  Pulsed field ionization of high-$n$ (90 $\leq n \leq$ 150) manifold states in
Rb Rydberg atoms has been investigated in high slew-rate regime. Two peaks in
the field ionization spectra were systematically observed for the investigated
$n$ region, where the field values at the lower peak do not almost depend on
the excitation energy in the manifold, while those at the higher peak increase
with increasing excitation energy. The fraction of the higher peak component to
the total ionization signals increases with increasing $n$, exceeding 80% at
$n$ = 147. Characteristic behavior of the peak component and the comparison
with theoretical predictions indicate that the higher peak component is due to
the tunneling process. The obtained results show for the first time that the
tunneling process plays increasingly the dominant role at such highly excited
nonhydrogenic Rydberg atoms.
