Session Q04: Ultrafast and Strong Field Physics

Delayed ionization and excitation dynamics in filament wake channels in dense gases and related transient optical nonlinearities.

The processes of subnanosecond evolution of filament wake channels in dense gases, which determine transient, inhomogeneous optical nonlinearities, are investigated theoretically for the case of high-pressure argon. The initial radial profiles of electron density and temperature, as well as the density of ions and excited neutral atoms are set by the energy intake and redistribution during the laser pulse, when the electrons are released via strong-field ionization and forcefully driven by the oscillating laser field, colliding with neighboring neutral atoms. The emerging electron gas thus gains energy via a short-range inverse Bremsstrahlung interaction and is engaged in collisional excitation and impact ionization of the atoms. A kinetic model predicts the prevalence of excited atoms over ionized atoms by the end of the laser pulse. Following fast thermalization in the pulse wake, the system evolution is determined by the continuing impact ionization (from the ground and from the excited states) and excitation of the residual ground-state neutral atoms by hot electrons, as well as by thermal conduction in the cooling electron gas. Solving numerically the system of diffusion-reaction equations, we obtain the spatio-temporal dependences of electron and excited-atom densities and the built-in radial electric field, which allow for tracing the wake channel evolution via linear and nonlinear light-scattering experiments.   

Intense-Field Photoionization of Analogous Molecules by Focused Ultrafast Pulses

The nature of a specific substituent in analogous molecules can dramatically affect their photodynamics. In previous work, we have shown that the dissociation rate for C-X bond breaking in the monohalobenzenes is strongly dependent of the value of X [\textit{Phys. Chem. Chem. Phys.}, \textbf{13}, 13783 (2011)]. We report on the ionization and fragmentation of analogous molecules in the focus of 50-fs, 800-nm laser pulses. Ion mass spectra are recorded as a function of intensity in the absence of the focal volume effect [\textit{Phys. Rev. Lett. }\textbf{100, }023002 (2008)]. Targets include carbon dioxide (CO$_{\mathrm{2}})$, carbon disulfide (CS$_{\mathrm{2}})$, and the substituted monoaromatics aniline (Ph-NH$_{\mathrm{2}})$ and nitrobenzene (Ph-NO$_{\mathrm{2}})$. We observe that the onset of ionization for CO$_{\mathrm{2}}$ and CS$_{\mathrm{2}}$ is dominated by parent ions. Ionization is insignificant for intensities that maximize alignment of CS$_{\mathrm{2}}$, which validates ultrafast electron diffraction experiments from aligned CS$_{\mathrm{2}}$ [\textit{Nature Comm. }\textbf{6, }8172 (2015)]. We observe REMPI in aniline including M$^{\mathrm{+}}$, M$^{\mathrm{2+}}$, \textellipsis with suppressed fragmentation. We observe minor amounts of parent ions and a propensity of fragmentation for nitrobenzene [T. Scarborough (in press) \textit{Phys. Chem. Chem. Phys.}]. Recent work at larger wavelengths will be discussed.   

Ultrafst coupled electron-nuclear dynamics following ionization of propiolic acid

Due to the electron correlation, the removal of an electron from a molecular orbital can trigger ultrafast, pure electron dynamics [1]. The hole charge created by the ionization can migrate throughout the molecule on a few-femtoseconds timescale even at frozen nuclei [2]. The slower nuclear dynamics is expected to dephase the pure electronic coherence at a later stage and trap the migrating charge. A full-dimensional quantum calculation of the concerted electron-nuclear dynamics following the outer-valence ionization of the propiolic acid molecule will be presented, showing that the charge created upon ionization of the HOMO will oscillate between the carbon triple bond and the carbonyl oxygen for more than 10 fs before getting trapped by the nuclear motion [3]. Contrary to recently reported calculations showing an ultrafast dephasing of the charge migration by the nuclear motion, the present results suggest that longer-lived electron coherences can exist even in polyatomics. \newline [1] L. S. Cederbaum and J. Zobeley, Chem. Phys. Lett. \textbf{307}, 205 (1999).\newline [2] A. I. Kuleff and L. S. Cederbaum, J. Phys. B \textbf{47}, 124002 (2014).\newline [3] V. Despr\'e, N. V. Golubev, and A. I. Kuleff, Phys. Rev. Lett. \textbf{121}, 203002 (2018).   

Macroscopic simulation of near-threshold high harmonic generation using microscopic TDSE calculations

Modeling strong-field induced radiation near the ionization threshold requires careful treatment of excited state dynamics. Solution of the time dependent Schrodinger equation (TDSE) provides accurate data in this regime, but the computational time needed prohibits direct calculation of the macroscopic response (e.g., from a gas jet) due to the range of intensities to be considered. We apply a method of interpolation of (precalculated) TDSE results as a function of laser intensity at a given wavelength to simulate the macroscopic propagation of the high harmonic signals using the discrete dipole approximation. This allows investigation of the angular dependence of harmonic and off-harmonic radiation near and below the ionization threshold. Results for hydrogen and helium will be presented.   

