Anthony F. Starace PublicationsCopyright (c) 2014 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsstarace
Recent documents in Anthony F. Starace Publicationsen-usSat, 13 Sep 2014 01:50:32 PDT3600Imaging 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.Potential-barrier effects in three-photon-ionization processes
http://digitalcommons.unl.edu/physicsstarace/207
http://digitalcommons.unl.edu/physicsstarace/207Mon, 04 Aug 2014 10:46:37 PDT
Resonance-like enhancements of generalized three-photon cross sections for extreme ultraviolet ionization of Ar, Kr, and Xe are demonstrated and analyzed within a single-active-electron, central-potential model. The resonant-like behavior is shown to originate fromthe potential barriers experienced by intermediate- and final-state photoelectron wave packets corresponding to absorption of one, two, or three photons. The resonance-like profiles in the generalized three-photon-ionization cross sections are shown to be similar to those found in the generalized two-photon-ionization cross sections [Phys. Rev. A 82, 053414 (2010)]. The complexity of Cooper minima in multiphoton-ionization processes is also discussed. Owing to the similar resonance-like profiles found in both two- and three-photon generalized cross sections, we expect such potential-barrier effects to be general features of multiphoton-ionization processes in most atoms with occupied p and d subshells.
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Liang-Wen Pi et al.Analytic model for the description of above-threshold ionization by an intense short laser pulse
http://digitalcommons.unl.edu/physicsstarace/206
http://digitalcommons.unl.edu/physicsstarace/206Tue, 08 Jul 2014 10:12:55 PDT
We present an analytic model for the description of above-threshold ionization (ATI) of an atom by an intense, linearly polarized short laser pulse. Our treatment is based upon a description of ATI by an infinitely long train of short laser pulses whereupon we take the limit that the time interval between pulses becomes infinite. In the quasiclassical approximation, we provide detailed quantum-mechanical derivations, within the time-dependent effective range (TDER) model, of the closed-form formulas for the differential probability P(p) of ATI by an intense, short laser pulse that were presented briefly by Frolov et al. [Phys. Rev. Lett. 108, 213002 (2012)] and that were used to describe key features of the high-energy part of ATI spectra for H and He atoms in an intense, few-cycle laser pulse, using a phenomenological generalization of the physically transparent TDER results to the case of real atoms. Moreover, we extend these results here to the case of an electron bound initially in a p state; we also take into account multiple-return electron trajectories. The ATI amplitude in our approach is given by a coherent sum of partial amplitudes describing ionization by neighboring optical cycles near the peak of the intensity envelope of a short laser pulse. These results provide an analytical explanation of key features in short-pulse ATI spectra, such as the left-right asymmetry in the ionized electron angular distribution, the multiplateau structures, and both large-scale and fine-scale oscillation patterns resulting from quantum interferences of electron trajectories. Our results show that the shape of the ATI spectrum in the middle part of the ATI plateau is sensitive to the spatial symmetry of the initial bound state of the active electron. This sensitivity originates from the contributions of multiple-return electron trajectories. Our analytic results are shown to be in good agreement with results of numerical solutions of the time-dependent Schrödinger equation for He and Ar atoms. Comparison of our results with those of quantitative rescattering theory is also discussed.
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M. V. Frolov et al.Resonant electron-atom bremsstrahlung in an intense laser field
http://digitalcommons.unl.edu/physicsstarace/205
http://digitalcommons.unl.edu/physicsstarace/205Sun, 09 Feb 2014 10:35:45 PST
We analyze a resonant mechanism for spontaneous laser-assisted electron bremsstrahlung (BrS) involving the resonant transition (via either laser-assisted electron-ion recombination or electron-atom attachment) into a laser-dressed intermediate quasibound state (corresponding, respectively, to either a field-free neutral atom or a negative-ion bound state) accompanied by ionization or detachment of this state by the laser field. This mechanism leads to resonant enhancement (by orders of magnitude) of the BrS spectral density for emitted photon energies corresponding to those for laser-assisted recombination or attachment. We present an accurate parametrization of the resonant BrS amplitude in terms of the amplitudes for nonresonant BrS, for recombination or attachment to the intermediate state, and for ionization or detachment of this state. The high accuracy of our general analytic parametrization of the resonant BrS cross section is shown by comparison with exact numerical results for laser-assisted BrS spectra obtained within time-dependent effective range theory. Numerical estimates of resonant BrS in electron scattering from a Coulomb potential are also presented.
