Ionization Dynamics Research Articles

Tracing the origins of an asymmetric momentum distribution for polar molecules in strong linearly polarized laser fields

We study the ionization dynamics of oriented ${\mathrm{HeH}}^{+}$ in strong linearly polarized laser fields by numerically solving the time-dependent Schr\"odinger equation. The calculated photoelectron momentum distributions for parallel orientation show a striking asymmetric structure. With a developed model pertinent to polar molecules, we trace the electron motion in real time. We show that this asymmetric structure arises from the interplay of the Coulomb effect and the permanent dipole in strong laser fields. This structure can be used to probe the degree of orientation which is important in ultrafast experiments for polar molecules. We also check our results for other polar molecules such as CO and BF.

Spectral control via multi-species effects in PW-class laser-ion acceleration

Laser-ion acceleration with ultra-short pulse, petawatt-class lasers is dominated by non-thermal, intra-pulse plasma dynamics. The presence of multiple ion species or multiple charge states in targets leads to characteristic modulations and even mono-energetic features, depending on the choice of target material. As spectral signatures of generated ion beams are frequently used to characterize underlying acceleration mechanisms, thermal, multi-fluid descriptions require revision for predictive capabilities and control in next-generation particle beam sources. We present an analytical model with explicit inter-species interactions, supported by extensive ab initio simulations. This enables us to derive important ensemble properties from the spectral distribution resulting from these multi-species effects for arbitrary mixtures. We further propose a potential experimental implementation with a novel cryogenic target, delivering jets with variable mixtures of hydrogen and deuterium. Free from contaminants and without strong influence of hardly controllable processes such as ionization dynamics, this would allow a systematic realization of our predictions for the multi-species effect.

Influence of tunnel ionization to third-harmonic generation of infrared femtosecond laser pulses in air

Here we present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5 ,upmu hbox {m}. At low intensities, longitudinal phase matching is dominating in the formation of 3rd harmonics, whereas at higher intensities locked X-waves are formed. We provide the arguments that the X-wave formation is governed mainly by the tunnel-like ionization dynamics rather than by the multiphoton one. Despite of this fact, the impact of the ionization-induced nonlinearity is lower than the one from bound–bound transitions at all intensities.

Amplification by stimulated emission of nitrogen-vacancy centres in a diamond-loaded fibre cavity

Abstract Laser threshold magnetometry using the negatively charged nitrogen-vacancy (NV−) centre in diamond as a gain medium has been proposed as a technique to dramatically enhance the sensitivity of room-temperature magnetometry. We experimentally explore a diamond-loaded open tunable fibre-cavity system as a potential contender for the realisation of lasing with NV− centres. We observe amplification of the transmission of a cavity-resonant seed laser at 721 nm when the cavity is pumped at 532 nm and attribute this to stimulated emission. Changes in the intensity of spontaneously emitted photons accompany the amplification, and a qualitative model including stimulated emission and ionisation dynamics of the NV− centre captures the dynamics in the experiment very well. These results highlight important considerations in the realisation of an NV− laser in diamond.

Modeling of capillary discharge plasmas for wakefield acceleration and beam transport

Next generation accelerators demand sophisticated beam sources to produce ultra-low emittances with large gradients. The subsequent beamline optics are equally critical to transporting these beams between accelerating stages or to interaction points. Capillary discharge plasmas may address each of these challenges. Capillaries have been demonstrated as sources capable of increasing the peak energy and beam quality of laser wakefield accelerators, and as active plasma lenses featuring orders-of-magnitude increases in peak magnetic field. These systems are sensitive to energy deposition, heat transfer, ionization dynamics, and magnetic field penetration; therefore, improved modeling will enable advances in capillary design. We present simulations of capillary discharge waveguides and active plasma lenses in using FLASH, a publicly-available multi-physics code in development at the University of Chicago. We report on the implementation of a 2D, cylindrically symmetric capillary model for capturing plasma density and temperature evolution with realistic conductivities and magnetic fields. We then illustrate the use of laser energy deposition to model low density channel formation for the matching and guiding of intense laser pulses. Lastly, we discuss simulations of active capillary plasmas with different fill species, which show agreement with experimental observations of nonlinearities in the current density profile and magnetic field.

