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Positron annihilation with halogenide ions
Volume 27A, number 8
PHYSICS
POSITRON
ANNIHILATION
LETTERS
WITH
9 September 1968
HALOGENIDE
IONS
G. GAMBARINI and L. ZAPPA Istituto di Fisica de1 Politecnico,
Gmppo Nazionale di Strutturn della Mater&,
Milmo,
Italy
Received 5 July 1968
The annihilation of positrons in substances of unsymmetrical valence type such as fluorite wss investigated. It was found that the lifetimes are nearly equal to those observed in alkali halides having the same anion.
The annihilation of positrons in alkali halides was recently investigated by Bussolati et al. [l]. Their results indicate that the lifetime spectrum is complex and that the negative ions play a predominant role while the positive ions have but a little influence on the annihilation features. Various models have been proposed in order to explain the basic characteristics of slow positron annihilation in ionic media [2]. In particular in media containing large concentrations of negative ions, the existence of a quasi-atomic system of the e+-anion type has been taken into consideration. Of course every model based on a positron bound to a negative ion must be substantiated by experimental results.on the annihilation features in ionic crystals other than alkali halides. For this reason we have measured the lifetimes of
positrons annihilating in substances of unsymmetrical valence type such as fluorite. The experimental procedure adopted for assembling positron source and crystalline specimens, the electronic equipment and the method for time spectrum analysis, were identical to those already used in previous investigations [1,3,4]; consequently they will not be reported here. Table 1 collects the results of our measurements; the stated errors are conservative estimates and do not arise only from the scatter of experimental results. In the table the lifetimes and intensities of the spectral components are arranged with the same criterion used by Bussolati et al. [l] to classify the components present in the time annihilation spectrum of alkali hali-
Table 1 Positron mean lives and intensities.
(A??)
71
(10-10s)
(l;-:os)
‘-“lo (10 s)
I1
IO
(%I
(56)
I2
I3
ckb,
(%I
MgF2 s.c.*
1.70
0.09
3.06 * 0.09
61
CaF2 S.C.
1.79 * 0.09
3.28 f 0.10
53 f 5
43.5
SrF2 S.C.
1.74 f 0.09
3.47 * 0.10
49 f 5
511 f 2.6
BaF2 S.C.
1.76 * 0.09
3.78 f 0.11
38 * 4
62.6 f 3.1
BaC12
2.93 f 0.15
5.73 f 0.17
83 f 7
17.1 * 0.9
CaBr2
2.62 * 0.13
6.23 f 0.19
40
38.5 f 1.9
SrBr2
2.72 * 0.14
5.39 f 0.16
3.87
7.41
Sr12
1.81 * 0.13
* S.C. = single crystal.
498
l
l
0.19
l
0.22
10.2
l
0.3
20 f 3
l
l
5
4
37.0 * 1.9 l
2.2
53 f 5
41.5
61 * 5
22.6 + 1.1
l
2.1
19.0 f 0.4
Volume 2 7A.
number 8
PHYSIC8
9
LETTERS
September1968
quite similar to those observed in alhali halides by Bussolati et al. A comparative examination allows us to draw the following two conclusions: a) the annihilation rates are prevailingly determined by the negative ion; b) to each negative ion one can associate, in a first approximation, the same values of the decay rates. This is true whatever the structure of the crystal may be.
