  Using the multi-dimensional wave function formalism, we investigate the
quantum state of a scalar field inside a true vacuum bubble nucleated through
false vacuum decay in flat spacetime. We developed a formalism which allows us
a mode-by-mode analysis. To demonstrate its advantage, we describe in detail
the evolution of the quantum state during the tunneling process in terms of
individual mode functions and interpret the result in the language of particle
creation. The spectrum of the created particles is examined based on quantum
field theory in the Milne universe.
