Anthony F. Starace PublicationsCopyright (c) 2017 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsstarace
Recent documents in Anthony F. Starace Publicationsen-usWed, 01 Nov 2017 01:58:05 PDT3600Doubly-excited state effects on two-photon double ionization of helium by time-delayed, oppositely circularly-polarized attosecond pulses
http://digitalcommons.unl.edu/physicsstarace/227
http://digitalcommons.unl.edu/physicsstarace/227Mon, 30 Oct 2017 11:14:18 PDT
We study two-photon double ionization (TPDI) of helium by a pair of time-delayed (non-overlapping), oppositely circularly-polarized attosecond pulses whose carrier frequencies are resonant with ^{1}P^{o} doubly-excited states. All of our TPDI results are obtained by numerical solution of the two-electron time-dependent Schrödinger equation for the six-dimensional case of circularly-polarized attosecond pulses, and they are analyzed using perturbation theory (PT). As compared with the corresponding nonresonant TPDI process, we find that the doubly-excited states change the character of vortex patterns in the two-electron momentum distributions for the case of back-to-back detection of the two ionized electrons in the polarization plane. The doubly-excited states also completely change the structure of fixed-energy, two-electron angular distributions. Moreover, both the fixed-energy and energy-integrated angular distributions, as well as the two-electron energy distributions, exhibit a periodicity with time delay τ between the two attosecond pulses of about 69 as, i.e. the beat period between the (2s2 p)^{1}P^{o}doubly-excited state and the He ground state. Using PT we derive an expression for an angle-integrated energy distribution that is sensitive to the slower beat period ∼1.2 fs between different doubly-excited states as well as to the long timescale ∼17 fs of autoionization lifetimes. However, with our current computer codes we are only able to study numerically the time-dependent phenomena occurring on an attosecond time scale.
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Jean Marcel Ngoko Djiokap et al.Imaging Electronic Motions by Ultrafast Electron Diraction
http://digitalcommons.unl.edu/physicsstarace/226
http://digitalcommons.unl.edu/physicsstarace/226Tue, 17 Oct 2017 08:50:37 PDT
Recently ultrafast electron diraction and microscopy have reached unprecedented temporal resolution, and transient structures with atomic precision have been observed in various reactions. It is anticipated that these extraordinary advances will soon allow direct observation of electronic motions during chemical reactions. We therefore performed a series of theoretical investigations and simulations to investigate the imaging of electronic motions in atoms and molecules by ultrafast electron diraction. Three prototypical electronic motions were considered for hydrogen atoms. For the case of a breathing mode, the electron density expands and contracts periodically, and we show that the time-resolved scattering intensities re ect such changes of the charge radius. For the case of a wiggling mode, the electron oscillates from one side of the nucleus to the other, and we show that the diraction images exhibit asymmetric angular distributions. The last case is a hybrid mode that involves both breathing and wiggling motions. Owing to the demonstrated ability of ultrafast electrons to image these motions, we have proposed to image a coherent population transfer in lithium atoms using currently available femtosecond electron pulses. A frequency-swept laser pulse adiabatically drives the valence electron of a lithium atom from the 2s to 2p orbitals, and a time-delayed electron pulse maps such motion. Our simulations show that the diraction images re ect this motion both in the scattering intensities and the angular distributions.
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Hua-Chieh Shao et al.Energy-resolved coherent diffraction from laser-driven electronic motion in atoms
http://digitalcommons.unl.edu/physicsstarace/225
http://digitalcommons.unl.edu/physicsstarace/225Tue, 17 Oct 2017 08:50:33 PDT
We investigate theoretically the use of energy-resolved ultrafast electron diffraction to image laser-driven electronic motion in atoms. A chirped laser pulse is used to transfer the valence electron of the lithium atom from the ground state to the first excited state. During this process, the electronic motion is imaged by 100-fs and 1-fs electron pulses in energy-resolved diffraction measurements. Simulations show that the angle-resolved spectra reveal the time evolution of the energy content and symmetry of the electronic state. The time-dependent diffraction patterns are further interpreted in terms of the momentum transfer. For the case of incident 1-fs electron pulses, the rapid 2s−2p quantum beat motion of the target electron is imaged as a time-dependent asymmetric oscillation of the diffraction pattern.
