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Many-electron character of two-photon above-threshold ionization of Ar
摘要： The absolute generalized cross sections and angular distribution parameters of photoelectrons for the two-photon above-threshold 3p ionization of Ar were calculated in the exciting photon energy range from 15.76 to 36 eV. The correlation function technique developed earlier was extended for the case when an intermediate-state function is of a continuum-type. We show that two-photon ionization of Ar near the 3p4 threshold to a large extent is determined by the (3p (cid:2)(cid:2)(cid:3) εd )2 two-photon absorption via the giant resonance. This many-electron correlation causes (i) an increase of the photoionization cross sections by more than a factor of 3; and (ii) the appearance of resonances in the exciting-photon energy range of the doubly excited states. The predictions are supported by a good agreement between length and velocity results obtained after taking into account the higher-order perturbation theory corrections.
关键词： angular distribution,two-photon ionization,above-threshold ionization,many-electron correlations,argon,generalized cross sections
更新于2025-09-23 15:23:52
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Polarization-Dependent Self-Injection by Above Threshold Ionization Heating in a Laser Wakefield Accelerator
摘要： We report on the experimental observation of a decreased self-injection threshold by using laser pulses with circular polarization in laser wakefield acceleration experiments in a nonpreformed plasma, compared to the usually employed linear polarization. A significantly higher electron beam charge was also observed for circular polarization compared to linear polarization over a wide range of parameters. Theoretical analysis and quasi-3D particle-in-cell simulations reveal that the self-injection and hence the laser wakefield acceleration is polarization dependent and indicate a different injection mechanism for circularly polarized laser pulses, originating from larger momentum gain by electrons during above threshold ionization. This enables electrons to meet the trapping condition more easily, and the resulting higher plasma temperature was confirmed via spectroscopy of the XUV plasma emission.
关键词： self-injection,Laser wakefield acceleration,XUV spectroscopy,above threshold ionization,polarization
更新于2025-09-23 15:19:57
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Above-threshold ionization driven by few-cycle spatially bounded inhomogeneous laser ?elds
摘要： In this work we study the main features of the photoelectrons generated when noble gas atoms are driven by spatially bounded inhomogeneous strong laser fields. These spatial inhomogeneous oscillating fields, employed to ionize and accelerate the electrons, result from the interaction between a pulsed low intensity laser and bow-tie shaped gold nanostructures. Under this excitation scheme, energy-resolved above-threshold ionization (ATI) photoelectron spectra have been simulated by solving the one-dimensional (1D) time-dependent Schr?dinger equation (TDSE) within the single active electron (SAE) approximation. These quantum mechanical results are supported by their classical counterparts, obtained by the numerical integration of the Newton-Lorentz equation. By using near infrared wavelengths (0.8 ? 3 μm) sources, our results show that very high energetic electrons (with kinetic energies in the keV domain) can be generated, far exceeding the limits obtained by using conventional, spatially homogeneous, fields. This new characteristic can be supported considering the non-recombining electrons trajectories, already reported by Neyra and coworkers (Neyra E, et al. 2018 J. Opt. 20 034002). In order to build a real representation of the spatial dependence of the plasmonic-enhanced field in an analytic function, we fit the generated 'actual' field using two Gaussian functions. We have further analyzed and explored this plasmonic-modified ATI phenomenon in a model argon atom by using several driven wavelengths at intensities in the order of 1014 W/cm2. Throughout our contribution we carefully scrutinize the differences between the ATI obtained using spatially homogeneous and inhomogeneous laser fields. We present the various physical origins, or correspondingly distinct physical mechanisms, for the ATI generation driven by spatially bounded inhomogeneous fields.
关键词： ultrafast optics,Above-threshold ionization,plasmonic fields
更新于2025-09-16 10:30:52
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Pulse-length effects in strong-field ionization of atoms by co-rotating and counter-rotating bicircular laser pulses
摘要： We study strong-?eld ionization of H and selected rare-gas atoms by solving the three-dimensional time-dependent Schr?dinger equation in the single-active-electron approximation with particular attention to the effect of pulse length on the total ionization yields (TIYs) produced by counter-rotating bicircular (CRBC) lasers compared to co-rotating bicircular (COBC) lasers. To this end, TIYs, photoelectron momentum distributions, and above-threshold ionization spectra are presented for various pulse durations and peak intensities. For pulses containing ?ve or more cycles of the fundamental near-infrared laser ?eld, the CRBC/COBC ratio of TIYs is affected due to ionization via excited states and multiphoton ionization channels. For pulses containing less than ?ve cycles, the CRBC/COBC ratio of TIYs depends on both the frequency combination and number of optical cycles: With the 1ω-2ω frequency combination, the ratio of TIYs is enhanced (reduced) for pulses with an odd (even) number of optical cycles. The 1ω-3ω frequency combination was considered for pulses containing two or three optical cycles, and for both pulse lengths the CRBC/COBC ratio of TIYs is less than unity.
关键词： photoelectron momentum distributions,bicircular laser pulses,time-dependent Schr?dinger equation,above-threshold ionization spectra,strong-?eld ionization
更新于2025-09-11 14:15:04
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Imaging charge migration in the asymmetric molecule with the holographic interference in strong-field tunneling ionization
摘要： Coherent superposition of several electronic states in molecules induces attosecond charge migration. We theoretically demonstrate a way to observe this attosecond charge migration in the asymmetric molecule HeH2+ using strong-field photoelectron holography, complementary to our previous work (He M et al. 2018 Phys. Rev. Lett. 120 133204) where the charge migration in the symmetric molecule was monitored. We disentangle the holographic interference from other types of interference in the photoelectron momentum distribution with the Fourier frequency filter method. By employing the holographic fringes originating from the ground state of HeH2+ as a reference, we develop a feasible approach to visualize the charge migration in the asymmetric molecule with attosecond temporal resolution.
关键词： strong laser field,above-threshold ionization,ultrafast phenomena
更新于2025-09-10 09:29:36
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High-order above-threshold ionization beyond the electric dipole approximation: Dependence on the atomic and molecular structure
摘要： We provide an in-depth analysis of high-order above-threshold ionization of atoms and molecules by strong laser pulses, using three different theoretical approaches beyond the electric dipole approximation: (i) the numerical solution of the time-dependent Schr?dinger equation, (ii) the classical three-step model, and (iii) the quantum-orbit model. In the classical and quantum-orbit models, we include an accurate description of the rescattering step by target-specific differential scattering cross sections. The considerable forward shift of the photoelectron momentum distributions along the laser propagation direction can be understood in terms of the nondipole electron motion after rescattering. An explanation of the additionally observed forward-backward asymmetry of the signal strength requires the accurate modeling of the rescattering step and the electron dynamics before the rescattering event. For the H2+ molecular ion, we compare the cases of parallel and perpendicular alignment of the molecular axis and we show that the interference pattern and its modification due to the nondipole effects are orientation dependent. Compared to atoms, the nondipole effects in molecular high-order above-threshold ionization appear more pronounced and amenable to experimental observation.
关键词： high-order above-threshold ionization,nondipole effects,photoelectron momentum distributions,quantum-orbit model,classical three-step model,H2+ molecular ion,electric dipole approximation,time-dependent Schr?dinger equation
更新于2025-09-04 15:30:14