des. The indexes 0,1,2,3 individuate the components in order of increasing lifetimes; those having the same index shall presumably have the same origin. For instance, if one attributes the various components to the annihilation of positrons from the first levels of a bound system, the same index shall individuate the same level. The rough model proposed by Bussolati et al. suggests that the components with index 1 and 2 may be ascribed to the annihilation of positrons from the ground and the first excited level, respectively, of the system e+-anion; as regards the 75 and 73 components, the proposed model is unable to explain their origin. We do not report in the table the “tail” component (whose intensity is lower than 1%) as its origin shall be attributed to positron annihilation in regions other than the homogeneous interior of the crystal [5,6]. We do not intend now to discuss a particular model for framing our present results but only to examine if they give further suggestions on positron annihilation in ionic media. The inspection of table 1 shows that the general trend of the time spectrum and the values of the lifetimes are
References A. Dupasquier and L. Zappa, Nuovo 1. C. Bussolati, Cimento 52B (1967) 529. Sov. Phys. 2. V.I. Gol’dsnskii and E. P..Prokop’ev, Solid State 8 (1966) 409. This article gives references to earlier works. S. Cova and L. Zappa, Nuovo Cimento 3: C. Bussolati, 50B (1967) 256. and L. Zappa, Nuovo Cimento 52B 4. M. Bertolaccini (1967) 487. 5. H.Weisberg and S.Berko, Phys.Rev.154 (1967) 249. to be published. 6. R. Paulin and G.Ambrosino,
*****
MEASUREMENT
OF
ELECTRON
ENERGY
LOSSES
IN STRONTIUM*
B. M. HARTLEY Department
of Physics,
University
Received
of Western Australia
11 July 1968
Surface and volume plasmon energies of 3.6 f 0.05 eV and 7.8 f 0.1 eV are measured for strontium. Energy losses due to NI and NHIH ionization are identified and a peak due to a polarization wave in the NHIII band is identified in the characteristic electron energy loss spectrum.
Electron energy loss spectra have been measured for metallic strontium over a range of bombarding energies using the reflection technique described previously [1,2]. Pure strontium was mounted as the target material and scraped in a vacuum of about 5 X 10-T Torr to produce a clean surface. No measurements of surface condition could be made but measured energy losses for each spectrum indicate that there is probably no significant contamination. During the running * Work supported by the U.S. Army Research the University of Western Australia.
Office and
of the spectra the strontium surface was renewed by continuous scraping. Fig. 1 shows the energy loss spectrum of strontium for primary energy 1000 V. The two lower energy losses in this spectrum are evaluated at 3.6 f 0.05 eV and 7.8 f 0.1 eV. The 3.6 eV energy loss is interpreted as being due to excitation of surface plasmons and the 7.8 eV loss as due to excitation of volume plasmons. This interpretation is consistent with the identification of similar peaks in calcium and barium [3]. Because of the natural width of the energy losses multiple volume and surface losses are difficult 499
PHYSICS
POSITRON
ANNIHILATION
LETTERS
WITH
9 September 1968
HALOGENIDE
IONS
G. GAMBARINI and L. ZAPPA Istituto di Fisica de1 Politecnico,
Gmppo Nazionale di Strutturn della Mater&,
Milmo,
Italy
Received 5 July 1968
The annihilation of positrons in substances of unsymmetrical valence type such as fluorite wss investigated. It was found that the lifetimes are nearly equal to those observed in alkali halides having the same anion.
The annihilation of positrons in alkali halides was recently investigated by Bussolati et al. [l]. Their results indicate that the lifetime spectrum is complex and that the negative ions play a predominant role while the positive ions have but a little influence on the annihilation features. Various models have been proposed in order to explain the basic characteristics of slow positron annihilation in ionic media [2]. In particular in media containing large concentrations of negative ions, the existence of a quasi-atomic system of the e+-anion type has been taken into consideration. Of course every model based on a positron bound to a negative ion must be substantiated by experimental results.on the annihilation features in ionic crystals other than alkali halides. For this reason we have measured the lifetimes of
positrons annihilating in substances of unsymmetrical valence type such as fluorite. The experimental procedure adopted for assembling positron source and crystalline specimens, the electronic equipment and the method for time spectrum analysis, were identical to those already used in previous investigations [1,3,4]; consequently they will not be reported here. Table 1 collects the results of our measurements; the stated errors are conservative estimates and do not arise only from the scatter of experimental results. In the table the lifetimes and intensities of the spectral components are arranged with the same criterion used by Bussolati et al. [l] to classify the components present in the time annihilation spectrum of alkali hali-
Table 1 Positron mean lives and intensities.
(A??)
71
(10-10s)
(l;-:os)
‘-“lo (10 s)
I1
IO
(%I
(56)
I2
I3
ckb,
(%I
MgF2 s.c.*
1.70
0.09
3.06 * 0.09
61
CaF2 S.C.
1.79 * 0.09
3.28 f 0.10
53 f 5
43.5
SrF2 S.C.