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A. N. Zheltukhin et al.Imaging coherent electronic motion in atoms by ultrafast electron diffraction
http://digitalcommons.unl.edu/physicsstarace/204
http://digitalcommons.unl.edu/physicsstarace/204Sun, 29 Dec 2013 11:18:37 PST
Ultrafast electron diffraction from time-varying coherent electronic states of the H atom is analyzed theoretically. This theoretical analysis identifies the conditions necessary to obtain time-resolved measurements. Electron diffraction from coherent electronic states exhibiting breathing and wiggling modes of electronic motion are simulated numerically in order to demonstrate the capability of attosecond electron pulses to image electron dynamics. The scattering patterns and their temporal behaviors are shown to differentiate the two kinds of target electronic motion. Moreover, our simulations show that inelastic processes contribute significantly to the diffraction patterns. Thus, although the diffraction patterns clearly distinguish different modes of target electronic motion, they cannot be easily related to the time-dependent target charge density.
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Hua-Chieh Shao et al.Harmonic generation spectroscopy with a
two-colour laser field having orthogonal
linear polarizations
http://digitalcommons.unl.edu/physicsstarace/203
http://digitalcommons.unl.edu/physicsstarace/203Fri, 22 Nov 2013 13:03:05 PST
The interpretation of many high-order harmonic generation (HHG) experiments is based on the assumption that the HHG yield of an atom can be factorized into (i) a laser-dependent ‘electron wave packet’ with rather simple properties, including a nearly universal shape, and (ii) an atomic photorecombination cross section. We show that this factorization is restricted to linearly polarized laser fields and fails in two-colour laser fields with orthogonal polarizations. At the same time, we show how two-colour HHG spectroscopy using orthogonally polarized intense fundamental and relatively weak second harmonic fields makes a complete experiment possible that enables the retrieval of the angle-resolved photorecombination cross sections for atomic p states.
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T. S. Sarantseva et al.Resonant enhancement of the harmonic-generation spectrum of beryllium
http://digitalcommons.unl.edu/physicsstarace/202
http://digitalcommons.unl.edu/physicsstarace/202Mon, 18 Nov 2013 06:27:33 PST
The high-order harmonic-generation (HHG) spectrum of Be is investigated in the multiphoton regime by solving the full-dimensional, two-active-electron, time-dependent Schr¨odinger equation in an intense (≈10^{13} W/cm^{2}), 30-cycle laser field. As the laser frequency ω_{L}varies from 1.7 to 1.8 eV (which is in the tunable range of a Ti:sapphire laser), the seventh harmonic becomes resonant sequentially with the transition between the ground state and two doubly excited autoionizing states, 2p4s(^{1}P) (at ω_{L}= 1.734 eV) and 2p5s(^{1}P) (at ω_{L}= 1.785 eV), while the third harmonic becomes resonant with the 2s2p(^{1}P) singly excited state (at ω_{L}= 1.766 eV). At each of these resonant frequencies, the HHG power spectrum is found to increase by an order of magnitude over a range of harmonics that form a plateau, extending from the resonant harmonic up to a cutoff at the 25th harmonic. In contrast to the well-known rescattering plateau cutoff law appropriate in the tunneling regime (which predicts a cutoff at the fifth or seventh harmonic), the multiphoton regime plateau we find for Be originates from atomic resonance effects. Off resonance, the Be HHG spectrum decreases monotonically with harmonic order. By taking the ratio of the integrated harmonic power of the seventh harmonic to that of the fifth harmonic, one can isolate the resonant effects of the two doubly excited states in the HHG spectrum from those of singly excited resonance states. These ratios exhibit resonance profiles for driving laser-pulse durations much longer than the lifetimes of these autoionizing states. The energy widths of these resonance features are comparable to the widths of the laser pulse and are much smaller than the autoionizing state widths. These results demonstrate an important role for electron correlations in enhancing harmonic-generation rates in the multiphoton regime.
