Barrier Suppression Ionization Research Articles

The Influence of the Effective Ionization Potential and Initial Momentum on the Barrier-Suppression Ionization Rate in a Circularly Polarized Laser Field

The Influence of the Effective Ionization Potential and Initial Momentum on the Barrier-Suppression Ionization Rate in a Circularly Polarized Laser Field

Transient Laser-Induced Breakdown of Dielectrics in Ultrarelativistic Laser-Solid Interactions

For high-intensity laser-solid interactions, the absolute density and surface density gradients of the target at the arrival of the ultrarelativistic laser peak are critical parameters. Accurate modeling of the leading edge-driven target preexpansion is desired to strengthen the predictive power of associated computer simulations. The transition from an initial solid state to a plasma state, i.e., the breakdown of the solid, defines the starting point of the subsequent target preexpansion. In this work, we report on the time-resolved observation of transient laser-induced breakdown (LIB) during the leading edge of high-intensity petawatt-class laser pulses with peak intensities of up to 5.7×1021W/cm2 in interaction with dielectric cryogenic hydrogen jet targets. LIB occurs much earlier than what is typically expected following the concept of barrier suppression ionization. The observation is explained by comparing a characterization study of target-specific LIB thresholds with laser contrast measurements. The results demonstrate the relevance of the laser pulse duration dependence of LIB for high-intensity laser-solid interactions. We provide an effective approach to determine the onset of LIB and thereby the starting point of target preexpansion in other laser-target systems.Received 28 July 2022Revised 2 December 2022Accepted 16 December 2022DOI:https://doi.org/10.1103/PhysRevApplied.19.014070Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsLight-matter interactionPlasma production & heating by laser beams, laser-foil, laser-clusterPhysical SystemsDielectricsRelativistic plasmasTechniquesFemtosecond laser irradiationLaser ablationOptical plasma measurementsPlasma PhysicsCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

The role of excited discrete atomic states in the formation of quantum vortices in barrier-suppression ionization of hydrogen-like atom by ultrashort laser pulse

We theoretically examine the quantum vortices creating through barrier-suppression ionization of two dimensional hydrogen atom by ultrashort laser pulse. We show that for the considered case of ionization, the excited discrete atomic states do not impact on the vortex formation, and the problem can be considered in the single-level atom approximation.

High-order harmonic generation in H2+ via multicolor beam superposition: barrier suppression ionization regime

Generation of high-order harmonics (HOHs) and attosecond pulses in the barrier suppression ionization regime for a target of hydrogen molecular ion, $${\text{H}}_{2}^{+}$$ , with large inter nuclear distance is introduced in this work. Some broadband femtosecon laser pulses are designed based on multicolor beam superposition method in order to interact with $${\text{H}}_{2}^{+}$$ . Behavioral study of harmonics cutoff, attosecond pulse widths and intensities, along with the maximum ionization intensity with respect to the spectral width of driving pulses was carried out. Molecular orbitals in this study are estimated via the linear combination of atomic orbital approximation, and temporal variations of these orbitals are studied in the various ionization regimes. It is seen in this work that although the cutoff harmonic is increased by the driving pulse intensity, but it is insensitive to the spectral driving pulsewidth. This study showed that intensities of the generated attosecond pulses gradually decreased by increasing the bandwidth of the driving laser pulse, but the duration of them is shown to be insensitive in this respect.

Magnetic quantum number dependence of hydrogen photoelectron spectra under circularly polarized pulse in barrier suppression ionization regime

Photoelectron spectra and transition mechanisms of a hydrogen atom in the n = 2 state under circularly polarized pulses in the barrier suppression ionization (BSI) regime, found by solving the time-dependent Schrödinger equation, are presented in detail. The photoelectron peak emerges from low-energy, in contrast to being centered at higher energy in the case of ionization from the ground state. We show that the magnetic quantum number dependence of photoelectron angular distributions on initial states yields 3- and 2-band structures for 2p− and 2p0 in zenith angle, respectively, and 1-band structures for both 2p+ and 2s. The intermediate transition pathways from these initial states to the corresponding final states are also revealed.

Coupled-cluster treatment of molecular strong-field ionization.

