Ionization Channels Research Articles

Control of frustrated double ionization channels with the carrier-envelope phase of few-cycle linearly polarized laser pulses

Using a three-dimensional classical method, we theoretically explore the frustrated double ionization (FDI) of Ar atoms under few-cycle linearly polarized laser pulses with different carrier-envelope phases (CEPs). Our calculated spectra are similar to the recent experimentally observed two peeks in the electron momentum distribution (Larimian et al 2020 Phys. Rev. Res. 2 013021). The dependence of FDI on laser CEPs is investigated, and the result reveals that the two different pathways of FDI can be controlled by CEPs. The information on which pathway leads to FDI is encoded in the electron-electron energy distribution shortly after recollision and momentum distribution at the ionization time after recollision.

Polarization-resolved analysis to solid high-order harmonic generation

We propose a quantitative polarization-resolved quantum trajectory approach to analyse the solid high-order harmonic generation (HHG). By using this approach, we can distinguish the ionization channels of different polarization components in HHG. We take the HHG in hBN under two counter-rotating circularly polarized fields as an example. We find that the right and left circular polarized high harmonics are contributed by the electron trajectories with the ionization channel around the high symmetry points K and K′ respectively. Furthermore, the intensity of the right and left circular polarization harmonics is determined by the interference between these electron trajectories. Our work offers a quantitatively polarization-resolved analysis to understand the underlying mechanism in HHG in the view of electron reciprocal space trajectory.

Characteristics of three-electrode pulsed surface dielectric barrier discharge: streamer-to-spark transition and hydrodynamic expansion

Streamer-to-spark transition and hydrodynamic expansion in a three-electrode pulsed surface dielectric barrier discharge are studied under atmospheric-pressure air. Three sequential discharge processes of the primary streamer, transitional streamer and spark phase during a single pulse are observed from the time-resolved plasma morphologies. The primary streamer and transitional streamer phases, with a rising voltage and low current, followed by a spark phase with a rapidly falling pulse and ascending current are characterized. Images of the discharge development show that the transitional streamer is maintained in the ionization channel after the primary streamer bridges the high-voltage electrode and the second grounded electrode. When the transitional streamer develops to a certain level, the streamer discharge transfers into the spark discharge. As a result, two shock waves are induced in the two exposed electrode domains, and then merge into a single ellipse during the process of hydrodynamic expansion. Boltzmann plots indicate that the electron temperature is 4.815 eV in the initial phase of spark discharge and gradually decreases in the spark phase. Stark broadening of the O atomic line shows that the electron density is 7.06 × 1017 cm−3 during the spark phase.

Numerical study of different electron density observed in Hydrogen and Deuterium ion source plasmas

Sequences of hydrogen (H) and deuterium (D) experiments have been done by NIFS research and development negative ion source (RNIS) for the deuterium NBI development. In the experiments, the co-extracted electron current with the negative ions and the electron density in the plasma generation region in the D experiment have been around three times higher than that in the H experiment. To explain the difference of the electron density in the RNIS driver region, a zero-dimensional numerical model is developed in the present study. The model only focuses on the isotope effect for vibrationally excited level of electronically grounded state molecules and its relevant cross-sections. The calculation results show that difference of the ionization channel numbers via molecular vibrationally excited states could be a reason to enhance ionization rate in D plasma.

On the size consistency problem for anti-symmetrised geminal power wave function ansatz

It is shown that the accepted proof of the anti-symmetrised geminal power (AGP) wave functions lack of size consistency is not general enough to constitute a proof for the size consistency of the AGP wave function. The origin of the size consistency problem for AGP wave function in previous proofs is shown to stem from the perceived notion that the natural orbitals of the AGP always can be localised or guessed a priori. We here show that by applying different constraints on a more general geminal coefficient matrix that the ionised/electron-attached determinants can be eliminated in different ways in a spin-restricted basis, which is not possible in the accepted proof. Furthermore it is shown how different constraints on the coefficients in the geminal coefficient matrix can lead to different ionisation channels upon dissociation. We discuss the consequences of the generation of natural orbitals from the solution of the AGP using a more general coefficient matrix. Finally the modern use of the natural AGP as a reference function for another correlation method is discussed where improvements to the orbitals used in the modern AGP are suggested.

