Session N04: Relativistic, Plasma, and X-Ray Processes

Live

At relativistic laser intensities ($I > 10^{18}$ W/cm$^2$) Thompson scattering becomes nonlinear, leading to emission of light deviating markedly from the nonrelativisitc regime. The relativistic motion of free electrons induces new dynamics, which manifest themselves in wavelength and angular shifts as well as harmonics that are intimately coupled with the intensity. Sarachik and Schappert [Phys. Rev. D 1 (1970)] showed the Doppler shifts of Relativistic Thomson Scattering (RTS) to be proportional to $I (1-\cos \theta)$, where $\theta$ is the observation angle relative to the laser propagation direction. Recently, Harvey [Phys. Rev. Accel. Beams 21 (2018)] explored more throughly the relationship between the angular distribution and the intensity theoretically. Both RTS features are calculable classically, making comparison with measurement straightforward. Previously, we showed the classical treatment of the Doppler shift to be in good agreement with measurement between $10^{18}$ and $10^{19}$ W/cm$^2$ at $\theta = 90^\circ$ [Optics Express 27, 30020]. In this presentation we will discuss our angular-distribution measurements between 30$^\circ$ and 130$^\circ$ in the 450 to 700 nm range for $I \sim 10^{18}$ to $10^{19}$ W/cm$^2$, and how they compare with numerical simulations.   

Live

The interaction between a non-relativistic charged particle and a free-space electromagnetic wave can be described by the ponderomotive potential. Although ponderomotive electron-laser interactions at relativistic velocities are important for emerging technologies from laser-based particle accelerators to laser-enhanced electron microscopy, the effects of special relativity on the interaction have only been studied theoretically. We use a transmission electron microscope (beam energy $\le $ 300 keV) to measure the position-dependent phase shift imparted to a relativistic electron wave function when it traverses a standing laser wave (continuous-wave intensity $=$ 175 GW/cm$^{\mathrm{2}})$. In contrast to the non-relativistic case, we demonstrate that the phase shift depends on both the electron velocity and the wave polarization. Remarkably, if the electron's speed is greater than 1/$\surd $2 of the speed of light, the phase shift at the electric field nodes of the wave can exceed that at the antinodes. In this case there exists a polarization such that the phase shifts at the nodes and antinodes are equal, and the electron does not experience Kapitza-Dirac diffraction.   

Live

We present the yields of k-shell hole creation due to laser rescattering in strong and ultrastrong fields. Laser driven rescattering at higher energies, where k-shell ionization can occur, involves relativistic effects and the Lorentz force from the laser magnetic field. The predicted demarcation of higher energy rescattering interactions has been described by a Lorentz deflection parameter \footnote{M. Klaiber, et al, \textbf{Phy. Rev. Lett} 118, 093001} in atomic units $\Gamma_R=U_p^{3/2} V_{IP}^{1/2}/(3 c^2 \omega) = 1$ for ionization of an electron from a binding energy $V_{IP}$ by an external field, frequency $\omega$ and ponderomotive energy $U_p$. Surprisingly, laser driven rescattering near $\Gamma_R \sim 1$ is able to create k-shell holes in all atoms from lithium to uranium and extends rescattering physics from the deep IR ($\lambda = 10 \mu m$) to 4th generation x-ray FEL sources ($\lambda= 1 nm$). Our results compare favorably with measurements in krypton and neon \footnote{Y. Deng, et al, \textbf{Phy. Rev. Lett} 116, 073901}. We report the laser intensity and wavelength needed to create the greatest number of k-shell holes, which can be as great as $10^{-4}$ (k-shell holes / optic cycle) and provide examples across the periodic table including Li, Ne, Kr and U.   

Live

Ionization and excitation of a dense gas medium during the filamenting ultrashort laser pulse determines the evolution of the electronic degrees of freedom in the filament wake channel and sets up the transient transformations of linear and nonlinear optical characteristics of the channel. During the pulse, the strong-field ionization of constituent atoms/molecules competes with impact ionization and collisional excitation by energetic free electrons, which are driven by the oscillating laser field and gain considerable energy via inverse Bremsstrahlung process while scattering on neighboring neutral atoms. This complex interplay determines the transverse profiles of the densities of ions and excited atoms, as well as of the electron density and temperature at the end of the pulse. Using a kinetic model of these processes, we explore sensitivity of the composition of thus formed inhomogeneous and highly nonequilibrium plasma to the envelope shape of the driving laser pulse. By considering a family of pulses that differ from one another by the asymmetry of their envelope functions but are normalized by the same cumulative strong-field ionization output, we show that asymmetric pulse envelopes skewed toward the earlier time result in considerably higher ratio of excited atoms to ionized atoms. Medium-specific estimates are made for high-pressure argon gas; they agree well with the results of recent experiments.   

Live

We develop the theoretical framework for the detection of hazardous molecules in the atmosphere 1 km away from the source using CARS (Coherent anti-Stokes Raman Spectroscopy) taking methanol as a prototype. The Maxwell-Liouville-von Neumann equations for chirped pulses of pump, Stokes, probe and anti-Stokes are derived for CARS scattering. A multilayer model of target molecule distribution with variable width is introduced considering the density distribution of molecules as Gaussian. The propagation of transform-limited pulses with pulse duration of 100 femtoseconds in the air is investigated and incorporated with the target molecule distribution considering the effects of scattering and absorption. The anti-Stokes pulses are significantly amplified to provide a detectable backscattered signal transferring energy from pump and probe pulses. The correlation of anti-Stokes amplification with coherence and population dynamics is examined and the effect of decoherence is studied. The conservation of energy and population during each scattering is verified for the model. The possibility of using deep learning to understand the phase changes of chirped pulses in the scattering process is investigated in order to further enhance the pulse intensity.   

