Yuta Ito

Deep Inner-Shell Multiphoton Ionization by Intense X-Ray Free-Electron Laser Pulses

We have investigated multiphoton multiple ionization dynamics of xenon atoms using a new x-ray free-electron laser facility, SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan, and identified that Xe(n+) with n up to 26 is produced at a photon energy of 5.5 keV. The observed high charge states (n≥24) are produced via five-photon absorption, evidencing the occurrence of multiphoton absorption involving deep inner shells. A newly developed theoretical model, which shows good agreement with the experiment, elucidates the complex pathways of sequential electronic decay cascades accessible in heavy atoms. The present study of heavy-atom ionization dynamics in high-intensity hard-x-ray pulses makes a step forward towards molecular structure determination with x-ray free-electron lasers.

Nanoplasma Formation by High Intensity Hard X-rays.

Using electron spectroscopy, we have investigated nanoplasma formation from noble gas clusters exposed to high-intensity hard-x-ray pulses at ~5 keV. Our experiment was carried out at the SPring-8 Angstrom Compact free electron LAser (SACLA) facility in Japan. Dedicated theoretical simulations were performed with the molecular dynamics tool XMDYN. We found that in this unprecedented wavelength regime nanoplasma formation is a highly indirect process. In the argon clusters investigated, nanoplasma is mainly formed through secondary electron cascading initiated by slow Auger electrons. Energy is distributed within the sample entirely through Auger processes and secondary electron cascading following photoabsorption, as in the hard x-ray regime there is no direct energy transfer from the field to the plasma. This plasma formation mechanism is specific to the hard-x-ray regime and may, thus, also be important for XFEL-based molecular imaging studies. In xenon clusters, photo- and Auger electrons contribute more significantly to the nanoplasma formation. Good agreement between experiment and simulations validates our modelling approach. This has wide-ranging implications for our ability to quantitatively predict the behavior of complex molecular systems irradiated by high-intensity hard x-rays.

Sequential multiphoton multiple ionization of atomic argon and xenon irradiated by x-ray free-electron laser pulses from SACLA

We have investigated multiphoton multiple ionization of argon and xenon atoms at 5 keV using a new x-ray free electron laser (XFEL) facility, the SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan. The experimental results are compared with the new theoretical results presented here. The absolute fluence of the XFEL pulse has been determined with the help of the calculations utilizing two-photon processes in the argon atom. The high charge states up to +22 observed for Xe in comparison with the calculations point to the occurrence of sequential L-shell multiphoton absorption and of resonance-enabled x-ray multiple ionization.

Charge and Nuclear Dynamics Induced by Deep Inner-Shell Multiphoton Ionization of CH3I Molecules by Intense X-ray Free-Electron Laser Pulses

In recent years, free-electron lasers operating in the true X-ray regime have opened up access to the femtosecond-scale dynamics induced by deep inner-shell ionization. We have investigated charge creation and transfer dynamics in the context of molecular Coulomb explosion of a single molecule, exposed to sequential deep inner-shell ionization within an ultrashort (10 fs) X-ray pulse. The target molecule was CH3I, methane sensitized to X-rays by halogenization with a heavy element, iodine. Time-of-flight ion spectroscopy and coincident ion analysis was employed to investigate, via the properties of the atomic fragments, single-molecule charge states of up to +22. Experimental findings have been compared with a parametric model of simultaneous Coulomb explosion and charge transfer in the molecule. The study demonstrates that including realistic charge dynamics is imperative when molecular Coulomb explosion experiments using short-pulse facilities are performed.

Controlling Low-Energy Electron Emission via Resonant-Auger-Induced Interatomic Coulombic Decay.

We have investigated interatomic Coulombic decay (ICD) after resonant Auger decay in Ar2, ArKr, and ArXe following 2p3/2 → 4s and 2p3/2 → 3d excitations in Ar, using momentum-resolved electron-ion-ion coincidence. The results illustrate that ICD induced by the resonant Auger decay is a well-controlled way of producing energy-selected slow electrons at a specific site.

