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SIMS characterization of hetero-isotopic cluster ions of lithium fluoride
Vacuum/volume 39/numbers Printed in Great Britain
24Jpages
0042-207X/89$3.00+.00 Pergamon Press plc
137 to 138/I 989
SIMS characterization ions of lithium fluoride Guang-hou
Wang.
state Physics,
Lie Dou, Zhi-gou
Naming
University,
of hetero-isotopic
Liu and Yi-zhang
Nanjing,
Zhu, Department
of Physics
cluster and Institute
of Solid-
PRC
and
Tao-nan
Zhao, Yan-hou
Jiang and Ji-hong
Yang, Modern
Analytical
Center,
Nanjing
University,
Nanjing,
PRC
Fast atom bombardment and secondary ion mass spectroscopy are used to study the characteristics of ion clusters sputtered from lithium fluoride crystal. Two types of cluster ions, Li(LiF) N and (LiF) N are identified. Large amounts of hetero-isotopic clusters are observed in both cases and isotopic effects exist in the formation of lithium fluoride ion clusters, another important property of alkali halides following #magic numbers’ in SIMS mass spectrum.
1. Introduction Clusters are microscopically small atomic aggregates with physical properties so unlike other forms of matter that they have been called the ‘fifth state of matter’ ‘, and play a central role in the vapor-to-condensed phase transition of all matter. Alkali halide clusters are widely studied systems because of their elementary electronic properties, ionic nature and low vapor pressure. However, up to now, little is known about their structure, stability and thermodynamic properties though the importance of the clusters continues to be investigdted2,‘. Recently, in the development of a high performance secondary ion mass spectrometer, Barlak et al“, observed that alkali halide cluster ions of the form M(MX),G were produced by ion bombardment and at certain ‘magic number’ N values the intensity of the cluster was enhanced. Some other experiments showed a monotonic decrease in the intensity of these clusters vs N (refs 5 and 6).
2. Experiments and results We used fast atom bombardment and secondary ion mass spectrometry (FABSIMS) to investigate the positive ion spectra of
high pure LiF crystal with 300 ppm impurity and (100) cleavage plane. Figure 1 shows one of the SIMS mass spectra of the LiF sample from 8 keV argon atom bombardment at I mA beam current, where some mass numbers are identified as those of the corresponding clusters of lithium fluoride whose relative yields are obviously larger than their neighbours. The molecular ion clusters are characteristic of two structural types : microcrystal-like Li(LiF),t and complex molecules (LiF),:. Tables I and 2 list some of these two types of cluster ions, their masses and their relative SIMS yields. The following features of cluster ion distribution are seen from Table I and Figure I.
(I) On the whole the intensities of Li(LiFj,; and (LiF),; decrease monotonically as a function of number N except where N = 7 and 13 for Li(LiF),G and N = 8 and 14 for (LiF),:. where the ion intensities are enhanced. (2) Hetero-isotopic cluster ions are produced preferentially in the emission of both Li(LiF),; and (LiF),: clusters. Especially at N = 13 we have observed only [Li(7),(6)hF, J’ with a mass of 339 au and an intensity of 0.40 for the ion cluster [Li(LiF) , J’ and, at N = 14 only [Li(7),(6),F,,]+ for the cluster (LiF):, with a mass of 357 au and an intensity of 0.3 I.