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Hua-Chieh Shao et al.Ionization enhancement and suppression by phase-locked ultrafast pulse pairs
http://digitalcommons.unl.edu/physicsstarace/224
http://digitalcommons.unl.edu/physicsstarace/224Tue, 29 Aug 2017 13:23:54 PDT
We present the results of a study of ionization of Xe atoms by a pair of phase-locked pulses, which is characterized by interference produced by the twin peaks. Two types of interference are considered: ordinary optical interference, which changes the intensity of the composite pulse and thus the ion yield, and a quantum interference, in which the excited electron wave packets interfere. We use the measured Xe+ yield as a function of the temporal delay and/or relative phase between the peaks to monitor the interferences and compare their relative strengths. We model the interference with a pulse intensity function and by calculating the ionization yield with the time-dependent Schrödinger equation. Our results provide insight into optimal control pulses generated with learning algorithms. The results also show that the relative phase between peaks of a control pulse, along with small features such as distortions and imperfections in thewings of an ideal shape, play a significant role in the control process.
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David B. Foote et al.Adiabatic-limit Coulomb factors for photoelectron and high-order-harmonic spectra
http://digitalcommons.unl.edu/physicsstarace/223
http://digitalcommons.unl.edu/physicsstarace/223Tue, 08 Aug 2017 11:57:05 PDT
A momentum-dependent Coulomb factor in the probability for nonlinear ionization of atoms by a strong low-frequency laser field is calculated analytically in the adiabatic approximation. Expressions for this Coulomb factor, valid for an arbitrary laser pulse waveform, are obtained and analyzed in detail for the cases of linear and circular polarizations. The dependence of the Coulomb factor on the photoelectron momentum is shown to be significant in both cases. Using a similar technique, the Coulomb factor for emission of high-order harmonics by an atom in a bichromatic laser field is also calculated. In contrast to the case of a single-frequency field, for bichromatic fields the Coulomb factor depends significantly on the harmonic energy.
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M. V. Frolov et al.Kinematical vortices in double photoionization of helium by attosecond pulses
http://digitalcommons.unl.edu/physicsstarace/222
http://digitalcommons.unl.edu/physicsstarace/222Mon, 17 Jul 2017 15:53:13 PDT
Two-armed helical vortex structures are predicted in the two-electron momentum distributions produced in double photoionization (DPI) of the He atom by a pair of time-delayed elliptically polarized attosecond pulses with opposite helicities. These predictions are based upon both a first-order perturbation theory analysis and numerical solutions of the two-electron, time-dependent Schrödinger equation in six spatial dimensions. The helical vortex structures originate from Ramsey interference of a pair of ionized two-electron wave packets, each having a total angular momentum of unity, and appear in the sixfold differential DPI probability distribution for any energy partitioning between the two electrons. The vortex structures are exquisitely sensitive to the time delay between the two pulses, their relative phase, their ellipticity, and their handedness; moreover, they occur in a variety of electron detection geometries. However, the vortex structures only occur when the angular separation β = cos^{−}^{1}( ˆp_{1} · ˆp_{2}) between the electron momenta p_{1} and p_{2} is held fixed. The vortex structures can also be observed in the fourfold differential DPI probability distribution obtained by averaging the sixfold differential probability over the emission angles of one electron. Such kinematical vortices are a general phenomenon that may occur in any ionization process, initiated by two time-delayed short pulses with opposite ellipticities, for particular detection geometries.
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Jean Marcel Ngoko Djiokap et al.Discontinuities in the electromagnetic fields of vortex beams in the complex source-sink model
http://digitalcommons.unl.edu/physicsstarace/221
http://digitalcommons.unl.edu/physicsstarace/221Tue, 09 May 2017 12:36:07 PDT
An analytical discontinuity is reported in what was thought to be the discontinuity-free exact nonparaxial vortex beam phasor obtained within the complex source-sink model. This discontinuity appears for all odd values of the orbital angular momentum mode. Such discontinuities in the phasor lead to nonphysical discontinuities in the real electromagnetic field components. We identify the source of the discontinuities, and provide graphical evidence of the discontinuous real electric fields for the first and third orbital angular momentum modes. A simple means of avoiding these discontinuities is presented.
