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 U5: Ionization and Dissociation in Strong-Fields |
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Chair: George Gibson, University of Connecticut Room: 310 |
Friday, June 9, 2017 10:30AM - 10:42AM |
U5.00001: Frequency Modulated Excitation of Rydberg Floquet States in Li Eric Magnuson, Tom Gallagher The Floquet structure of atomic states perturbed by oscillating electric fields is well understood from high resolution spectroscopy. The effect of the oscillating fields is to frequency modulate the atomic states. Here we use a frequency modulated (FM) diode laser to excite the Floquet structure of Li Rydberg states perturbed by synchronous microwave (MW) fields. The probability of excitation is dependent on the phase shift between the laser and state modulation. We observe the phase dependence of the excitation probability, and compare our results to the Floquet description to produce an absolute measure of the phase shift between the frequency modulation of the laser and the MW field at the atoms. This can be used as a reference to determine the absolute phase shift of previous phase dependent ionization experiments, which tests the predictions of the classical electron motion model. [Preview Abstract] |
Friday, June 9, 2017 10:42AM - 10:54AM |
U5.00002: Strong-field electron tunnelling dynamics in atomic hydrogen Robert Sang, U.Satya Sainadh, Han Xu, Atia-Tul Noor, William Wallace, Xiaoshan Wang, Anatoli Kheifets, Igor Ivanov, Klaus Bartschat, Nicolas Douguet, Igor Litvinyuk An atom in the presence an ultra-short pulse of light can significantly distort the binding potential and it is possible for an electron to tunnel though the atomic binding potential. The 'Attoclock' technique, proposed by [Nat.Phys \textbf{4}, 565 (2008)] has facilitated the experimental measurements of tunnelling dynamics, such as tunneling time of an electron through the binding atomic potential. Previous attoclock experiments hinted at zero-tunnelling delays, however, exact theoretical solutions were not available to study the ionization dynamics in detail. Atomic Hydrogen (H) is the simplest atomic system and can be solved exactly using 3D-TDSE. We use the attoclock technique to investigate tunnelling dynamics with H using COLTRIMS and 770nm, 6fs pulses at intensities from 0.165-0.39 PW/cm$^{\mathrm{2}}$. We compare these results with the full solution of 3D-TDSE. [Preview Abstract] |
Friday, June 9, 2017 10:54AM - 11:06AM |
U5.00003: Correlated two-electron quantum dynamics in intense laser fields Dieter Bauer, Martins Brics, Julius Rapp, Adrian Hanusch Two electrons interacting with each other, a binding potential, and a laser field turn out to be a real buzzkill for time-dependent many-body approaches. In particular, TDDFT with known and practicable exchange-correlation potentials fails in reproducing typical strong-field laser-atom phenomena of current interest such as photoelectron or transient absorption spectra. Even high-harmonics spectra, commonly believed to be an ``easy" observable for TDDFT, can be qualitatively wrong. Failures of that kind will be illustrated in the talk. The challenge is to overcome these troubles with TDDFT. Working methods like MCTDHF or TDCI are quite expensive. As TDDFT is based on the single-particle density and MCTDHF/TDCI on the wavefunction, the obvious idea is to go just one ``small" step beyond TDDFT and consider the one-body reduced density matrix (1-RDM) as the basic variable. Deriving equations of motion for the eigenfunctions and eigenvalues of the 1-RDM leads to time-dependent natural orbital theory (TDNOT). It will be shown in the talk that TDNOT indeed overcomes all the problems TDDFT has with two electrons in intense laser fields. [Preview Abstract] |
Friday, June 9, 2017 11:06AM - 11:18AM |
