  Accurate rates for energy-degenerate l-changing collisions are needed to
determine cosmological abundances and recombination. There are now several
competing theories for the treatment of this process, and it is not possible to
test these experimentally. We show that the H I two-photon continuum produced
by astrophysical nebulae is strongly affected by l-changing collisions. We
perform an analysis of the different underlying atomic processes and simulate
the recombination and two-photon spectrum of a nebula containing H and He. We
provide an extended set of effective recombination coefficients and updated
l-changing 2s-2p transition rates using several competing theories. In
principle, accurate astronomical observations could determine which theory is
correct.