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Andrew Vikartofsky et al.Enhancing high-order-harmonic generation by time delays between two-color, few-cycle pulses
http://digitalcommons.unl.edu/physicsstarace/220
http://digitalcommons.unl.edu/physicsstarace/220Thu, 16 Mar 2017 09:58:09 PDT
Use of time delays in high-order-harmonic generation (HHG) driven by intense two-color, few-cycle pulses is investigated in order to determine means of optimizing HHG intensities and plateau cutoff energies. Based upon numerical solutions of the time-dependent Schrõdinger equation for the H atom as well as analytical analyses, we show that introducing a time delay between the two-color, few-cycle pulses can result in an enhancement of the intensity of the HHG spectrum by an order of magnitude (or more) at the cost of a reduction in the HHG plateau cutoff energy. Results for both positive and negative time delays as well as various pulse carrier-envelope phases are investigated and discussed.
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Dian Peng et al.Imaging population transfer in atoms with ultrafast electron pulses
http://digitalcommons.unl.edu/physicsstarace/219
http://digitalcommons.unl.edu/physicsstarace/219Mon, 26 Sep 2016 14:15:50 PDT
We propose the use of ultrafast electron diffraction (UED) to image a controllable, laser-driven coherent electron population transfer in lithium atoms with currently available femtosecond electron pulses. Our simulations demonstrate the ability of ultrafast electrons to image such an electronic population transfer, thus validating UED as a direct means of investigating electron dynamics. Provided the incident electron pulses have sufficient temporal resolution, the diffraction images are shown to resolve also the relative phases of the target electronic wave functions.
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Hua-Chieh Shao et al.Multistart spiral electron vortices in ionization by circularly polarized UV pulses
http://digitalcommons.unl.edu/physicsstarace/218
http://digitalcommons.unl.edu/physicsstarace/218Tue, 12 Jul 2016 14:37:07 PDTMultistart spiral vortex patterns are predicted for the electron momentum distributions in the polarization plane following ionization of the helium atom by two time-delayed circularly polarized ultrashort laser pulses. For two ultraviolet (UV) pulses having the same frequency (such that two photons are required for ionization), single-color two-photon interferometry with corotating or counter-rotating time-delayed pulses is found to lead respectively to zero-start or four-start spiral vortex patterns in the ionized electron momentum distributions in the polarization plane. In contrast, two-color one-photon plus two-photon interferometry with time-delayed corotating or counter-rotating UV pulses is found to lead respectively to one-start or three-start spiral vortex patterns. These predicted multistart electron vortex patterns are found to be sensitive to the carrier frequencies, handedness, time delay, and relative phase of the two pulses. Our numerical predictions are obtained by solving the six-dimensional two-electron time-dependent Schrödinger equation (TDSE). They are explained analytically using perturbation theory (PT). Comparison of our TDSE and PT results for single-color two-photon processes probes the role played by the time-delay-dependent ionization cross channels in which one photon is absorbed from each pulse. Control of these cross channels by means of the parameters of the fields and the ionized electron detection geometries is discussed.
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Jean Marcel Ngoko Djiokap et al.Atomic photoionization experiment by harmonic-generation spectroscopy
http://digitalcommons.unl.edu/physicsstarace/217
http://digitalcommons.unl.edu/physicsstarace/217Thu, 17 Mar 2016 14:26:39 PDT
Measurements of the high-order-harmonic generation yield of the argon (Ar) atom driven by a strong elliptically polarized laser field are shown to completely determine the field-free differential photoionization cross section of Ar, i.e., the energy dependence of both the angle-integrated photoionization cross section and the angular distribution asymmetry parameter.