100
go- Li(7),F 33
[LIILIF~]+ L1(7),F,
80 70 -
9 >, ? $
d
(LIF):
59 L1(7l,F, /II
6050-
L1(7),(6l,F~
40-
30-
26
L1C7),F3
LiC7kF5 137 162
L1(7),F, 208
L1(71,(6),F,,
(LIF),,
20 -
0
50
100
I50
200
250
300
350
400
Mass
Figure1. Secondary
ion mass spectrum
of LiF crystal
by 8 keV Ar atom bombardment. 137
Guang-hou
Wang et a/: SIMS characterization
Table I. Ion clusters Lt(LiF),
produced
of lithium fluoride
hq bombardment
of X keV Ar
~itOm5
.v
Ion clusters
Isotopes
I
Li(LiF)
2
Li(LiF),
Li(7),F Ll(7),(6),F Li(6) .F Li(7)$. Li(7),th),T, Li(7),(6),1-. Li(h),l:? Li(7),F, Li(7),(6),F, Li(7),(6):1’, Li(7),(6),F, L1(6),F i Li(7),F, Li(7),(6),F, Li(7)\(6)$, Li(7),(6),F, Li(7),(6),F, Li(6),F,
3
1
Li(LiF)
Li(LIF),
i
Rcl,ltivc L yields
Mass
Li(7)F Li(6)F Li(7),F, Li(7),(6),1‘: Li(h),F. Li(7),Fi Li(7)?(6),F, Li(7),(h)‘F, Li(6),F,
26 25 52 51 50 78 77 76 75 IO4 103 IO2 101
(LiF)
91.50 100.0 46. I6 17.x7 6.91 15.25 3.67 3 05
2
(Lit-‘):
7
(LiF)
4
(LiF),
j
I .25 I .A5 2.v I .67
3. Discussion The preferential emission of hetero-isotopic cluster ions is due to the isotopic effect in the process ofcluster formation since lithium has two isotopes ‘Li and ‘Li. and may be closely related to the isotopic fractionation in the sputtering of solid-state materials with different isotopes such as C:I, Si. MO. Cu and U (ref 7). This has been confirmed by copper ion clusters produced by sputtering”. The enhanced ion intensity at N = I3 for the cluster [Li(LiF) , ?]’ is related to the ‘cubic-hkc’ structure 3 x 3 x 3, which is similar to the cluster ion [CS(CSI),~]’ where the magic numbers N = 13.22,37 and 62 corresponding to the structures of3 x 3 x 3, 3 x 3 x 5. 3 x 5 x 5 and 5 x 5 x 5 have been observcdJ. The ion clusters (LiF),:, are the complex molecular ions with electronic properties more or less similar to those of inert gas elements which have close-compact structures when they are aggregated’.
by bombardment
Isotopes
I
I.71 0.64 0.2x 0 20
produced
Ion clusters
IOO.0 IWO
These indicate that (a) there is prcl’erential formation of the hetero-isotopic cluster ions in the sputtering process. and (b) the ion clusters with certain numbers such as N = I3 or 14 are more stable.
138
Table 2. Ion clusters (LiF), neutrnl atoms
Li(7),F, Li(7),(6),F, Li(7)?(6),F, Li(7),(6),F, Li(G),F,
100
01‘ 8 kcV AI
Relative yields 27.53 5.06 1.75 4.03 2.56 1.13 2.69 I I9
1.23 0.8I 0.86 0.5 I 0.46 0.64
The reason for the enhanced intensity of (LiF),: . either due to ionization or due to the isotopic effect, or both. is not clear now and needs further investigation. Acknowledgements This work was financially supported by the Laboratory of Solid State Microstructure at Nanjing University and the National Natural Science Foundation, China. The authors are grateful to Professor Duan Feng, Chang-de Gong and Hong-ru Zhe for their support of our research. References
’ G D Stein. P/IJ..s TN& 17, 503 (1979). ‘J E Campana. T M Barlak, R J Colton. J J DeCorpo. J R Wyatt and B L Dunlap. Pl+v Rw h/r, 47, 1046 (I98 I). ‘G-h Wang. L Dou, J Z Pang. L Z Zhou and C D Gong. f’r’rr)qPfzm 7, 1 (1987). “T M Barlak. J E Campana, R J Colton, J J Decorpo and J R Wyatt, .I PhK\ C‘hrw, 85, 3x40 (1981); J Am Chem sot, 104, 1212 (1982). ‘J A Taylor and J W Rabalais, .%/ix, Sri, 74, 229 (I 978). “F Honda, Y Fukuda and J W Rabalais, J C’hwn P/I~.v. 70, 4834 (1979). ‘W A Russell, D A Papanastassion and T A Tomhrello. Rrrrlirrt E//cct.c, 52, 44 ( 1980). “G-h Wang. L Dou, Z G Liu, T N Zhao. Y H Jiang and J f1 Yang, P/I,I,.c Rcz, B37, 9093 (19X8). “J Z Pan&. G H Wang, L Dou :tnd C D Gong. PIIJ:Y I,(,//. Al 17, I I5 (19%)
24Jpages
0042-207X/89$3.00+.00 Pergamon Press plc
137 to 138/I 989
SIMS characterization ions of lithium fluoride Guang-hou
Wang.
