Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session M5: Ultrafast Dynamics and New Short Pulse Light Sources |
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Chair: Zenghu Chang, University of Central Florida Room: 310 |
Thursday, June 8, 2017 8:00AM - 8:12AM |
M5.00001: Spectral Broadening and Pulse Compression of a High Average Power Yb:KGW Laser John Beetar, Shima Gholam-Mirzaei, Sean Buczek, Steven Solis, Michael Chini We investigate the spectral broadening and temporal compression of 20 W average power, 50 kHz repetition rate near infrared pulses from a Yb:KGW laser amplifier. The spectrum of the 280 fs pulses centered at 1030 nm is broadened through self-phase modulation in a set of thin fused silica windows. We characterize the potential of the multi-plate continuum setup for achieving a broad supercontinuum spectrum, as well as the energy throughput and stability of the system. We further characterize the spectral phase of the pulses in order to determine the most appropriate approach for pulse compression. Our results suggest that the multi-plate continuum technique is a viable route to obtaining few cycle pulses at high average powers. [Preview Abstract] |
Thursday, June 8, 2017 8:12AM - 8:24AM |
M5.00002: Characterizing attosecond x-ray pulses Stefan Pabst, Marcus Dahlström Attosecond x-ray pulses offer unprecedented opportunities for probing and triggering new types of ultrafast motion. At the same time, their characterization faces new challenges that do not exist in the UV regime. Inner-shell ionization is the dominant ionization mechanism in the x-ray regime and triggers secondary processes like fluorescence, Auger decay, and shake-up. We show that these secondary events create additional delay-dependent modulations. Our recently proposed wavepacket-based characterization scheme can eliminate the impact of these unwanted side effects and is a reliable method for reconstructing attosecond x-ray pulses. [Preview Abstract] |
Thursday, June 8, 2017 8:24AM - 8:36AM |
M5.00003: A Statistical Model of Capillary-Based High-Harmonic Generation Peter Horak, Arthur Degen-Knifton, William S. Brocklesby We present a novel, computationally fast method for estimation of high-harmonic spectra generated from ultrashort laser pulses propagating through gas-filled capillaries. In the regime of high pulse intensities, ionization-induced nonlinearities break up the pump pulse into a train of short sub-pulses. We show that a statistical analysis of the number, peak intensities, and temporal widths of these sub-pulses allows us to calculate approximate high-harmonic spectra via numerical simulations that run up to 100 times faster than full explicit simulations. While our current model does not include all aspects of high-harmonic phase matching and is therefore limited in predicting absolute output powers, we validate the model by comparison with full explicit simulations and with previous experimental results and find good qualitative agreement in features such as spectral broadening with pump pulse energy and gas pressure, photon flux versus capillary length, and spectral shaping by selectively exciting discrete harmonics. [Preview Abstract] |
Thursday, June 8, 2017 8:36AM - 8:48AM |
M5.00004: Attosecond Molecular-Frame Angular Distribution of Electronic Coherence Shungo Miyabe, R. Lucchese, C. W. McCurdy Using \textit{ab initio} electron-ion scattering calculations, it is demonstrated that for a molecule, oriented in space and excited by an attosecond pulse, the degree of electronic coherence left in the ion depends sensitively not only on the orientation of the electric field polarization vector in the molecular frame, but also on the details of the angular distribution in the molecular-frame of electrons ejected in different ionization channels. Accurate modeling of the degree of coherence induced by attosecond ionization in the molecular ion can require a coupled-channel electron-ion scattering wavefunction, which takes interactions of various ionization channels into account and also the inclusion of electron correlation in the wavefunctions of the ionic states, both of which make a notable difference the computed results presented here for the water and glycine molecules. Numerical simulations reported here are based on one-photon single ionization amplitudes calculated using the complex-Kohn variational method, and the amount of coherence in the ion is expressed in terms of the $N$-electron reduced density matrix of the full $(N+1)$-electron system of the ion plus ionized electron. [Preview Abstract] |
