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On the nature of emission bands of self-trapped excitons in solid xenon
Solid State Communications, Vol. 32, pp. 787—790. Pergamon Press Ltd. 1979. Printed in Great Britain. ON THE NATURE OF EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOUD XENON LYa. Fugol, A.G. Belov, E.I. Tarasova Physico-Technical Institute of Low Temperatures, UkrSSR Academy of Sciences, Kharkov, USSR (Received 6 April 1979 by E.A. Kraner) The paper presents the analysis of experimental data on the temperature dependence of luminescence spectra of solid xenon. The mechanisms of exciton self-trapping down to quasi-molecular states under different conditions are discussed.A new treatment of spectral distribution of intensity through quasi-molecular luminescence bands is proposed; according to the treatment, at T < 55 K the emission is associated with the transition from the lowest vibrational relaxation excited state while at T> 60K from the term ‘.E~.Spectral redistribution of intensities in the 60—150K ranp is due to an increase in the rate of vibrational relaxation in the state 1~ with increasing the crystal temperature. ~
THE LOW-LYING collective states in cryocrystals of Xe, Kr, Ar, Ne rare gases are exciton bands r’(l / 2), T(3/2) and r’(3/2). In the lifetime of exciton states, self-trapped states may be formed under the exciton—phonon interaction. The probability of self-trapping increases in the series of Xe to Ne [1]. Hence, the luminescence of free excitons is the most intensive in Xe, less intensive in Ar and absent in Ne. Besides the exciton luminescence,
consistent data on an unusual temperature dependence of luminescence bands over the whole temperature range of the Xe crystals existence (4—165K) (Fig. 3). If at 4—50 K the spectral bands M and Eex are dominant, for 50—70 K the M-band intensity decreases appreciably, the exciton luminescence Eex is quenched, and a 7.6 eV band W appears. As the temperature increases further (90—165 K), a new long-wave band M1 appears and its
cryocrystals of rare gases display the emission of selftrapped states of a quasi-atomic (one-centered) or a quasi-molecular (two-centered)type. For Xe, only quasimolecular self-trapped states are formed because of a weak exciton—phonon interaction. The formation ofa Xe~molecule within the crystal lattice is shown schematically in Fig. 1. As two centers are brought closer together forming the molecule, the self-trapping, at first, results in the formation of a vibration-excited molecule, and then the relaxation of vibrational energy occurs. The position of quasi-molecular luminescence bands is dependent on the vibrational relaxation rate. A self-trapped luminescence spectrum of Xe contains two broad maxima: Mat 7.2 eV and W at 7.6 eV, the nature of which (in particular, of the latter) produces different interpretations. The paper presents the analysis of different formation mechanisms of quasi-molecular bands in solid Xe, the estimates of energy shifts for an excited state ~ of Xe~in the lattice, and the treatment of a self-trapped luminescence spectrum. A general scheme of radiative transitions for the lowest exciton and selftrapped states is shown in Fig. 2 that is in agreement with the treatment proposed. The 7.6 eV emission of Xe was first described by Basov et al. [2] and Jortner et al. [3]. Since then the self-trapped luminescence spectra of Xe have been studied repeatedly [1, 4, 5], and at present, there are
intensity becomes gradually dominant. Note, the band M1 is a new band different from M. In the temperature range of 4 to 50 K the band M shifts with temperature towards the red (AEM /~T~ 0.003 eV deg’). The ternperature dependence ofM is completely reversible and was measured for annealed samples with large crystallites. The data given are in agreement with the measurement results from [5]. Extrapolation of the M-band position at T> 57 K is shown in Fig. 3 by a dashed line and, as seen, does not coincide with the band M1. For unannealed specimens of a poor structure, the maximum M has a red shift of about 0.1 eV. For liquid, at 165 K the emission of Xe~is observed at EM ~ 7.05 eV. However, at temperatures above 100 K even a vibrational level v’ = 1 of the excited molecule is populated. When the spectral decomposition of the maximum observed is made, it appears that for liquid the emission band from v’ = 0 is to be located at EM 7.0 eV. The M-band position in gas is dependent on both temperature and pressure. At T ~ 300 K, the emission should be produced by all three lowest vibrational levels of both terms 1, 3~ and be directed onto a repulsive term ‘.~. In order to separate from the complex band an emission component from V’ = 0, the shape and position of emission bands of the isolated molecule Xe~have been calculated in the 100—400 K temperature range under population of some vibrational
787
788
EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOLID XENON
Vol. 32, No.9
~~~iiioIui~ ®~ciiIiiiiik~~~ / D
~
~7\
gas
II~\~quid
0)
‘M1~
Fig. 1. A scheme of formation of quasi-molecular centres of Xe~in a f.c.c. crystal lattice.