Ionization rates and Stark shifts of H2, CO, O2, H2O, and CH4 in static electric fields have been computed with coupled-cluster methods in a basis set of atom-centered Gaussian functions with a complex-scaled exponent. Consideration of electron correlation is found to be of great importance even for a qualitatively correct description of the dependence of ionization rates and Stark shifts on the strength and orientation of the external field. The analysis of the second moments of the molecular charge distribution suggests a simple criterion for distinguishing tunnel and barrier suppression ionization in polyatomic molecules.

Theoretical derivation of laser-dressed atomic states by using a fractal space

The derivation of approximate wave functions for an electron submitted to both a Coulomb and a time-dependent laser electric fields, the so-called Coulomb-Volkov (CV) state, is addressed. Despite its derivation for continuum states does not exhibit any particular problem within the framework of the standard theory of quantum mechanics (QM), difficulties arise when considering an initially bound atomic state. Indeed the natural way of translating the unperturbed momentum by the laser vector potential is no longer possible since a bound state does not exhibit a plane wave form explicitly including a momentum. The use of a fractal space permits to naturally define a momentum for a bound wave function. Within this framework, it is shown how the derivation of laser-dressed bound states can be performed. Based on a generalized eikonal approach, a new expression for the laser-dressed states is also derived, fully symmetric relative to the continuum or bound nature of the initial unperturbed wave function. It includes an additional crossed term in the Volkov phase which was not obtained within the standard theory of quantum mechanics. The derivations within this fractal framework have highlighted other possible ways to derive approximate laser-dressed states in QM. After comparing the various obtained wave functions, an application to the prediction of the ionization probability of hydrogen targets by attosecond XUV pulses within the sudden approximation is provided. This approach allows to make predictions in various regimes depending on the laser intensity, going from the non-resonant multiphoton absorption to tunneling and barrier-suppression ionization.

Search for the best analytical formula in the tunneling and the barrier-suppression ionization regimes

In this work we derive an analytical formula for the ionization rate of the hydrogen atom in its ground state in the strong linearly polarized laser field. The derivation utilizes Keldysh-like probability amplitude in the length gauge in the dipole approximation including Coulomb effects in the final state of the ionized electron. We apply a properly chosen pre-exponential factor into the Gordon–Volkov wave function. We compare our theory with many previously known numerical or theoretical results for the field parameters for which (the laser frequency is less than the binding energy of the atom; usually ) and ( is the peak laser field). Thus, we consider both the tunneling and the barrier-suppression ionization regimes. It appears that in most cases ionization rates computed here are very close to those obtained long ago within the Perelomov, Popov and Terent’ev theory or its later modification by Popruzhenko et al. However, including Coulomb effects in the final state of the ionized electron in the present Keldysh-like theory leads to non-trivial changes (in relation to ordinary Keldysh theory without these Coulomb effects) in photoelectron energy spectra or angular distributions.

Laser Field Ionization Rates in the Barrier-Suppression Regime

We analyze the influence of ponderomotive and Stark shifts on the barrier suppression transition rate for noble and alkali atoms for a low-frequency linearly-polarized laser field. We consider three approaches to the barrier suppression ionization (BSI) rate: through a critical field, an ion charge, and the Airy function. We show that ponderomotive and Stark shifts affect the rate differently in these three approaches. Generally, the rate curve of K is more strongly lowered than the one of Ar.

Pre-excitation studies for rubidium-plasma generation

The key element in the Proton-Driven-Plasma-Wake-Field-Accelerator (PWFA) project is the generation of highly uniform plasma from Rubidium vapor. A scientifically straightforward, yet highly challenging way to achieve full ionization is to use high power laser which can assure the barrier suppression ionization (BSI) along the 10m long active region. The Wigner-team in Budapest is investigating an alternative way of uniform plasma generation. The proposed Resonance Enhanced Multi-Photon Ionization (REMPI) scheme can be probably realized by much less laser power. In the following we plan to investigate the resonant pre-excitations of the Rb atoms, both theoretically and experimentally. In the following our theoretical framework is presented together with the status report about the preparatory work of the planned experiment.

Importance of quantum orbits in continuum–continuum harmonic generation

The ab initio trajectory analysis in the few cycle laser pulse presents a clear contribution of all quantum orbits in the continuum–continuum harmonic generation via interference of associated wave-packets. Based on the empirical formulas, the relative intensities of these harmonics are estimated which are in good agreement with the results of simulation. Using time-frequency analysis and by means of the classical model, the role of higher-order return trajectories in an intense infrared laser can clearly be seen in the barrier-suppression ionization regime.

