Ionization Regime Research Articles

Strong-Field Ionization Phenomena Revealed by Quantum Trajectories.

We investigate the photoionization dynamics of atoms subjected to intense, ultrashort laser pulses through the use of quantum trajectories. This method provides a unique and consistent framework for examining electron dynamics within a time-dependent potential barrier. Our findings demonstrate that quantum trajectories offer additional insights into several key aspects of strong-field ionization, including the transition between ionization regimes, nonadiabatic effects under the barrier, the impact of the shape of the electronic potential, and the efficiency of over-the-barrier ionization.

Anisotropic effects in the nondipole relativistic photoionization of hydrogen

In the nonrelativistic and dipole regime of multiphoton ionization, spherical symmetry in all but the polarization direction of the laser pulse ensures that directional dependency in the photoelectron spectra is limited to the laser polarization direction, with the final distribution exhibiting no asymmetry along the propagation direction of the laser. When relativistic effects and spatial dependency in the external potential are accounted for however, the addition of time dilation and radiation pressure both impose anisotropic effects. Previously we have found that nondipole effects induce a redshift in the photoelectron energy distribution, while conversely relativistic effects induce a blueshift, with the net effect of an apparent near-cancellation of the two. In this work we study these effects further. By examining photoelectron momentum distributions acquired from simulations with the time-dependent Dirac equation we propose explanatory models for both phenomena and present a simplified model of the shifts as a function of the angle relative to the propagation direction of the laser pulse. It is found that both nondipole and relativistic effects must be accounted for on an equal footing in order to correctly describe the photoelectron momentum distribution in the high-intensity regime.

Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS)

We present the Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS) project, designed for the spatial and spectral study of the diffuse ionised gas (DIG) in a selection of nearby spiral galaxies observed with the MUSE integral-field spectrograph. Our primary objective is to investigate the various ionisation mechanisms at play within the DIG. We analysed the distribution of high- and low-ionisation species in the optical spectra of the sample on a spatially resolved basis. We introduced a new methodology for spectroscopically defining the DIG, optimised for galaxies of different resolutions. Firstly, we employed an innovative adaptive binning technique on the observed datacube based on the spectroscopic signal-to-noise ratio (S/N) of the collisional [S II] line to increase the S/N of the rest of the lines including [O III], [O I], and He I. Subsequently, we created a DIG mask by eliminating the emissions associated with both bright and faint H II regions. We also examined the suitability of using Hα equivalent width (EWHα) as a proxy for defining the DIG and its associated ionisation regime. Notably, for EWHα < 3 Å – the expected emission from hot low-mass evolved stars (HOLMES) – the measured value is contingent on the chosen population synthesis technique performed. Our analysis of the showcase sample reveals a consistent cumulative DIG fraction across all galaxies in the sample, averaging around 40%–70%. The average radial distribution of the [N II]/Hα, [S II]/Hα, [O I]/Hα, and [O III]/Hβ ratios are enhanced in the DIG regimes (up to 0.2 dex). It follows similar trends between the DIG regime and the H II regions, as well as the Hα surface brightness (ΣHα), indicating a correlation between the ionisation of these species in both the DIG and the H II regions. The DIG loci in typical diagnostic diagrams are found, in general, within the line ratios that correspond to photoionisation due to the star formation. There is a noticeable offset correspondent to ionisation due to fast shocks. However, an individual diagnosis performed for each galaxy reveals that all the DIG in these galaxies can be attributed to photoionisation from star formation. The offset is primarily due to the contribution of Seyfert galaxies in our sample, which is closely aligned with models of ionisation from fast shocks and galactic outflows, thus mimicking the DIG emission. Our results indicate that galaxies exhibiting active galactic nucleus (AGN) activity should be considered separately when conducting a general analysis of the DIG ionisation mechanisms, since this emission is indistinguishable from high-excitation DIG.

Entanglement in photo-ionization process.

We report a study of the entanglement between the quantized photon field and an atom arising in the photo-ionization process. Our approach is based on an ab initio solution of the time-dependent Schrödinger equation (TDSE) describing the quantum evolution of a bipartite system consisting of the atom and the quantized electromagnetic field. Using the solution of the TDSE, we calculate the reduced photon density matrix, which we subsequently use to compute entanglement entropy. We explain some properties of the entanglement entropy and propose an approximate formula for the entanglement entropy based on the analysis of the density matrix and its eigenvalues. We present the results of a comparative study of the entanglement in the photo-ionization process for various ionization regimes, including the tunneling and the multiphoton ionization regimes.