Attoclock setup with negative ions: A possibility for experimental validation.

The presumed connection in attoclock setups between the electron tunneling time and its asymptotic momentum has triggered vigorous debates (e.g., [1-3]). In neutral atomic systems investigated so far, including Ar and Kr [4] as well as atomic hydrogen [5], the action of the long-range Coulomb potential on the electron momentum hinders extracting the effect of the tunneling process on the offset angle of the asymptotic electron momentum. We propose and investigate an attoclock setup using F$^-$ or Cl$^-$ to circumvent this difficulty. Our calculations, performed with realistic laser parameters in the tunneling regime, predict essentially a ``zero'' offset angle with no detectable effect of polarization. Most importantly, the predictions could be checked directly against experiment. [1] A. N. Pfeiffer {\it et al.}, Nat. Phy. {\bf 8} (2012) 76. [2] L. Torlina {\it et al.}, Nat. Phy. {\bf 11} (2015) 503. [3] H. Ni, U. Saalmann, and J.-M. Rost, Phys. Rev. Lett. {\bf 117} (2016) 023002. [4] N. Camus {\it et al.}, Phys. Rev. Lett. {\bf 119} (2017) 023201. [5] U. S. Sainadh {\it et al.}, Nature (2019), in press; arXiV:1707.05445. (2018).   

Nonadiabatic effect in laser-induced quantum tunneling

The nonadiabaticity of quantum tunneling through an evolving barrier is relevant to resolving laser-driven dynamics of atoms and molecules at an attosecond timscale. We propose and demonstrate a novel scheme to detect the nonadiabatic behavior of tunnel ionization studied in an attoclock configuration, without counting on the laser intensity calibration or the modeling of the Coulomb effect. In this scheme, the degree of nonadiabaticity for tunneling scenarios in elliptically polarized laser fields can be steered continuously simply with the pulse ellipticity, while the critical instantaneous vector potentials remain identical. We observe the characteristic feature of the measured photoelectron momentum distributions, which matches the distinctive prediction of nonadiabatic theories. Our experiments demonstrate that the nonadiabatic initial transverse momentum at the tunnel exit is approximately proportional to the instantaneous effective Keldysh parameters in the tunneling regime. Our study clarifies a long-standing controversy over the validation of the adiabatic approximation and will substantially advance studies of laser-induced ultrafast dynamics in experiments.   

Trojan Wave Packets in Helium atom in configuration space - highly correlated laser-augmented two-electron motions of strongly driven Helium atom

Some time ago [1] we have extended our theory of the Trojan Wave Packets to the two electron wave packets in Helium atom where the two Trojan Wave Packets on a parallel orbits are mutually stabilizing themselves in so-called Langmuir "hoop earrings" configurations. Unlike for the original Trojan Packets in the Hydrogen atom it requires to put the Helium atom in the magnetic field perpendicular to both orbits in addition to the Circularly Polarized (CP) field to stabilize the classical motion. Noticing that the 1-dimensional model of the Helium atom in the electric field of the linearly polarized electromagnetic wave is similar to the 2D model of the Hydrogen atom in such field and that the superposed hyper-dimensional 2D time dependent electric field has two counter-rotating CP field components we discover stable non-dispersing shape invariant Trojan Wave Packets in configuration space. They correspond to highly correlated electron motions in the physical space when one electron approaches the helium nucleus while the other escapes far. Within the full physical 6D Helium model they correspond to the low angular momentum highly correlated electron wave packet motions along the field polarization axes. [1] M. Kalinski, et al., Phys. Rev. Lett. {\bf 95}, 103001, (2005)   

A semi-classical approach for solving the time-dependent Schroedinger equation (TDSE) in inhomogeneous electromagnetic fields.

We are developing a semi-classical approach to solve the TDSE in spatially inhomogeneous time-dependent electromagnetic fields, extending the complex-trajectory scheme developed by Boiron and Lombardi for time-independent scalar potentials [1]. Our numerical implementation for XUV-IR streaked photoemission from hydrogen atoms results in better agreement with benchmark Crank Nicholson grid-propagation calculations than calculations based on the standard strong-field-approximation (SFA) method [2]. Applications to the reconstruction of spatiotemporally resolved plasmonic fields at the surface of Au nanospheres suggested in [3], allow the highly accurate of reconstruction of the induced transient plasmonic field distribution at the nanoparticle surface. [1] M. Boiron, and M. Lombardi, J. Chem. Phys. 108, 3431 (1998). [2] F. Krausz, and M. Ivanov, Rev. Mod. Phys. 81, 163 (2009). [3] J. Li, E. Saydanzad, and U. Thumm, Phys. Rev. Lett. 120, 223903 (2018) .