Transition limit regimen between multiphoton and barrier suppression ionization. Analysis of Benzene, Coronene and Circumcoronene

Transition limit between multiphoton and total barrier suppression as a function of the electric field of radiation at intensities of the order of 109 to 1013 W.cm−2 and the molecular sizes was analyzed.

Inner ionization mechanisms of methane clusters driven by an ultrashort intense laser pulse

By use of three-dimensional particle dynamics simulations, the explosion dynamics of methane clusters driven by an ultrashort intense laser pulse has been analyzed. Especially the influences of some classical inner ionization mechanisms on the charged state and kinetic energy of the ions produced in the laser-cluster interaction process have been discussed in detail. It is presented that when the laser intensity is low, the atoms are ionized by the laser field mainly through the tunnel ionization while the barrier suppression ionization dominates for the inner ionization as the laser intensity increases. Under the irradiation of the same laser pulse with high intensity, the cluster can attain highly-ionized charged states by the barrier suppression ionization rather than by the tunnel ionization. After the cluster is ionized, the ignition ionization effect from the inner Coulomb field of the cluster and the electron impact ionization effect from the free electrons resided inside the cluster will facilitate the process that the cluster can be ionized further.

Extension of the cut-off in high-harmonic generation using two delayed pulses of the same colour

We demonstrate a novel technique for an extension of the cut-off frequency in high-harmonic generation using a shaped field, consisting of two delayed pulses, identical in all parameters except a carrier envelope phase shift of π. We obtain high harmonics up to an energy of 5.5Up + Ip, surpassing the conventional limit of 3.17Up + Ip. For high intensities approaching the threshold for barrier suppression ionization, beyond which no harmonics are generated, this mechanism acquires practical importance as a means to overcome the limit imposed by saturation. We analyse the underlying mechanism using both classical and quantum approaches, and highlight the role of the shaped field in modifying the electron trajectories in the continuum to enhance the collection of kinetic energy from the driving field. Of greatest importance are the trajectories describing electrons ionized across the first peak of the laser field which receive the enhanced kinetic energies responsible for the cut-off extension above the conventional limit. Interestingly, the most energetic trajectories do not contribute to the harmonic cut-off in the single pulse case. We conclude with a discussion of the sub-cycle structure of the emitted harmonic radiation and possible applications of the technique.

Landau-Dykhne approach for relativistic electron momentum and angular distributions for the ionization of multicharged atomic ions by superintense laser fields

Analytic expressions are obtained for relativistic electron momentum and angular distributions in the tunneling and barrier-suppression ionization of multicharged atomic ions by a superintense linearly polarized field. The Landau-Dykhne approach is used for derivation of these distributions. The condition for applicability of this adiabatic approximation requires that an ejected electron should have sufficiently high kinetic energy so that the ionization probability should be exponentially small. The electron momentum can be both less and larger than the relativistic quiver momentum. In the vicinity of the quiver momentum we found a new peculiarity: sharp cusp in the momenum distribution along the polarization of laser radiation. It is absent at the nonrelativistic ionization of atomic ions by moderate laser fields. The electron momentum spectra along the laser electric field and along the laser magnetic field do not depend on the sign of momentum. In contrast, the relativistic momentum spectra along the direction of propagation of laser radiation and along in the opposite sense are significantly different.

Single attosecond burst generation during ionization of excited atoms by intense ultrashort laser pulses

We develop an analytical approach to describing the generation of a single attosecond burst during barrier-suppression ionization of a hydrogen atom by an intense laser pulse. We derive analytical expressions that describe the evolution of the electron wave packet in the time interval between the detachment from the atom and the collision with the parent ion for an arbitrary initial atomic state by assuming the atom to be fully ionized in one laser-field half-period. For various s-states, we derive expressions for the profile of the attosecond burst generated when the electron packet collides with the ion and analyze the dependence of its generation efficiency on the principal quantum number n of the initial atomic state. The results obtained are compared with the results of three-dimensional numerical calculations. We show that the attosecond pulse generation efficiency can be several orders of magnitude higher than that in the case of ionization from the ground state when pre-excited atomic states are used.

