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 G8: Time-Resolved Electron Dynamics and Attosecond Spectroscopy |
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Chair: William McCurdy, University of California, Davis Room: 555AB |
Wednesday, May 25, 2016 8:00AM - 8:12AM |
G8.00001: Birth of a resonant attosecond wavepacket L. Argenti, V. Gruson, L. Barreau, A. Jimenez-Galan, F. Risoud, J. Caillat, A. Maquet, B. Carre, F. Lepetit, J.-F. Hergott, T. Ruchon, R. Taieb, F. Martin, P. Salieres Both amplitude and phase are needed to characterize the dynamics of a wavepacket. However, such characterization is difficult when both attosecond and femtosecond timescales are involved, as it is the case for broadband photoionization to a continuum encompassing autoionizing states. Here we demonstrate that Rainbow RABBIT, a new attosecond interferometry, allows the measurement of amplitude and phase of a photoelectron wavepacket created through a Fano resonance with unprecedented precision. In the experiment, a tunable attosecond pulse train is combined with the fundamental laser pulse to induce two-photon transitions in helium via an intermediate autoionizing state. From the energy and time-delay resolved signal, we fully reconstruct the resonant electron wavepacket as it builds up in the continuum. Measurements accurately match the predictions of a new time-resolved multi-photon resonant model, known to reproduce ab initio calculations. This agreement confirms the potential of Rainbow RABBIT to investigate photoemission delays in ultrafast processes governed by electron correlation, as well as to control structured electron wavepackets. [Preview Abstract] |
Wednesday, May 25, 2016 8:12AM - 8:24AM |
G8.00002: Attosecond pulse characterization with Rydberg wavepackets Stefan Pabst, Eva Lindroth, Marcus Dahlström For attosecond dynamics, it is crucial to know the exact properties of isolated and trains of attosecond pulses. A full pulse characterization is, therefore, of high interest. Here, we propose a new method of fully characterizing these pulses by photoionizing an electronically excited Rydberg wavepackets. The different energy levels of the Rydberg states make it possible to interfere different spectral components with each other. The dipole phases, which normally enter in the ionization step, are eliminated here so that the spectral phase can be uniquely determined. This method is very versatile and offers several advantages that will be discussed. [Preview Abstract] |
Wednesday, May 25, 2016 8:24AM - 8:36AM |
G8.00003: Phase dependent excitation of Rydberg atoms in non-zero average fields. Eric Magnuson, Vincent Carrat, Tom Gallagher The final energy of an electron excited to a high lying Rydberg state in the presence of a microwave (MW) field shows a dependence on the phase of the field at which the excitation occurs. This phase dependence is comparable to that seen in strong field experiments using attosecond pulses to probe systems perturbed by intense infrared (IR) fields. In zero average field, final energies exhibit a phase dependence at twice the frequency of the MW field. We show a phase dependence at the same frequency as the MW field emerges in the presence of a non-zero average field, parallel to the MW polarization. To isolate phase dependence at the MW frequency, we amplitude modulate the IR excitation laser and phase lock this modulation to the MW field. Li atoms are excited to states near the ionization limit in the presence of a MW field, and bound Rydberg states (n>150) are detected. In an applied average field, we observe modulation of the Rydberg signal at the MW frequency. This modulation vanishes as the average field is zeroed, but persists even in fields large enough to ionize most of the population. We compare these results to symmetry arguments and a model of classical Rydberg orbits. An experiment to determine the absolute phase of the modulation relative the MW field is discussed. [Preview Abstract] |
Wednesday, May 25, 2016 8:36AM - 8:48AM |