Live

Photoelectron holography has proven itself as a powerful tool for imaging molecular dynamics on an attosecond time scale. Electrons undergoing strong-field ionization take many paths to the detector, some of which will interact strongly with the core and others that will not, the interference between these trajectories can be used to image the target. This is known an photoelectron holography. We identify, in experiment and theory, an overlooked holographic interference structure in strong-field ionization, dubbed ``the spiral'', deriving from two electron paths for which both the binding potential and the laser field are equally important. We show the spiral is the true origin of interference carpets that were previous associated with direct above-threshold ionization trajectories, which we show can not properly replicate the inteference pattern. Through an analytical derivation in multiple models we link the formation of the interference carpets to a fundamental symmetry of the laser field. The two trajectories that make up the spiral are sensitive to structural phases from the binding potential. In addition, the symmetry of the bound-state is shown to be encoded in the spiral. All this makes the spiral an ideal structure for photoelectron holography and imaging.   

Live

Frequency resolved x-ray scattering (FRXS) analysis of ultrafast time-resolved internal motion in molecules is especially useful in identifying low signal-to-noise features such as dissociation. Weak vibrations and terminal dissociation speeds, as well as dissociation time shifts, have been reported using this method. Here we show that FRXS can also reveal accelerations during dissociation, which appear as diffuse features with characteristic fringe patterns in the FRXS map. Simulations of non-ballistic iodine trajectories are compared to iodine photodissociation x-ray scattering experiments, revealing evidence for non-ballistic dissociation trajectories subject to both repulsive and attractive forces.   

On Demand

We investigate the double photoionization process in the $X$ = He, Li$^+$, C$^{4+}$, Be, B$^+$ and Ne$^{6+}$ targets triggered by the absorption of a single photon of energy in the extreme UV/soft X-ray spectral region. Our theoretical method consists in solving the time-dependent Schr\"odinger equation with a spectral method of configuration interaction type \footnote{S. Barmaki et al., \textbf{J. Phys. B} 51, 105002 (2018)}, \footnote{S. Barmaki et al., \textbf{Phys. Rev. A} 89, 063406 (2014)}. The probe of the electron dynamics in the different systems shows that the way the outer electrons will leave their parent $X$ system is systematically dictated by the amount of excess photon energy available to them relative to the ionization potential of the corresponding $X^{+}$ ion \footnote{S. Barmaki et al., \textbf{Chem. Phys.} 517, 24 (2019)}.   

On Demand

We present accurate strong-field simulation results based on novel one-dimensional (1D) atomic model potentials that we derive from the corrections proposed earlier using the reduced ground state density of a three-dimensional (3D) single-active-electron atom [Sz. Majorosi et.al., Phys. Rev. A, 98 (2018) 023401]. The correction involves a change of the asymptotics of the 1D model potentials while maintaining the correct ground state energy. We construct correct 1D models of the H and He atoms and of $\mathrm{H}_{2}^{+}$ and $\mathrm{H}_{2}$ using improved parameters of existing soft-core Coulomb potential forms [Sz. Majorosi et.al., Phys. Rev. A, (2020) accepted (arXiv:1907.13619)]. We test these 1D models by comparing the corresponding simulation results with their 3D counterparts in typical strong-field physics scenarios with near- and mid-infrared laser pulses, having peak intensities in the $10^{14}-10^{15}\,\mathrm{W/cm}^2$ range, and we find an impressively increased accuracy in the dynamics of the most important atomic quantities on the time scale of the excitation. We also present the high-order harmonic spectra of H, He and Ne, computed using our 1D atomic model potentials. They show a very good match with the structure and phase obtained from the 3D simulations.   

Trojan Wave Packets in the Circularly Polarized and the Magnetic Fields on the Langmuir type $(1)$ Helium trajectories extended for the $2N$ electron atoms

We extend the concept of the Langmuir type $(1)$ "Hoop Earrings" rotating Helium-like model trajectories [1] used in the early attempts to impose the Hydrogen Bohr atom quantization to more electron atoms to the $2N$ electrons. While the original Helium trajectories consist of two electron points moving in phase on the two parallel circles those correspond to two $N$ electron configurations placed at the vertexes of angles of regular polygons also parallel in space, placed symmetrically on two parallel planes with respect the nucleus and also one being the perpendicular projection of the other. The addition of the Circularly Polarized electromagnetic field with the electric field rotating in planes of the field free electrons is causing the shape polarization distortion from regularity of the resulting polyhedron. The classical stabilization of the trajectories by the combination of fields further leads to the existence of non-dispersing localized wave packets moving around the trajectories. The time dependent Hartree simulations confirming existence of such Wave Packets in a selected cases as well as the simulations using our recently developed Time Dependent Quantum Diffusion Monte Carlo Method are conducted. [1] M. Kalinski, et al., Phys. Rev. Lett. {bf 95}, 103001, (2005).