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Jean Marcel Ngoko Djiokap et al.Carrier-envelope-phase-induced asymmetries in double ionization of helium by an intense few-cycle XUV pulse
http://digitalcommons.unl.edu/physicsstarace/201
http://digitalcommons.unl.edu/physicsstarace/201Wed, 13 Nov 2013 14:57:24 PST
The carrier-envelope-phase (CEP) dependence of electron angular distributions in double ionization of He by an arbitrarily polarized, few-cycle, intense XUV pulse is formulated using perturbation theory (PT) in the pulse amplitude. Owing to the broad pulse bandwidth, interference of first- and second-order PT amplitudes produces asymmetric angular distributions sensitive to the CEP. The PT parametrization is shown to be valid by comparing with results of solutions of the full-dimensional, two-electron time-dependent Schrödinger equation for the case of linear polarization.
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Jean Marcel Ngoko Djiokap et al.Use of Attosecond Electron Pulses to Image Electronic
Motion in Atoms and Molecules
http://digitalcommons.unl.edu/physicsstarace/200
http://digitalcommons.unl.edu/physicsstarace/200Fri, 11 Oct 2013 08:45:46 PDT
We investigate theoretically the direct imaging of coherent electronic motion in atoms and molecules using attosecond electron pulses. The theories of time-resolved ultrafast electron diffraction and (e, 2e) momentum spectroscopy as well as the requisite conditions for carrying out time-resolved measurements to obtain timedependent images are discussed. Results of simulations showing images of the motions of coherent superposition states in both the hydrogen atom and the hydrogen molecular ion are shown, thus indicating the capability of ultrafast electron pulses to investigate time-dependent electron dynamics.
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Hua-Chieh Shao et al.Zero-range-potential model for strong-field molecular processes: Dynamic polarizability and photodetachment cross section
http://digitalcommons.unl.edu/physicsstarace/199
http://digitalcommons.unl.edu/physicsstarace/199Wed, 11 Sep 2013 08:53:48 PDT
We develop a general theoretical framework for the analytical description of the interaction of a model diatomic molecular system with an intense, arbitrarily polarized monochromatic laser field. The model molecule comprises an electron in the field of two zero-range potentials separated by the internuclear distance. This model has an exact analytical solution within the theoretical framework of the quasistationary quasienergy (Floquet) approach. In addition to the development of this general framework, we also present a detailed analysis of the weak-field limit, within which we obtain both the frequency-dependent polarizability and the angle-resolved photodetachment cross section for the model system. These fundamental properties are analyzed for both homonuclear and heteronuclear molecular systems in a linearly polarized laser field, for both ground and excited electronic states, and for arbitrary orientation of the molecular axis relative to the polarization vector of the laser field. The analytical expressions for the polarizability and angle-resolved photoelectron spectra exhibit characteristic double-slit interference patterns, allowing one to study their dependence on the parameters of the problem beyond the level of the Born approximation.
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S. V. Borzunov et al.Validity of Factorization of the High-Energy Photoelectron Yield in Above-Threshold Ionization of an Atom by a Short Laser Pulse
http://digitalcommons.unl.edu/physicsstarace/198
http://digitalcommons.unl.edu/physicsstarace/198Thu, 18 Jul 2013 13:21:21 PDT
An analytic description for the yield, P(p), of high-energy electrons ionized from an atom by a short (few-cycle) laser pulse is obtained quantum mechanically. Factorization of P(p) in terms of an electron wave packet and the cross section for elastic electron scattering (EES) is shown to occur only for an ultrashort pulse, while in general P(p) involves interference of EES amplitudes with laser-field-dependent momenta. The analytic predictions agree well with accurate numerical results.
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M. V. Frolov et al.Corrigendum to "Effect of elliptically polarised light on the angular distribution of photoelectrons"
http://digitalcommons.unl.edu/physicsstarace/197
http://digitalcommons.unl.edu/physicsstarace/197Thu, 18 Jul 2013 12:36:22 PDT
Correction to article published in J. Phys. B: Atom. Molec. Phys. 8 (1975), pp.1806-9

Re-states the definition of the polarisation p in equation (5), the association of E_{b} and E_{a} with E_{y} and E_{x}, and the interchange of subscripts y and x in Equation 11.

The substance and conclusions of the papers are unchanged.

We wish to thank Dr Keh-Ning Huang for pointing out this error.

