Double Ionization Channels Research Articles

Control of Frustrated Nonsequential Double Ionization Channels With the Carrier-Envelope Phase of Elliptically Polarized Laser Pulses

Abstract Using a classical approach, we theoretically investigate the frustrated double ionization (FDI) of Ar atoms under elliptically polarized (EP) laser fields. The FDI yield is calculated as a function of laser intensity with different carrier-envelope phases (CEPs), and, similar to the carrier-envelope phase (CEP) dependence of nonsequential double ionization (NSDI), several knee-shaped structures appear. Moreover, both recollision and direct ionization trajectories are observed in FDI events for the “knee” structure regime. The footprints of which channel leads to FDI are encoded in the photoelectron momentum distributions. FDI events related to recollision excitation with subsequent ionization channels and recollision impact ionization channels can be controlled by CEP in EP laser fields.

Nonsequential double ionization channels control of CO2 molecules with counter-rotating two-color circularly polarized laser field by laser wavelength

Abstract Using the classical ensemble model, we investigate the effect of laser wavelength on the electron dynamics process of nonsequential double ionization (NSDI) for linear triatomic molecules driven by a counter-rotating two-color circularly polarized laser field. Based on the delay time between recollision and final double ionization, two particular ionization channels are separated: recollision-impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI). Numerical results show that with the increase of the laser wavelength, the triangle structure of the ion momentum distribution becomes more obvious, which indicates that the electron–electron correlation of NSDI is enhanced. In addition, we find that the ratio of the RESI channel gradually decreases with the increase of the laser wavelength, while the ratio of the RII channel is opposite. However, the dominant channel is still RESI. It means that the two ionization channels can be controlled effectively by changing the laser wavelength.

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.

Mechanisms and dynamics of the NH+ 2 + H+ and NH+ + H+ + H fragmentation channels upon single-photon double ionization of NH3

We present state-selective measurements on the + H+ and NH+ + H+ + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH3, where the two photoelectrons and two cations are measured in coincidence using 3D momentum imaging. Three dication electronic states are identified to contribute to the + H+ dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited fragment with roughly 1 eV of internal energy. The NH+ + H+ + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the + H+ channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states.

Threshold photodissociation dynamics of NO2 studied by time-resolved cold target recoil ion momentum spectroscopy.

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both "above threshold" (dissociating) and "below threshold" (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving "hot" neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO-O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled "inner region" to a loosely coupled "outer region" where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.

Coulomb exploded directional double ionization of N2O molecules in multicycle femtosecond laser pulses**Project supported by the National Key R&D Program of China (Grant No. 2018YFA0306303), the National Natural Science Fundation of China (Grant Nos. 11425416, 11834004, and 11761141004), and the 111 Project of China (Grant No. B12024).

We experimentally investigate Coulomb exploded directional double ionization of N2O molecules in elliptically polarized femtosecond laser pulses. The denitrogenation and deoxygenation channels are accessed via various pathways. It leads to distinct asymmetries in directional breaking of the doubly ionized N2O molecules versus the instantaneous laser field vector, which is revealed by tracing the sum-momentum spectra of the ionic fragments as a recoil of the ejected electrons. Our results demonstrate that the accessibility of the Coulomb exploded double ionization channels of N2O molecules are ruled by the detailed potential energy curves, and the directional emission of the fragments are governed by the joint effects of the electron localization-assisted enhanced ionization of the stretched molecules and the profiles of the molecular orbitals.

Strong-field-induced bond rearrangement in triatomic molecules

A comparative study of bond rearrangement is reported for the double ionization of three triatomic molecules: carbon dioxide, carbonyl sulfide, and water (${\mathrm{D}}_{2}\mathrm{O}$). Specifically, we study the formation of the molecular cation ${\mathrm{AC}}^{+}$ from the edge atoms of a triatomic molecular dication ${\mathrm{ABC}}^{2+}$ following double ionization by intense, short (23 fs, 790 nm) laser pulses. The comparison is made using the double ionization branching ratio of each molecule, thereby minimizing differences due to differing ionization rates. The rearrangement branching ratio is highest for water, which has a bent initial geometry, while ${\mathrm{CO}}_{2}$ and OCS are linear molecules. The angular distribution of $\mathrm{O}{}_{2}{}^{+}$ fragments arising from ${\mathrm{CO}}_{2}$ is essentially isotropic, while ${\mathrm{SO}}^{+}$ from OCS and ${\mathrm{D}}_{2}^{+}$ from ${\mathrm{D}}_{2}\mathrm{O}$ are aligned with the laser polarization. In the ${\mathrm{CO}}_{2}$ and ${\mathrm{D}}_{2}\mathrm{O}$ cases, the angular distributions of the bond rearrangement channels are different from the angular distributions of the dominant dissociative double ionization channels ${\mathrm{CO}}^{+}+{\mathrm{O}}^{+}$ and ${\mathrm{OD}}^{+}+{\mathrm{D}}^{+}$. Only the angular distribution of ${\mathrm{SO}}^{+}$ from OCS is both aligned with the laser polarization and similar to the angular distribution of the largest dissociative channel, ${\mathrm{CO}}^{+}+{\mathrm{S}}^{+}$. The mixed behavior observed from the angular distributions of the different molecules stands in contrast to the relative consistency of the magnitude of the bond rearrangement branching ratio.

