Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session J5: Strong Field Ionization |
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Chair: Thomas Weinacht, Stony Brook University Room: Fairfield |
Wednesday, June 10, 2015 2:00PM - 2:12PM |
J5.00001: Atomic tunneling ionization in a photon picture Yujun Wang, B.D. Esry Above-threshold ionization (ATI) and high-harmonic generation (HHG) are studied by the photon-phase formalism [1] in the tunneling regime. Different from the commonly used three-step model for understanding such strong-field phenomena, we show that each order of the ATI or HHG peaks is strongly associated with a single ``photon channel'' in the photon-phase picture. This simplicity allows an identification of pathways for each of the orders. This picture not only provides a convenient means to understand the electron dynamics in the strong field, but also gives insights that may help engineer laser pulses to manipulate the output of the ATI or HHG. We apply this method to quantify the strong-field-induced ionization threshold shift and study the carrier-envelope phase dependence of the HHG. \\[4pt] [1] V. Roudnev and B. D. Esry, Phys. Rev. Lett. 99, 220406 (2007); J. J. Hua and B. D. Esry, J. Phys. B 42, 085601 (2009). [Preview Abstract] |
Wednesday, June 10, 2015 2:12PM - 2:24PM |
J5.00002: Analytic Model for Description of Above-Threshold Ionization by an Intense, Short Laser Pulse Anthony F. Starace, M.V. Frolov, D.V. Knyazeva, N.L. Manakov, J.-W. Geng, L.-Y. Peng We present an analytic model for above-threshold ionization (ATI) of an atom by an intense, linearly-polarized short laser pulse.\footnote{M.V. Frolov \emph{et al.}, Phys. Rev. A \textbf{89}, 063419 (2014).} Our quantum analysis provides closed-form formulas for the differential probability of ATI, with amplitudes 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 analytic results explain key features of short-pulse ATI spectra, such as the left-right asymmetry in the ionized electron angular distribution, the multi-plateau structures, and both large-scale and fine-scale oscillation patterns resulting from quantum interferences of electron trajectories. The ATI spectrum in the middle part of the ATI plateau is shown to be sensitive to the spatial symmetry of the initial bound state of the active electron owing to contributions from multiple-return electron trajectories. An extension of our analytic formulas to real atoms provides results that are in good agreement with results of numerical solutions of the time-dependent Schr\"{o}dinger equation for He and Ar atoms. [Preview Abstract] |
Wednesday, June 10, 2015 2:24PM - 2:36PM |
J5.00003: Phase dependence in above threshold ionization Vincent Carrat, Eric Magnuson, Tom Gallagher Exciting an atom with high-frequency radiation in the presence of a low frequency field can result in energy transfer between the photoelectron and the low frequency field, depending on the phase of the low frequency field when the excitation occurs. We excite Li atoms with IR lasers in the presence of a microwave field. In a previous experiment, detection of highly excited states with excitation by a ps laser tuned above the ionization limit clearly showed a phase dependence. The variation of the signal due to a phase change reached 0.1\% of the total excitation. We are using a new excitation scheme with a CW amplitude modulated laser, where the modulation is phase locked to the microwaves. We observe a signal variation of 10\% of the total excitation. The modulated laser is frequency tuned closer to the limit, explaining some of this increase. Additionally while a ps pulse spreads the population over a broad energy spectrum, the modulated excitation keeps it in narrow bands. By tuning the laser frequency we can couple efficiently those bands to the highly excited states, increasing the collection efficiency. The modulated laser allows the observation of phase dependent transfer to both higher and lower energies. The observations can be described with relatively simple models. [Preview Abstract] |
Wednesday, June 10, 2015 2:36PM - 2:48PM |
J5.00004: Studies of Keldysh Scaled Systems with Ultrafast Strong-Field Sources Urszula Szafruga, Cosmin Blaga, Junliang Xu, Anthony DiChiara, Emily Link, Pierre Agostini, Louis DiMauro Keldysh theory [1] suggests that we can control the multiphoton/tunneling ionization mechanism by choosing appropriate laser parameters and target atoms. The Keldysh parameter values for the noble gases at near-infrared wavelengths (0.8 micron) are similar to those of the alkali metal atoms in strong mid-infrared (3-4 micron) laser fields. By studying atomic species with similar Keldysh parameters and different electronic structures (noble gases vs alkali metals) we aim to expand our understanding of the global, Keldysh invariant, and atom specific ionization features. Further, since alkali metal atoms have a single valence electron they may provide a more appropriate test of theories based off of the single-active-electron approximation. In this work we measured photoelectron spectra and ion yields of Sodium, Potassium and Cesium spanning the range of multiphoton and tunneling ionization regimes. Our findings are discussed in relation to previous results in noble gas/800nm experiments and compared to ADK, SFA and TDSE calculations. \\[4pt] [1] L. V. Keldysh, ``Ionization in the field of a strong electromagnetic wave'', Zh. Eksp. Teor. Fiz. \textbf{47}, 1945 (1964). [Sov. Phys. JETP \textbf{20}, 1307 (1965)]. [Preview Abstract] |
Wednesday, June 10, 2015 2:48PM - 3:00PM |
