Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session C9: Strong-Field Physics in Atoms, Molecules, and Clusters |
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Chair: Itzik Ben-Itzhak, Kansas State University Room: 556AB |
Tuesday, May 24, 2016 2:00PM - 2:12PM |
C9.00001: Nonlinear optical response of multiply ionized noble-gas atoms Maryam Tarazkar(1,3), Dmitri Romanov(2,3), Robert Levis(1,3) Calculation of dynamic polarizabilities and hyperpolarizabilities of ionized species using \textit{ab initio} methods presents computational and conceptual difficulties, as these ionized species often have open-shell electronic system. We use multi-configurational self-consistent field (MCSCF) method with extended basis sets for calculating dynamic polarizability and second-order hyperpolarizabilities of atomic noble gases and their multiply charged cations in non-resonant regime. The calculations were performed at wavelengths ranging from about 100 nm to the red of the first multi-photon resonance all the way toward the static regime. The results were benchmarked to those of CCSD calculations for ions of even-number charge. The second-order hyperpolarizability coefficients were found to decrease when the electrons are progressively removed from the system. At higher ionization states, these coefficients become less dispersive as a function of wavelength. The values and even the signs of the $\gamma^{(2)}$ coefficients were found to depend on the spin of the ionic quantum state. Thus, for Ne$^{\mathrm{+3}}$ and Ne$^{\mathrm{+4}}$, in low-spin states ($^{\mathrm{2}}$P$_{u}$, and $^{\mathrm{1}}$S$_{g}$, respectively) the sign of $\gamma^{(2)}$is positive, whereas in high-spin states ($^{\mathrm{4}}$S$_{u}$, and $^{\mathrm{3}}$P$_{g})$ the sign is negative. The calculated hyperpolarizabilities of multiply ionized atoms relate to experiments on very bright high-order harmonic generation in multiply ionized plasmas (D.~Popmintchev \textit{et al.}, \textit{Science}, \textbf{350} (6265), 1225 (2015)). [Preview Abstract] |
Tuesday, May 24, 2016 2:12PM - 2:24PM |
C9.00002: Intense Laser Ionization and Acceleration of Electrons in Highly-Charged Ions Using Vortex Laser Beams Liang-Wen Pi, Andrew Vikartofsky, Anthony F. Starace Recent advances in laser technology have led to the development of high-power petawatt lasers, making possible laser intensities of the order of $\mathrm{10^{22}~W/cm^2}$. An electron in a highly-charged ion can be ionized in a laser field at its peak intensity and swiftly accelerated to GeV energies. Our prior investigation of laser acceleration of electrons using linearly-polarized Gaussian beams (with zero orbital angular momentum) has revealed that the final-state energies and ejection angles of the electrons depend on the initial target ion positions relative to the laser focus.\footnote{L.-W. Pi, S. X. Hu, and A. F. Starace, Phys. Plasmas \textbf{22}, 093111 (2015).} We report here recent simulations of laser ionization and acceleration of electrons using linearly-polarized vortex laser beams (i.e., Laguerre-Gaussian beams), which carry orbital angular momentum and can spin microscopic objects. These simulations show that the inherent spiral phase structure of the vortex beams leads to improved final-state energy and ejection angle distributions of the electrons. [Preview Abstract] |
Tuesday, May 24, 2016 2:24PM - 2:36PM |
C9.00003: Strong-field ionization inducing multi-electron-hole coherence probed by attosecond pulses Jing Zhao, Jianmin Yuan, Zengxiu Zhao Recent advances in attosecond spectroscopy has enabled resolving electron-hole dynamics in real time [1]. The correlated electron-hole dynamics and the resulted coherence are directly related to how fast the ionization is completed. How the laser-induced electron-hole coherence evolves and whether it can be utilized to probe the core dynamics are among the key questions in attosecond physics or even attosecond chemistry. In this work, we propose a new scenario to apply IR-pump-XUV-probe schemes to resolving strong field ionization induced and attosecond pulse driven electron-hole dynamics and coherence in real time. The coherent driving of both the infrared laser and the attosecond pulse correlates the dynamics of the core-hole and the valence-hole which leads to the otherwise forbidden absorption and emission of XUV photon. An analytical model is developed based on the strong-field approximation by taking into account of the essential multielectron configurations. The emission spectra from the core-valence transition and the core-hole recombination are found modulating strongly as functions of the time delay between the two pulses, which provides a unique insight into the instantaneous ionization and the interplay of the multi-electron-hole coherence. (arXiv: 1510.07947 (2015)) [Preview Abstract] |