Angular distribution parameters in two-photon double ionization of helium beyond the dipole approximation

We study nondipole effects in the two-photon double ionization of helium by solving the full-dimensional time-dependent Schr\"odinger equation (TDSE). The angular distributions of the photoelectrons in TDSE calculations are parametrized by several angular distribution parameters. The electron correlation effects are revealed and discussed by analyzing angular distribution parameters of the photoelectrons. Compared with the linearly polarized laser, the values of dipole angular parameters for circularly polarized laser pulses in the nonsequential double-ionization regime are closer to the pure dipole emission of the one-photon single ionization, which indicates a weaker electron correlation for circular pulses. For the nondipole angular parameters, in the sequential double-ionization regime, we find that the parameters show peak structures from the sequential ionization channel over the energy sharing between two electrons and these peak positions are slightly shifted by the electron correlation. In general, due to the nondipole effects, the angular distributions of photoelectrons are shifted towards the direction of laser propagation, due to the transfer of the photon linear momentum.

ФИЗИЧЕСКИЕ ПРОЦЕССЫ В ИОНИЗОВАННОЙ ЛАЗЕРНЫМ ИЗЛУЧЕНИЕМ АТМОСФЕРЕ ПРИ СОЗДАНИИ ЛИНИИ РАДИОСВЯЗИ

The results of studies of ionization processes in the atmosphere that occur under the action of laser radiation make it possible to consider the possibility of using directed laser beams for location or formation of a communication channel. To do this, with the help of laser radiation in the atmosphere, a channel with a weakly ionized wall is formed, into which radio emission is introduced. The mini mum electron concentration Ne in the ionized channel, which ensures the reflection of the transmitted electromagnetic wave, depends on its frequency. If the frequency of electromagnetic radiation propagating in the channel is less than the frequency of plasma oscillations in the medium, then the waves are completely reflected from the ionized layer. In the range of long waves (1000-3000 m), the effect of self-radiation of plasma centers can be used to transmit information. By changing the dimensions of the plasma source in space, by modulating the laser radiation, it is possible to vary the frequency and amplitude of the emitted electromagnetic waves in a given range. The main mechanisms of creation of weak ionization channels in the atmosphere by laser radiation are considered. A scheme for the transmission of electromagnetic energy is proposed and its parameters are calculated. A variant of electromagnetic energy transfer by creating a plasma mirror in the atmosphere is considered. The frequency, duration, and energy of laser pulses necessary to create weak ionization channels in the atmosphere and ionosphere are estimated. The calculations and experiments carried out showed that an IR laser sub nanosecond radiation source with a pulse repetition rate of 108 Hz and providing a breakdown or pre-breakdown ionization mode can be used to transfer electromagnetic microwave energy in the atmosphere. It has been established that in the atmosphere it is possible to use both the breakdown mode and the pre-breakdown mode for generating and transmitting long-wave and microwave electromagnetic radiation. It is shown that in the pre-breakdown mode of aerosol ionization, anomalous scattering on plasma particle halos may occur due to the formation of plasmons, which increase the scattering intensity by 2-3 orders of magnitude.

Probing aqueous ions with non-local Auger relaxation.

Non-local analogues of Auger decay are increasingly recognized as important relaxation processes in the condensed phase. Here, we explore non-local autoionization, specifically Intermolecular Coulombic Decay (ICD), of a series of aqueous-phase isoelectronic cations following 1s core-level ionization. In particular, we focus on Na+, Mg2+, and Al3+ ions. We unambiguously identify the ICD contribution to the K-edge Auger spectrum. The different strength of the ion–water interactions is manifested by varying intensities of the respective signals: the ICD signal intensity is greatest for the Al3+ case, weaker for Mg2+, and absent for weakly-solvent-bound Na+. With the assistance of ab initio calculations and molecular dynamics simulations, we provide a microscopic understanding of the non-local decay processes. We assign the ICD signals to decay processes ending in two-hole states, delocalized between the central ion and neighbouring water. Importantly, these processes are shown to be highly selective with respect to the promoted water solvent ionization channels. Furthermore, using a core-hole-clock analysis, the associated ICD timescales are estimated to be around 76 fs for Mg2+ and 34 fs for Al3+. Building on these results, we argue that Auger and ICD spectroscopy represents a unique tool for the exploration of intra- and inter-molecular structure in the liquid phase, simultaneously providing both structural and electronic information.

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study.

The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00-10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH3OH)nH+. However, the stable dimer parent ion (CH3OH)2+ is readily detected below the dissociation threshold to yield the dominant CH3OH2+ fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH3OH)2+ parent ion dissociates via five parallel channels at low internal energies. The loss of both CH2OH and CH3O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH3(H)OH+⋯CH2OH ion, the precursor to the CH2OH-, H2O-, and CH3-loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH3OH2+, which explains the dominance of the protonated methanol fragment ion in the mass spectrum.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate.

