Bulletin of the American Physical Society
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 56, Number 5
Monday–Friday, June 13–17, 2011; Atlanta, Georgia
Session H5: Ultrafast Molecular Ionization and HHG |
Hide Abstracts |
Chair: Jason Jones, University of Arizona Room: A705 |
Wednesday, June 15, 2011 8:00AM - 8:12AM |
H5.00001: Multi-photon ionization of the H$_2^+$ molecule by a laser pulse Ethan Secor, Xiaoxu Guan, Klaus Bartschat, Barry I. Schneider We solve the time-dependent Schr\"odinger equation in prolate spheroidal coordinates to calculate the angle-differential cross section of the electron ejected from an aligned H$_2^+$ ion exposed to laser pulses with central photon energies of 40 and 50~eV. A finite-element discrete-variable representation is used to discretize the problem. The work was motivated by discrepancies in the predictions from time-dependent close-coupling~[1] and time-independent exterior complex scaling methods~[2]. As in those calculations, we employed the fixed-nuclei approximation. In the weak-field approximation, the dependence of the ionization amplitude on the relative angles between the laser polarization and the molecular axis can be separated in terms of parallel and perpendicular geometries. The angle-integrated cross section for two- and three-photon absorption was also investigated for central photon energies from $10-30$~eV. We compare these results with those obtained from time-independent perturbation theory.\\[4pt] [1] M. Foster, J. Colgan, O. Al-Hagan, J. L. Peacher, D. H. Madison, and M. S. Pindzola, Phys. Rev. A {\bf 75} (2007) 062707.\\[0pt] [2] T. N. Rescigno, D. A. Horner, F. L. Yip, and C. W. McCurdy, Phys. Rev. A {\bf 72} (2005) 052709. [Preview Abstract] |
Wednesday, June 15, 2011 8:12AM - 8:24AM |
H5.00002: Non-dissociative and dissociative ionization of a CO$^{+}$ beam in intense ultrashort laser pulses B. Gaire, U. Ablikim, M. Zohrabi, S. Roland, K.D. Carnes, I. Ben-Itzhak We have investigated the ionization of CO$^{+}$ beams in intense ultrashort laser pulses. With the recent upgrades to our coincidence three-dimensional momentum imaging method we are able to measure both non-dissociative and dissociative ionization of the molecular-ion beam targets. Using CO$^{+}$ as an example, we have found that non-dissociative ionization (leading to the metastable dication CO$^{2+})$ involves a direct transition, i.e. the molecule is ionized with little or no internuclear distance stretch. Dissociative ionization (C$^{+}$+O$^{+})$ occurs both directly and indirectly, stretching first and then ionizing. Our results show that the yield of dissociative ionization is higher than that of non-dissociative ionization and can be manipulated with the laser pulse duration by suppressing the indirect ionization path using ultrashort pulses ($\le $10 fs). [Preview Abstract] |
Wednesday, June 15, 2011 8:24AM - 8:36AM |
H5.00003: Ultrafast molecular photoionization using a complete Multiconfiguration Time-dependent Hartree Fock Approach Keith V. Lawler, Daniel J. Haxton, C. William McCurdy A MCTDHF method treating electrons and nuclei on the same dynamical footing has been implemented using prolate spheroidal coordinates for the electrons. Many-electron diatomics in fields are treated in full dimensionality. Ionization of diatomics by short pulses is described by combining exterior complex scaling (ECS) and the projected flux method of analyzing the $N$-electron time-dependent MCTDHF wavefunction $ket{\Psi(t)}$. The photoionization cross section from a short pulse calculation is \begin{equation} \sigma(E) = \frac{2 \pi \alpha \omega_{fi}}{| F_{pulse}(E)|^2} \int_{0}^{T}{dt \int_{0}^{T}{dt' \langle\Psi(t')| i (\hat{H}_{ECS}- \hat{H}_{ECS}^\dag)| \Psi(t)\rangle e^{\imath E (t-t')}}} \end{equation} where $H$ is nonhermitian only on the complex of the ECS contour. The pulse ends at $t=0$, and $F_{pulse}(E)$ is its Fourier transform. By projecting the $N$-electron time-dependent wave function onto an electronic state of the $(N-1)$-electron cation, we calculate excitation ionzation cross sections. This approach is applied to channel-specific single photoionization of H$_2$, He$_2$, and N$_2$ including correlation among all electrons. [Preview Abstract] |