Quantum chemical molecular dynamics analysis of the effect of oxygen vacancies and strain on dielectric characteristic of hfo 2-x films

The effect of strain and intrinsic defects on both electronic and structural characteristics of HfO2-x used for sub-100-nm semiconductor devices was analyzed by a quantum chemical molecular dynamics analysis. The magnitude of the band gap of HfO2 decreases by about 10% under the applied strain of 5%. The stable crystallographic structure of the monoclinic HfO2 changes to a cubic-like structure under the strain. The magnitude of the band gap of the HfO2-x decreases drastically from 5.7 eV to about 1.0 eV due to the formation of an electronic state within the band gap when an oxygen vacancy is introduced to the perfect HfO2. In the HfO2-x film, oxygen atoms near the oxygen vacancy can move drastically at temperatures higher than 800 K. Therefore, it is very important to control the chemical composition of the hafnium oxide film and to optimize the annealing condition to maintain both the high reliability and performance of the gate oxide film

Extracting conformational structure information of benzene molecules via laser-induced electron diffraction

We have measured the angular distributions of high energy photoelectrons of benzene molecules generated by intense infrared femtosecond laser pulses. These electrons arise from the elastic collisions between the benzene ions with the previously tunnel-ionized electrons that have been driven back by the laser field. Theory shows that laser-free elastic differential cross sections (DCSs) can be extracted from these photoelectrons, and the DCS can be used to retrieve the bond lengths of gas-phase molecules similar to the conventional electron diffraction method. From our experimental results, we have obtained the C-C and C-H bond lengths of benzene with a spatial resolution of about 10 pm. Our results demonstrate that laser induced electron diffraction (LIED) experiments can be carried out with the present-day ultrafast intense lasers already. Looking ahead, with aligned or oriented molecules, more complete spatial information of the molecule can be obtained from LIED, and applying LIED to probe photo-excited molecules, a “molecular movie” of the dynamic system may be created with sub-Ångström spatial and few-ten femtosecond temporal resolutions.

Electron spectroscopy of rare-gas clusters irradiated by x-ray free-electron laser pulses from SACLA

We have measured electron energy spectra and asymmetry parameters of Ar clusters and Xe clusters illuminated by intense x-rays at 5 and 5.5 keV. A velocity map imaging spectrometer was developed for this purpose and employed at an x-ray free-electron laser facility, SACLA in Japan. The cluster size dependence and the peak fluence dependence of the electron spectra and asymmetry parameters are discussed.

Strain Dependence of Dielectric Properties and Reliability of High-k Thin Films

The effect of strain on both electronic and structural characteristics of HfO2 used for sub-50-nm semiconductor devices was analyzed by a quantum chemical molecular dynamics analysis. The band structure of HfO2 is strongly affected by strain in the film. Based on the change rate of band gap obtained from our simulations, the strain sensitivity of the relative permittivity of HfO2 film is predicted to about 2.4%/1%-strain. The predicted strain sensitivity was validated quantitatively by measuring the change of the capacitance of MOS capacitors using a HfO2 gate dielectric film. It is very important, therefore, to minimize the mechanical strain in the dielectric film to assure the reliability of MOS transistor.

Influence of Oxygen Composition and Carbon Impurity on Electronic Reliability of HfO2

The effect of point defects such as oxygen vacancy and carbon interstitial on both electronic and structural characteristics of hafnium dioxide was analyzed by a quantum chemical molecular dynamics method. When a carbon atom as the impurity is introduced in hafnium dioxide, carbon impurity states (donor and acceptor) are formed in the band gap of hafnium dioxide. The band gap calculated from the energy difference between the donor and acceptor decreases to 1.6 eV. We conclude therefore, it is very important to control the composition of HfO2 films in order to assure the electronic performance and reliability of hafnium dioxide film.