Ionization Cross Sections for Rydberg-Atom-Rydberg-Atom Collisions

A classical-trajectory Monte Carlo method has been applied to collisions of two Rydberg atoms. Numerical calculations were made for velocities v=0.01vₑ to 10vₑ, where the Rydberg electron's velocity vₑ(a.u.)=1/n and n is the principal quantum number of the Rydberg atom. The total ionization cross sections scale as n⁴ and show a v^-0.65 dependence at low v, a slight maximum around vₑ, and a rapid decrease at high v. The cross sections are almost an order of magnitude larger than πn⁴a₀² at thermal energies.