5~M 1~~’
~
_____
/ ,‘/
‘,/
E~ w °‘
~JJ~ MWEe
re
internuclear distance Fig. 2. A general scheme of radiation transitions in the Xe crystal. levels (v’ = 0, 1, 2, 3). The calculations make it possible to establish the fact that the radiation transition from the lowest state is associated with the band maximum at ~‘M = 6.9 ±0.05 eV. It should be noted that the band width observed in gas and in liquid is 0.6eV and does not permit the contributions of transitions ~ -+ ~ and ~ -+ ~ to be separated (the band splitting is no more than 0.2eV). Measurements of the kinetics ofintensity indicate, however, independent contributions of these two transitiong to the emission of gas and liquid. In gas, one can observe two typical times of quenching r1 = 2 nsec and = 100 nsec for states ~ and respectively [6]. In liquid, two times (r1 = 2 nsec and r2 = 27 nsec) were also observed in experiments with an excess electric field to separate the recombination radiation [7]. For solid, ~
‘,\
~
E1
7 energy,ev 8
9
(4—150K), liquid (165 K) and gaseous (300K) xenon Fig. [1—7].Temperature 3. Intensity distribution of samples in spectra is presented of solid by hon. zontal lines straight line drawn (0.003from eV deg~inclination) the emission maximum. displaysAa reversible temperature displacement of band M in the 4—60 K range; its interpolation to T> 60K is denoted by a dotted-dashed line. A dashed curve shows the position of the M-band maximum under heating of the unannealed sample. there are no reliable data on lifetime involving the finite time of self-trapping (in solid xenon the latter factor may prove to be comparable to the lifetime of singlet states). A number of experimental data indicate that the M.band at 4K corresponds to the self-trapped emission of Xe crystals from v’ = 0 of state ~ (i) the obvious similarity to gas or liquid spectrum and the strict sequence when comparing to other rare elements; (ii) a small effect of the crystal structure defects and the sample thickness of the M-band intensity and position; (iii) the invariable position of band M in the spectrum of pure Xe and in spectra of solid solutions of other rare elements with a small impurity of Xe. As to band W, there are many different interpretations. Despite the discrepancy with certain facts, several authors assume that the pure W-band is an emission 3E~in a “perfect” crystal, while thefrom M-band state
Vol. 32, No.9
EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOLID XENON
789
estimate V~ 0.12G (G ~ 1 eV, the exciton bond energy) [10]. From this it follows that the excited molecule—lattice interaction can be assumed to be weak. In Fig. 1 four types of the lattice sites are shown. Of these only plane A contains atoms entering into the nearest surroundings of both centers of the molecule. The effect of the rest (B, C and D-type) is considered in terms of the interaction with the nearest center. Note,
o
that as two centers are brought together producing the formation of the Xe~molecule in the crystal, the separation between these centers and the A atoms decreases as compared to an equilibrium separation d, and that between the centers and atoms of other type increases. Each of the A atoms, therefore, should make a much larger contribution to VL than that from any other atom. The required quantities V~, V~,Vj)~,° and V~° were calculated by using the Lennard—Johns potential, the excited state potential being averaged as in the calculations for a molecular ion [11]. Since the precise configuration of distorted lattice around the excited molecule is unknown, possible extreme changes in V~
I
E~I
~ I C
Q)
-
5
I
~ntera~micdistance