Double ionization of nitrogen molecules in orthogonal two-color femtosecond laser fields

Double ionization of nitrogen molecules in orthogonally polarized two-color femtosecond laser fields is investigated by varying the relative intensity between the fundamental wave (FW) and its second harmonic (SH) components. The yield ratios of the double ionization channels, i.e., the non-dissociative and Coulomb exploded (N+, N+), to the singly charged N2+ channel exhibit distinct dependences on the relative strength between the FW and SH fields. As the intensity ratio of SH to FW increases, the yield ratio of (N+, N+)/N2+ gradually increases, while the ratio of /N2+ first descends and then increases constituting a valley shape which is similar to the behavior of Ar2+/Ar+ observed in the same experimental condition. Based on the classical trajectory simulations, we found that the different characteristics of the two doubly ionized channels stem from two mechanisms, i.e., the is mostly accessed by the (e, 2e) impact ionization while the recollision-induced excitation with subsequent ionization plays an important role in producing the (N+, N+) channel.

Imaging of strong field dissociative single and double ionization channels of N2O

In this paper, we study single and double ionizations of N2O in a short elliptically polarized 800 nm laser pulse using the COLTRIMS technique. The molecular-frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionizations of N2O molecules are reported for the single ionization dissociative channel NO[Formula: see text] + N and the double ionization dissociative channel NO[Formula: see text] + N[Formula: see text]. The ionizations of multiple orbitals for the two studied dissociative channels were identified via studying the orientation dependent ionization rates for their KERs. The results show that the shape of the ionizing orbitals governs the single and double ionization processes of N2O.

Nonsequential double ionization channels control of Ar with few-cycle elliptically polarized laser pulse by carrier-envelope-phase.

The carrier-envelop-phase (CEP) dependence of nonsequential double ionization (NSDI) of atomic Ar with few-cycle elliptically polarized laser pulse is investigated using 2D classical ensemble method. We distinguish two particular recollision channels in NSDI, which are recollision-impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI). We separate the RII and RESI channels according to the delay time between recollision and final double ionization. By tracing the history of the trajectories, we find the electron correlation spectra as well as the competition between the two channels are sensitively dependent on the laser field CEP. Finally, control can be achieved between the two channels by varying the CEP.

Double ionization of water molecules induced by swift protons

Experimental cross sections for single and double ionization of ${\mathrm{H}}_{2}\mathrm{O}$ by swift ${\mathrm{H}}^{+}$ with energy ranging from 0.3 to 2.0 MeV are reported. In this energy range the ionization is the dominant collision process and charge transfer reactions can be disregarded. A multihit coincidence technique is used to measure the ${\text{H}}^{+}+{\text{OH}}^{+}$ and ${\text{H}}^{+}+{\text{O}}^{+}$ fragmentation channels. Single- and double-hit differential measurements together with a semiempirical calculation allow separating quantitatively the prompt and Auger-like decay contributions to fragmentation following a vacancy in the $2{a}_{1}$ molecular orbital. Concerning the double-ionization channel, it is found that for lower energies the mechanism of a sequential double-electron removal, known as TS2, dominates. For energies above above 750 keV/u ionization resulting from a single vacancy followed by an Auger like deexcitation takes over the TS2, becoming the main contribution to the double-ionization cross section. Our results are compared to the electron-impact data within the same velocity range and also with theoretical calculations available in the literature.

Electron dynamics of molecular double ionization by elliptically polarized few-cycle laser pulses

Using the classical ensemble method, we have investigated double ionization (DI) of diatomic molecules driven by elliptically polarized few-cycle laser pulses. The results show that DI channel depends strongly on internuclear distances (R), which is dominated by nonsequential double ionization (NSDI) for small and large R, while sequential double ionization (SDI) for mediate R. By tracing NSDI trajectories, we find that NSDI mainly originates from recollision process for small R and collision process for large R. Moreover, the correlated momentum distributions along the long axis strongly depend on the carrier-envelope-phase (CEP), and this phase dependence is affected by R.

Insights from the zero-angular-momentum wave in single and double ionization of He by fast electrons

In a recent contribution our group proposed a perturbative approach to deal with the open problem of double ionization of helium by electron impact. The goal of this work is to delve into the physics of the scattering function, and provide evidence of the nature of the single-ionization channels present in the three-body wave function. We show that the fast oscillating parts of the wave function are indeed the single-ionization channels, which appear coupled with the double continuum ones. We found a particular type of basis set within the Generalized Sturmian method that is better suited to isolate single from double-ionization channels.

Gas-Phase Protein Inner-Shell Spectroscopy by Coupling an Ion Trap with a Soft X-ray Beamline.