state Physics,
Lie Dou, Zhi-gou
Naming
University,
of hetero-isotopic
Liu and Yi-zhang
Nanjing,
Zhu, Department
of Physics
cluster and Institute
of Solid-
PRC
and
Tao-nan
Zhao, Yan-hou
Jiang and Ji-hong
Yang, Modern
Analytical
Center,
Nanjing
University,
Nanjing,
PRC
Fast atom bombardment and secondary ion mass spectroscopy are used to study the characteristics of ion clusters sputtered from lithium fluoride crystal. Two types of cluster ions, Li(LiF) N and (LiF) N are identified. Large amounts of hetero-isotopic clusters are observed in both cases and isotopic effects exist in the formation of lithium fluoride ion clusters, another important property of alkali halides following #magic numbers’ in SIMS mass spectrum.
1. Introduction Clusters are microscopically small atomic aggregates with physical properties so unlike other forms of matter that they have been called the ‘fifth state of matter’ ‘, and play a central role in the vapor-to-condensed phase transition of all matter. Alkali halide clusters are widely studied systems because of their elementary electronic properties, ionic nature and low vapor pressure. However, up to now, little is known about their structure, stability and thermodynamic properties though the importance of the clusters continues to be investigdted2,‘. Recently, in the development of a high performance secondary ion mass spectrometer, Barlak et al“, observed that alkali halide cluster ions of the form M(MX),G were produced by ion bombardment and at certain ‘magic number’ N values the intensity of the cluster was enhanced. Some other experiments showed a monotonic decrease in the intensity of these clusters vs N (refs 5 and 6).
2. Experiments and results We used fast atom bombardment and secondary ion mass spectrometry (FABSIMS) to investigate the positive ion spectra of
high pure LiF crystal with 300 ppm impurity and (100) cleavage plane. Figure 1 shows one of the SIMS mass spectra of the LiF sample from 8 keV argon atom bombardment at I mA beam current, where some mass numbers are identified as those of the corresponding clusters of lithium fluoride whose relative yields are obviously larger than their neighbours. The molecular ion clusters are characteristic of two structural types : microcrystal-like Li(LiF),t and complex molecules (LiF),:. Tables I and 2 list some of these two types of cluster ions, their masses and their relative SIMS yields. The following features of cluster ion distribution are seen from Table I and Figure I.
(I) On the whole the intensities of Li(LiFj,; and (LiF),; decrease monotonically as a function of number N except where N = 7 and 13 for Li(LiF),G and N = 8 and 14 for (LiF),:. where the ion intensities are enhanced. (2) Hetero-isotopic cluster ions are produced preferentially in the emission of both Li(LiF),; and (LiF),: clusters. Especially at N = 13 we have observed only [Li(7),(6)hF, J’ with a mass of 339 au and an intensity of 0.40 for the ion cluster [Li(LiF) , J’ and, at N = 14 only [Li(7),(6),F,,]+ for the cluster (LiF):, with a mass of 357 au and an intensity of 0.3 I.