Fig. 4. A scheme of potentials of interaction between 9uasi-molecule Xe~and its surroundings in the ground ~‘~) and excited (lIZ) states. is due to that transition near the vacancy [4]. Other authors, with due regard to arguments (i)—(iii), assume the possibility of appearance of a new crystal structure with a hindered medium relaxation at T~70K [8]. There is, however, no support for this assumption. It has been also assumed that the W-band is associated with one of the higher terms [5]; the assumption is very tempting, but requires recalculation of the terms of the molecule Xe~[9]. To resolve these discrepancies, we estimate theoretically the lattice effect on term positions in the Xe~ quasi-molecule. The shift of self-trapped bands in crystal, EM, with respect to that in the isolated molecule, i’M, is E —6 9 ,~ —
M
‘
—
—
L
~‘
L’
‘~ ‘
where V~,V~is the shift of intramolecular potentials for the ground and excited.states that is equal to molecule—lattice interaction potentials, and ~‘M = 6.9 eV. Considering the weakness of the Van der Waals forces of lattice atom interactions, we assume that the main contribution to V~and ~ is due to a sum of pairwise interactions with nearest neighbours. For the ground state this assumption is well tested. For the excited state, we can refer to the experimental data [1] and to the
and V~with varying the deformation parameters are calculated. An upper limit to the energy of the interaction between the excited molecule and nearest neighbours can be compared to the hypothetical model where with shifting two atoms forming the molecule their neighbours remain “anchored” at the perfect lattice sites. It is this configuration that is shown in Fig. 1. In this case the minimum of the internuclear potential of the molecule incorporated in a “undeformed” crystal would decrease only by 0.1 eV, and V~ V~= 0.45 eV. According to (1), this shift would be followed by the emission band EM = 7.35 eV. The lowest value of the V~energy occurs when all atoms of the surroundings are removed from the molecular centers by the distance Rmjn corresponding to a minimum position of the VZ pairwise potential. In that case VZ=—0.3eV, V~=—0.S3eVandhence, V~ = 0.2 ±0.05 eV that would correspond to a 7.1 eV emission band. If we introduce some averaged (for atoms of all types) distance R between the atom center of the molecule and its surroundings (the cavity radius), the energy of molecule—lattice interaction can be given as a function of coordinate V1~’°(R) (Fig. 4). Taking into account the lattice elasticity, the cavity radius R is smaller than Rmjn. It is seen from Fig. 4 that the value of V~ VZ remains negative increasing its absolute value. This implies that the molecular transition shift in the crystal with respect to that in the free mole. cule is sure to be blue and of the order of 0.2—0.3 eV. From the calculations made and the general physical considerations it is obvious that the model of the molecule formation in a crystal without distorted —
—
—
—
—
—
790
EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOLID XENON
Vol. 32, No.9
surrounding lattice is worthless. Moreover, for the anchored surroundings a high barrier h > 0.2 eV appears that closes the channel of exciton self-trapping. The vacancy in the nearest surroundings can produce only a slight decrease in the matrix shift and in the self-trapping barrier. For B, C, D vacancies the decrease does not exceed (V2 VZ)/22 ~ 0.01 eV. The interaction with A-atoms is two or three times stronger and can produce a 0.05 eV decrease in the barrier. For the undistorted lattice, the barrier cannot become surmounted because of this decrease. When considering the medium relax-