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James A. R. Samson et al.Asymmetries in production of He<sup>+</sup>(<i>n</i> = 2) with an intense few-cycle attosecond pulse
http://digitalcommons.unl.edu/physicsstarace/196
http://digitalcommons.unl.edu/physicsstarace/196Thu, 18 Jul 2013 11:02:36 PDT
By solving the two-electron time-dependent Schrödinger equation, we study carrier-envelope-phase (CEP) effects on ionization plus excitation of He to He^{+}(n = 2) states by a few-cycle attosecond pulse with a carrier frequency of 51 eV. For most CEPs the asymmetries in the photoelectron angular distributions with excitation of He^{+}(2s) or He^{+}(2p) have opposite signs and are two orders of magnitude larger than for ionization without excitation. These results indicate that attosecond pulse CEP effects may be significantly amplified in correlated two-electron ionization processes.
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Jean Marcel Ngoko Djiokap et al.Time-resolved ultrafast electron (<i>e</i>,2<i>e</i>) momentum spectroscopy
http://digitalcommons.unl.edu/physicsstarace/195
http://digitalcommons.unl.edu/physicsstarace/195Thu, 16 May 2013 07:14:07 PDT
The (e,2e) process is analyzed for the case of an ultrafast electron pulse incident upon a target prepared in a time-varying, coherent superposition of states. Conditions under which time-resolved target momentum densities can be obtained from experimental measurements are discussed. Results for coherent electronic motions in both the H atom and the H_{2}^{+} molecule are used to illustrate the capability of an ultrafast electron pulse to image time-dependent target electron dynamics.
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Hua-Chieh Shao et al.Control of atomic dynamics in laser-assisted electron-atom scattering through the driving-laser ellipticity
http://digitalcommons.unl.edu/physicsstarace/194
http://digitalcommons.unl.edu/physicsstarace/194Wed, 20 Mar 2013 14:33:04 PDT
Orders of magnitude increases of the cross sections are predicted for laser-assisted low-energy electron-atom scattering (accompanied by absorption of laser photons) as the laser ellipticity is increased. These ellipticity-controlled enhancements are manifestations of the field-free electron-atom scattering dynamics, such as the Ramsauer-Townsend effect in low-energy elastic electron-atom scattering. The strong sensitivity of laser-assisted scattering cross sections to this dynamics and the laser ellipticity is illustrated for e-Ne and e-Ar scattering in both midinfrared (λ = 3.5 μm) and CO_{2} (λ = 10.6 μm) laser fields of moderate intensities.
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A. V. Flegel et al.Analytic description of elastic electron-atom scattering in an elliptically polarized laser field
http://digitalcommons.unl.edu/physicsstarace/193
http://digitalcommons.unl.edu/physicsstarace/193Mon, 07 Jan 2013 06:43:26 PST
An analytic description of laser-assisted electron-atom scattering (LAES) in an elliptically polarized field is presented using time-dependent effective range (TDER) theory to treat both electron-laser and electron-atom interactions nonperturbatively. Closed-form formulas describing plateau features in LAES spectra are derived quantum mechanically in the low-frequency limit. These formulas provide an analytic explanation for key features of the LAES differential cross section. For the low-energy region of the LAES spectrum, our result generalizes the Kroll-Watson formula to the case of elliptic polarization. For the high-energy (rescattering) plateau in the LAES spectrum, our result generalizes prior results for a linearly polarized field valid for the high-energy end of the rescattering plateau [Flegel et al., J. Phys. B 42, 241002 (2009)] and confirms the factorization of the LAES cross section into three factors: two field-free elastic electron-atom scattering cross sections (with laser-modified momenta) and a laser field-dependent factor (insensitive to the scattering potential) describing the laser-driven motion of the electron in the elliptically polarized field. We present also approximate analytic expressions for the exact TDER LAES amplitude that are valid over the entire rescattering plateau and reduce to the three-factor form in the plateau cutoff region. The theory is illustrated for the cases of e-H scattering in a CO_{2}-laser field and e-F scattering in a midinfrared laser field of wavelength λ = 3.5 μm, for which the analytic results are shown to be in good agreement with exact numerical TDER results.
