Resonant impact ionization (RII) spectroscopy, a new technique for studying low concentrations of trap levels in narrow-gap Hg_1-xCd_xTe has been used to investigate impurity and defect levels in both bulk and liquid phase epitaxy (LPE) crystals of this ternary material. Mercury interstitials, deliberately introduced into bulk samples with x approximately=0.22 and x approximately=0.24, provide direct evidence for the formation of trap levels near 45 and 61 meV above the valence band edge for these x-value samples. The 61 meV level in the x approximately=0.24 (E_g=121 meV at 5 K) sample is seen to enhance the transition strength of resonant two-photon magneto-absorption (TPMA) observed at high fields by acting as a near-resonant intermediate state for the two-photon process. In the LPE material, samples which have received just a post-anneal in an Hg-saturated atmosphere show strong RII structure due to defect levels at approximately=60 meV above the valence band, whereas samples which are both doped with indium and post-annealed show only intrinsic one-photon magneto-absorption structure.