smaller than that in state Therefore, at 70K the vibrational relaxation is not managed to occur and the W-band (7.6 eV) is observed that corresponds to the emission from higher vibrational states of v’ > 5. It should be noted that for great vibrational numbers the emission from a right-hand revolution point of the potential is the most probable (see Fig. 2). A further increase in temperature accelerates vibrational relaxation in the molecule a red wavelength shift of the band is observed. For 150K, the M1 -band emission is associated with transitions from
ation at h 5 0.05 eV, the vacancies, however, can favour the exciton self-trapping having practically no influence on the position of the self-trapped luminescence line. A new treatment of the nature of bands Wand M1 and the temperature dependence of luminescence of solid xenon is as follows. Let us consider the scheme of levels in Xe crystal (Fig. 2). The lowest exciton bands [‘(3/2) and r’(3/2), a half-width being the same (B 1 eV), are known to have a small splitting 3P (~E 3F ~ 0.15 eV) similar to that of atomic levels 1 and 2. The absorption, however, is observed only to the band [‘(3/2) because of the latter has a high impurity of singlet state [‘(1/2) (up to 50%). At the same time [“(3/2) is not essentially observed in spectra and has no singlet impurity. Because the singlet term forms only a repulsive molecular state, its impurity raiseXe~.Selfthe barrier 1E~of up to the bound self-trapped term trapping from r’(3/27to term ~ appears to be more suitable for excitons at the band bottom. For low temperatures (below 50—60 K), the barrier hinders self-trapping from the band [‘(3/2). The emission of free excitons Eex and the exciton relaxation into band r’(3/2) are observed. (The latter process may be favoured by the symmetry distortion in the lattice and a partial removing of the restriction.) The excition lifetime in state [“(3/2) is much larger than that in [‘(3/2) and the barrier to self-trapping is lower. This favours the formation of the quasi-molecular state 3E~the emission of which is observed as band M. As the temperature increases (T 60K), a new, more effective channel for exciton relaxation in [‘(3/2)
both revolution points of lower vibrational levels v’ = 0 and v’ = 1 of the state ‘~. The treatment proposed is supported by the nature of photoinduced spectra of bands M and W near the exciton absorption edge in Xe [12]. The 0.15 eV shift of the excitation energy of band W with respect to band Mis clearly seen. A similar behaviour of the photoinduced spectrum is observed also for two self-trapped bands of solid argon (121, which were earlier treated in the same framework as bands M and W in solid xenon [1].
—
appears the excitions begin to surmount the barrier for the state ‘~. In this case the emission of free excitons and self-trapped excitons from the state ~ vanishes. A radiation lifetime in singlet state is —
‘~
~
—
REFERENCES 1. 2. ~
I.Ya. Fugol, Adv. Phys. 27, 1 (1978). N.G. Basov, O.V. G.N. Bogdankevich, Danilychev, A.G. Devyatkov, KashnikovV.A. & N.P. Lantsov, Firma v ZhETF 7,404(1968). J. Jorther, J. Meyer, S.A. Rice & E.G. Wilson, J. Chem. Phys. 42,4250 (1965). 4. R.A. Kink, A.E. Lykhmus & M.V. Selg, Firma v ZhETF 28, 505 (1978). 5. R. 363Heumllller (1978). & M. Creuzberg, Opt. Comm. 26, 6. J.W. Keto, R.E. Gleason & G.K. Walters, Phys. Rev. Lett. 33, 1365 (1974). 7. 5. Kubota, M. Hishida, M. Suzuki & J. Raun, Rup792, Rikkyo Univ~Tokyo (1979). 8. V. Yakhot,Phys. Sol. (b)Pitzer, 74,451(1976). 9. W.C. Ermler, Y.S.Stat. Lee & K.S. J. Chem. Phys. 69, 976 (1978). 10. A.G. Molchanov, Preprint 113, Fizicheskiy Inst. USSR Acad. Sci., Moscow (1971). 11. SD. Druger & R.S. Knox, J. Chem. Phys. 50, 3143 (1969). 12. C. Ackermann, R. Haensel, U. Hahn, R. Brodmann, G. Tolkiehn & G. Zimmerer, DESYSR-75/l 1 (1975).