U5.00004: Cation dynamics of molecular Hydrogen in the presence of a strong laser field, preliminary results A. Gatton, E. Champenois, K. Larsen, N. Shivaram, S. Bakhti, W. Iskander, T. Sievert, D. Reedy, M. Weller, J.B. Williams, A. Landers, Th. Weber We present preliminary results from a new 2-color laser+synchrotron Cold Target Recoil Ion Momentum Spectrometer (COLTRIMS) experiment in which we overlap a pulsed laser ($1030nm$, $12ps$, $5*10^{11}W/cm^2$) with light from beamline $10.0.1$ ($18.56eV$, $80ps$, $50meV$ resolution) at the Advanced Light Source at Lawrence Berkeley National Lab. The data (absent the laser) shows asymmetric proton emission in the fragmenting hydrogen cation due to the retro-action of the photoelectron Coulomb potential, as reported recently by Waitz et. al. (PRL 116, 043001 (2016)). Preliminary analysis hints that this effect exists and may even be enhanced in the laser dressed states of the dissociating cation. Of even more interest, preliminary analysis hints at the signature of light induced conical intersections in the dissociation of the laser dressed hydrogen cations, as recently reported by Natan et. al. (PRL 116, 143004 (2016)). [Preview Abstract] |
Friday, June 9, 2017 11:18AM - 11:30AM |
U5.00005: Angular-momentum-assisted dissociation of CO in strong optical fields Amy Mullin, Hannah Ogden, Matthew Murray, Qingnan Liu, Carlos Toro Filaments are produced in CO gas by intense, chirped laser pulses. Visible emission from C$_{\mathrm{2}}$ is observed as a result of chemical reactions of highly excited CO. At laser intensities greater than 10$^{\mathrm{14}}$ W cm$^{\mathrm{-2}}$, the C$_{\mathrm{2}}$ emission shows a strong dependence on laser polarization. Oppositely chirped pulses of light with $\omega _{\mathrm{0}}=$800 nm are recombined spatially and temporally to generate angularly accelerating electric fields (up to 30 THz) that either have an instantaneous linear polarization or act as a dynamic polarization grating that oscillates among linear and circular polarizations. The angularly accelerating linear polarization corresponds to an optical centrifuge that concurrently drives molecules into high rotational states (with J$\approx $50) and induces strong-field dissociation. Higher order excitation is observed for the time-varying laser polarization configuration that does not induce rotational excitation. The results indicate that the presence of rotational angular momentum lowers the threshold for CO dissociation in strong optical fields by coupling nuclear and electronic degrees of freedom. [Preview Abstract] |
Friday, June 9, 2017 11:30AM - 11:42AM |
U5.00006: Angle-dependence of strong-field ionization of singly- and doubly-charged carbonyl sulfide Peter Sandor, Robert R. Jones, Adonay Sissay, Paul Abanador, Fran\c{c}ois Mauger, Mette Gaarde, Kenneth J. Schafer, Kenneth Lopata We have studied the ionization probability of OCS molecules exposed to intense 780 nm laser pulses as a function of the angle between the molecular axis and the linear laser polarization. The molecules are exposed to two laser pulses. The first induces no ionization but, instead, creates a rotational wave packet within each molecule that exhibits preferential alignment in the laboratory frame at specific time delays. We measure the variation in the single and double ionization yield as a function of the delay between the two pulses. We obtain the angular dependent ionization probability by fitting the observed delay-dependent yield to moments of the angular distribution of the rotational wavepacket which can be accurately calculated. The experimentally determined angular distributions are compared to results of new time-dependent density functional theory predictions as well as previous measurements and calculations performed at somewhat lower laser intensities [1]. Accurate molecular ionization rate anisotropies are an important pre-requisite to utilizing strong-field techniques, such as high-harmonic spectroscopy, to probe of intramolecular electron dynamics. [1] J.L. Hansen et al., J. Phys. B 45, 015101 (2012); R. Johansen et al., J. Phys. B 49, 205601 (2016) [Preview Abstract] |
Friday, June 9, 2017 11:42AM - 11:54AM |
