  We study the decay of gap plasmons localized between a scanning tunneling
microscope tip and metal substrate, excited by inelastic tunneling electrons.
The overall excited energy from the tunneling electrons is divided into two
categories in the form of resistive dissipation and electromagnetic radiation,
which together can further be separated into four different channels, including
SPP channel on the tip, SPP channel on the substrate, air mode channel and
direct quenching channel. We find that most of the excited energy goes to
surface plasmon polaritons on the metallic STM tip, rather than that on the
substrate. The direct quenching in the apex of tip also takes a considerable
portion especially in high frequency region.
