Direct Ionization Channels Research Articles

Evidence of ac-Stark-shifted resonances in intense two-color circularly polarized laser fields

We report on the appearance of a structure at low energies in the photo-electron momentum distribution of the hydrogen atom exposed to two-color counter-rotating bi-circular laser fields. These structures, which arise due to AC-Stark shifted resonances, break the three-fold symmetry, typical for the $\omega-2\omega$ bi-circular fields. We discuss the physical origin of this structure in terms of partial-wave interference between direct ionization channels and a resonant pathway, that passes through the AC-Stark shifted state and show how the underlying Rydberg state population depend on the field strength and pulse duration.

Plasmon resonant (e, 2e) spectroscopy on Be(0001)

We investigated the mechanisms of secondary electron (SE) emission from Be(0001) by impact of 100 and 150 eV electrons. We made use of (e, 2e) spectroscopy to disentangle the different SE production mechanisms. We observed a large increase in the SE yield when the energy loss of the primary electron equals the characteristic energy of volume and surface plasmons. The line shape of the SE spectrum associated with plasmon excitation reveals that one relevant emission mechanism corresponds to direct single-particle excitation in which the plasmon energy and momentum are transferred to a valence band electron of the solid. The contributions to the SE yield associated with surface and volume plasmon excitation are comparable in the case of specular geometry, where the projectile momentum is mainly transferred perpendicular to the surface. On the contrary, the emission of SEs associated with surface plasmon excitation is significantly enhanced when the exchanged momentum lies close to the surface plane and electrons are emitted from Be surface state. This reflects the increased sensitivity to surface modes of the latter geometry. Finally, the coupling between the direct ionization channel and the plasmon-assisted one results in a resonant increase of the secondary emission.

Strong field control of the interatomic Coulombic decay process in quantum dots

In recent years the laser-induced interatomic Coulombic decay (ICD) process in paired quantum dots has been predicted (Bande, 2013). In this work we target the enhancement of ICD by scanning over a range of strong-field laser intensities. The GaAs quantum dots are modeled by a one-dimensional double-well potential in which simulations are done with the space-resolved multi-configuration time-dependent Hartree method including antisymmetrization to account for the fermions. As a novelty a complementary state-resolved ansatz is developed to consolidate the interpretation of transient state populations, widths obtained for the ICD and the competing direct ionization channel, and Fano peak profiles in the photoelectron spectra. The major results are that multi-photon processes are unimportant even for the strongest fields. Further, below-π to π pulses display the highest ICD efficiency while the direct ionization becomes less dominant.

Harmonic Generation and Nonlinear Propagation: When Secondary Radiations Have Primary Consequences

In this Letter, it is experimentally and theoretically shown that weak odd harmonics generated during the propagation of an infrared ultrashort ultra-intense pulse unexpectedly modify the nonlinear properties of the medium and lead to a strong modification of the propagation dynamics. This result is in contrast with all current state-of-the-art propagation model predictions, in which secondary radiations, such as third harmonic, are expected to have a negligible action upon the fundamental pulse propagation. By analysing full three-dimensional ab initio quantum calculations describing the microscopic atomic optical response, we have identified a fundamental mechanism resulting from interferences between a direct ionization channel and a channel involving one single ultraviolet photon. This mechanism is responsible for wide refractive index modifications in relation with significant variation of the ionization rate. This work paves the way to the full physical understanding of the filamentation mechanism and could lead to unexplored phenomena, such as coherent control of the filamentation by harmonic seeding.

A photoionization investigation of small, homochiral clusters of glycidol using circularly polarized radiation and velocity map electron–ion coincidence imaging

A detailed study of the valence photoionization of small homochiral glycidol (C3O2H6) clusters is carried out with the help of circularly-polarized VUV synchrotron radiation by recording photoionization-based spectroscopic data detected by velocity map electron imaging with coincidence ion selection. We show that information on the stability of cationic as well as neutral chiral clusters can be obtained with enhanced sensitivity by examining the chiral fingerprint encapsulated in Photoelectron Circular Dichroism (PECD) spectra. In particular, by varying the clustering conditions we demonstrate that the PECD signal effectively carries the signature of the neutral precursor species, prior to any fragmentation of the ion, as may be inferred from the below-threshold monomer measurements (including ion imaging). Here the monomer's direct ionization channel is closed and the monomer ion hence must result exclusively as a fragment from dissociative ionization of the dimer (or higher) clusters. At higher photon energies, the mass-selection on the electron spectroscopy data, achieved through filtering the electron images in coincidence with selected ion masses, evidently succeeds in providing a degree of size-selection on the neutral clusters being ionized with, in particular, a clear differentiation of monomer and dimer PECD, showing the strong sensitivity of this chiroptical effect to the non-local long-range molecular potential.

