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

At each of three frequencies {ω}/2{π}=26.43, 30.36, and 36.02 GHz, we observe 3d hydrogen atoms with certain principal quantum numbers n₀{}1 to be more stable against microwave ionization. Which n₀'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 n₀³{ω}. Our data give strong support for the enhanced stability being due to scarred 3d wave functions for the strongly driven atoms.