WordNet (r) 3.0 (2006):

subatomic
    adj 1: of or relating to constituents of the atom or forces
           within the atom; "subatomic particles"; "harnessing
           subatomic energy"
    2: of smaller than atomic dimensions

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "subatomic":
   atomatic, atomic, atomiferous, atomistic, corpuscular, cyclic,
   diatomic, dibasic, embryonic, evanescent, germinal, granular,
   heteroatomic, heterocyclic, hexatomic, homocyclic, impalpable,
   imperceptible, imponderable, inappreciable, indiscernible,
   infinitesimal, intangible, invisible, isobaric, isocyclic,
   isoteric, isotopic, microcosmic, microscopic, molecular, monatomic,
   pentatomic, tenuous, tetratomic, thin, triatomic, tribasic,
   ultramicroscopic, unseeable