Quantum-Mechanical Phase Jumps in Collision-Induced Coherent Excitation

We present experimental and theoretical excitation probabilities $\mathcal{P}$ and coherence parameters $\lambda$ and $\chi$ for ${\mathrm{Be}}^{+}$($2s\to 2p$) excitation in 2-15-keV ${\mathrm{Be}}^{+}$-Ne collisions with impact parameters near 1.6 a.u. The phase difference $\chi$ between the quantum-mechanical amplitudes for $M_L=0\mathrm{}$ and $M_L=1\mathrm{}$ substate excitation exhibits a rapid change near 4 keV, where $\lambda$, which measures the relative size of those amplitudes, approaches its maximum value of unity. Impact-parameter calculations of $\mathcal{P}$, $\lambda$, and $\chi$ predict the trends found experimentally.