Abstract
Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (, ) is presented. Resonances to the doubly excited states converging to the level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schrödinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.
- Received 23 July 2011
DOI:https://doi.org/10.1103/PhysRevLett.107.243003
© 2011 American Physical Society