Observation of Mixed-Parity Electric-Dipole Oscillations in Charge Transfer to then=2Hydrogen Levels by Fast Protons in Gases

We find evidence for strong, collision-averaged excitation coherence between $2s$ and $2p_0$ amplitudes in charge transfer by fast protons ($v_i\approx 2-3$ a.u.) undergoing single collisions in He, Ar, and ${\mathrm{O}}_2$. Quantum beat difference signals in $\mathrm{Ly}\alpha$ (Eck beats) induced by reversible electric fields have amplitudes very similar to those seen earlier in C targets, supporting a "last-layer" (gas-layer?) hypothesis.