We study how weak broadband noise affects 30.36 GHz 10%-''ionization''
thresholds F(10%) (that produce ''ionization'' probability P(ion) = 1
0%) of no = 54, ..., 72 H atoms. Calculations confirm that increased n
oise lowers F(10%) for most n0's, raises some, but affects others litt
le. We show noise to be a useful spectroscopic tool for uncovering nov
el quantal phenomena such as scarred states, which we link semiclassic
ally to approximately adiabatic evolution near the separatrix of a non
linear resonance island. Our results apply generally to the quantal be
havior of classical mixed phase space systems.