  Nonleptonic weak decays of Xi_c into pi^+ and a meson (M)-baryon (B) final
state, MB, are analyzed from the viewpoint of probing S=-2 baryon resonances,
i.e. Xi(1620) and Xi(1690), of which spin-parity and other properties are not
well known. We argue that the weak decay of Xi_c is dominated by a single
quark-line diagram, preferred by the Cabibbo-Kobayashi-Maskawa coefficient,
color recombination factor, the diquark correlation, and the kinematical
condition. The decay process has an advantage of being free from meson
resonances in the pi^+M invariant mass distribution. The invariant mass
distribution of the meson-baryon final state is calculated with three different
chiral unitary approaches, assuming that the Xi(1620) and Xi(1690) resonances
have J^P=1/2^-. It is found that a clear peak for the Xi(1690) is seen in the
piXi and KbarLambda spectra. We also suggest that the ratios of the piXi,
KbarLambda and KbarSigma final states are useful to distinguish whether the
peak is originated from the Xi(1690) resonance or it is a KbarSigma threshold
effect.