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A. V. Flegel et al.High-order-harmonic-generation spectroscopy with an elliptically polarized laser field
http://digitalcommons.unl.edu/physicsstarace/192
http://digitalcommons.unl.edu/physicsstarace/192Thu, 13 Dec 2012 14:02:52 PST
Analytic formulas describing high-order-harmonic generation (HHG) by atoms in an intense laser field with small ellipticity are obtained quantum mechanically in the tunneling limit. The results show that factorization of the HHG yield in terms of an electron wave packet and the photorecombination cross section (PRCS) is valid only for s states of a bound atomic electron, whereas the HHG yield for p states involves two different atomic parameters. For the latter case, elliptic HHG spectroscopy enables one to retrieve both the energy and angular dependence of the PRCS of the target atom, as we illustrate for the case of HHG by Xe in a midinfrared laser field.
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M. V. Frolov et al.Threshold effects in strong-field ionization: Energy shifts and Rydberg structures
http://digitalcommons.unl.edu/physicsstarace/191
http://digitalcommons.unl.edu/physicsstarace/191Mon, 19 Nov 2012 10:52:10 PST
The behavior of strong-field ionization rates of neutral atoms in the vicinity ofmultiphoton ionization thresholds is analyzed using formal collision theory.Our approach,which accounts nonperturbatively for effects of an intense laser field, shows that the ionization rates have a nearly constant behavior below and above each multiphoton threshold and that between such thresholds there are an apparently finite number of rapid oscillations due to resonances with laser-field-modified Rydberg states. This pattern is typical for any atomic target, as we illustrate specifically for hydrogen and neon atoms. The flat behavior of the ionization yield near multiphoton thresholds gives the appearance of an energy shift of the ionization thresholds, which have been postulated in a number of recent studies concerning diverse aspects of above-threshold ionization and high-harmonic generation of atoms. The flat behaviors of the rates near threshold exhibit only a rather weak dependence on the laser-field intensity. Other aspects of the near-threshold behavior of ionization rates and their dependence on the laser-field parameters are also discussed.
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Katarzyna Krajewska et al.Enhanced asymmetry in few-cycle attosecond pulse
ionization of He in the vicinity of autoionizing
resonances
http://digitalcommons.unl.edu/physicsstarace/190
http://digitalcommons.unl.edu/physicsstarace/190Fri, 14 Sep 2012 10:10:57 PDT
By solving the two-active-electron, time-dependent Schrödinger equation in its full dimensionality, we investigate the carrier-envelope phase (CEP) dependence of single ionization of He to the He^{+}(1s) state triggered by an intense few-cycle attosecond pulse with carrier frequency ω corresponding to the energy ћω = 36 eV. Effects of electron correlations are probed by comparing projections of the final state of the two-electron wave packet onto field-free highly correlated Jacobi matrix wave functions with projections onto uncorrelated Coulomb wave functions. Significant differences are found in the vicinity of autoionizing resonances. Owing to the broad bandwidths of our 115 and 230 as pulses and their high intensities (1–2 PWcm^{−2}), asymmetries are found in the differential probability for ionization of electrons parallel and antiparallel to the linear polarization axis of the laser pulse. These asymmetries stem from interference of the one- and two-photon ionization amplitudes for producing electrons with the same momentum along the linear polarization axis. Whereas these asymmetries generally decrease with increasing ionized electron kinetic energy, we find a large enhancement of the asymmetry in the vicinity of two-electron doubly excited (autoionizing) states on an energy scale comparable to the widths of the autoionizing states. The CEP dependence of the energy-integrated asymmetry agrees very well with the predictions of time-dependent perturbation theory (Pronin et al 2009 Phys. Rev. A 80 063403).
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Jean Marcel Ngoko Djiokap et al.Resonant phenomena in laser-assisted radiative
attachment or recombination
http://digitalcommons.unl.edu/physicsstarace/189
http://digitalcommons.unl.edu/physicsstarace/189Tue, 10 Apr 2012 08:36:04 PDT
Resonant enhancements are predicted in cross sections σn for laser-assisted radiative attachment or electron–ion recombination accompanied by absorption of n laser photons. These enhancements occur for incoming electron energies at which the electron can be attached or recombined by emitting μ laser photons followed by emission of a spontaneous photon upon absorbing n + μ laser photons. The close similarity between rescattering plateaus in spectra of resonant attachment/recombination and of high-order harmonic generation is shown based on a general parametrization for σ_{n} and on numerical results for e−H attachment.
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A. N. Zheltukhin et al.