The influence of ion gratings on rotational wavepacket dynamics in H 2

We generalize earlier work [V.G. Stavros, E. Harel, S.R. Leone, J. Chem. Phys. 122 (2005) 064301] by illustrating the plausible role of ion gratings in time-dependent degenerate four-wave mixing (TD-DFWM) experiments in H 2. We postulate that at high laser intensities (10 14–10 15 W/cm 2), H 2 + /H + ions generate a static ion grating, it’s signature manifested in the transformation from homodyne- to heterodyne-detection of the TD-DFWM signal, depending on laser intensity. The change in signal detection agrees with the calculated intensity for barrier suppression ionization (BSI) in H 2 and the reported onset of saturation for H 2 + and H +, pointing towards the likely role of ion gratings in intense laser field, FWM experiments.

Extreme multielectron ionization of elemental clusters in ultraintense laser fields

AbstractIn this paper we present computational and theoretical studies of extreme multielectron ionization in Xen clusters (n = 55‐2171, initial cluster radii R0 = 8.7‐31.0 Å) driven by ultraintense Gaussian infrared laser fields (peak intensity IM = 1015‐1020 W cm−2, temporal pulse length τ = 10‐100 fs, and frequency v = 0.35fs−1). The microscopic approach, which rests on three sequential‐parallel processes of inner ionization, nanoplasma formation, and outer ionization, properly describes the high ionization levels (with the formation of {Xeq+}n with q = 5‐36), the inner/outer cluster ionization mechanisms, and the nanoplasma response. The cluster size and laser intensity dependence of the inner ionization levels are determined by a complex superposition of laser‐induced barrier suppression ionization (BSI), with the contributions of the inner field BSI manifesting ignition enhancement and screening retardation effects, together with electron impact ionization. The positively charged nanoplasma produced by inner ionization reveals intensity‐dependent spatial inhomogeneity and spatial anisotropy, and can be either persistent (at lower intensities) or transient (at higher intensities). The nanoplasma is depleted by outer ionization that was semiquantitatively described by the cluster barrier suppression electrostatic model, which accounts for the cluster size, laser intensity, and pulse length dependence of the outer ionization yield.

Extreme ionization of Xe clusters driven by ultraintense laser fields

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xe(n) clusters (n=2-2171, initial cluster radius R(0)=2.16-31.0 A) driven by ultraintense infrared Gaussian laser fields (peak intensity I(M)=10(15)-10(20) W cm(-2), temporal pulse length tau=10-100 fs, and frequency nu=0.35 fs(-1)). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of [Xe(q+)](n) with q=2-36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (I(M)=10(18)-10(20) W cm(-2)) inner ionization is dominated by BSI. At lower intensities (I(M)=10(15)-10(16) W cm(-2)), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the [Xe(q+)](n) ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe(2171) at tau=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at I(M)=10(15)-10(16) W cm(-2) establishes an ultraintense laser pulse length control mechanism of extreme ionization products.

Control of cluster multielectron ionization in ultraintense laser fields

We performed classical molecular dynamics simulations to explore the controllability of the inner ionization process in Xen clusters (n = 2−2171), driven by ultraintense infrared Gaussian laser fields (peak intensity IM = 1015−1018 W cm−2, temporal pulse length τ = 10−100 fs, and frequency ν = 0.35 fs−1). Controllability of ion charge abundances and of their spatial distributions inside the cluster emerges from the different pulse length dependences of classical barrier suppression ionization (BSI) and of electron impact ionizations (EII), as well as from the time scale of the Coulomb explosion (CE). For large clusters (Xe2171), low intensities (1015 W cm−2), and long pulses (τ = 100 fs), EII is the dominating ionization channel, which favors the formation of maximum charged ions (Xe10+, Xe11+) in the cluster center. In contrast, BSI forms an inverse radial charge ordering with the highest charges in the exterior cluster shells. This suggests that the production of the two inverse radial charge distributions with an equal average ion charge can be forced by the choice of multiple pulses with different intensities and pulse lengths. At high intensities (1017−1018 W cm−2), where EII is insignificant and CE sets in much earlier, the BSI radial charge ordering and the enhancement of the ion charges beyond the single-atom limit by the ignition effect is observed only for short pulses.