Classical scaling of nonclassical stability in microwave ionization of excited 3d H atoms

At each of three frequencies ω/2π=26.43, 30.36, and 36.02 GHz, we observe 3d hydrogen atoms with certain principal quantum numbers ${\mathit{n}}_0$≫1 to be more stable against microwave ionization. Which ${\mathit{n}}_0$’s are more stable differs for each ω. Even when the enhanced stability is not found in 3d classical calculations, we show that it scales classically: This quantal effect occurs at certain values of the classically scaled frequency ${\mathit{n}}_0^3$ω. Our data give strong support for the enhanced stability being due to scarred 3d wave functions for the strongly driven atoms.