Attosecond four-wave mixing spectroscopy using an XUV pulse and two
noncollinear near-infrared pulses is employed to measure Rydberg wavepacket
dynamics resulting from extreme ultraviolet excitation of a 3s electron in
atomic argon into a series of autoionizing 3s-1np Rydberg states around 29 eV.
The emitted signals from individual Rydberg states exhibit oscillatory
structure and persist well beyond the expected lifetimes of the emitting
Rydberg states. These results reflect substantial contributions of longer-lived
Rydberg states to the four wave mixing emission signals of each individually
detected state. A wavepacket decomposition analysis reveals that coherent
amplitude transfer occurs predominantly from photoexcited 3s-1(n+1)p states to
the observed 3s-1np Rydberg states. The experimental observations are
reproduced by time-dependent Schr\"odinger equation simulations using
electronic structure and transition moment calculations. The theory highlights
that coherent amplitude transfer is driven non-resonantly to the 3s-1np states
by the near-infrared light through 3s-1(n+1)s and 3s-1(n-1)d dark states during
the four-wave mixing process