Fine 3D control of THz emission in air with dual femtosecond laser pre-pulses at tunnelling ionisation regime

Fine 3D control of THz emission in air with dual femtosecond laser pre-pulses at tunnelling ionisation regime

Ultrafast dynamics of fluorene initiated by highly intense laser fields.

We present an investigation of the ultrafast dynamics of the polycyclic aromatic hydrocarbon fluorene initiated by an intense femtosecond near-infrared laser pulse (810 nm) and probed by a weak visible pulse (405 nm). Using a multichannel detection scheme (mass spectra, electron and ion velocity-map imaging), we provide a full disentanglement of the complex dynamics of the vibronically excited parent molecule, its excited ionic states, and fragments. We observed various channels resulting from the strong-field ionization regime. In particular, we observed the formation of the unstable tetracation of fluorene, above-threshold ionization features in the photoelectron spectra, and evidence of ubiquitous secondary fragmentation. We produced a global fit of all observed time-dependent photoelectron and photoion channels. This global fit includes four parent ions extracted from the mass spectra, 15 kinetic-energy-resolved ionic fragments extracted from ion velocity map imaging, and five photoelectron channels obtained from electron velocity map imaging. The fit allowed for the extraction of 60 lifetimes of various metastable photoinduced intermediates.

Role of excitation in the electron rotational asymmetry in circularly polarized laser fields

We comparatively study electron rotational asymmetries in photoionization of excited hydrogen atoms and photodetachment of the fluorine anions in single circularly polarized laser fields by numerically solving the three-dimensional time-dependent Schrödinger equation. We first show that single-photon ionization (or detachment) is more favorable as the electron initially corotates with respect to the direction of the laser field. More interestingly, we systematically demonstrate that the electron rotational asymmetry of the excited hydrogen atom is completely contrary to that of the fluorine anion in the multiphoton ionization (or detachment) regime, which can be attributed to the role played by excited bound states by analyzing the total excitation probability in the excited hydrogen atom. Furthermore, we also confirm that the electron rotational asymmetry of and degenerate orbitals in the excited hydrogen atom changes when the ionization mechanism switches from three-photon to over-the-barrier.

Wavelength scaling of photoionization circular dichroism of atomic electron ring currents and its effect on photoelectron spin polarization

Circularly polarized orbitals in atoms are referred to as electron ring currents with a helical angular phase and thus manifest circular dichroism in photoionization by circularly polarized light fields. Opposite circular dichroism has been revealed in single-photon and nonadiabatic tunneling ionization regimes. The transition of the circular dichroism and its underlying mechanism are still not clear. Here, we systematically study the wavelength scaling of photoionization circular dichroism of electron ring currents. We present a universal description of the dichroic photoelectron energy spectra and the spin polarization from the infrared to the extreme ultraviolet regions. We reveal that the channel interference and competition via the intermediate states with different angular quantum numbers in a few-photon ionization regime give rise to the transition of the circular dichroism and the spin polarization of photoelectrons.

Exceeding the Boundaries of the Paraxial Spatiotemporal Nonlinear Optics and Filamentation for Ultrashort Laser Pulses.

An impressive phenomenon of observed plasma instability and conical emission under the propagation of ultrashort laser pulses in the air is reported. The discussed novel findings demonstrating nonlinear effects are incapable to be explained in the standard spatiotemporal paraxial optics. Three main mechanisms are investigated. The first one is related to the nonlinear nonparaxial mechanisms for waveguiding of femtosecond pulses, and the second one considers the mechanism of single filament formation at weak ionization. The third mechanism demonstrates a new physical effect leading to collision ionization with intensities in the range of 1010-1011 W/cm2. Furthermore, a new ionization regime of instability is suggested at intensities below the critical thresholds for multiphoton and tunnel ionization. The experimental results and findings are supported by theoretical analyses and numerical simulations.

Strong field non-Franck–Condon ionization of H$$_2$$: a semi-classical analysis

Single ionization of H $$_2$$ molecules exposed to strong and short laser pulses is investigated by a semi-classical method. Three laser characteristics are considered: (i) The carrier-wave frequency corresponds to wavelengths covering and bridging the two ionization regimes: From tunnel ionization (TI) at 800 nm to multiphoton ionization (MPI) at 266 nm. (ii) Values of the peak intensity are chosen within a window to eliminate competing double ionization processes. (iii) Particular attention is paid to the polarization of the laser field, which can be linearly or circularly polarized. The results and their interpretation concern two observables, namely the end-of-pulse total ionization probability and vibrational distribution generated in the cation H $$_2^+$$ . The most prominent findings are an increased ionization efficiency in circular polarization and a vibrational distribution of the cation that favors lower-lying levels than those that would be populated in a vertical (Franck–Condon) ionization, leading to non-Franck–Condon distributions, both in linear and circular polarizations.