G8.00004: Tunneling ionization time-resolved by backpropagation Hongcheng Ni, Ulf Saalmann, Jan M. Rost We determine the ionization time in tunneling ionization by an elliptically polarized light pulse relative to its maximum. This is achieved by a full quantum propagation of the electron wave function forward in time, followed by a classical backpropagation to identify tunneling parameters, in particular the fraction of electrons that has tunneled out. We find, that the ionization time is close to zero for single active electrons in helium and in hydrogen if the fraction of tunneled electrons is large. We expect our analysis to be essential to quantify ionization times for correlated electron motion. [Preview Abstract] |
Wednesday, May 25, 2016 8:48AM - 9:00AM |
G8.00005: Circularly Polarized MHOHG with Bichromatic Circularly Polarized Laser Pulses Andre D Bandrauk, Francois Mauger, Turgay Uzer Circularly polarized MHOHG-Molecular High Order Harmonic Generation is shown to occur efficiently with intense ultrashort bichromatic circularly polarized pulses due to frequent electron-parent -ion recollision with co-or counter-rotating incident circular pulses as predicted in 1995[1]..We show in this context that molecules offer a very robust and efficient frameworkfor the production of circularly polarized harmonics for the generation of single circularly polarized ``attosecond'' pulses.[2].The efficiency of such new MHOHG is shown to depend on the compatibility of the symmetry of the molecular medium with the net electric field generated by the combination of the laser pulses.Using a time-dependent symmetry analysis with concrete examples such as H2$+$ vs H3$+$ we show how all the features(harmonic order and \textasciicircum polarization) of MHOHG can be explained and predicted. [1] T Zuo,AD Bandrauk,J Nonl Opt Phys Mat 4,533-546(1995);AD Bandrau,HZ Lu,Phys Rev A 68,043408(2003) [2] KJ Yuan,AD Bandrauk, Phys Rev Lett 110,023003(2013) [Preview Abstract] |
Wednesday, May 25, 2016 9:00AM - 9:12AM |
G8.00006: Inequivalence of Phase and Time Delay in High Harmonic Generation with Short Pulses Dian Peng, Liangwen Pi, Anthony Starace When mixing two (or more) laser pulses, the phase difference and the time delay are two crucial parameters. For long pulses, the relative phase and the time delay are equivalent: for example, $\cos (\omega_{1}t)+\cos (\omega_{2}t+\phi)=\cos (\omega_{1}t) + \cos [\omega_{2}(t+\phi/\omega_{2})]$, i.e. in the extreme case of infinitely long pulses, the phase $\phi$ can be viewed as a time delay $\phi/\omega_{2}$ between the two pulses. However, for ultra short pulses, this equivalence breaks down: the carrier-envelope phase can't be viewed as equivalent to a time delay between two pulse envelopes. Our quantum simulations show that the inequivalence of the phase and the time delay in short pulses can result in significantly different high-order harmonic generation spectra, with up to an order of magnitude difference in intensity and up to about 10 harmonic orders of difference in cutoff energy. Further analysis shows the underlying physics of such difference. Exposing this inequivalence directly for the first time, our work provides new insights into pulse shaping and related issues for both experimentalists and theorists. [Preview Abstract] |
Wednesday, May 25, 2016 9:12AM - 9:24AM |
G8.00007: Angular dependence of the attosecond time delay in the H$_2^+$ ion Anatoli Kheifets, Vladislav Serov Angular dependence of attosecond time delay relative to polarization of light can now be measured using combination of RABBITT and COLTRIMS techniques [1]. This dependence brings particularly useful information in molecules where it is sensitive to the orientation of the molecular axis [2]. Here we extend the theoretical studies of [2] and consider a molecular ion H$_2^+$ in combination of an attosecond pulse train and a dressing IR field which is a characteristic set up of a RABBIT measurement. We solve the time-dependent Schr\"odinger equation using a fast spherical Bessel transformation (SBT) for the radial variable [3], a discrete variable representation for the angular variables and a split-step technique for the time evolution. The use of SBT ensures correct phase of the wave function for a long time evolution which is especially important in time delay calculations. To speed up computations, we implement an expanding coordinate (EC) system [4] which allows us to reach space sizes and time periods unavailable by other techniques. [1] S. Heuser {\em et al} arxiv:1503.08966, 2015, Nat. Phys. submitted [2] V. V. Serov {\em et al} Phys. Rev. A, {\b 87}:063414, 2013 [3] V. V. Serov arXiv:1509.07115, 2015 [4] V. V. Serov {\em et al}, Phys. Rev. A, {\b 75}:012715, 2007 [Preview Abstract] |