Ionization of two-electron atom (xenon) studied by Bohmian mechanics theory* *Project supported by the Jilin Provincial Science and Technology Development Plan Program for Excellent Youth Talents, China (Grant No. 20180520174JH) and the National Natural Science Foundation of China (Grant Nos. 11704145, 11904050, 11774129, 11747007, 11534004, and 12074145).

The ionization dynamics of two-electron atom in an intense laser field is studied by the Bohmian mechanics (BM) theory, and the xenon atomic potential function is used as a model. The single ionization process and double ionization process are calculated by the BM theory and their results are in good agreement with those calculated by numerically solving the time-dependent Schrödinger equation. The analyses of the types, trajectories, and forces of Bohmian particles (BPs) undergoing the single and double ionizations indicate that the re-collision process accounts for a considerable proportion in the singly ionized cases. Furthermore, the analysis of the work done by the external force acting on the BPs shows that the quantum force plays an important role in the re-collision process. This work is helpful in understanding the ionization of two-electron atom in an intense laser field.

Field-Induced Tunneling Ionization and Terahertz-Driven Electron Dynamics in Liquid Water.

Liquid water at ambient temperature displays ultrafast molecular motions and concomitant fluctuations of very strong electric fields originating from the dipolar H2O molecules. We show that such random intermolecular fields induce the tunnel ionization of water molecules, which becomes irreversible if an external terahertz (THz) pulse imposes an additional directed electric field on the liquid. Time-resolved nonlinear THz spectroscopy maps charge separation, transport, and localization of the released electrons on a few-picosecond time scale. The highly polarizable localized electrons modify the THz absorption spectrum and refractive index of water, a manifestation of a highly nonlinear response. Our results demonstrate how the interplay of local electric field fluctuations and external electric fields allows for steering charge dynamics and dielectric properties in aqueous systems.

Population change of OH and H2O in water vapor glow discharge measured using concentration modulation spectroscopy

A distributed feedback laser with a wavelength of 2.8 µm was used to measure the species produced by water vapor glow discharge. Only the absorption spectra of OH radicals and transient H2O molecules were observed using concentration modulation (CM) spectroscopy. The intensities and orientations of the absorption peaks change with the demodulation phase, but the direction of one absorption peak of H2O is always opposite to the other peaks. The different spectral orientations of OH and H2O reflect the increase or the decrease of the number of particles in the energy levels. If more transient species can be detected in the discharge process, the dynamics of excitation, ionization, and decomposition of H2O can be better studied. This study shows that the demodulation phase relationship of CM spectrum can be used to study the population change of molecular energy levels.

Autoionization dynamics of helium nanodroplets resonantly excited by intense XUV laser pulses

The ionization dynamics of helium droplets irradiated by intense, femtosecond extreme ultraviolet (XUV) pulses is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited to atomic-like 2p states with a photon energy of 21.5 eV and autoionize by interatomic Coulombic decay (ICD). A complex evolution of the electron spectra as a function of droplet size (250 to 106 He atoms per droplet) and XUV intensity (109–1012 W cm−2) is observed, ranging from narrow atomic-like peaks that are due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of a numerical simulation based on rate equations taking into account all relevant processes—multi-step ionization, electronic relaxation, ICD, secondary inelastic collisions, desorption of electronically excited atoms, and collective autoionization (CAI).

Photoelectron ionization time of aligned molecules clocked by attosecond angular streaking

Attoclock is a powerful technique to resolve laser-driven electron dynamics on an attosecond time scale. Previously, it was mainly limited to atomic targets. Here we apply the attoclock technique to aligned molecules to study the subcycle ionization dynamics. By controlling the alignment of the molecule relative to the major axis of a strong elliptically polarized laser field, we demonstrate that the electron ionization time in aligned molecules can be precisely resolved by the attoclock technique. We find that the ionization time corresponding to the peak of the photoelectron momentum distribution shifts several tens of attoseconds relative to the laser field peak depending on the molecular orientation and molecular orbital structure. A significant difference between the measurement and molecular Ammosov-Delone-Krainov model is found. We further show that the photoelectron ionization time of molecules is sensitive to angular-dependent ionization rate, opening up opportunities for probing attosecond electron dynamics in complex polyatomic molecules.