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M. V. Frolov et al.Control of threshold enhancements in harmonic generation by atoms in a two-color laser field with orthogonal polarizations
http://digitalcommons.unl.edu/physicsstarace/216
http://digitalcommons.unl.edu/physicsstarace/216Tue, 01 Mar 2016 10:57:37 PST
Threshold phenomena (or channel-closing effects) are analyzed in high-order harmonic generation (HHG) by atoms in a two-color laser field with orthogonal linearly polarized components of a fundamental field and its second harmonic. We show that the threshold behavior of HHG rates for the case of a weak second harmonic component is sensitive to the parity of a closing multiphoton ionization channel and the spatial symmetry of the initial bound state of the target atom, while for the case of comparable intensities of both components, suppression of threshold phenomena is observed as the relative phase between the components of a two-color field varies. A quantum orbit analysis as well as phenomenological considerations in terms of Baz’ theory of threshold phenomena [Zh. Eksp. Teor. Fiz. 33, 923 (1957)] are presented in order to describe and explain the major features of threshold phenomena in HHG by a two-color field.
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M. V. Frolov et al.Favorable target positions for intense laser acceleration of electrons in hydrogen-like, highly-charged ions
http://digitalcommons.unl.edu/physicsstarace/215
http://digitalcommons.unl.edu/physicsstarace/215Mon, 14 Sep 2015 14:14:27 PDT
Classical relativistic Monte Carlo simulations of petawatt laser acceleration of electrons bound initially in hydrogen-like, highly-charged ions show that both the angles and energies of the laser-accelerated electrons depend on the initial ion positions with respect to the laser focus. Electrons bound in ions located after the laser focus generally acquire higher (≈GeV) energies and are ejected at smaller angles with respect to the laser beam. Our simulations assume a tightly-focused linearly-polarized laser pulse with intensity approaching 10^{22}W/cm^{2}. Up to fifth order corrections to the paraxial approximation of the laser field in the focal region are taken into account. In addition to the laser intensity, the Rayleigh length in the focal region is shown to play a significant role in maximizing the final energy of the accelerated electrons. Results are presented for both Ne^{9+} and Ar^{17+} target ions.
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Liang-Wen Pi et al.Electron Vortices in Photoionization by Circularly Polarized Attosecond Pulses
http://digitalcommons.unl.edu/physicsstarace/214
http://digitalcommons.unl.edu/physicsstarace/214Thu, 10 Sep 2015 11:43:36 PDT
Single ionization of He by two oppositely circularly polarized, time-delayed attosecond pulses is shown to produce photoelectron momentum distributions in the polarization plane having helical vortex structures sensitive to the time delay between the pulses, their relative phase, and their handedness. Results are obtained by both ab initio numerical solution of the two-electron time-dependent Schrödinger equation and by a lowest-order perturbation theory analysis. The energy, bandwidth, and temporal duration of attosecond pulses are ideal for observing these vortex patterns.
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Jean Marcel Ngoko Djiokap et al.Scaling laws for high-order-harmonic generation with midinfrared laser pulses
http://digitalcommons.unl.edu/physicsstarace/213
http://digitalcommons.unl.edu/physicsstarace/213Mon, 10 Aug 2015 10:04:45 PDT
We derive an analytic expression for thewavelength scaling of the high-order-harmonic generation (HHG) yield induced by midinfrared driving laser fields. It is based on a quasiclassical description of the returning electron wave packet, which is shown to be largely independent of atomic properties. The accuracy of this analytic expression is confirmed by comparison with results of numerical solutions of the time-dependent Schr¨odinger equation for wavelengths in the range of 1.4 μm ≤ λ ≤ 4 μm. We verify the wavelength scaling of the HHG yield found numerically for midinfrared laser fields in a recent paper by Le et al. [
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M. V. Frolov et al.Photodetachment of a model molecular system by an elliptically polarized field
http://digitalcommons.unl.edu/physicsstarace/212
http://digitalcommons.unl.edu/physicsstarace/212Sun, 26 Jul 2015 17:08:25 PDT
The differential cross section for one-photon molecular detachment by an elliptically polarized field is analyzed for a one-electron molecular model comprised of an electron in the field of two (generally nonequivalent) attractive zero-range potentials (ZRPs) separated by the distance R. A phenomenological parametrization of the photodetachment cross section for a fixed-in-space molecular system in terms of two scalar dynamical parameters is presented and circular dichroism effects are discussed. Analytic results for the dynamical molecular parameters within the ZRP molecular model are used to analyze interference phenomena (including two-center interference) and dichroic effects in the detached electron angular distributions and their dependence on the interatomic distance R and on the orientation of the molecular axis with respect to the polarization plane. Numerical ZRP results for angular distributions are presented for both symmetric and asymmetric molecules in an elliptically polarized field.
