DEPLETION OF HARMONIC OSCILLATOR STATES BY PERIODIC KICKS N.~Sato$^{A}$, S.~Watanabe$^{A}$, M.~Matsuzawa$^{A}$, S.~Yoshida$^{B}$, C.~O.~Reinhold$^{B,C}$, J.~Burgd\"orfer$^{B,C,D}$ $^{{A}}$Department of Applied Physics and Chemistry, U.~of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan $^{{B}}$Department of Physics, University of Tennessee, Knoxville, U.S.A. $^{{C}}$Physics Division, Oak Ridge National Laboratory, Oak Ridge, U.S.A. $^{{D}}$Institute for Theoretical Physics, Vienna University of Technology, Austria Ionization of high-lying Rydberg states of an atom by periodic impulsive kicks differs from that of low-lying states. The reason appears to be due to the nearly constant energy spacing of Rydberg states. Exploiting a simple harmonic oscillator as a model, we account for the formation of depletion dips and metastable features.