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Femtosecond is equal to 10-15second (unit of time), comes from a combination of the metric prefix femto (f) with the second (s). Plural name is femseconds with symbol is fs.
Attosecond is equal to 10 second (unit of time), comes from a combination of the metric prefix atto (a) with the second (s). Plural name is attoseconds with symbol is as.