I can't say whether the root morpheme in "absent" is "ab-" or "abs-". (From an etymological point of view, ab- appears to be a negative prefix -- from M.Fr. absent (O.Fr. ausent), from L. absentem (nom. absens), prp. of abesse "be away from, be absent" -- from O.Fr. absence, from L. absentia, noun of state from absentem (nom. absens), prp. of abesse "be away from, be absent," from ab- "away" + esse "to be").
On the one hand, "absent" may be analyzed and compared with "present" (pre-/ ab- sent), but in this case it's clear that we can't consider -s- as a common root for those semantically related words, because it doesn't seem to be the lexical core of these words at all. On the other hand in the case of "abnormal" and "normal" this negative prefix ab- is easily identified.