Thursday, June 8, 2017 8:48AM - 9:00AM |
M5.00005: Ultrafast Dynamics in Molecular Systems Studied Using Polarization Spectroscopy Niranjan Shivaram, Elio Champenois, Said Bakhti, Richard Thurston, Pavan Muddukrishna, Ali Belkacem Studying ultrafast dynamics in complex polyatomic molecules using photo-electron and photo-ion spectroscopy can be very challenging due to the presence of many competing processes. Photon-in photon-out methods like the widely used transient absorption spectroscopy can be very helpful in isolating some of these processes. Here, we discuss a different approach to measure ultrafast dynamics in molecules using the technique of polarization spectroscopy which is a special case of four-wave mixing. Two pulses (drive and probe) with a relative polarization of 45 degrees interact via the third order susceptibility of a medium. The signal generated along the probe direction with a polarization orthogonal to the input probe polarization is measured using a crossed polarizer. We first demonstrate the method using two weak near infra-red (IR) femtosecond pulses from a Titanium Sapphire laser system to study the electronic response in ultraviolet (UV) grade fused silica. We then discuss the extension of this method to study ultrafast excited state dynamics in molecules like O-nitrophenol using UV and two IR pulses. This method has the potential to be more sensitive than transient absorption by 2 to 3 orders of magnitude without the requirement of resonant transitions to probe the dynamics. [Preview Abstract] |
Thursday, June 8, 2017 9:00AM - 9:12AM |
M5.00006: Dynamic modification of optical nonlinearities related to femtosecond laser filamentation in gases Dmitri Romanov (1,3), Maryam Tarazkar (2,3), Robert Levis (2,3) During and immediately after the passing of a filamenting laser pulse through a gas-phase medium, the nonlinear optical characteristics of the emerging filament-wake channel undergo substantial transient modification, which stems from ionization and electronic excitation of constituent atoms/molecules. We calculate the related hyperpolarizability coefficients of individual ions, and we develop a theoretical model of filament channel evolution applicable to atmospheric-pressure and high-pressure gases. The evolution is mediated by energetic free-electron gas that results from the strong-field ionization and gains considerable energy via inverse Bremsstrahlung process. The ensuing impact ionization and excitation of the residual neutral atoms/molecules proceeds inhomogeneously both inside the channel and on its surface, being strongly influenced by the thermal conduction of the electron gas. The model shows critical importance of channel-surface effects, especially as regards the effective electron temperature. The calculated spatial-temporal evolution patterns ultimately determine the transient modifications of linear and nonlinear optical properties of filament wake channels. Medium-specific estimates are made for atmospheric- and high-pressure argon, as well as for molecular nitrogen gas. [Preview Abstract] |
Thursday, June 8, 2017 9:12AM - 9:24AM |
M5.00007: Ultrafast double hydrogen migration in ethanol Nora G. Kling, Razib Obaid, Sergio Diaz-Tendero, Hui Xiong, Margaret Sundberg, Soroush Khosravi, Michael Davino, Ann Marie Carroll, Timur Osipov, Fernando Martin, Nora Berrah Hydrogen migration is ubiquitous in nature. Strong-field induced single hydrogen migration in small hydrocarbons has been studied with a variety of light sources, and, for acetylene and allene, has even been controlled via the carrier-envelope phase of a laser pulse. Previous strong field laser experiments have also shown that for more complex targets, such as ethanol, two hydrogen atoms can migrate, producing the H$_{\mathrm{3}}$O$^{\mathrm{+}}$ hydronium ion. Here we use 35 fs, 790 nm, mid-10$^{\mathrm{14\thinspace }}$W/cm$^{\mathrm{2}}$ laser pulses, to induce double hydrogen migration in ethanol and record the resulting ionic fragments with a cold-target recoil ion momentum spectrometer (COLTRIMS) apparatus. Following Coulomb explosion, the molecules fragment into many channels, including the coincident H$_{\mathrm{3}}$O$^{\mathrm{+}} \quad +$ C$_{\mathrm{2}}$H$_{\mathrm{3}}^{\mathrm{+\thinspace }}$channel of interest. Theoretical support indicates that the first hydrogen comes from the terminal carbon, and the second comes from the adjacent carbon, occurring on a 10's to 100's of fs timescale. [Preview Abstract] |
