XUV-FEL spectroscopy: He two-photon ionization cross-sections (Vol. 42, No. 5)

image The time-of-flight mass spectra of He+ and H2+ recorded at λ = 61.4, 58.4, 56.0 and 53.4 nm.

Non-linear optical processes of atoms and molecules such as multiphoton absorption and tunnelling ionization are very attractive issues in current atomic, molecular and optical sciences. The recent development of free electron laser (FEL) sources enabled us to investigate such non-linear optical processes in the extreme ultraviolet (XUV) wavelength regions Our group demonstrated that we can determine absolute values of a two-photon ionization cross section of atomic species and its wavelength dependence by using an XUV FEL light source. This was achieved by introducing an internal reference for the cross section measurements and by the frequency tunability of the FEL light source.

The FEL light source we used is the SPring-8 Compact SASE Source test accelerator in RIKEN, Harima Institute, equipped with a couple of compact vacuum undulators, having a unique advantage of its high peak intensity and frequency tunability in the 50 ~ 62 nm region.

We measured the wavelength dependence and the light field intensity dependence of the absolute values of two-photon ionization cross section of He at 53.4, 58.4, 56.0 and 61.4 nm, covering the 1s2p and 1s3p resonances in the light field intensities range of 5×1012 ~ 5×1013 W/cm2 by measuring simultaneously one-photon ionization signal of H2 mixed in the sample as reference.

We showed through the critical comparison with the theoretically obtained cross sections that, in the resonance wavelength regions, dressed state formation through the strong coupling between the intermediate 1snp resonance state and the 1s2 ground state needs to be taken into account if the XUV light field intensity becomes larger than ~1012 W/cm2. We are now entering into the stage of quantitative non-linear spectroscopy in the XUV wavelength region.

Determination of absolute two-photon ionization cross section of He by XUV Free Electron Laser
T. Sato, A. Iwasaki, I. Kazuki, T. Okino, K. Yamanouchi, J. Adachi, A. Yagishita, H. Yazawa, F. Kannari, M. Aoyma, K. Yamakawa, K. Midorikawa, H. Nakano, M. Yabashi, M. Nagasono, A. Higashiya and T. Ishikawa, J. Phys. B: At. Mol. Opt. Phys., 44, 161001 (2011)