Radiation forces and confinement of neutral particles into the pulse envelope. New regime of collision ionization

For a long time, transverse optical forces with continuous-wave laser beams have been used for non-contact manipulation of micro-scale objects. By replacing the continuous-wave lasers with pulse-mode lasers many novel physical effects related with the influence of Poynting vector on the longitudinal part of the optical force can be observed. The following question arises: What is the impact of this longitudinal addition to the optical force, connected with the flow of energy, on the ensemble of thousand particles in continuous media? The aim of this work is to find analytical expressions of the radiation force and potential densities arising from a laser pulse propagating in a dielectric media. This allows us to find an effective averaged longitudinal real force at the level of the laser pulse’s spot. The obtained force is proportional to the initial pulse energy and inversely proportional to its time duration. In the femtosecond region the force becomes strong enough to confine neutral particles into the pulse envelope and translate them with the group velocity in gases. Additionally, if the trapped particles into the pulse’s spot are with high density, the probability of collision with the free atoms and molecules in air become significant. The collision energies are in the range of 12–24 GeV and high enough to ionize the neutral atoms. Thus, a new type of collision ionization can be observed, when powerful femtosecond pulses propagate in gaseous media.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements.

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 1013 cm-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

Multiphoton ionization of standard optical fibers

Atoms ionization by the simultaneous absorption of multiple photons has found applications in fiber optics, where it leads to unique nonlinear phenomena. To date, studies of the ionization regime have been limited to gas-filled hollow-core fibers. Here, we investigate multiphoton ionization of standard optical fibers, where intense laser pulses ionize the atoms constituting the fiber structure itself, instead of that of the filling gas. We characterize material modifications produced by optical breakdown. Their formation affects laser beam dynamics over hours long temporal scales. The damage features are studied by means of optical microscopy and X-ray microtomography. In the framework of glass photonics, our results pave the way for a novel glass waveguide micromachining technique.

Anomalous Ionization Regime in a Forced Electron Beam Induced Arc Discharge Ion Source for Singly Charged Radioactive Ion Beam Production

At TRIUMF’s ISAC facility, 1+ Radioactive Ion Beams (RIBs) of noble gases, halogens, and molecules are created using a Forced Electron Beam Induced Arc Discharge (FEBIAD) ion source. Reported ionization efficiencies for FEBIADs range from 10% to 25% for 40Ar+, while TRIUMF-FEBIAD ionization efficiency seems to be < 1% with a 90% emittance <15 μm. As RIB ion sources aim for a high ionization efficiency, an experimental and numerical campaign was conducted to investigate the comparably low efficiencies observed. The experimental results for 40Ar+ indicate that up to a 10% ionization efficiency is possible by operating the source at different parameters. The measurements agree with theoretical estimations; however, certain combinations of parameters produce an anomalously high electron current which enhances the ionization efficiency threefold. Present investigations aim to characterize and model the anomalous electron current because, if proven reliable, the argon ionization efficiency could reach 30% with no significant impact on beam emittance.

Laser-Induced Electron Fresnel Diffraction in Tunneling and Over-Barrier Ionization

Photoelectron momentum distribution in strong-field ionization has a variety of structures that reveal the complicated dynamics of this process. Recently, we identified a low-energy interference structure in the case of a super-intense extreme ultraviolet (XUV) laser pulse and attributed it to the laser-induced electron Fresnel diffraction. This structure is determined by the laser-induced electron displacement [Geng et al. Phys. Rev. A 104 (2021) L021102]. In the present work, we find that the Fresnel diffraction picture also appears in the tunneling and over-barrier regime of ionization by short pulses. However, the electron displacement is now induced by the electric field component of the laser pulse rather than the magnetic field component in the case of the super-intense XUV pulse. After corresponding modifications to our quantum and semiclassical models, we find that the same physical mechanism of the Fresnel diffraction governs the low-energy interference structures along the laser polarization. The results predicted by the two models agree well with the accurate results from the numerical solution to the time-dependent Schrödinger equation.

Electron Dynamics and Correlations During High-Order Harmonic Generation in Be.

