Probing quantal dynamics of mixed phase space systems with noise

We study how weak broadband noise affects 30.36 GHz 10%-''ionization'' thresholds F(10%) of n0=54,...,72 H atoms. Calculations confirm that increased noise lowers F(10%) for most n0's raises some, but affects others little. We show noise to be a useful spectroscopic tool for uncovering novel quantal phenomena such as scarred states, which we link semiclassically to approximately adiabatic evolution near the separatrix of a nonlinear resonance island. Our results apply generally to the quantal behavior of classical mixed phase space systems