Quantum excitations of intrinsic degrees of freedom in multi-dimensional tunneling systems

We extend a useful formalism to construct a wave function in a multi-dimensional tunneling system and to investigate the quantum excitations of an intrinsic degree of freedom, which is a harmonic oscillator linearly coupled to a tunneling degree of freedom. To demonstrate the usefulness of the formalism, we construct a wave function in an analytic form for a simple tunneling system with a cubic potential. Based on the wave function, we discuss interesting features of the multi-dimensional tunneling system in which the quantum excitations play an important role. It is shown that the quantum excitations of the linearly coupled harmonic oscillators increase the decay width in comparison with the case of no excitations.