  Angle-resolved photoemission spectroscopy with low-energy tunable photons
along the nodal direction of oxygen isotope substituted Bi2Sr2CaCu2O8+delta
reveals a distinct oxygen isotope shift near the electron-boson coupling "kink"
in the electronic dispersion. The magnitude (a few meV) and direction of the
kink shift are as expected due to the measured isotopic shift of phonon
frequency, which are also in agreement with theoretical expectations. This
demonstrates the participation of the phonons as dominant players, as well as
pinpointing the most relevant of the phonon branches.
