  Collisions between electrically charged particles and neutral atoms are
central for understanding the dynamics of neutral gases and plasmas in a
variety of physical situaziones of terrestrial and astronomical interest.
Specifically, redistribution of angular momentum states within the degenerate
shell of highly excited Rydberg atoms occurs efficiently in distant collisions
with ions. This process is crucial in establishing the validity of the local
thermal equilibrium assumption and may also play a role in determining a
precise ionization fraction in primordial recombination. We provide an accurate
expression for the non-perturbative rate coefficient of collsions between
protons and H(n_l) ending in a final state H(n_l'), with n being the principal
quantum number and l,l' the initial and final angular momentum quantum numbers,
respectively. The validity of this result is confirmed by results of classical
trajectory Monte Carlo simulations. Previous results, obtained by Pengelly and
Seaton only for dipole-allowed transitions, l--->l+-1, overestimate the
l-changing collisional rate approximately by a factor of six, and the physical
origin of this overestimation is discussed.