C, N, and O near-edge ion yield spectroscopy of 8+ selected electrosprayed cations of cytochrome c protein (12 kDa) has been performed by coupling a linear quadrupole ion trap with a soft X-ray beamline. The photoactivation tandem mass spectra were recorded as a function of the photon energy. Photoionization of the precursor, accompanied by CO2 loss, is the dominant relaxation process, showing high photoion stability following direct or resonant photoionization. The partial ion yields extracted from recorded mass spectra show significantly different behaviors for single and double ionization channels, which can be qualitatively explained by different Auger decay mechanisms. However, the single ionization spectra reveal characteristic structures when compared to existing near-edge X-ray absorption fine structure (NEXAFS) spectra from thin films of peptides and proteins. Therefore, the present experiment opens up new avenues for near-edge X-ray spectroscopy of macromolecules in the gas phase, overcoming the radiation damage issue or the environmental effects as due to the surface, intermolecular interactions, and solvent.

Direct two-XUV-photon double ionization in xenon

We report the observation of the direct two-XUV-photon double ionization of xenon by energetic coherent XUV continuum radiation. The spectrum of the XUV continuum spans from ∼15 to ∼23 eV, for which the two-photon sequential double ionization channel is partially open. The two-XUV-photon process is exploited in a second-order autocorrelation measurement of the temporal width of the continuum radiation, revealing energetic XUV pulses at the border between the atto- and femto-second scales.

Dissociative double ionization of 1-bromo-2-chloroethane irradiated by an intense femtosecond laser field

The dissociative double ionization and multi-photon ionization of 1-bromo-2-chloroethane (BCE) irradiated by the 800 nm femtosecond laser field have been investigated by dc-slice imaging technology. The charged parent ion ratio [BCE(2+)]/[BCE(+)] was measured, and the corresponding ionization process including non-sequential double ionization and sequential double ionization was analyzed. The sliced images of different photo-dissociated ions were detected, and the corresponding kinetic energy release (KER) distributions were calculated and extracted. Furthermore, the dissociative double ionization channels, attributed to the cleavage of the C-C, C-Br, and C-Cl bonds by the Coulombic repulsive forces, were discussed, and the revised equilibrium distance R(e)*, the energy ratio E(exp)/E(coul), and the value a=√(R(e)*)/(E(exp)/E(coul)) were calculated.

Redistribution of Ce 4d oscillator strength observed in photoionization of endohedral Ce@C+82 ions

Cerium atoms and ions with a filled 4d subshell are expected to have a 4d photoabsorption oscillator strength of 10 according to the number of equivalent electrons in that shell. While this expectation was confirmed for free atomic cerium ions by observing the single and double photoionization channels, measurements with Ce ions encapsulated within a C82 Fullerene cage showed that the single and double ionization channels contain far less than 50% of the Ce 4d oscillator strength.

Orientation dependence in the double ionization of HD molecules

We are currently investigating the level of isotropy in the dissociative double ionization of HD molecules by fast ions as a function of projectile ion velocity. The light ions used to doubly ionize ground state HD molecules originate at the Auburn University Accelerator, which is capable of producing ions with velocities ranging from 3 to 13 atomic units. The molecule fragment momenta are measured using Cold Target Recoil Momentum Spectroscopy (COLTRIMS), allowing for the isolation of the double ionization channel and determination of the internuclear axis relative to the ion beam.

Observation and analysis of 3s–np resonance excitation in Cr, Mn and Fe atoms

Photoabsorption and the subsequent decay processes of Cr, Mn and Fe atoms have been studied for the 3s-excitation region using a charge-separated-photoion-yield method combined with monochromatized synchrotron radiation. Two weak peak-type features, which can be attributed to the first members of 3s−1np resonance excitation series, are observed in the double ionization channel of Cr, in the double and triple ionization channels of Mn, and in the triple ionization channel of Fe. The observed resonances have been fitted to Beutler–Fano profiles to obtain resonance parameters, and compared with MCDF calculations to identify those resonance states. The decay processes of these resonance states are discussed. The spectator Auger process plays a central role in the decay of 3s−1np excitation states. In particular, the triple ionizations can be exclusively explained by spectator Auger processes.

Photoabsorption and subsequent decay of Na and Mg atoms in the 2s–np autoionizing resonance region

Photoabsorption and subsequent decay of Na and Mg atoms have been studied experimentally in the vicinity of the 2s excitation region. The charge-separated photoion-yield method using monochromatized synchrotron radiation was employed to detect the resonance excitation features separately in the single and double ionization channels. The photoion-yield spectrum of each atomic species consists of asymmetric features from the 2s−1np autoionizing resonance series below the 2s ionization limit and several weak features from the 2s−13s−1npn′s shake-up processes above the ionization limit. The observed resonances were analysed by multi-configuration Dirac–Fock calculation and by least-squares fitting to the Beutler–Fano profile. The decay processes of the resonance states are discussed. The Na 2s−13p resonance can end up with Na2+ ions by one-step double autoionization with branching ratio about 0.05.