Introduction

Abstract

Multiphoton transitions are found whenever electrons bound in atoms or molecules interact with sufficiently intense electromagnetic radiation. Multiphoton ionization is the ultimate outcome of multiphoton transitions provided the radiation is reasonably intense, which we will take to mean about 10⁶ W/cm², or more, at optical frequencies. With intense sources and interaction times of the order of a nanosecond or more, ionization may be expected for atomic, and dissociation with varying degrees of ionization for molecular gases. For still higher intensities (about 10¹³–10¹⁴ W/cm²), ionization of anything in the interaction region can be predicted with certainty.