We calculate the Doppler broadening of the $W^-$ resonance produced in $bar{ u}_e e^-$ collisions of cosmic anti-neutrinos with $E_{ u}approx 6.3 PeV$ with electrons in atoms up to Iron. Revisiting this issue is prompted by recent observations of PeV neutrinos by Ice-Cube. Despite its poor energy resolution, the $20%$ Doppler broadening of the resonance due to electronic motions can produce observable effects via non-linear neutrino absorption near the resonance. The attendant suppression of the peak cross section allows $bar{ u}_e$ to travel correspondingly longer distances. While this effect is unlikely to be directly detected in the near future, it may facilitate terrestrial tomography at depths of $sim 10 km$, complementing deeper explorations using the more frequent nuclear interactions at lower energies.
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