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M. V. Frolov et al.Nonlinear Dichroism in Back-to-Back Double Ionization of He by an Intense Elliptically Polarized Few-Cycle Extreme Ultraviolet Pulse
http://digitalcommons.unl.edu/physicsstarace/211
http://digitalcommons.unl.edu/physicsstarace/211Thu, 02 Apr 2015 13:54:35 PDT
Control of double ionization of He by means of the polarization and carrier-envelope phase (CEP) of an intense, few-cycle extreme ultraviolet (XUV) pulse is demonstrated numerically by solving the six-dimensional two-electron, time-dependent Schrödinger equation for He interacting with an elliptically polarized XUV pulse. Guided by perturbation theory (PT), we predict the existence of a nonlinear dichroic effect (∝ I^{3/2}) that is sensitive to the CEP, ellipticity, peak intensity I, and temporal duration of the pulse. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) originates from interference of first- and second-order PT amplitudes, allowing one to probe and control S- and D-wave channels of the two-electron continuum.We show that the back-to-back in-plane geometry with unequal energy sharing is an ideal one for observing this dichroic effect that occurs only for an elliptically polarized, few-cycle attosecond pulse.
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Jean Marcel Ngoko Djiokap et al.Rescattering effects in laser-assisted electron-atom
bremsstrahlung
http://digitalcommons.unl.edu/physicsstarace/210
http://digitalcommons.unl.edu/physicsstarace/210Tue, 17 Mar 2015 14:39:54 PDT
Rescattering effects in non-resonant spontaneous laser-assisted electron–atom bremsstrahlung (LABrS) are analyzed within the framework of time-dependent effective-range (TDER) theory. It is shown that high energy LABrS spectra exhibit rescattering plateau structures that are similar to those that are well-known in strong field laser-induced processes as well as those that have been predicted theoretically in laser-assisted collision processes. In the limit of a low-frequency laser field, an analytic description of LABrS is obtained from a rigorous quantum analysis of the exact TDER results for the LABrS amplitude. This amplitude is represented as a sum of factorized terms involving three factors, each having a clear physical meaning. The first two factors are the exact field-free amplitudes for electron–atom bremsstrahlung and for electron– atom scattering, and the third factor describes free electron motion in the laser field along a closed trajectory between the first (scattering) and second (rescattering) collision events. Finally, an extension of these TDER results to the case of LABrS in a Coulomb field is discussed.
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A. N. Zheltukhin et al.Comment on “Universality of Returning Electron Wave
Packet in High-Order Harmonic Generation with
Midinfrared Laser Pulses”
http://digitalcommons.unl.edu/physicsstarace/209
http://digitalcommons.unl.edu/physicsstarace/209Thu, 12 Feb 2015 08:58:48 PST
In conclusion, we have shown that when the same definition for the HHG yield is used [cf. Eq. (1)], the results of Ref. [1] give the same scaling law found earlier in Refs. [2–5] for wavelengths λ ≤ 2 μm. We note that this latter scaling law can be obtained analytically by using results of the model developed in Ref. [6] for the description of short-pulse HHG spectra. These analytic results as well as new numerical TDSE results for longer wavelengths, λ ≤ 4 μm, will be published elsewhere.
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M. V. Frolov et al.Imaging electronic motions in atoms by energy-resolved ultrafast electron diffraction
http://digitalcommons.unl.edu/physicsstarace/208
http://digitalcommons.unl.edu/physicsstarace/208Thu, 11 Sep 2014 21:55:34 PDT
We propose energy-resolved ultrafast electron diffraction as a means of directly imaging target electronic motions whose space, time, and energy information can be simultaneously retrieved from time-resolved diffraction measurements. The energy-resolved diffraction images are simulated for breathing, wiggling, and hybrid modes of electronic motion in the H atom. The simulations demonstrate the capabilities of ultrafast electron diffraction to image and distinguish different kinds of electronic motion. The theoretical analysis of the scattering process identifies the requirements for time- and state-resolved imaging of electronic motion and provides interpretations of the results.
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Hua-Chieh Shao et al.