K. Shimada

Frustration of direct photoionization of Ar clusters in intense extreme ultraviolet pulses from a free electron laser

We have measured the kinetic energies of fragment ions from Ar clusters (average cluster size N~ 10?600) exposed to intense extreme ultraviolet free electron laser pulses (? ~ 61 nm, I~ 1.3? 1011 W cm?2). For small clusters (N 200), the average kinetic energy of ions strongly increases with increasing the cluster size, indicating a promotion of the multiple ionization, whereas the average kinetic energy is observed to be saturated for N 200. Considering how many photoelectrons can escape from the cluster, it was found that the size dependence of the ion kinetic energy exhibited the frustration of direct photoionization, which resulted from the strong Coulomb potential of the highly ionized cluster.

Strong-field electron spectra of rare-gas atoms in the rescattering regime: enhanced spectral regions and a simulation of the experiment

We report experimental and theoretical laser-induced photoelectron spectra of Ne, Ar, Kr and Xe in the direction of the laser polarization at laser intensities in the region 0.5–3.2 × 1014Wcm−2. For Ar and Xe, spectra at different angles are also presented for the laser intensities of 2.4 × 1014Wcm−2 for Ar and 1.3 × 1014Wcm−2 for Xe. Resonance-like structures evolve as a function of the laser intensity in the longitudinal spectra, which become suppressed at large angles. The simulations are in the context of the strong-field approximation, which is augmented to account for rescattering and the atom-specific electron–positive-ion scattering potentials, and they include focal averaging as well as saturation. Agreement between the data and these simulations is generally fair, especially for intensities below the saturation regime.

Angle-resolved high-order above-threshold ionization spectra for N2 and O2: measurements and the strong-field approximation

Angle-resolved high-order above-threshold-ionization spectra are presented which cover the rescattering plateau and its cutoff. The data are compared with calculations based on a recent version of the molecular strong-field approximation, which was originally formulated for direct ionization and subsequently extended to include rescattering.

Extraction of electron?ion differential scattering cross sections for C2H4 by laser-induced rescattering photoelectron spectroscopy

We have measured angle-resolved rescattering electron momentum distributions for C2H4 generated by intense infrared laser pulses and extracted large-angle elastic differential cross sections (DCSs) for electrons scattering from C2H4+. The angle-dependent ionization rate describing the initial single-ionization step favours ionization from molecules that have their molecular plane aligned perpendicular to the laser polarization direction. The extracted DCSs are well reproduced by the theoretical calculation. We demonstrate that DCSs for electron–ion scattering of the nonlinear polyatomic molecule can be extracted from the laser-induced rescattering electron spectra.

Probing molecular symmetry effects in the ionization of N2 and O2 by intense laser fields.

High-resolution electron spectroscopy is used to explore the role played by molecular symmetry in determining the morphology of the energy spectra of electrons ejected when N2 and O2 are irradiated by intense laser fields. In O2, the low-energy part of the electron spectrum is curtailed due to the destructive interference brought about by the antibonding nature of the O2 valence orbital. The high-energy tail of the spectrum is also suppressed by virtue of electron rescattering being of little consequence in O2. In contrast, in N2, which has a bonding valence orbital, the electron dynamics follow the pattern that has been established for atomic ionization in strong optical fields.

Frustration of direct photoionizations of rare gas clusters in intense extreme ultraviolet free-electron laser pulses

We performed a momentum imaging measurements of Ar clusters irradiated by extreme ultraviolet free electron laser pulses (61nm 1.3?1011W/cm2) for various cluster sizes. Dominant fragment ions were Ar+ and Ar2+ ions with significant kinetic energy (up to 40eV). This indicates that Ar clusters absorbed many photons, become highly ionized and finally dissociated into many fragments through Coulomb explosion. The size dependence of the ion kinetic energy distribution indicates that larger clusters significantly suffer from the frustration of direct photoelectron emissions and multiple ionizations.

Momentum spectroscopy of fragment ions emitted from Xe clusters irradiated by EUV-FEL at SPring-8

We have studied multiple ionization of Xe clusters by 61-nm 1011−1012 W/cm2extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan. Kinetic energy distributions of Xe+, Xe+2, Xe+3, and Xe+4 ions emitted from Xe clusters were measured.

Two-source double-slit interference in angle-resolved high-energy above-threshold ionization spectra of diatoms

When an electron from a diatomic molecule undergoes tunneling-rescattering ionization, a novel form of destructive interference can be realized that involves all four geometric orbits that are available to the electron when it is freed, because both ionization and rescattering may take place at the same or at different centers. We find experimentally and confirm theoretically that in orientation-averaged angle-resolved high-order above-threshold ionization spectra the corresponding destructive interference is visible for O_{2} but not for N_{2}. This effect is different from the suppression of ionization that is well known to occur for O_{2}.

Electron-Ion Differential Cross Section extracting from the High-order Above-Threshold Ionization spectroscopy of C2H4 and C2H6

We measured the angle-resolved High-order Above-Threshold Ionization spectroscopy of C2H4 and C2H6 with near-IR laser pulse, and extracted the electron-ion differential cross sections of those molecules from the electron spectroscopy by applying the quantitative rescattering theory. The differential cross sections show dramatically different distributions for C2H4 and C2H6, and this difference is believed to arouse from their different HOMO symmetry.

Frustration of direct photoionizations of Argon clusters irradiated with intense EUV-FEL pulses

We have measured the kinetic energies of fragment ions from Ar clusters (average cluster size ~ 10 to 600) exposed to intense extreme ultraviolet free-electron laser pulses (? ~ 61nm, I ~ 1.3?1011 [W/cm2]). We found that the average kinetic energy of fragment ions was saturated for 200.