Wednesday, June 15, 2011 8:36AM - 8:48AM |
H5.00004: Ellipticity dependence of high harmonics generated using 400 nm driving lasers Yan Cheng, Sabih Khan, Kun Zhao, Baozhen Zhao, Michael Chini, Zenghu Chang High order harmonics generated from 400 nm driving pulses hold promise of scaling photon flux of single attosecond pulses by one to two orders of magnitude. We report ellipticity dependence and phase matching of high order harmonics generated from such pulses in Neon gas target and compared them with similar measurements using 800 nm driving pulses. Based on measured ellipticity dependence, we predict that double optical gating (DOG) and generalized double optical gating (GDOG) can be employed to extract intense single attosecond pulses from pulse train, while polarization gating (PG) may not work for this purpose. [Preview Abstract] |
Wednesday, June 15, 2011 8:48AM - 9:00AM |
H5.00005: Spectral splitting and quantum path identification of high harmonic generation from a semi-infinite gas-cell Wei Cao, Guillaume Laurent, Hui Li, Itzik Ben-Itzhak, Lew Cocke High Harmonic Generation (HHG) from a semi-infinite gas-cell is investigated. By properly tuning the phase matching condition, a double-peak structure in each harmonic order is observed. Using a SFA model calculation, we are able to identify the two peaks as due short and long electronic trajectories. In order to characterize the HHG field, a delayed IR pulse is synchronized to the harmonic field to perform an XUV-IR cross-correlated experiment on a second gas target. The resulting photoelectron spectrum is analyzed to yield the relative phases of the harmonics for each peak separately. This work is supported by Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy; The U.S. Army Research Office under grant number W911NF-07-1-0475 and the National Science Foundation under CHE-0822646. [Preview Abstract] |
Wednesday, June 15, 2011 9:00AM - 9:12AM |
H5.00006: Separation of target structure and medium propagation effects in high-harmonic generation of Ar and N$_{2}$ Cheng Jin, Anh-Thu Le, C.D. Lin We calculate high-harmonic generation (HHG) by intense infrared lasers in atomic and molecular targets taking into account the macroscopic propagation of both fundamental and harmonic fields. We demonstrate that these ab initio calculations are capable of accurately reproducing available experimental results for isotropic and aligned target media despite the sensitivity of HHG spectra to the experimental conditions. We further show that the simulated (or experimental) HHG spectra can be factored out as a product of a ``macroscopic wave packet'' and photo-recombination transition dipole moment where the former depends on the laser properties and the experimental conditions, while the latter is the property of the target only. The factorization makes it possible to extract target structure from experimental HHG spectra, and for ultrafast dynamic imaging of transient molecules. [Preview Abstract] |
Wednesday, June 15, 2011 9:12AM - 9:24AM |
H5.00007: Evidence of the $2s2p(^{1}P)$ Doubly-Excited State in the Harmonic Generation Spectrum of Helium Jean Marcel Ngoko Djiokap, Anthony F. Starace By solving the two-active-electron time-dependent Schr\"{o} dinger equation in an intense, ultrashort laser field, we investigate effects of electron correlation and of doubly excited states of helium on high-order harmonic generation. For a fundamental frequency of the driving laser in the range of the third and the fourth harmonics of the Ti:Sapphire laser, one can put several harmonics (e.g., the 9th, 11th, and 13th) in resonance with the transition between the ground state and the isolated $2s2p(^{1}P)$ autoionizing state of helium. For a moderate peak laser intensity of $10^{14}$ W/cm$^{2}$, we analyze the role of the monochromatic driving laser pulse shape on the intensity of the generated harmonics. In a multiphoton regime in which intermediate Rydberg states are accessible, the intensity of harmonics in resonance with the He$(2s2p)$ autoionizing state is found to increase by a factor of more than 3. For the case of the resonant 11th harmonic, we find that strong coupling between autoionizing states can reduce its enhancement in the vicinity of the $2s2p(^{1}P)$ resonance. [Preview Abstract] |
