  We report on the determination of the spin Hall angle and the identification
of the role of the interface roughness for the inverse spin Hall effect (ISHE)
in ultra-clean, defect-reduced epitaxial Pt films. By applying vector network
analyzer ferromagnetic resonance spectroscopy to a series of single crystalline
Fe (12 nm) /Pt (t$_\text{Pt}$) bilayers we determine the real part of the spin
mixing conductance $(4.4\:\pm\:0.2)\cdot 10^{19}\: $m$^{-2}$ and reveal a very
small spin diffusion length in the epitaxial Pt $(1.1\:\pm\:0.1)$ nm film. We
investigate the spin pumping and ISHE in a stripe microstucture excited by a
microwave coplanar waveguide (CPW) antenna. By using their different angular
dependencies, we distinguish between spin rectification effects and the inverse
spin Hall effect. The relatively large value of the spin Hall angle $(5.7 \pm
1.4)\: \% $ hints that interface engineering by epitaxially grown non-magnetic
materials can increase the spin-to-charge current conversion efficiency.
