  The properties of bosonic Ytterbium photoassociation spectra near the
intercombination transition $^{1}S_{0}$--$^{3}P_{1}$ are studied theoretically
at ultra low temperatures. We demonstrate how the shapes and intensities of
rotational components of optical Feshbach resonances are affected by mass
tuning of the scattering properties of the two colliding ground state atoms.
Particular attention is given to the relationship between the magnitude of the
scattering length and the occurrence of shape resonances in higher partial
waves of the van der Waals system. We develop a mass scaled model of the
excited state potential that represents the experimental data for different
isotopes. The shape of the rotational photoassociation spectrum for various
bosonic Yb isotopes can be qualitatively different.