1.74 f 0.09
3.47 * 0.10
49 f 5
511 f 2.6
BaF2 S.C.
1.76 * 0.09
3.78 f 0.11
38 * 4
62.6 f 3.1
BaC12
2.93 f 0.15
5.73 f 0.17
83 f 7
17.1 * 0.9
CaBr2
2.62 * 0.13
6.23 f 0.19
40
38.5 f 1.9
SrBr2
2.72 * 0.14
5.39 f 0.16
3.87
7.41
Sr12
1.81 * 0.13
* S.C. = single crystal.
498
l
l
0.19
l
0.22
10.2
l
0.3
20 f 3
l
l
5
4
37.0 * 1.9 l
2.2
53 f 5
41.5
61 * 5
22.6 + 1.1
l
2.1
19.0 f 0.4
Volume 2 7A.
number 8
PHYSIC8
9
LETTERS
September1968
quite similar to those observed in alhali halides by Bussolati et al. A comparative examination allows us to draw the following two conclusions: a) the annihilation rates are prevailingly determined by the negative ion; b) to each negative ion one can associate, in a first approximation, the same values of the decay rates. This is true whatever the structure of the crystal may be.
des. The indexes 0,1,2,3 individuate the components in order of increasing lifetimes; those having the same index shall presumably have the same origin. For instance, if one attributes the various components to the annihilation of positrons from the first levels of a bound system, the same index shall individuate the same level. The rough model proposed by Bussolati et al. suggests that the components with index 1 and 2 may be ascribed to the annihilation of positrons from the ground and the first excited level, respectively, of the system e+-anion; as regards the 75 and 73 components, the proposed model is unable to explain their origin. We do not report in the table the “tail” component (whose intensity is lower than 1%) as its origin shall be attributed to positron annihilation in regions other than the homogeneous interior of the crystal [5,6]. We do not intend now to discuss a particular model for framing our present results but only to examine if they give further suggestions on positron annihilation in ionic media. The inspection of table 1 shows that the general trend of the time spectrum and the values of the lifetimes are
References A. Dupasquier and L. Zappa, Nuovo 1. C. Bussolati, Cimento 52B (1967) 529. Sov. Phys. 2. V.I. Gol’dsnskii and E. P..Prokop’ev, Solid State 8 (1966) 409. This article gives references to earlier works. S. Cova and L. Zappa, Nuovo Cimento 3: C. Bussolati, 50B (1967) 256. and L. Zappa, Nuovo Cimento 52B 4. M. Bertolaccini (1967) 487. 5. H.Weisberg and S.Berko, Phys.Rev.154 (1967) 249. to be published. 6. R. Paulin and G.Ambrosino,
*****
MEASUREMENT
OF
ELECTRON
ENERGY
LOSSES
IN STRONTIUM*
B. M. HARTLEY Department
of Physics,
University
Received
of Western Australia
11 July 1968
Surface and volume plasmon energies of 3.6 f 0.05 eV and 7.8 f 0.1 eV are measured for strontium. Energy losses due to NI and NHIH ionization are identified and a peak due to a polarization wave in the NHIII band is identified in the characteristic electron energy loss spectrum.
Electron energy loss spectra have been measured for metallic strontium over a range of bombarding energies using the reflection technique described previously [1,2]. Pure strontium was mounted as the target material and scraped in a vacuum of about 5 X 10-T Torr to produce a clean surface. No measurements of surface condition could be made but measured energy losses for each spectrum indicate that there is probably no significant contamination. During the running * Work supported by the U.S. Army Research the University of Western Australia.
Office and
of the spectra the strontium surface was renewed by continuous scraping. Fig. 1 shows the energy loss spectrum of strontium for primary energy 1000 V. The two lower energy losses in this spectrum are evaluated at 3.6 f 0.05 eV and 7.8 f 0.1 eV. The 3.6 eV energy loss is interpreted as being due to excitation of surface plasmons and the 7.8 eV loss as due to excitation of volume plasmons. This interpretation is consistent with the identification of similar peaks in calcium and barium [3]. Because of the natural width of the energy losses multiple volume and surface losses are difficult 499