100
go- Li(7),F 33
[LIILIF~]+ L1(7),F,
80 70 -
9 >, ? $
d
(LIF):
59 L1(7l,F, /II
6050-
L1(7),(6l,F~
40-
30-
26
L1C7),F3
LiC7kF5 137 162
L1(7),F, 208
L1(71,(6),F,,
(LIF),,
20 -
0
50
100
I50
200
250
300
350
400
Mass
Figure1. Secondary
ion mass spectrum
of LiF crystal
by 8 keV Ar atom bombardment. 137
Guang-hou
Wang et a/: SIMS characterization
Table I. Ion clusters Lt(LiF),
produced
of lithium fluoride
hq bombardment
of X keV Ar
~itOm5
.v
Ion clusters
Isotopes
I
Li(LiF)
2
Li(LiF),
Li(7),F Ll(7),(6),F Li(6) .F Li(7)$. Li(7),th),T, Li(7),(6),1-. Li(h),l:? Li(7),F, Li(7),(6),F, Li(7),(6):1’, Li(7),(6),F, L1(6),F i Li(7),F, Li(7),(6),F, Li(7)\(6)$, Li(7),(6),F, Li(7),(6),F, Li(6),F,
3
1
Li(LiF)
Li(LIF),
i
Rcl,ltivc L yields
Mass
Li(7)F Li(6)F Li(7),F, Li(7),(6),1‘: Li(h),F. Li(7),Fi Li(7)?(6),F, Li(7),(h)‘F, Li(6),F,
26 25 52 51 50 78 77 76 75 IO4 103 IO2 101
(LiF)
91.50 100.0 46. I6 17.x7 6.91 15.25 3.67 3 05
2
(Lit-‘):
7
(LiF)
4
(LiF),
j
I .25 I .A5 2.v I .67
3. Discussion The preferential emission of hetero-isotopic cluster ions is due to the isotopic effect in the process ofcluster formation since lithium has two isotopes ‘Li and ‘Li. and may be closely related to the isotopic fractionation in the sputtering of solid-state materials with different isotopes such as C:I, Si. MO. Cu and U (ref 7). This has been confirmed by copper ion clusters produced by sputtering”. The enhanced ion intensity at N = I3 for the cluster [Li(LiF) , ?]’ is related to the ‘cubic-hkc’ structure 3 x 3 x 3, which is similar to the cluster ion [CS(CSI),~]’ where the magic numbers N = 13.22,37 and 62 corresponding to the structures of3 x 3 x 3, 3 x 3 x 5. 3 x 5 x 5 and 5 x 5 x 5 have been observcdJ. The ion clusters (LiF),:, are the complex molecular ions with electronic properties more or less similar to those of inert gas elements which have close-compact structures when they are aggregated’.
by bombardment
Isotopes
I
I.71 0.64 0.2x 0 20
produced
Ion clusters
IOO.0 IWO
These indicate that (a) there is prcl’erential formation of the hetero-isotopic cluster ions in the sputtering process. and (b) the ion clusters with certain numbers such as N = I3 or 14 are more stable.
138
Table 2. Ion clusters (LiF), neutrnl atoms
Li(7),F, Li(7),(6),F, Li(7)?(6),F, Li(7),(6),F, Li(G),F,
100
01‘ 8 kcV AI
Relative yields 27.53 5.06 1.75 4.03 2.56 1.13 2.69 I I9
1.23 0.8I 0.86 0.5 I 0.46 0.64
The reason for the enhanced intensity of (LiF),: . either due to ionization or due to the isotopic effect, or both. is not clear now and needs further investigation. Acknowledgements This work was financially supported by the Laboratory of Solid State Microstructure at Nanjing University and the National Natural Science Foundation, China. The authors are grateful to Professor Duan Feng, Chang-de Gong and Hong-ru Zhe for their support of our research. References
’ G D Stein. P/IJ..s TN& 17, 503 (1979). ‘J E Campana. T M Barlak, R J Colton. J J DeCorpo. J R Wyatt and B L Dunlap. Pl+v Rw h/r, 47, 1046 (I98 I). ‘G-h Wang. L Dou, J Z Pang. L Z Zhou and C D Gong. f’r’rr)qPfzm 7, 1 (1987). “T M Barlak. J E Campana, R J Colton, J J Decorpo and J R Wyatt, .I PhK\ C‘hrw, 85, 3x40 (1981); J Am Chem sot, 104, 1212 (1982). ‘J A Taylor and J W Rabalais, .%/ix, Sri, 74, 229 (I 978). “F Honda, Y Fukuda and J W Rabalais, J C’hwn P/I~.v. 70, 4834 (1979). ‘W A Russell, D A Papanastassion and T A Tomhrello. Rrrrlirrt E//cct.c, 52, 44 ( 1980). “G-h Wang. L Dou, Z G Liu, T N Zhao. Y H Jiang and J f1 Yang, P/I,I,.c Rcz, B37, 9093 (19X8). “J Z Pan&. G H Wang, L Dou :tnd C D Gong. PIIJ:Y I,(,//. Al 17, I I5 (19%)