Tuesday, May 24, 2016 2:36PM - 2:48PM |
C9.00004: Role of high-order dispersion on strong-field laser-molecule interactions Marcos Dantus, Muath Nairat Strong-field (10$^{\mathrm{12}}$-10$^{\mathrm{16}}$ W/cm$^{\mathrm{2}})$ laser-matter interactions are characterized by the extent of fragmentation and charge of the resulting ions as a function of peak intensity and pulse duration. Interactions are influenced by high-order dispersion, which is difficult to characterize and compress. Fourth-order dispersion (FOD) causes a time-symmetric pedestal, while third-order dispersion (TOD) causes a leading (negative) or following (positive) pedestal. Here, we report on strong-field interactions with pentane and toluene molecules, tracking the molecular ion and the doubly charged carbon ion C$^{\mathrm{2+}}$ yields as a function of TOD and FOD for otherwise transform-limited (TL) 35fs pulses. We find TL pulses enhance molecular ion yield and suppress C$^{\mathrm{2+}}$ yield, while FOD reverses this trend. Interestingly, the leading pedestal in negative TOD enhances C$^{\mathrm{2+}}$ yield compared to positive TOD. Pulse pedestals are of particular importance in strong-field science because target ionization or alignment can be induced well before the main pulse arrives. A pedestal following an intense laser pulse can cause sequential ionization or accelerate electrons causing cascaded ionization. Control of high-order dispersion allows us to provide strong-field measurements that can help address the mechanisms responsible for different product ions in the presence and absence of pedestals. [Preview Abstract] |
Tuesday, May 24, 2016 2:48PM - 3:00PM |
C9.00005: Investigation of the single ionization of molecular iodine using velocity map imaging Dale Smith, Vincent Tagliamonti, James Dragan, George Gibson We study the strong-field single ionization of iodine using velocity map imaging and find several distinct dissociation pathways leading to $I_{2} \to I^{+} + I$. To identify the molecular orbital from which the electron is removed we measured the kinetic energy release of the dissociation pathways as a function of laser wavelength, intensity, and polarization. We find that the many of these channels are not consistent with ionization from the first three valence orbitals of $I_{2}$. [Preview Abstract] |
Tuesday, May 24, 2016 3:00PM - 3:12PM |
C9.00006: Inner-orbital ionization of iodine George Gibson, Dale Smith, Vincent Tagliamonti, James Dragan Many coincidence techniques exist to study multiple ionization of molecules by strong laser fields. However, the first ionization step is critical in many experiments, although it is more difficult to obtain information about this initial step. We studied the single electron ionization of $I_2$, as it presents interesting opportunities in that it is heavy and does not expand significantly during the laser pulse. Moreover, there are several distinct low-lying valence orbitals from which the electron may be removed. Most importantly, the kinetic energy release of the $I^+ + I$ dissociation channel can be measured and should correspond to well-known valence levels and separated atom limits. As it turns out, we must invoke deep valence orbits, built from the 5s electrons, to explain our data. Ionization from deep orbitals may be possible, as they have a smaller critical internuclear separation for enhanced ionization. [Preview Abstract] |
Tuesday, May 24, 2016 3:12PM - 3:24PM |