Thursday, June 8, 2017 9:24AM - 9:36AM |
M5.00008: Mechanisms and time-resolved dynamics for trihydrogen cation (H$_{\mathrm{3}}^{\mathrm{+}})$ formation from organic molecules in strong laser fields N. Ekanayake, M. Nairat, B. Kaderiya, P. Feizollah, B. Jochim, T. Severt, B. Berry, Kanaka Raju P., K. D. Carnes, S. Pathak, D. Rolles, A. Rudenko, I. Ben-Itzhak, J. E. Jackson, B. G. Levine, M. Dantus Strong-field laser-matter interactions often lead to exotic chemical reactions. H$_{\mathrm{3}}^{\mathrm{+}}$ formation from organic molecules is one such case which requires multiple bonds to break and form. Here, we present the first experimental evidence for the existence of two different reaction mechanisms for H$_{\mathrm{3}}^{\mathrm{+}}$ formation from organic molecules irradiated by a strong-field laser. The assignment of the two different mechanisms was accomplished through the strong-field ionization of methanol isotopomers, ethylene glycol, and acetone. Our findings are supported by femtosecond time-resolved measurements, coincidence measurements, and \textit{ab initio} calculations with the most plausible transition states involved in the two mechanisms. This exotic chemical reaction is important as it shows that a strong laser field can not only selectively break multiple bonds but also can lead to the formation of multiple new bonds within an extremely short timescale, on the order of 100 femtoseconds. This work is supported by the U.S. Department of Energy under Grants DOE SISGR (DE-SC0002325) and DE-FG02-86ER13491. [Preview Abstract] |
Thursday, June 8, 2017 9:36AM - 9:48AM |
M5.00009: Multimode Vibrational Wave Packet Dynamics of Strong-Field-Ionized Methyl Iodide Probed by Femtosecond XUV Absorption Spectroscopy Zhi-Heng Loh, Zhengrong Wei, Jialin Li Studies of vibrational wave packets (VWPs) created on the neutral electronic ground-state by intense laser fields have identified $\it{R}$-selective depletion (RSD) as the dominant mechanism for their generation. Another mechanism that is proposed to give rise to VWPs, bond softening (BS), remains hitherto unobserved. Here, we employ femtosecond XUV absorption spectroscopy to investigate the VWP dynamics of CH$_{3}$I induced by intense laser fields. Analysis of the first-moment time traces computed about the neutral depletion region reveals both the fundamental and the hot bands of the C—I stretch mode. The initial oscillation phases of these vibrations distinguishes the contributions of RSD and BS to the generation of the VWP in the neutral species. The relative oscillation amplitudes that are associated with the two phases suggest that the C—I VWP is generated predominantly by BS. In the case of the CH$_{3}$I$^{+}$ $\tilde{X}$ $^{2}\it{E}_{3/2}$ ion state, VWP motion along the C—I stretch mode is dominant over the CH$_{3}$ umbrella mode. Moreover, the amplitudes of the VWPs are only 1 pm (C—I distance) and 1$^{\circ}$ (H–C–I bond angle). The ability to resolve such VWP dynamics points to the exquisite sensitivity of femtosecond XUV absorption spectroscopy to structural changes. [Preview Abstract] |
Thursday, June 8, 2017 9:48AM - 10:00AM |
M5.00010: Strong-field fragmentation of diiodomethane studied with time-resolved three-body Coulomb explosion Balram Kaderiya, Y. Malakar, Kanaka Raju P., T. Severt, X. Li, W.L. Pearson, F. Ziaee, K. Jensen, J. Rajput, I. Ben-Itzhak, D. Rolles, A. Rudenko Laser Coulomb explosion imaging (CEI) is an efficient tool for mapping time-dependent changes of molecular geometry in many light-induced bond breaking or rearrangement processes. Here, we apply the three-body CEI technique to map in space and time nuclear wave packets created in strong-field ionization and dissociation of diiodomethane molecules. Analyzing coincident three-particle momentum maps in the triply ionized final state, we disentangle different ionization and break-up pathways and trace the time evolution of both, bond lengths and angles for major reaction channels. By combining different representations of the three-body breakup (kinetic energy release vs. relative ion emission angle, Dalitz plots, Newton diagrams etc.), we identify contributions due to bound and dissociating parts of the nuclear wave packet in the singly charged ionic state, observe signatures of I$_{\mathrm{2}}$/I$_{\mathrm{2}}^{\mathrm{+}}$ elimination and highlight the role of intermediate long-lived doubly charged states. [Preview Abstract] |
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