476,1502,1532 and 1671A. Relative intensities of the components depend on the Kr concentration and are somewhat different for different samples. The nature of this band is not clear. In the region of 1 500A the molecular emission of Kr 2 must be observed. But further the presence of structure in the K-band stimulates detailed investigations of the origin of this band,

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FIG. 4. Luminescence spectrum of Kr—Ar solid solution. (a) concentration of Kr is 0.25 per cent; (b) concentration of Kr is I per cent; (c) concentration of Kr is 15 per cent. It should be noted that absorption spectra of rare-gas solid solutions contain no analogues of molecular or heteroatomic bands. The observed bands of 6 are of a quite different character atomic absorption as compared with the emission A bands and are shifted considerably to the violet side. The difference in the absorption and luminescence spectra seems to be due to configuration instability of the crystal lattice at excitation and indicates the relaxation that occurs near emission centers by the beginning of luminescence.

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CATHODO-LUMINESCENCE OF RARE-GAS SOLID SOLUTIONS

Vol. 12, No. 1

REFERENCES 1.

JORTNER J., MEYER L., RICES. A. and WILSON E.G., J. Chem. Phys. 42,4250(1965).

2. 3.

BASOV N. G. et al., Fis’ma ZhETF 7,404 (1968);J. Lumin. 1,834 (1970). BELOV A. G., SAVCHENKO E. V. and FUGOL’ I. YA., III Vsesoyuznaya konferentsiya p0 spektroskopii VUF, Tezisy dokl., 35, Kharkov, FTINT AN UkSSR (1972).

4. BELOV A.G., SAVCHENKO E, V. and FUGOL’ I. YA., Trudy FTINT AN UkSSR, Fizika kondensirovannogo sostoyaniya, vyp. X, 229 (1970). 5. MULLIKEN R. S., J. Chem. Fhys. 52, 5170 (1970). 6.

BALDINI G. and KNOX R. S., Phys. Rev. Lett. 11, 127 (1963); BALDINI G., Fhys. Rev. 137, 508 (1965)

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