摘要
Ⅰ. INTRODUCTION Since the ionizing potential of uranium is 49958 cm<sup>-1</sup>, at least three photons of wavelengths in visible region are required to ionize an uranium atom from its ground state or one of the low-lying metastable states. When a single-colour laser beam acts with the uranium vapor, the uranium atom can sequentially absorb three photons and ionize, but the cross sec-
