Bunching and Focusing of Tunneling Wave Packets in Enhancement of High-Order Above-Threshold Ionization

Numerical solution of the time-dependent Schrödinger equation for a model argon atom subject to an intense laser pulse quantitatively reproduces the resonance enhancements seen in the high-order part of the above-threshold ionization (ATI) electron spectrum in earlier experiments. The calculation reveals that the envelope of the ATI spectrum is due to bunching of the ionized wave packet on the first reencounter with its parent ion core, followed by backscattering of such bunches on subsequent encounters. The resonant structure is caused by interference between bunches produced in different cycles.