title: Two-photon inner-shell ionization in the extreme ultraviolet.
abstract: We have observed the simultaneous inner-shell absorption of two extreme-ultraviolet photons by a Xe atom in an experiment performed at the short-wavelength free electron laser facility FLASH. Photoelectron spectroscopy permitted us to unambiguously identify a feature resulting from the ionization of a single electron of the 4d subshell of Xe by two photons each of energy (93±1)  eV. The feature's intensity has a quadratic dependence on the pulse energy. The results are discussed and interpreted within the framework of recent results of ion spectroscopy experiments of Xe obtained at ultrahigh irradiance in the extreme-ultraviolet regime.
authors.affiliation: School of Physical Sciences and NCPST, Dublin City University, Dublin 9, Ireland.
authors.forename: V
authors.initials: V
authors.lastname: Richardson
authors.forename: J T
authors.initials: JT
authors.lastname: Costello
authors.forename: D
authors.initials: D
authors.lastname: Cubaynes
authors.forename: S
authors.initials: S
authors.lastname: Düsterer
authors.forename: J
authors.initials: J
authors.lastname: Feldhaus
authors.forename: H W
authors.initials: HW
authors.lastname: van der Hart
authors.forename: P
authors.initials: P
authors.lastname: Juranić
authors.forename: W B
authors.initials: WB
authors.lastname: Li
authors.forename: M
authors.initials: M
authors.lastname: Meyer
authors.forename: M
authors.initials: M
authors.lastname: Richter
authors.forename: A A
authors.initials: AA
authors.lastname: Sorokin
authors.forename: K
authors.initials: K
authors.lastname: Tiedke