The size distribution of iron-sulfur binary cluster ions, Fe
nS
m+ (
n=1-36,
m=1-23), was investigated using secondary ion mass spectrometry (SIMS). The dissociation patterns,
n=2-26,
m=1-17, were also investigated by varying the acceleration voltage of secondary ions step by step. Iron-sulfur binary cluster ions were produced by 10 keV Xe ion bombardment on a pyrite (FeS
2) sheet and were mass-analyzed by a sector type mass spectrometer. Judging from the size distribution observed in mass spectra, the composition ratio
n/m of the cluster ions gradually tended to increase as the cluster size became larger. The dissociation patterns of small size tended to yield the Fe
nS
n+ (
n=2, 4, 6) and Fe
nS
n-1+ (
n=7, 8, 9) which might be more stable structure. As the cluster size became larger, the dissociation mainly occurred by emitting Fe or S
2. Dissociation patterns of Fe
nS
m+ suddenly changed at Fe
nS
M+ which had maximum peak intensity in the series of Fe
nS
m+ for the fixed
n. In case of
m≤M, Fe emission occurred and S
2 emission for
m>M
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