We show that a small number of intentionally introduced defects can be usedas a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe$_{2}$,that we measure by scanning tunneling spectroscopic imaging. We show from themomentum and energy dependence of the quasiparticle interference that Fermisurface nesting is inconsequential to charge density wave formation in2H-NbSe$_{2}$. We demonstrate that by combining quasiparticle interference datawith additional knowledge of the quasiparticle band structure from angleresolved photoemission measurements, one can extract the wavevector and energydependence of the important electronic scattering processes thereby obtainingdirect information both about the fermiology and the interactions. In2H-NbSe$_{2}$, we use this combination to show that the importantnear-Fermi-surface electronic physics is dominated by the coupling of thequasiparticles to soft mode phonons at a wave vector different from the CDWordering wave vector.
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