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At least 883 records · Page 49Linked to original sources
PubMed2001

Manifestation of charge-density fluctuations in metal clusters: suppression of the ionization channel.

The fluctuations in the electronic charge density of metallic clusters in response to an approaching electron suppress the single-ionization channel. This conclusion is made on the basis of numerical calculations for the total ionization cross sections using the random-phase approximation with exchange to describe the particle-hole (de)excitations. The general trends can be understood by means of the Thomas-Fermi model. The present theory explains, for the first time, the behavior of the measured total ionization cross section for C(60). The interplay between finite size and non-local screening effects is studied by tracing the changes in the ionization cross sections for Li clusters with an increasing cluster radius.

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PubMed2002

Correlation between double and nonresonant single ionization.

We performed simultaneous measurements of electrons and ions produced in Xe photoionization driven by an 800-nm, 100-fs laser pulse. The obtained energy and angular resolved electron spectra allow the identification of the electronic states populated during the ionization. Xe2+ ions appear at the same laser intensity as electrons emerging from a nonresonant 9-photon ionization process of Xe. Similar to optical tunneling ionization, the nonresonant ionization delivers low-energy electrons needed for the formation of Xe2+ by a backscattering process.

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PubMed2003

Observation of a transition in the dynamics of strong-field double ionization.

The double ionization of argon and xenon in an intense laser field has been studied in detail using an electron-ion coincidence technique. The observed double ionization electron spectra in xenon show resonancelike structures here resolved for the first time. In argon, the featureless spectra are consistent with rescattering. This represents a clear transition in the dynamics of strong-field double ionization, analogous to the well-known transition between the tunneling and multiphoton regimes in single ionization.

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PubMed2004

Carrier phase dependence in the ionization of Rydberg atoms by short radio-frequency pulses: a model system for high order harmonic generation.

We report time-resolved electron emission in experiments on ionization of rubidium Rydberg atoms (n=90) by few-cycle radio-frequency (RF) (1-10 MHz) pulses. The electron emission occurs in multiple bursts and strongly depends on the carrier-envelope phase as well as the duration and amplitude of the RF pulses. Remarkably, ionization is observed during a series of cycles with the same amplitude. Even at the low RF frequencies, ionization is not completed in a single cycle. Remixing of the states at the zero crossing of the field is believed to play an essential role. Similarities with the ionization process leading to high order harmonic generation are discussed.

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PubMed2004

Multielectron ionization of CdSe quantum dots in intense femtosecond ultraviolet light.

Multielectron ionization of colloidal CdSe quantum dots under intense femtosecond UV excitation has been studied. By directly probing the absorption from the ionized electron, quantitative measurements of the yield and dynamics of the ionization have been made as a function of excitation fluence and variations of size and potential structure of quantum dots. The results have been explained by an ionization mechanism involving resonant two-photon absorption.

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PubMed2004

Fully differential rates for femtosecond multiphoton double ionization of neon.

We have investigated the full three-dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the recollision energy of the first electron is on the order of the ionization potential. The momentum correlation in the direction perpendicular to the laser field depends on the time difference of the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize.

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PubMed2004

Dynamic polarization in the strong-field ionization of small metal clusters.

We report on the strong field ionization of small transition metal clusters (nickel, Ni(n) n=1-36) within the quasistatic regime at an infrared wavelength of 1.5 microm and at intensities up to 2 x 10(14) W/cm(2). From ion yields in a constant axial intensity beam, we obtained saturation intensities for the individual Ni(n) clusters. As compared to quasistatic, single active electron calculations, a dramatic suppression of ionization was observed. Dynamic polarization in the laser field likely leads to strong multielectron screening of the "active" electron. Representing the metal clusters as classical conducting spheres, we obtained, via a barrier suppression calculation, the classical ionization rates. Agreement was obtained for larger clusters with n>10 when the dynamic polarization was taken into account, emphasizing the multielectron nature of the ionization suppression.

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PubMed2004

Absolute ionization rates of multielectron transition metal atoms in strong infrared laser fields.

We report on nonresonant strong field ionization of the multielectron transition metal atoms V, Nb, Ta, Ni, and Pd. Operating in the adiabatic regime (lambda = 1.5 microm), we quantitatively determined both (i) the first charge state saturation intensities and (ii) the absolute ionization rates for intensities ranging from threshold up to 3 x 10(14) W/cm2. We observed a dramatic suppression of ionization relative to single active electron approximation expectations. We suggest that this derives from dynamic polarization or screening effects within the multielectron atom, stressing a need for many-body theories of strong field ionization.

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PubMed2005

Controlling attosecond double ionization dynamics via molecular alignment.

We investigate the dynamics of double ionization in aligned nitrogen molecules. An ultrashort, weak laser pulse creates an aligned ensemble of molecules that is ionized with a subsequent, strong probe pulse. We find that the two electrons involved in nonsequential double ionization more likely exit the molecule in the same direction if it is parallel to the probe laser polarization, indicating that they are ejected within a few hundred attoseconds of each other. Double ionization is less probable and takes longer for perpendicular molecules.

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PubMed2005

Landau quantization and time dependence in the ionization of cold, strongly magnetized Rydberg atoms.