Wednesday, June 15, 2011 9:24AM - 9:36AM |
H5.00008: Attosecond Coherent Electron WavePackets-CEWP's-Effect of Nuclear Motion Andre D. Bandrauk Attosecond(10$^{-18}$ s)laser pulses allow for the creation of CEWP's-Coherent Electron Wavepackets as time dependent superpositions of delocalized molecular orbitals [1]. The evolution and detection of CEWP,s during photodissociation can be simulated from non-BornOppenheimer time dependent Schroedinger equations, TDSE's on large memory supercomputers, including coupled elelctron-nuclear motion. Two methods will be proposed for studying the dynamics of CEWP's:asymmetric photionization [2-3] or molecular high order harmonic generation, MHOHG [4]on attosecond time scales.\\[4pt] [1] S.Chelkowski, G.L.Yudin, A.D.Bandrauk, J Phys B 39, S409(2006)\\[0pt] [2] A.D.Bandrauk, N H Shon,Intl.J.Quantum Chem (2004)\\[0pt] [3] A.D.Bandrauk, S.Chelkowski, J.Manz, P.B.Corkum, J.Phys B42,134001(2009)\\[0pt] [4] A.D.Bandrauk, S.Chelkowski, T.Bredtman, in preparation [Preview Abstract] |
Wednesday, June 15, 2011 9:36AM - 9:48AM |
H5.00009: Short pulse phenomena produced with long pulses J.V. Hern\'andez, B.D. Esry We have found a way to produce to phenomena usually associated with ultrashort laser pulses using surprisingly long pulses. For example, the spatial asymmetry of a dissociating molecule has been observed to vary with the CEP for very short intense pulses [1-3]. By using chirped pulses with large bandwidths, however, our calculations show large, CEP-dependent asymmetry in dissociating H$_2^+$ even for 100 fs long pulses. This effect is also shown in the spatial asymmetry of an ionized atomic target. We also find varying the bandwidth and the direction of the chirp of the pulse can affect another multi-photon process, zero-photon dissociation of H$_2^+$ [4]. While it is not obvious how these unexpected phenomena can be explained in the usual time-dependent, field-based picture, their explanation in terms of a photon picture is rather straightforward, underscoring its utility even in the strong field regime. [1] M.~F.~Kling {\it et al.}, Science {\bf 312}, 246 (2006). [2] M.~Kremer {\it et al.}, Phys.~Rev.~Lett. {\bf 103}, 213003 (2009). [3] V.~Roudnev, B.~D.~Esry, and I.~Ben-Itzhak, Phys.~Rev.~Lett. {\bf 93}, 163601 (2004). [4] J.~H.~Posthumus {\it et al.} J.~Phys.~B: At.~Mol.~Opt.~Phys. {\bf 28}, 623 (2004). [Preview Abstract] |
Wednesday, June 15, 2011 9:48AM - 10:00AM |
H5.00010: Sub-IR-cycle time-resolved AC Stark shifts in helium Uwe Thumm, Feng He, Camilo Ruiz, Andreas Becker The exposure of atoms to strong infrared (IR) laser fields periodically shifts their energy levels. We discuss a strategy for observing the instantaneous atomic level shifts in helium by analyzing measured delay-dependent XUV pump - IR probe ionization spectra based on numerical solutions of the time-dependent Schr\"odinger equation (SE). For IR pulse intensities that are weak enough to not excite or ionize helium, our scheme exploits that a single spectrally broad attosecond pulses will populate a distribution of excited states out of which the atom can be ionized by the weak IR pulse. Since the ionization probability depends on the magnitude of the IR electric field at the time of XUV excitation, detection of the ionization probability as a function of the center frequency of the XUV pulse and the time delay between the XUV and IR pulses, allows us to deduce instantaneous Stark shift with sub-IR-cycle resolution. We verify that this strategy works after taking into account electron correlation by solving the SE equation for two electrons. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700