C9.00007: Sequential three-body breakup of a CO$_{\mathrm{2}}^{\mathrm{+}}$ beam JYOTI RAJPUT, U. ABLIKIM, M. ZOHRABI, BETHANY JOCHIM, BEN BERRY, K. D. CARNES, B. D. ESRY, I. BEN-ITZHAK The dissociative double ionization of a CO$_{\mathrm{2}}^{\mathrm{+}}$ beam leading to the three-body fragmentation channel C$^{\mathrm{+}} \quad +$ O$^{\mathrm{+}} \quad +$ O$^{\mathrm{+}}$ can have its origin in either a sequential or concerted process. In case of the sequential mechanism, the first step is a two-body breakup into CO$^{\mathrm{2+}} \quad +$ O$^{\mathrm{+}}$, followed by a second step wherein CO$^{\mathrm{2+}}$ further fragments into C$^{\mathrm{+}} \quad +$ O$^{\mathrm{+}}$. The rotation of the CO$^{\mathrm{2+}}$ formed during the first step [1] has been used to discriminate between the sequential and non-sequential mechanisms in experiments which employ multi-coincidence momentum imaging techniques for detecting recoil fragments [2,3]. We propose a novel way to look at this discriminating feature in terms of the angle of rotation of the CO$^{\mathrm{2+}}$ intermediate. We will also discuss the implications on the measured momentum distribution of detecting indistinguishable fragments in a coincidence measurement. [1] E. Krishnakumar \textit{et al.} ,Phys. Rev. A \textbf{44}, R4098 (1991). [2] N. Neumann \textit{et al.} ,Phys. Rev. Lett. \textbf{104}, 103201 (2010). [3] C. Wu \textit{et al.} ,Phys. Rev. Lett. \textbf{110}, 103601 (2013). [Preview Abstract] |
Tuesday, May 24, 2016 3:24PM - 3:36PM |
C9.00008: Nonadiabatic dynamics in strong field molecular ionization with few cycle laser pulses Vincent Tagliamonti, P\'eter S\'andor, Arthur Zhao, Tam\'as Rozgonyi, Philipp Marquetand, Thomas Weinacht We study strong field ionization in several small molecules using few (4-10) cycle laser pulses. Using a supercontinuum light source, we are able to tune the laser wavelength (photon energy) over $\sim$200 nm (500 meV). We measure the photoelectron spectrum as a function of laser intensity, frequency, and bandwidth and demonstrate some control over the final state of the molecule in the ionization process. We find that intermediate multiphoton resonances and coupled electron nuclear dynamics result in ionization to different ionic continua. Interestingly, not only do these resonances strongly influence the final states produced in the cation, they can also dominate the PES whether the bandwidth is broad or narrow. [Preview Abstract] |
Tuesday, May 24, 2016 3:36PM - 3:48PM |
C9.00009: Strong Field Double Ionization of Conjugated Molecules Arthur Zhao, Peter Sandor, Vincent Tagliamonti, Thomas Weinacht, Spiridoula Matsika We use ultrafast (sub-10 fs) pulses and coincidence velocity map imaging to study strong field double ionization of molecules. We observe an enhancement of the double ionization yield for conjugated molecular systems. This enhancement persists even with elliptically polarized light, which excludes the possibility of re-scattering. Fragment ions resulting from Coulomb explosion of the dications are observed with high kinetic energy, which suggests the removal of deeply bound electrons. This hypothesis is corroborated by electronic structure calculations. [Preview Abstract] |
Tuesday, May 24, 2016 3:48PM - 4:00PM |
C9.00010: Imaging Anisotropic Nanoplasma Dynamics in Superfluid Helium Droplets Camila Bacellar, Adam Chatterley, Florian Lackner, Sri Pemmaraju, Rico Tanyag, Charles Bernando, Deepak Verma, Sean O’Connell, Timur Osipiv, Dipanwita Ray, Kenneth Ferguson, Tais Gorkhover, Michele Swiggers, Maximilian Bucher, Andrey Vilesov, Christoph Bostedt, Oliver Gessner The dynamics of strong-field induced nanoplasmas inside superfluid helium droplets are studied using single-shot, single-particle femtosecond time-resolved X-ray coherent diffractive imaging (CDI) at the Linac Coherent Light Source (LCLS). Intense (\textasciitilde 10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}}$, \textasciitilde 50 fs) 800~nm laser pulses are employed to initiate nanoplasma formation in sub-micron (200 nm -- 600 nm) sized helium droplets. The dynamics of the nanoplasma formation and subsequent droplet evolution are probed by x-rays pulses (\textasciitilde 100 fs, 600 eV) that are delayed with respect to the near-infrared (NIR) pulses by 10's of femtoseconds to hundreds of picoseconds. Pump-probe time-delay dependent effects in the CDI patterns reveal distinct dynamics evolving on multiple timescales. Very fast (\textless 100 fs) appearing features are possibly indicative of electronic dynamics, while slower ($\ge $1 ps) dynamics are likely associated with structural changes correlated to nuclear motion including droplet disintegration. In particular, the CDI images exhibit strong indications for anisotropic dynamics governed by the NIR polarization axis, providing previously inaccessible insight into the mechanisms of nanoplasma formation and evolution. [Preview Abstract] |
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