U5.00007: Strong Field Probes of Ultrafast Molecular Dynamics: Dissociation of NO$_{2}$ Ruaridh Forbes, Andrey E. Boguslavskiy, Iain Wilkinson, Jonathan G. Underwood, Albert Stolow Strong laser-field based methods such as high-harmonic generation and strong field ionization (SFI) are considered novel probes of ultrafast excited state molecular dynamics. We present an experimental femtosecond time-resolved SFI study of the excited state dynamics of NO$_{2}$ using channel-resolved above-threshold ionization (CRATI) as the probe technique. CRATI makes use of PhotoElectron-PhotoIon COincidence (PEPICO) spectroscopy to study correlations in fragmentation dynamics in molecular systems. The use of PEPICO and covariance methods allows us to correlate ATI photoelectrons associated with a particular ionic fragment and, hence, SFI electron orbital ionization channel. In all ionization channels considered, we observed variations in the ion and photoelectron yields as a function of pump-probe delay as well as the observation of persistent ATI combs at long time delays. In disentangling the excited state dynamics of NO$_{2}$, we examine the complex roles of one-photon excitation, multiphoton excitation to higher lying excited neutral states, non-adiabatic excited state dynamics and several neutral and ionic dissociation channels. [Preview Abstract] |
Friday, June 9, 2017 11:54AM - 12:06PM |
U5.00008: Double Ionization of Water in Strong NIR Fields Greg McCracken, Chelsea Liekhus-Schmaltz, Andreas Kaldun, Philip Bucksbaum Strong field ionization of molecules is more complex than its atomic counterpart due to nuclear motion. This is particularly true in a molecule such as water, which has vibrational half periods on the order of a few optical cycles. In this work, we study the double ionization of H$_{\mathrm{2}}$O in 40 fs, 800 nm pulses at intensities ranging from 10$^{\mathrm{14}}$ W/cm$^{\mathrm{2}}$ to 10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}}$. Single OH$+$-H$+$ dissociations are fully reconstructed using a time and position sensitive ion detector in ultra-high vacuum. We build a 2D map of the kinetic energy release and angular distribution of the dissociation. The map reveals a wealth of different ionization pathways including tunnel ionization from multiple orbitals, bond softening, and enhanced ionization. The fast unbending of the water molecule caused by some pathways is also apparent in rovibrational structure of the OH$+$ fragments seen in the 2D map. [Preview Abstract] |
Friday, June 9, 2017 12:06PM - 12:18PM |
U5.00009: Ionization and fragmentation of methane with intense mid-infrared fields Yu Hang Lai, Junliang Xu, Kaikai Zhang, Xiaowei Gong, Kent Talbert, Pierre Agostini, Cosmin Blaga, Louis DiMauro We investigated the ionization and fragmentation rate of methane (CH4) at several wavelengths between 3 and 4 um. We found that the amount of fragmenting ions relative to the intact molecular ions exhibit a pronounced wavelength dependence and is peaked at around 3.3 to 3.6 $\mu $m. In contrast, the feature is absent in the same measurements with deuterated methane (CD4). The results suggested that the resonance of C-H bond stretching mode is playing a significant role in the dissociation processes. Moreover, by comparing the total ion yields of CH4 with that of CD4, we found that the overall ionization rate of CH4 is also enhanced at around 3.3 to 3.6 $\mu $m. This result has important implications in understanding tunnel ionization in the presence of vibrational resonance. [Preview Abstract] |
Friday, June 9, 2017 12:18PM - 12:30PM |
U5.00010: Internal conversion and strong-field molecular ionization with shaped few-cycle laser pulses Vincent Tagliamonti, Arthur Zhao, Brian Kaufman, Tamas Rozgonyi, Philipp Marquetand, Lea Ibele, Thomas Weinacht We study strong-field molecular ionization and internal conversion dynamics using few-cycle laser pulses tailored with an AOM based pulse shaper. The pulse shapes we used were chosen to illuminate internal conversion dynamics taking place during resonance enhanced strong-field molecular ionization. We measure the photoelectron spectrum as a function of pulse shape and use a recently developed model to interpret the pulse shape dependent molecular dynamics. Our results highlight the subtle interplay of electronic and nuclear dynamics involved in strong-field ionization. [Preview Abstract] |
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