Updating of ionization data for ionization balance evaluations of atoms and ions for the elements hydrogen to germanium

Atomic data for electron impact ionization of all the elements from H to Ge are reviewed, the rates for these processes needing to be regularly updated following the publication of new experimental data and new theoretical calculations. Experimental cross sections, along with specific theoretical calculations when experimental data are missing, are fitted as functions of the electron energy, and from these fits ionization rate coefficients can be evaluated. It has been possible to take into account all elements but not all charge states of every element. Since the purpose of the paper is to update the ionization data evaluated and proposed in previous review papers, it is discussed if modifications are needed for the ions not considered. For highly ionized ions starting from the Ne-like iso-electronic sequence corrections do not appear necessary. On the other hand, except for Fe, for slightly ionized ions, specifically below the S-like iso-electronic sequence, the previously proposed data often underestimate the total ionization cross section, since only direct ionization channels have been considered and indirect processes have been neglected. Multiplicative correction coefficients are given to agree with recently published theoretical calculations. Experimental ionization data are considered, even when the presence of populated metastable levels (related to the electron density inside the source) is reported in the ion beams involved in the cross-section data measurements. We deem such a procedure acceptable when the proposed rates have to be included in codes that simulate the impurity behaviour in magnetic-confinement fusion devices, i.e., when radial transport is added to ionization and recombination to predict spatially resolved charge-state distributions. On the other hand, for astrophysical plasmas the contributions of metastable levels to the experimental data may represent a serious problem since, generally, the values of the electron densities that are involved are much lower than those in the ion sources. However, we critically investigated this problem and we found that the presence of metastables does not significantly modify the rates of most of the ions apart from a dozen. For this set of ions we provide different, corrected rates. Recombination is not considered since a review has been recently published.

Dissociative and non-dissociative ionization of the N2 molecule by the impact of 0.75–3.5 MeV He+

Total projectile electron loss and absolute dissociative and non-dissociative ionization cross sections of the N2 molecule in coincidence with the final projectile charge states have been measured for the impact of 0.75–3.5 MeV He+. Comparison is made with proton, He+ and electron impact data available in the literature for the direct ionization channels. The fragmentation fractions for singly-charged products follow a scaling law indicating that the main dynamical variables behind the fractions are the momentum transfer at intermediate velocities and the energy transfer at high velocities.

Multiple Ionization of Ne by H2+ Ions with Correlated Projectile Electron Loss

We consider multiple ionization of Ne by fast H2+, with and without projectile electron loss. Measurements of the total cross section for the direct ionization channel support the similarity of ionization of Ne by an H2+ projectile that does not break and Ne ionization by an equi-velocity proton. The multiple target ionization with projectile electron loss is analyzed within a proposed model, adapted from the independent particle model, taking into account post-collisional decay channels following the ionization of Ne 2s electrons. We point out that, for the H2+ + Ne collision system, the post-collisional decay channels have a prominent role in the multiple target ionization associated with the antiscreening mode (two-center electron-electron correlation).

Partial double- and single-differential cross-sections for CO 2 by electron collision

This Letter reports the partial double-differential cross-section (PDDCS) and the partial single-differential cross-sections (PSDCS) for the CO 2 molecule by electron collision at 100 and 500 eV. The two direct ionization channels producing CO 2 + and CO 2 2+ and the four dissociative channels producing CO +, C +, O + and C 2+ are examined. The calculations as a function of energy loss were made by employing a semi-empirical formula which requires the oscillator strengths as input data. For the evaluation of PDDCS with respect to exit electron(s) energy and the scattering angle, the differential generalized oscillator strengths were used in the optical limit. The partial ionization cross-sections (PICS) are also calculated, with electron energies varying from ionization threshold to 10 keV.

