Tunable femtosecond vacuum-ultraviolet radiation in the range omega(D) = 102-124 nm has been generated by two-photon-resonant and near-resonant four-wave difference frequency mixing (omega(D) = 2 omega(p) - omega(I)) in krypton and argon by use of intense 250-fs ArF laser pulses (omega(p)) and tunable femtosecond signal and idler pulses (omega(I)) generated by an optical parametric generator. (C) 1996 Optical Society of America