THE LOW-LYING collective states in cryocrystals of Xe, Kr, Ar, Ne rare gases are exciton bands r’(l / 2), T(3/2) and r’(3/2). In the lifetime of exciton states, self-trapped states may be formed under the exciton—phonon interaction. The probability of self-trapping increases in the series of Xe to Ne [1]. Hence, the luminescence of free excitons is the most intensive in Xe, less intensive in Ar and absent in Ne. Besides the exciton luminescence,
consistent data on an unusual temperature dependence of luminescence bands over the whole temperature range of the Xe crystals existence (4—165K) (Fig. 3). If at 4—50 K the spectral bands M and Eex are dominant, for 50—70 K the M-band intensity decreases appreciably, the exciton luminescence Eex is quenched, and a 7.6 eV band W appears. As the temperature increases further (90—165 K), a new long-wave band M1 appears and its
cryocrystals of rare gases display the emission of selftrapped states of a quasi-atomic (one-centered) or a quasi-molecular (two-centered)type. For Xe, only quasimolecular self-trapped states are formed because of a weak exciton—phonon interaction. The formation ofa Xe~molecule within the crystal lattice is shown schematically in Fig. 1. As two centers are brought closer together forming the molecule, the self-trapping, at first, results in the formation of a vibration-excited molecule, and then the relaxation of vibrational energy occurs. The position of quasi-molecular luminescence bands is dependent on the vibrational relaxation rate. A self-trapped luminescence spectrum of Xe contains two broad maxima: Mat 7.2 eV and W at 7.6 eV, the nature of which (in particular, of the latter) produces different interpretations. The paper presents the analysis of different formation mechanisms of quasi-molecular bands in solid Xe, the estimates of energy shifts for an excited state ~ of Xe~in the lattice, and the treatment of a self-trapped luminescence spectrum. A general scheme of radiative transitions for the lowest exciton and selftrapped states is shown in Fig. 2 that is in agreement with the treatment proposed. The 7.6 eV emission of Xe was first described by Basov et al. [2] and Jortner et al. [3]. Since then the self-trapped luminescence spectra of Xe have been studied repeatedly [1, 4, 5], and at present, there are
intensity becomes gradually dominant. Note, the band M1 is a new band different from M. In the temperature range of 4 to 50 K the band M shifts with temperature towards the red (AEM /~T~ 0.003 eV deg’). The ternperature dependence ofM is completely reversible and was measured for annealed samples with large crystallites. The data given are in agreement with the measurement results from [5]. Extrapolation of the M-band position at T> 57 K is shown in Fig. 3 by a dashed line and, as seen, does not coincide with the band M1. For unannealed specimens of a poor structure, the maximum M has a red shift of about 0.1 eV. For liquid, at 165 K the emission of Xe~is observed at EM ~ 7.05 eV. However, at temperatures above 100 K even a vibrational level v’ = 1 of the excited molecule is populated. When the spectral decomposition of the maximum observed is made, it appears that for liquid the emission band from v’ = 0 is to be located at EM 7.0 eV. The M-band position in gas is dependent on both temperature and pressure. At T ~ 300 K, the emission should be produced by all three lowest vibrational levels of both terms 1, 3~ and be directed onto a repulsive term ‘.~. In order to separate from the complex band an emission component from V’ = 0, the shape and position of emission bands of the isolated molecule Xe~have been calculated in the 100—400 K temperature range under population of some vibrational
787
788
EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOLID XENON
Vol. 32, No.9
~~~iiioIui~ ®~ciiIiiiiik~~~ / D
~
~7\
gas
II~\~quid
0)
‘M1~
Fig. 1. A scheme of formation of quasi-molecular centres of Xe~in a f.c.c. crystal lattice.
