  We have determined the electron-coupling spectrum of superconducting
Bi2Sr2CaCu2O8+d from high-resolution angle-resolved photoemission spectra by
two deconvolution-free robust methods. As hole concentration decreases, the
coupling spectral weight at low energies ~<15 meV shows a twofold and nearly
band-independent enhancement, while that around ~65 meV increases moderately,
and that in ~>130 meV decreases leading to a crossover of dominant coupling
excitation between them. Our results suggest the competition among multiple
screening effects, and provide important clues to the source of sufficiently
strong low-energy coupling, {\lambda}LE ~ 1, in underdoped system.
