| Summary: | We investigate l-mixing processes in collisions of excited positronium (Ps) atoms with protons in the range of the principal quantum number n of the Ps atom between 4 and 8. We show first that results of the threshold theory agree very well with convergent close-coupling calculations. We then compare quantum cross sections with results of semiclassical and classical theories. In the semiclassical theory, the Ps atom is treated quantum-mechanically, whereas the relative Ps-p motion is described classically. For l-mixing processes there is a good agreement between quantum and semiclassical results, whereas classical theory strongly underestimates the l-mixing cross sections. The conclusions are important for the interpretation of Classical Trajectory Monte Carlo simulations and the use of their data in the design of experiments.
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