5~M 1~~’
~
_____
/ ,‘/
‘,/
E~ w °‘
~JJ~ MWEe
re
internuclear distance Fig. 2. A general scheme of radiation transitions in the Xe crystal. levels (v’ = 0, 1, 2, 3). The calculations make it possible to establish the fact that the radiation transition from the lowest state is associated with the band maximum at ~‘M = 6.9 ±0.05 eV. It should be noted that the band width observed in gas and in liquid is 0.6eV and does not permit the contributions of transitions ~ -+ ~ and ~ -+ ~ to be separated (the band splitting is no more than 0.2eV). Measurements of the kinetics ofintensity indicate, however, independent contributions of these two transitiong to the emission of gas and liquid. In gas, one can observe two typical times of quenching r1 = 2 nsec and = 100 nsec for states ~ and respectively [6]. In liquid, two times (r1 = 2 nsec and r2 = 27 nsec) were also observed in experiments with an excess electric field to separate the recombination radiation [7]. For solid, ~
‘,\
~
E1
7 energy,ev 8
9
(4—150K), liquid (165 K) and gaseous (300K) xenon Fig. [1—7].Temperature 3. Intensity distribution of samples in spectra is presented of solid by hon. zontal lines straight line drawn (0.003from eV deg~inclination) the emission maximum. displaysAa reversible temperature displacement of band M in the 4—60 K range; its interpolation to T> 60K is denoted by a dotted-dashed line. A dashed curve shows the position of the M-band maximum under heating of the unannealed sample. there are no reliable data on lifetime involving the finite time of self-trapping (in solid xenon the latter factor may prove to be comparable to the lifetime of singlet states). A number of experimental data indicate that the M.band at 4K corresponds to the self-trapped emission of Xe crystals from v’ = 0 of state ~ (i) the obvious similarity to gas or liquid spectrum and the strict sequence when comparing to other rare elements; (ii) a small effect of the crystal structure defects and the sample thickness of the M-band intensity and position; (iii) the invariable position of band M in the spectrum of pure Xe and in spectra of solid solutions of other rare elements with a small impurity of Xe. As to band W, there are many different interpretations. Despite the discrepancy with certain facts, several authors assume that the pure W-band is an emission 3E~in a “perfect” crystal, while thefrom M-band state
Vol. 32, No.9
EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOLID XENON
789
estimate V~ 0.12G (G ~ 1 eV, the exciton bond energy) [10]. From this it follows that the excited molecule—lattice interaction can be assumed to be weak. In Fig. 1 four types of the lattice sites are shown. Of these only plane A contains atoms entering into the nearest surroundings of both centers of the molecule. The effect of the rest (B, C and D-type) is considered in terms of the interaction with the nearest center. Note,
o
that as two centers are brought together producing the formation of the Xe~molecule in the crystal, the separation between these centers and the A atoms decreases as compared to an equilibrium separation d, and that between the centers and atoms of other type increases. Each of the A atoms, therefore, should make a much larger contribution to VL than that from any other atom. The required quantities V~, V~,Vj)~,° and V~° were calculated by using the Lennard—Johns potential, the excited state potential being averaged as in the calculations for a molecular ion [11]. Since the precise configuration of distorted lattice around the excited molecule is unknown, possible extreme changes in V~
I
E~I
~ I C
Q)
-
5
I
~ntera~micdistance
Fig. 4. A scheme of potentials of interaction between 9uasi-molecule Xe~and its surroundings in the ground ~‘~) and excited (lIZ) states. is due to that transition near the vacancy [4]. Other authors, with due regard to arguments (i)—(iii), assume the possibility of appearance of a new crystal structure with a hindered medium relaxation at T~70K [8]. There is, however, no support for this assumption. It has been also assumed that the W-band is associated with one of the higher terms [5]; the assumption is very tempting, but requires recalculation of the terms of the molecule Xe~[9]. To resolve these discrepancies, we estimate theoretically the lattice effect on term positions in the Xe~ quasi-molecule. The shift of self-trapped bands in crystal, EM, with respect to that in the isolated molecule, i’M, is E —6 9 ,~ —
M
‘
—
—
L
~‘
L’
‘~ ‘
where V~,V~is the shift of intramolecular potentials for the ground and excited.states that is equal to molecule—lattice interaction potentials, and ~‘M = 6.9 eV. Considering the weakness of the Van der Waals forces of lattice atom interactions, we assume that the main contribution to V~and ~ is due to a sum of pairwise interactions with nearest neighbours. For the ground state this assumption is well tested. For the excited state, we can refer to the experimental data [1] and to the
