Resolving an electronic wavepacket composed of dark autoionizing neutral states in Argon

We study the dynamics of autoionizing Rydberg states in Argon using time-resolved photoelectron spectroscopy and demonstrate a novel technique of energy- and time-resolving electron wavepackets in atoms and molecules. An autoionizing $(3p)^{-1}nf^\prime$ wavepacket is excited using a two-color pulse and is probed by an ionizing time-delayed IR pulse. The differential changes in the electron yield due to the action of the IR pulse are energy and angle analyzed using a velocity map imaging spectrometer. In addition to measuring quantum beats in the photoelectron signal, we also observe strong quantum beating in the autoionization channel modulated by a two IR photon process. The latter channel is analyzed energetically with few-meV resolution while retaining femtosecond time-resolution, which is in contrast to traditional photoelectron measurements where it is not possible to obtain both high energy and time-resolution. The experimental results show good agreement with time-dependent perturbation theory calculations.