Wednesday, May 25, 2016 9:24AM - 9:36AM |
G8.00008: Attosecond time delays in the nuclear dynamics of strong-field molecular dissociation Greg Armstrong The relative time delay in the photoemission from neighboring atomic valence sub-shells has become an area of considerable recent interest, with delays of tens of attoseconds reported in pump-probe experiments for a number of atomic targets. Such delays may be extracted, for example, from phase differences in the photoelectron energy spectra for the different sub-shells as a function of delay between pump and probe pulses. The focus of such experiments has, to date, been atomic targets, on the assumption that only electronic motion can lead to delays on the attosecond scale.We investigate the molecular analogue of such studies by calculating the kinetic-energy release (KER) spectrum for neighboring vibrational states as a function of pump-probe delay time. In particular, we focus on molecular targets where electronic excitation is negligible, and show that attosecond time delays are also possible for purely nuclear motion. We will present evidence of these attosecond delays derived from both numerical solutions of the time-dependent Schr\"{o}dinger equation and experiment. We analyze and understand the observed shifts using the photon-phase formalism [1,2]. [1]V. Roudnev and B. D. Esry, Phys.Rev.Lett. 99 220406 (2007). [2] J.J.Hua and B. D. Esry, J. Phys.B 42 085601 (2009). [Preview Abstract] |
Wednesday, May 25, 2016 9:36AM - 9:48AM |
G8.00009: Observing Superexcited State Dynamics in Atomic and Molecular Systems by Attosecond Transient Absorption Chen-Ting Liao, Xuan Li, Daniel Haxton, Robert Lucchese, C. William McCurdy, Arvinder Sandhu The electron dynamics of superexcited atomic and molecular states are studied experimentally and theoretically. For atomic systems, such as $3s3p^6 5p$ autoionizing state in argon, delay-dependent optical density (OD) only shows positive changes. However, molecular system such as Oxygen shows positive OD in the case of $ns\sigma_g$ and $ns\pi_g$ autoionizing states of oxygen and negative OD for $nd\sigma_g$ states. The negative OD change corresponds to the reduced absorption comparing to the static continuum background. We investigated this electron symmetry dependent effect in depth experimentally by attosecond transient absorption spectroscopy, and theoretically by multiconfiguration time-dependent Hartree-Fock (MCTDHF) method. [Preview Abstract] |
Wednesday, May 25, 2016 9:48AM - 10:00AM |
G8.00010: Studies in Above- and Below-Threshold Harmonics in Argon with an Infrared Femtosecond Laser. Andrew Chew, Yanchun Yin, Jie Li, Xiaoming Ren, Eric Cunningham, Yi Wu, Zenghu Chang We investigate and compare the above- and below-threshold harmonics in Argon gas using our recently-developed 1 kHz, two-cycle (11.4 fs), 3mJ, and carrier-envelope-phase(CEP)-stable laser at 1.6 $\mu $m. Such ultraviolet pulses can serve as pump or probe for studying dynamics in atoms and molecules. Unlike high harmonics with photon energy well above the ionization potential, the mechanism for generating harmonics near the ionization threshold is still under intense investigation. Previous work by Chini et. al. on below-threshold harmonics was done using a 0.8 $\mu $m few-cycle Ti:Sapphire spectrally-broadened source with energy up to 300 $\mu $J. It has been predicted by theory that free-free transitions dominate the below threshold harmonic generation as the laser wavelength increase from near infrared to mid-infrared. We are therefore interested in investigating how using a longer wavelength laser might lead to changes to the behavior of below-threshold harmonics when we vary various parameters. We report the $\pi $-periodity CEP dependence and ellipticity dependence of the above- and below-threshold harmonics. [Preview Abstract] |
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