and V~with varying the deformation parameters are calculated. An upper limit to the energy of the interaction between the excited molecule and nearest neighbours can be compared to the hypothetical model where with shifting two atoms forming the molecule their neighbours remain “anchored” at the perfect lattice sites. It is this configuration that is shown in Fig. 1. In this case the minimum of the internuclear potential of the molecule incorporated in a “undeformed” crystal would decrease only by 0.1 eV, and V~ V~= 0.45 eV. According to (1), this shift would be followed by the emission band EM = 7.35 eV. The lowest value of the V~energy occurs when all atoms of the surroundings are removed from the molecular centers by the distance Rmjn corresponding to a minimum position of the VZ pairwise potential. In that case VZ=—0.3eV, V~=—0.S3eVandhence, V~ = 0.2 ±0.05 eV that would correspond to a 7.1 eV emission band. If we introduce some averaged (for atoms of all types) distance R between the atom center of the molecule and its surroundings (the cavity radius), the energy of molecule—lattice interaction can be given as a function of coordinate V1~’°(R) (Fig. 4). Taking into account the lattice elasticity, the cavity radius R is smaller than Rmjn. It is seen from Fig. 4 that the value of V~ VZ remains negative increasing its absolute value. This implies that the molecular transition shift in the crystal with respect to that in the free mole. cule is sure to be blue and of the order of 0.2—0.3 eV. From the calculations made and the general physical considerations it is obvious that the model of the molecule formation in a crystal without distorted —
—
—
—
—
—
790
EMISSION BANDS OF SELF-TRAPPED EXCITONS IN SOLID XENON
Vol. 32, No.9
surrounding lattice is worthless. Moreover, for the anchored surroundings a high barrier h > 0.2 eV appears that closes the channel of exciton self-trapping. The vacancy in the nearest surroundings can produce only a slight decrease in the matrix shift and in the self-trapping barrier. For B, C, D vacancies the decrease does not exceed (V2 VZ)/22 ~ 0.01 eV. The interaction with A-atoms is two or three times stronger and can produce a 0.05 eV decrease in the barrier. For the undistorted lattice, the barrier cannot become surmounted because of this decrease. When considering the medium relax-
smaller than that in state Therefore, at 70K the vibrational relaxation is not managed to occur and the W-band (7.6 eV) is observed that corresponds to the emission from higher vibrational states of v’ > 5. It should be noted that for great vibrational numbers the emission from a right-hand revolution point of the potential is the most probable (see Fig. 2). A further increase in temperature accelerates vibrational relaxation in the molecule a red wavelength shift of the band is observed. For 150K, the M1 -band emission is associated with transitions from
ation at h 5 0.05 eV, the vacancies, however, can favour the exciton self-trapping having practically no influence on the position of the self-trapped luminescence line. A new treatment of the nature of bands Wand M1 and the temperature dependence of luminescence of solid xenon is as follows. Let us consider the scheme of levels in Xe crystal (Fig. 2). The lowest exciton bands [‘(3/2) and r’(3/2), a half-width being the same (B 1 eV), are known to have a small splitting 3P (~E 3F ~ 0.15 eV) similar to that of atomic levels 1 and 2. The absorption, however, is observed only to the band [‘(3/2) because of the latter has a high impurity of singlet state [‘(1/2) (up to 50%). At the same time [“(3/2) is not essentially observed in spectra and has no singlet impurity. Because the singlet term forms only a repulsive molecular state, its impurity raiseXe~.Selfthe barrier 1E~of up to the bound self-trapped term trapping from r’(3/27to term ~ appears to be more suitable for excitons at the band bottom. For low temperatures (below 50—60 K), the barrier hinders self-trapping from the band [‘(3/2). The emission of free excitons Eex and the exciton relaxation into band r’(3/2) are observed. (The latter process may be favoured by the symmetry distortion in the lattice and a partial removing of the restriction.) The excition lifetime in state [“(3/2) is much larger than that in [‘(3/2) and the barrier to self-trapping is lower. This favours the formation of the quasi-molecular state 3E~the emission of which is observed as band M. As the temperature increases (T 60K), a new, more effective channel for exciton relaxation in [‘(3/2)
both revolution points of lower vibrational levels v’ = 0 and v’ = 1 of the state ‘~. The treatment proposed is supported by the nature of photoinduced spectra of bands M and W near the exciton absorption edge in Xe [12]. The 0.15 eV shift of the excitation energy of band W with respect to band Mis clearly seen. A similar behaviour of the photoinduced spectrum is observed also for two self-trapped bands of solid argon (121, which were earlier treated in the same framework as bands M and W in solid xenon [1].
—
appears the excitions begin to surmount the barrier for the state ‘~. In this case the emission of free excitons and self-trapped excitons from the state ~ vanishes. A radiation lifetime in singlet state is —
‘~
~
—
REFERENCES 1. 2. ~
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