Delayed ionization and excitation dynamics in a filament wake channel in a dense-gas medium

A unified theoretical description is developed for the formation of an ionized filament channel in a dense-gas medium and the evolution of electronic degrees of freedom in this channel in the laser pulse wake, as illustrated on an example of high-pressure argon. During the laser pulse, the emerging free electrons gain energy via inverse Bremsstrahlung on neutral atoms, enabling impact ionization and extensive collisional excitation of the atoms. A kinetic model of these processes produces the radial density distributions in the immediate wake of the laser pulse. After the pulse, the thermalized electron gas drives the system evolution via impact ionization (from the ground and excited states) and collisional excitation of the residual neutral atoms, while the excited atoms are engaged in Penning ionization. The interplay of these three processes determines the electron gas cooling dynamics. The local imbalance of the free-electron and ion densities induces a transient radial electric field, which depends critically on the electron temperature. The evolving radial profiles of the electron, ion, and excited-atom densities, as well as the profiles of electron temperature and induced electric field are obtained by solving the system of diffusion-reaction equations numerically. All these characteristics evolve with two characteristic timescales, and allow for measuring the electronic stage of the wake channel evolution via linear and nonlinear light-scattering experiments.

Analysis of the multielectron dynamics in intense laser-induced ionization of CO by the time-dependent effective potentials for natural orbitals

We simulated the multielectron dynamics of a CO molecule irradiated by near-IR λ = 760 nm two-cycle pulses with different carrier envelope phases by using the multiconfiguration time-dependent (TD) Hartree–Fock (MCTDHF) method. The ionization rate estimated from the simulations is higher when the laser electric field ɛ(t) points from C to O than in the opposite case, in agreement with the results of two-color experiments. The mechanism of the directional anisotropy in tunnel ionization of CO was examined by converting the obtained multielectron dynamics to the representation in terms of TD natural orbitals {ϕj(t)}. Within the framework of MCTDHF, we derived the equations of motion for {ϕj(t)}. From the derived equations, we defined the TD effective potentials that govern the dynamics of ϕj(t). consists of the one-body part v1(t) including the interaction with ɛ(t) and the two-body part v2,j(t) originating from electron–electron interaction. In for the 5σ HOMO, a narrow hump associated with the increase in v2,5σ(t) is surmounted on the field-distorted barrier of v1(t) + v2,5σ(0) near the nucleus C when ɛ(t) points from C to O, which results in the anisotropic ionization of CO. For 4σ, a high barrier that suppresses ionization is formed in when ɛ(t) points from C to O, which suggests that dynamical correlation exists between 4σ and 5σ electrons on route to ionization. For the opposite phase, becomes barrierless, enhancing high-harmonic generation through 4σ. We also simulated the dynamics for a λ = 380 nm pulse to investigate how reflects the nonadiabatic electronic response to the pulse. We found that the height of the hump in v1(0) + v2,5σ(t) is nearly proportional to the induced dipole moment of ϕ5σ(t), irrespective of whether the response is adiabatic or not. The for LiH is also presented to demonstrate the ubiquity of hump structures.

Atomic photoionization dynamics in ultrashort cycloidal laser fields

We present numerical simulations of ultrafast multiphoton ionization dynamics in a two-dimensional atomic model driven by co- and counterrotating circularly polarized single-color and bichromatic carrier envelope phase (CEP) stable ultrashort laser pulse sequences. Taking into account phase variations due to CEP fluctuations and the Gouy phase, our results accurately reproduce recently measured photoelectron momentum distributions [Pengel et al., Phys. Rev. Lett. 118, 053003 (2017), Phys. Rev. A 96, 043426 (2017); Kerbstadt et al., Nat. Comm. 10, 685 (2019), Adv. Phys. X 4, 1672583 (2019)]. The time evolution of the complex-valued electron wave function in coordinate and momentum space is calculated to study the bound state- and the vortex formation dynamics. The non-vanishing azimuthal probability current density proves the vortex nature of electron wave packets with odd-numbered rotational symmetry. Their angular momentum expectation value assumes half-integer values of 3.5 (corotating) and 0.5 (counterrotating). Knowledge of the wave function allows us to analyze the photoionization dynamics and to validate the physical pictures proposed in previous experimental studies. As an outlook, we investigate how electron vortices develop from the multiphoton- to the tunnel regime with increasing laser intensity.