The effect of electronic autoionization on vibrational branching ratio in (1 + 1')-photon resonance-enhanced multiphoton ionization of molecules: dependence on laser intensity

We have studied the laser intensity effect on the vibrational branching ratio (VBR) in -photon resonance-enhanced two-photon ionization of molecules, considering the electronic autoionization through the lowest autoionizing (AI) state of symmetry. In this ab initio calculation we have studied resonant two-photon transitions from the ground state to the doubly excited autoionizing state of symmetry as well as to the electronic continuum (:) of molecules via the resonant intermediate levels. Transitions to two different continuum energies (-0.4500 au and -0.5853 au) have been considered; the first one is 1.36 eV above the dissociation threshold (-0.50 au) of the ion and the second one is 0.048 eV above the vibrational level of the ion in the ground state . Parallel transitions through other intermediate near-resonant rovibrational levels v = 0-3 and 5-20 with the allowed values of total angular momentum j of the state have been included in the calculation. Besides the effect of interference of direct ionization with autoionization channels, we have considered here (i) the effects of mutual interference among parallel autoionization channels and that among parallel ionization channels via the near-resonant and resonant rovibrational levels of the state and (ii) the effect of the two-photon coupling between intermediate rovibrational levels via the autoionizing state and the continuum in order to study the laser intensity dependence of vibrational distribution of ions in the ground state. It has been shown that these two effects (channel interference effect and two-photon coupling effect) lead to non-Franck-Condon type VBR and the values differ from those obtained by considering the direct ionization channels alone. The laser intensity dependence of VBR is also different from that obtained by neglecting the autoionization channels. For transitions to two different continuum energies these effects become prominent at two different laser intensities. Furthermore, it has been found that the transitions through different intermediate resonant rotational levels of the state give rise to different vibrational branching ratios.

SCORPION: A system for coincidences between recoil and projectile ions at NSC, New Delhi

An on-line facility to measure coincidences between the recoil ions and the scattered projectiles (SCORPION) has been designed, fabricated and commissioned at Nuclear Science Centre (NSC), New Delhi. The facility consists of a four jaw slit assembly, a time of flight (TOF) spectrometer, a parallel plate electrostatic charge analyser and a one dimensional position sensitive parallel plate avalanche counter (PPAC). Details of the design and working principles of various components and the test results obtained for the Si q+-Ar collision system are presented to highlight the performance of the system. A multiple loss of up to four electrons has been observed for 60 MeV Si4+ ions colliding with argon atoms in a single collision condition. Spectra of recoil ions detected in coincidence with a particular charge state of the scattered projectile show a bell shaped distribution as a function of the recoil charge state (r) for the electron loss events. However, the yield of recoil ions drops asr increases for the direct ionization channel. Also for electron loss, the peak of the recoil ion distribution is seen to shift to a higher recoil charge state as the number of lost electrons from the projectile increases.

Near threshold effects in positron-O2 scattering.

Structure has been observed in the total ionization cross section of ${\mathrm{O}}_{2}$ above 8 eV positron impact energy. A comparison with available data on the excitation to the Schumann-Runge continuum indicates that coupling effects between the positronium formation, excitation, and direct ionization channels may be responsible.

Completing the iron period: double-resonance, fluorescence-dip Rydberg spectroscopy and ionization potentials of titanium, vanadium, iron, cobalt, and nickel

We have used the fluorescence-dip technique to obtain double-resonance spectra of transition-metal atoms produced in a rf glow-discharge sputtering machine. Rydberg spectra of the neutral elements Ti, V, Fe, Co, and Ni have been analyzed to yield ionization potentials accurate to 1 cm−1, completing the table of values for the iron period (K through Zn). The 63 737-cm−1 value obtained for Fe agrees with a recently reported result, and new ionization potentials for Ti, V, Co, and Ni are 55 073, 54 413, 63 565 and 61 619 cm−1, respectively. Quantum defects for nd and ns series were obtained. Series converging to some excited ion core levels do not show detectable s–d mixing, which would provide a direct ionization channel and lead to the asymmetric line shapes that are characteristic of autoionization. Fano q parameters were determined for states autoionizing without a change in core configuration. Resolved spectra of low-lying Rydberg states display a complicated level structure not observed in studies of alkali or rare-gas atoms.

Spin polarization by selective laser-induced interference

States behaving like autoionizing states can be selectively induced by laser radiation into one of the continuum spin channels in the photoionization of polarized excited alkaki atoms. As a result of destructive or constructive interference between the direct ionization channel and those introduced by the dressing laser radiation, the cross section of this specific spin component is completely suppressed or enhanced, while leaving the other spin channel unaffected. The $q$ parameter which determines the line shape of the Fano-type resonance can be resonantly tuned as a function of the dressing laser frequency.