Abstract
Eiji Shigemasa (IMS) and co-workers have uncovered how argon (Ar) atoms are photoionized in an intense extreme ultraviolet (EUV) laser field, by using a free electron laser (FEL). They demonstrate that the two-photon ionization is significantly enhanced by intermediate resonances in singly charged ion states.
An intense light source such as a laser can modify the properties of matter, and be used to achieve effects not observable by using ordinary light sources. Features owing to multiphoton processes are essentially nonlinear phenomena. Shigemasa and co-workers observed electrons emitted during the photoionization process of Ar atoms in intense EUV laser fields, which were measured after each ultra-short FEL pulse. Since the FEL spectrum and its power are fluctuating at every shot, the fine details of the photoionization process are blurred during the course of a measurement. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse.
The experimental results revealed that the dominant ionization pathway of Ar atoms irradiated with intense EUV-FEL pulses has two steps: in a first step, a single laser photon is absorbed by Ar leading to the production of a singly-charged argon ion (Ar+), and then two more photons are absorbed by Ar+, as a second step, leading to the production of a doubly-charged ion (Ar2+). It was also found that this pathway is strongly encouraged to occur when the energy levels of the intermediate Ar+ states coincide with the incident FEL photon energy. The fluctuation in mean photon energy of the FEL pulses is utilized to study the photon energy dependence of the double ionization process, which clearly unveiled this resonance effect.
Pictorial drawings for illustrating the importance and role of a resonance in three-photon double ionization processes.
Presenters
Eiji Shigemasa (Institute for Molecular Science, National Institutes of Natural Sciences)
Yasumasa. Hikosaka (Department of Environmental Science, Niigata University)
Akiyoshi Hishikawa (Department of Chemistry, Graduate School of Science, Nagoya University)
Mitsuru Nagasono (RIKEN, XFEL Project Head Office)
Haruhiko Ohashi (Japan Synchrotron Radiation Research Institute)
Paper Information
Jornal: Physical Review Letters, vol.105, 133001 (2010)
Title: Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy
Author: Y. Hikosaka, M. Fushitani, A. Matsuda, C.-M. Tseng, A. Hishikawa, E. Shigemasa,,
M. Nagasono, K. Tono, T. Togashi, H. Ohashi, H. Kimura, Y. Senba, M. Yabashi, and T. Ishikawa
1131