J5.00005: Laser pulse duration dependence of the low-energy structure in strong field ionization Yu Hang Lai, Kaikai Zhang, Cosmin Blaga, Junliang Xu, Pierre Agostini, Louis DiMauro, Bruno Schmidt, Fran\c{c}ois L\'egar\'e Low-energy structure (LES) in strong field ionization is a spike-like feature appearing in the low energy part (a few eV) of photoelectron spectra along the laser polarization.\footnote{C. I. Blaga et al, Nat. Phys. \textbf{5}, 335 (2009); W. Quan et al, Phys. Rev. Lett. \textbf{103}, 093001 (2009) } It has been observed in rare gas atoms and diatomic molecules. In the classical picture, the formation of LES is due to the Coulomb interaction between the ionized electron and its parent ion via the process of multiple forward scattering, which can happen only if the electron is ionized with a small drift momentum. We have studied the LES in rare gas atoms with few-cycle laser pulses centered at 1800nm. We observed that the LES peak shifts to lower energy as the pulse duration decreases from 5 down to 2 optical cycles, which is in qualitative agreement with classical-trajectory Monte Carlo simulations. Classically, the shift could be attributed to the dependence of the ratio between the field amplitude of the central cycle and the adjacent cycle on the pulse duration.\footnote{A. Kastner, U. Saalmann, and J. M. Rost, J. Phys. B \textbf{45}, 074011 (2012)} Our data support the classical nature of the LES. [Preview Abstract] |
Wednesday, June 10, 2015 3:00PM - 3:12PM |
J5.00006: Breakdown of the Strong-Field Approximation for Transverse Electron Momentum Distributions in Strong-Field Ionization Robert Sang, J.E. Calvert, S. Goodall, X. Wang, H. Xu, A.J. Palmer, I.A. Ivanov, A.S. Kheifets, D. Kielpinski, I.V. Litvinyuk We investigated the transverse electron momentum distributions for the strong field ionization of atoms by laser pulses with varying ellipticity. We investigated two ionization regimes; tunelling and over the barrier ionization regimes. The over the barrier regime was accessed by using neon atoms in excited atomic metastable states and is the first such strong-field experiment to use such an atomic species. We will show that the transverse momentum distributions evolve in qualitatively different when the ellipticity of the driving laser pulses is varied. [Preview Abstract] |
Wednesday, June 10, 2015 3:12PM - 3:24PM |
J5.00007: Strong Field Ionization of small molecules measured with few-cycle ultrafast pulses and Velocity Map Imaging Peter Sandor, Vincent Tagliamonti, Tamas Rozgonyi, Thomas Weinacht Using few-cycle pulses produced through filamentation in Argon, we study strong field ionization of small molecules with velocity map imaging to detect the photoelectrons. We discriminate between the direct removal of electrons from different valence orbitals (`direct ionization') and post-ionization transitions in the ion (`indirect ionization') using coincidence detection of the electron and ion produced from each molecule. We study how direct and indirect ionization vary with pulse duration, intensity and pulse shape, and interpret the pulse shape dependence of the photoelectron spectrum. [Preview Abstract] |
Wednesday, June 10, 2015 3:24PM - 3:36PM |
J5.00008: Strong-field enhanced ionization of molecules Wei Lai, Chunlei Guo Enhanced ionization (EI) of molecules has been extensively studied over the past two decades as a common process in molecular dissociative ionization in strong laser fields. However, experimental study of EI is far from being complete. In this work, we perform a systematic experimental investigation of EI in several commonly-studied small molecules, including N2, O2, and CO. Our results show that double-ionization induced EI occurs only in a newly-discovered channel with a lower kinetic energy release instead of the commonly-seen channel. Furthermore, the dependence of EI on laser intensity and laser polarization is explored. Lastly, the comparison between N2 and O2 shows that molecular orbital shape plays a key role in EI. [Preview Abstract] |
Wednesday, June 10, 2015 3:36PM - 3:48PM |
J5.00009: Fully angle-resolved strong-field ionization and dissociation of ethylene from rotational wavepacket dynamics Vinod Kumarappan, Xiaoming Ren, Anh-Thu Le, Varun Makhija We obtain the full orientation dependence of strong field ionization and dissociation of ethylene, an asymmetric top molecule, by a linearly polarized laser pulse. The molecules are set into complex rotational motion by the non-resonant laser pulse and subsequently ionized or fragmented by a more intense probe pulse. By decomposing the delay dependent yields of ionization dissociation products in a suitable basis set, we obtain the orientation dependences of both processes and show that HOMO and HOMO-1 orbitals contribute to the ionization signal and that ionization from HOMO-1 and HOMO-2 lead to emission of a hydrogen atom. The time-dependent angular distribution and the initial rotational temperature of the molecules are also obtained from the same analysis. [Preview Abstract] |
Wednesday, June 10, 2015 3:48PM - 4:00PM |
J5.00010: Alignment- and orientation-dependent strong-field ionization of molecules: Field-induced orbital distortion effects Maciej Dominik Spiewanowski, Lars Bojer Madsen Strong-field ionization (SFI) is a starting point for many strong-field phenomena, e.g., high-order harmonic generation, as well as a source of fundamental information about the ionized target. Therefore, investigation of SFI of atoms and molecules has been the aim for research since the first strong laser pulses became available. We present a recently developed method, adiabatic strong-field approximation, to study ionization yields as a function of alignment angle for CO2, CO, and OCS molecules. We show that orbital distortion plays an important role in explaining the position and relative strength of maxima in the yields for both polar and nonpolar molecules, even for targets with low polarizabilities at low laser intensities. In particular, we report that for ionization of CO2 the maximum in ionization yield shifts towards the experimentally-measured maximum with respect to the strong-field approximation. For ionization of the CO molecule, not only does the theory predict the preferred direction of ionization correctly, but also the ratio between yields for the two molecular orientations where the electric field points either towards the C or towards the O end. Finally, we find that ionization of OCS is more probable for the laser pointing from the O end towards the S end. [Preview Abstract] |
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