We investigate theoretically the high-order harmonic generation in beryllium atom irradiated by a short 1850 nm linearly polarized laser pulse in the intermediate strong-field ionization regime with the Keldysh parameter of 0.85. To this end, the respective time-dependent Schrödinger equation is solved by the time-dependent restricted-active-space configuration-interaction (TD-RASCI) method. By systematically increasing the active space of included configurations, we demonstrate an individual effect of different physical processes evoked by the pulse, which, all together, significantly enrich and extend the computed high-order harmonic generation spectrum.

Evolution of the frequency-comb structure and coherence from a Keldysh multiphoton into a tunneling regime.

We present a theoretical study of the characteristics of the frequency-comb structure and coherence via high-order harmonic generation (HHG) driven by the laser pulse trains when the ionization process is pushed from Keldysh multiphoton into tunneling regime. HHG is obtained by solving accurately the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We find that the nested comb structures are formed from each harmonic order in the Keldysh multiphoton ionization regime. But it is severely suppressed or even disappeared in the Keldysh tunneling ionization regime. It implies that the temporal coherence of the emitted frequency comb modes is very sensitive to the Keldysh ionization regime. To understand the evolution of frequency-comb structure and coherence, we perform the calculation of the time-dependent ionization probability and the spectral phase of frequency-comb HHG. We find that the frequency-comb HHG driven by the laser pulse trains in the Keldysh multiphoton regime has a good coherence because the ionization probability of the atom driven by each laser pulse is stable, leading to a phase-coherent frequency-comb structure rather than those cases in the Keldysh tunneling regime with high laser intensity. Our results shed light on current interest and significance to the experimental realization of controllable and frequency-comb vacuum-ultraviolet light sources.

Propensity rules for photoelectron circular dichroism in strong field ionization of chiral molecules.

Chiral molecules ionized by circularly polarized fields produce a photoelectron current orthogonal to the polarization plane. This current has opposite directions for opposite enantiomers and provides an extremely sensitive probe of molecular handedness. Recently, such photoelectron currents have been measured in the strong-field ionization regime, where they may serve as an ultrafast probe of molecular chirality. Here we provide a mechanism for the emergence of such strong-field photoelectron currents in terms of two propensity rules that link the properties of the initial electronic chiral state to the direction of the photoelectron current.

Frequency shiftand control ofhigh-order harmonicsof H atom driven by anasymmetric laser pulse

A scheme of the large frequency shift for high-order harmonic generation (HHG) produced by atomic gas driven by an asymmetric laser pulse is proposed in the tunneling ionization regime. By numerically solving the three-dimensional time-dependent Schrodinger equation in the dipole approximation, we theoretically investigate the characteristics of HHG emitted from hydrogen atom driven by the laser pulse with different rising and falling times. Our results show that the HHG spectra of atomic H in cutoff region present a strong redshift and blueshift. The shift can be adjusted by varying the rising time or falling time of the laser pulse. The time frequency analysis, reveals that the reason for the frequency shift comes from different contributions from the rising time or falling time in the asymmetric laser pulse. If the contributed harmonics during the falling time is larger than that during the falling time, the red shift of HHG occurs. otherwise the blue shift appears. Therefore, by shaping the laser pulse waveform, the frequency of atomic HHG for a given order in the cutoff region in the tunneling ionization regime is tunable, which can cover the frequency range from the odd order to the adjacent even order.

High‐Harmonic Generation from 2D Monolayer Electrides

AbstractThis work reports high‐harmonic generation (HHG) from 2D monolayer electrides investigated by using ab‐initio approach based on time‐dependent density functional theory, mainly focusing on monolayer Mg2N. Due to low ionization energy and wide potential well, they exhibit distinctively high HHG efficiency and high cutoff energy for out‐of‐plane pumping, although they also show atomic‐like behavior. For out‐of‐plane pumping with a peak intensity of 40 TW cm−2 at 1550 nm, high harmonics up to 120th order are generated with efficiency 4 orders‐of‐magnitude higher than monolayer h‐BN. For pump intensity below the direct above‐threshold ionization (ATI) regime, the tunneling from the ground state and ATI from one‐ or two‐photon resonant state simultaneously contribute to HHG. In the other subnitride monolayers of alkaline earth with larger atomic sizes (Ca2N and Sr2N), further higher harmonic generation efficiencies are obtained due to the lower work functions and wider potential wells. For in‐plane pumping, the intraband transition is the major source of the HHG due to the metallic in‐plane responses. The presented results will attract great attention due to the peculiar HHG signature from fundamental aspects and pave the way of obtaining coherent extreme ultraviolet (EUV)/soft‐X‐ray with high efficiency by using monolayer electrides.