Introduction

Abstract

Tunneling phenomena are one of the most significant features of quantum mechanics that is drastically different from classical mechanics. Figure 1.1 shows a schematic model of a classical process and a tunneling process. A particle A with kinetic energy W collides with a potential energy barrier, whose height is E ₀. When W is larger than E ₀, the particle can enter the final state by passing over the potential energy barrier, which is known as a classical process (cf. the solid line in Fig. 1.1). But, when W is smaller than E ₀, the particle cannot go to the final state. This is predicted by classical mechanics and is clearly known by our everyday experience with macroscopic objects. In the microscopic world of atoms and molecules, however, a quite different situation takes place by quantum mechanics since all particles have a wave character. Even if the kinetic energy (W) of a particle like an atom is smaller than the barrier height (E ₀), it can go to the final state by passing through the barrier due to its wave character, depicted by the dashed line in Fig. 1.1. This is known as the tunneling effect, which is described in detail in the general textbooks on quantum mechanics.