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C5H8O2) and methyl butyrate (MB, C5H10O2) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO2 is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E0 = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO2 loss as 66.6 ± 4.3 kJ mol-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E0 = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at ΔfHo0K[CH2C(OH)OCH3+] = 502 ± 6 kJ mol-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH3 and CO2 loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

Influence of Vibrational Excitation and Nuclear Dynamics in Multiphoton Photoelectron Circular Dichroism of Fenchone.

We report photoelectron circular dichroism of S-(+)-fenchone enantiomers recorded with state-state vibrational level resolution using picosecond laser (2 + 1) resonance enhanced multiphoton ionization via 3s and 3p Rydberg intermediate states. The 3p state decays to the 3s state on a picosecond time scale so that, above the 3p Rydberg excitation threshold, ionization of vibrationally hot 3s states competes with direct 3p-1 ionization. Complex vibronic dynamics of the 3p → 3s internal conversion weaken the Rydberg Δv = 0 propensity rule in both the 3p-1 and 3s-1 ionization channels. Large variations of the forward-backward chiral asymmetry factors are observed between the Δv = 0 and Δv > 0 vibrational transitions, including dramatic swings from up to ±17%. Such changes of sign indicate complete reversal of the preferred direction for photoelectron emission in the laboratory frame, associated with vibrational motion. These asymmetry switches easily exceed the amplitude and frequency of such vibrationally induced flips previously observed in single photon ionization.

Benchmark PhotoIonization Cross-Sections of Neutral Scandium from the Ground and Excited States

The B-spline R-matrix method has been used to investigate cross-sections for photoionization of neutral scandium from the ground and excited states in the energy region from the 3d and 4s valence electron ionization thresholds to 25 eV. The initial bound states of Sc and the final residual Sc+ ionic states have been accurately calculated by combining the multiconfiguration Hartree-Fock method with the frozen-core close-coupling approach. The lowest 20 bound states of Sc I belonging to the ground 3d4s2 and excited 3d24s, 3d24p, 3d4s4p, 4s24p, and 3d3 configurations have been considered as initial states. The 81 LS final ionic states of Sc+ belonging to the terms of 3p63d2, 3p63d4l (l = 0–3), 3p63d5l (l = 0–3), 3p63d6s, 3p64s2, 3p64s4l (l = 0–3), 3p64s5l (l = 0–1), and 3p64p2 configurations have been included in the final-state close-coupling expansion. The cross-sections are dominated by complicated resonance structures in the low energy region converging to several Sc+ ionic thresholds. The inclusion of all these final ionic states has been noted to significantly impact the near-threshold resonance structures and background cross-sections. The important scattering channels for leaving the residual ion in various final states have been identified, and the 3d electron ionization channels have been noted to dominate the cross-sections at higher photon energies.

Femtosecond 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy of the C-State in Molecular Oxygen.

Coincidence electron-cation imaging is used to characterize the multiphoton ionization of O2 via the v = 4,5 levels of the 3s(3Πg) Rydberg state. A tunable 100 fs laser beam operating in the 271–263 nm region is found to cause a nonresonant ionization across this wavelength range, with an additional resonant ionization channel only observed when tuned to the 3Πg(v = 5) level. A distinct 3s → p wave character is observed in the photoelectron angular distribution for the v = 5 channel when on resonance.

Nondipole photoelectron momentum shifts in strong-field ionization with mid-infrared laser pulses of long duration

We consider atomic strong-field ionization in the long-pulse limit for linearly polarized infrared laser fields. We show how nondipole effects in the plane formed by the propagation and polarization directions lead to (i) a shift of the origin of the rings describing the energetically allowed final electron momenta for individual above-threshold ionization (ATI) channels and (ii) a redistribution in the continuum population on each ring. In this manner, the photoelectron energy in a given ATI channel depends on the direction of the ejected electron.

Interference effects on intense XUV pulse induced resonant transitions between core ionized states of helium

In the context of core-resonant ionization of helium atom the inclusion of the He–He+(2p) ionization channel sets off a quantum mechanical interference between two competing pathways, He–He+(1s)–He+(2p) and He–He+(2p) (He–He+(1s) and He–He+(2p)–He+(1s)), thereby significantly modifying the line shape of the photoelectron spectrum. The signature of this interference arises as an asymmetry in the line shape of the multi-peak photoelectron spectra. On account of the discrepancies over the magnitude of the two photon photoionization cross section for the He–He+(2p) ionization channel the calculations have been performed for three different magnitudes of this photoionization cross section and the findings confirm that asymmetry in the line shape is prevalent for all the chosen cross sections. However, the magnitude of asymmetry depends on the magnitude of the photoionization cross section for the He–He+(2p) ionization channel. It is also interesting to note that at the considered laser peak intensities in this work an additional ionization channel from excited ionic state of helium plays a significant role in influencing the line shape of the photoelectron spectrum. The effects of pulse duration on the line shape of the photoelectron spectrum and the possibility to control the interference induced asymmetry in the multi peak line shape has also been investigated.

