Quantum-Mechanical Phase Jumps in Collision-Induced Coherent Excitation

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