  We show that a small number of intentionally introduced defects can be used
as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe$_{2}$,
that we measure by scanning tunneling spectroscopic imaging. We show from the
momentum and energy dependence of the quasiparticle interference that Fermi
surface nesting is inconsequential to charge density wave formation in
2H-NbSe$_{2}$. We demonstrate that by combining quasiparticle interference data
with additional knowledge of the quasiparticle band structure from angle
resolved photoemission measurements, one can extract the wavevector and energy
dependence of the important electronic scattering processes thereby obtaining
direct information both about the fermiology and the interactions. In
2H-NbSe$_{2}$, we use this combination to show that the important
near-Fermi-surface electronic physics is dominated by the coupling of the
quasiparticles to soft mode phonons at a wave vector different from the CDW
ordering wave vector.