The electric-field-ionization and autoionization behavior of cold Rydberg atoms of 85Rb in magnetic fields up to 6 T is investigated. Multiple ionization potentials and field-ionization bands reflecting the Landau energy quantization of the quasifree Rydberg electron are observed. The time-resolved and state-selective field-ionization study provides evidence of mixing and spin flips of the Rydberg electron. Spin-orbit coupling combined with mixing gives rise to a Feshbach-type autoionization of metastable positive-energy atoms.

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PubMed2006

Enhancement and control of H2 dissociative ionization by femtosecond VUV laser pulses.

We report ab initio calculations of H2 ionization by VUV/fs 10(12) W/cm2 laser pulses including correlation and all electronic and vibrational degrees of freedom (DOF). Inclusion of the nuclear DOF leads to a substantial increase of resonance enhanced multiphoton ionization. By varying pulse duration, it is possible to control the ratio of dissociative to nondissociative ionization as well as the final H+(2) vibrational distribution. For pulses longer than 10 fs and proportional to omega>0.46 a.u., dissociative ionization entirely dominates, which is a very unusual situation in photoionization studies.

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PubMed2006

Selective ionization of oriented nonpolar molecules with asymmetric structure by phase-controlled two-color laser fields.

We report on the selective ionization of oriented nonpolar molecules with asymmetric structure by using phase-controlled two-color omega + 2omega laser pulses with an intensity of 1.0 x 10(13) W/cm(2) (tunneling ionization regime) and a pulse duration of 130 fs. The orientation of 1-bromo-2-chloroethane was monitored by the directional asymmetries of the forward-backward emission in dissociative ionization. The observed direction of orientation clearly confirms that molecular orientation is induced not by dynamic orientation but by selective ionization of oriented molecules, which reflects the structure of the highest occupied molecular orbital. This method can be applied for the vast majority of molecules.

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PubMed2006

Kinematically complete study of dissociative ionization of by ion impact.

We present a kinematically complete study of dissociative ionization of D(2) by 13.6 MeV/u S(15+) ions. The experiment allows us to unravel the competing mechanisms, namely, direct single ionization, autoionization of doubly excited states, ionization excitation, and double ionization, and to analyze the corresponding electron angular distribution from fixed-in-space molecules. The conclusions are supported by theoretical calculations in which the correlated motion of all electrons and nuclei and the interferences between them are described from first principles.

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PubMed2006

Effects of the carrier-envelope phase in the multiphoton ionization regime.

We theoretically investigate the effects of the carrier-envelope phase of few-cycle laser pulses in the multiphoton ionization regime. For atoms with low ionization potential, total ionization yield barely exhibits phase dependence, as expected. However, population of some bound states clearly shows phase dependence. This implies that the measurement of the carrier-envelope phase would be possible through the photoemission between bound states without energy-and-angle-resolved photoelectron detection. The considered scheme could be particularly useful to measure the carrier-envelope phase for a light source without an amplifier, such as a laser oscillator, which cannot provide sufficient pulse energy to induce tunneling ionization.

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PubMed2006

Electron gyroharmonic effects in ionization and electron acceleration during high-frequency pumping in the ionosphere.

Optical emissions and incoherent scatter radar data obtained during high-frequency electromagnetic pumping of the ionospheric plasma from the ground give data on electron energization in an energy range from 2 to 100 eV. Optical emissions at 4278 A from N2+ that require electrons with energies above the 18 eV ionization energy give the first images ever of pump-induced ionization of the thermosphere. The intensity at 4278 A is asymmetric around the ionospheric electron gyroharmonic, being stronger above the gyroresonance. This contrasts with emissions at 6300 A from O(1D) and of electron temperature enhancements, which have minima at the gyroharmonic but have no apparent asymmetry. This direct evidence of pump-induced ionization contradicts previous indirect evidence, which indicated that ionization is most efficiently produced when the pump frequency was below the gyroharmonic.

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PubMed2006

Adiabatic approximation of the correlation function in the density-functional treatment of ionization processes.

The ionization of a one-dimensional model helium atom in short laser pulses using time-dependent density-functional theory is investigated. We calculate ionization probabilities as a function of laser intensity by approximating the correlation function of the system adiabatically with an explicit dependence on the fractional number of bound electrons. For the correlation potential we take the derivative discontinuity at integer numbers of bound electrons explicitly into account. This approach reproduces ionization probabilities from the solution of the time-dependent Schrödinger equation, in particular, the so-called knee due to nonsequential ionization.

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PubMed2001

Theoretical studies of core excitation and ionization in molecular systems.

Photoionization cross sections have been computed using correlated initial and final target states obtained with standard ab initio techniques and using a numerical representation of the continuum scattering orbitals. Here, core ionization of small molecular systems will be focused on. A brief review of previous results obtained using these methods is given. New results are then given for the N 1s ionization of NO and the N 1s ionization of N2. In the study of NO, the spin-coupling scheme which best describes the shape-resonant state and the energy dependence of the branching ratios for the singlet and triplet final ion states will be considered. In the study of N2 core ionization. the effects of the coupling of nearly degenerate core levels and the effects of correlation on the importance of interchannel coupling will be considered.

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PubMed1985

Plasma ionized calcium during open-heart surgery.

Plasma ionized calcium has been measured in eighteen patients undergoing open heart surgery. No change in ionized calcium concentration was found during the period of cardiopulmonary bypass. In the early postbypass period, a wide range of ionized calcium concentrations was found, but this did not obviously affect myocardial performance. Factors influencing the ionized calcium concentration under these circumstances are discussed.

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