Charged particle dynamics and distribution functions in low pressure dual-frequency capacitively coupled plasmas operated at low frequencies and high voltages

In high aspect ratio (HAR) dielectric plasma etching, dual-frequency capacitively coupled radio-frequency plasmas operated at low pressures of 1 Pa or less are used. Such plasma sources are often driven by a voltage waveform that includes a low-frequency component in the range of hundreds of kHz with a voltage amplitude of 10 kV and more to generate highly energetic vertical ion bombardment at the wafer. In such discharges, the energetic positive ions can overcome the repelling potential created by positive wall charges inside the etch features, which allows high aspect ratios to be reached. In order to increase the plasma density a high-frequency driving component at several 10 MHz is typically applied simultaneously. Under such discharge conditions, the boundary surfaces are bombarded by extremely energetic particles, of which the consequences are poorly understood. We investigate the charged particle dynamics and distribution functions in this strongly non-local regime in argon discharges by particle-in-cell simulations. By including a complex implementation of plasma-surface interactions, electron induced secondary electron emission (δ-electrons) is found to have a strong effect on the ionization dynamics and the plasma density. Due to the high ion energies at the electrodes, very high yields of the ion induced secondary electron emission (γ-electrons) are found. However, unlike in classical capacitive plasmas, these γ-electrons do not cause significant ionization directly, since upon acceleration in the high voltage sheaths, these electrons are too energetic to ionize the neutral gas efficiently. These γ- and δ-electrons as well as electrons created in the plasma bulk and accelerated towards the electrodes to high energies by reversed electric fields during the local sheath collapse are found to induce the emission of a high number of δ-electrons, when they hit boundary surfaces. This regime is understood fundamentally based on the following approach: first, dual-frequency discharges with identical electrode materials are studied at different pressures and high-frequency driving voltages. Second, the effects of using electrodes made of different materials and characterized by different secondary electron emission coefficients are studied. The electron dynamics and charged particle distribution functions at boundary surfaces are determined including discharge asymmetries generated by using different materials at the powered and grounded electrodes.

Surface processes in low-pressure capacitive radio frequency discharges driven by tailored voltage waveforms

Particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are performed to investigate the sputtering and the secondary electron emission (SEE) in geometrically symmetric capacitively coupled Ar discharges with Cu electrodes driven by tailored voltage waveforms (TVWs). The driving voltage waveform is composed of multiple consecutive harmonics (1 ⩽ N ⩽ 4) of the fundamental frequency (13.56 MHz) and is tailored by adjusting the identical phases (θ) of the even harmonics. The simulations are based on a discharge model in which realistic approaches are implemented for the description of the SEE induced by electrons and heavy-particles at the electrodes, as well as for the sputtering of the electrodes by heavy-particles. In case of applying a single frequency, the voltage amplitude is varied (250 V ⩽ ϕ1 ⩽ 2500 V), while in case of applying multi-frequency TVWs, N and θ are varied at a fixed total voltage amplitude (ϕtot = 1000 V) at a low pressure of 0.5 Pa. By applying more than one harmonic to drive the discharge, the mean energy of Ar+ ions and fast Ar atoms at the electrodes can be controlled by changing the phase angles. Due to the dependence of the sputtering yield on the heavy-particle energies, the flux of sputtered atoms can as well be controlled by the phase angles at both electrodes. The domain over which the sputtered atom flux can be varied is enlarged by adding more harmonics to the driving voltage waveform. For all conditions investigated, electron induced SEs (δ-electrons) induce strong ionization in the α-mode and dominate the ionization dynamics at high voltage amplitudes.

