If the nearest available gap in the acceptor is at lower energy, then the electron must lose some of its energy to make the jump. This process is called inelastic tunneling. But the dumped energy needs to go somewhere, otherwise the electron can’t tunnel. If a chemical is placed in the gap between the plates, then an electron can tunnel across so long as it is able to donate its excess energy to the chemical—which it can do so long as the molecules in the gap have bonds capable of vibrating at just the right frequency, corresponding to that of the dumped energy.