Energy and angular distributions of electrons from sodium atoms photo-ionized with femtosecond laser pulses

We studied photoelectron angular distributions (PADs) and energy spectra of sodium atoms photo-ionized with intense short laser pulses in the near-infrared region. Energy spectra of photoelectrons due to multistep ionization of sodium atoms through intermediate levels 5p, 6p, 7p, and 4f were observed in above threshold ionization (ATI) at laser intensities of 2 × 1013 W cm−2 and 1 × 1013 W cm−2. The PADs from the 5p(4f) state for zero, first, second and third order ATI peaks were measured. We observed that for the two selected laser intensities the angular distribution of photoelectrons showed only slight variations. The PADs exhibit main lobes within ∼60° degrees from the laser polarization and additional lobes that become more pronounced with the increasing ATI peak number. Besides a general analytical representation of the observed angular dependences, the experimental PADs for each ATI peak were fitted with Legendre polynomials accounting for different ionization channels. The good agreement between the experimental data and fitting functions allowed to evaluate the contributions of the ionization channels to the observed ATI orders. The calculations of the photoelectron spectra by numerically solving the time-dependent Schrödinger equation in a single active electron approximation produced agreement with the experiment in the main features of the spectra, while differences in more subtle details were observed.

Confinement enhanced spin–orbit interchannel coupling effect on the atomic photoionization

The photoionization cross-section of an atom trapped inside a fullerene anion in the near-threshold region is dominated by the Coulomb confinement resonances. These prominent structures facilitate the possibility of an enhanced spin–orbit interaction activated interchannel coupling (SOIAIC) leading to significant modifications of the photoionization parameters even for low-Z confined atomic systems. The existence of such confinement enhanced SOIAIC structures are demonstrated for the relativistically split 2p ionization channels of Ar trapped inside fullerene anion. Dramatic modifications are observed not only in the cross-sections but also in the angular distributions, and relative phases between the degenerate channels present. Similar structures are found for the case of confined Kr as well. A detailed analysis of these interesting features is carried out which showed the importance of correlation and relativistic effects on the photoionization dynamics of confined atomic systems.

Phase delay effect of nonsequential double ionization of Ar atoms in a 45°cross-linearly-polarized two-color laser field

We theoretically investigate the nonsequential double ionization (NSDI) of Ar atoms by a three-dimensional (3D) classical ensemble method in a 45∘ cross-linearly-polarized two-color (CTC) laser field. Our results show that the phase delay of the synthesized two-color laser field can influence the NSDI process. By tracing the classical trajectories of the NSDI process, we find that the single-recollision (SR) and the double-recollision (DR) both occur. Whether the recollision processes happen once or twice, the NSDI predominantly experience the recollision-induced excitation with subsequent ionization (RESI) channel. And the phase delay of the CTC pulse could influence the probability of the double ionization and the energy transfer in NSDI process. This provides a method to obtain the information about the dynamics of the recollision process.

Photoionization dynamics of Ar trapped in a fullerene anion: Coulomb confinement resonances in 2s subshell and its impact on the 3s subshell

The photoionization dynamics of the inner subshell of an endohedral anion, using relativistic random phase approximation (RRPA), is investigated. The charged nature of the fullerene induces Coulomb confinement resonances (CCRs) not only on the 2s cross-section but also on other dynamical attributes such as angular distribution asymmetric parameter, phase shift, and time delay of the same subshell. More importantly, we find that the 2s CCRs resurrect confinement resonances into the photoionization dynamics of 3s ionization channels at energies far away from the 3s threshold. The impacts of the revivification of these resonances on the 3s cross-section, phase shift, and time delay are explored in this work.

Mean Free Paths and Cross Sections for Electron Scattering from Liquid Water.

Electron collision with liquid water is theoretically investigated and reported in this article. The range of projectile energy considered is 10-5000 eV, covering all major channels, viz., ionization, inelastic, elastic, and total scattering. The liquid phase electron charge density and static potential are generated and used in the calculation under a spherical complex optical potential formalism to achieve the goals. For the ionization channel, the complex scattering potential-ionization contribution method is used. The agreement with available theoretical data is satisfactory. The study on the total electron scattering from liquid water, using a common method for elastic and inelastic cross sections, is new and requires further attempts to support the reported data.