Origin of strong-field-induced low-order harmonic generation in amorphous quartz

Kerr-type nonlinearities form the basis for our physical understanding of nonlinear optical phenomena in condensed matter, such as self-focusing, solitary waves and wave mixing1–3. In strong fields, they are complemented by higher-order nonlinearities that enable high-harmonic generation, which is currently understood as the interplay of light-driven intraband charge dynamics and interband recombination4–6. Remarkably, the nonlinear response emerging from the subcycle injection dynamics of electrons into the conduction band, that is from ionization, has been almost completely overlooked in solids and only partially considered in the gas phase7–10. Here, we reveal this strong-field-induced nonlinearity in a-SiO2 as a typical wide-bandgap dielectric by means of time-resolved, low-order wave-mixing experiments, and show that, close to the material damage threshold, the so far unexplored injection current provides the leading contribution. The sensitivity of the harmonic emission to the subcycle ionization dynamics offers an original approach to characterize the evolution of laser-induced plasma formation in optical microprocessing. Strong-field-induced nonlinearities from the injection of electrons into the conduction band contribute to harmonic generation in amorphous quartz. Close to the damage threshold, they dominate over intraband and interband contributions.

Ultrafast Creation of Overlapping Rydberg Electrons in an Atomic BEC and Mott-Insulator Lattice.

We study an array of ultracold atoms in an optical lattice (Mott insulator) excited with a coherent ultrashort laser pulse to a state where single-electron wave functions spatially overlap. Beyond a threshold principal quantum number where Rydberg orbitals of neighboring lattice sites overlap with each other, the atoms efficiently undergo spontaneous Penning ionization resulting in a drastic change of ion-counting statistics, sharp increase of avalanche ionization, and the formation of an ultracold plasma. These observations signal the actual creation of electronic states with overlapping wave functions, which is further confirmed by a significant difference in ionization dynamics between a Bose-Einstein condensate and a Mott insulator. This system is a promising platform for simulating electronic many-body phenomena dominated by Coulomb interactions in the condensed phase.

The impact of electron–electron correlation in ultrafast attosecond single ionization dynamics

The attosecond ultrafast ionization dynamics of correlated two- or many-electron systems have, so far, been mainly addressed investigating atomic systems. In the case of single ionization, it is well known that electron–electron correlation modifies the ionization dynamics and observables beyond the single active electron picture, resulting in effects such as the Auger effect or shake-up/down and knock-up/down processes. Here, we extend these works by investigating the attosecond ionization of a molecular system involving correlated two-electron dynamics, as well as non-adiabatic nuclear dynamics. Employing a charge-transfer molecular model system with two differently bound electrons, a strongly and a weakly bound electron, we distinguish different pathways leading to ionization, be it direct ionization or ionization involving elastic and inelastic electron scattering processes. We find that different pathways result in a difference in the electronic population of the parent molecular ion, which, in turn, involves different subsequent (non-adiabatic) postionization dynamics on different time scales.

Angle-resolved nonresonant two-photon single ionization of argon using 9.3-eV photons produced via high-order harmonic generation

We present an experimental study on the photoionization dynamics of non-resonant one-color two-photon single valence ionization of neutral argon atoms. Using 9.3 eV photons produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and ions produced from non-resonant two-photon ionization in coincidence. Photoionization from the $3p$ orbital produces a photoelectron scattering wave function with $p$ and $f$ partial wave components, which interfere and result in a photoelectron angular distribution with peak amplitude perpendicular to the VUV polarization. The comparison between the present results and two previous sets of theoretical calculations [Pan, C. & Starace, A. F. (1991). $\textit{Physical Review A}$, 44(1), 324., and Moccia, R., Rahman, N. K., & Rizzo, A. (1983). $\textit{Journal of Physics B: Atomic and Molecular Physics}$, 16(15), 2737.] indicates that electron-electron correlation contributes appreciably to the two-photon ionization dynamics.

Fragmentation dynamics of positron-impact hydrogen ionization at near-threshold energies

Fragmentation dynamics of positron-impact hydrogen ionization at near-threshold energies