Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session Q1: High Field, Short Pulse Physics |
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Chair: Lew Cocke, Kansas State University Room: Nittany Lion Inn Ballroom CDE |
Friday, May 30, 2008 2:00PM - 2:36PM |
Q1.00001: Attosecond Precision Pump-Probe Experiments: Visualizing Molecular and Electronic Dynamics Invited Speaker: A series of time-resolved experiments is reported with the goal (i) to map, completely characterize and finally to control femtosecond nuclear motion in simple molecules, (ii) to generate and observe ultra-fast electronic wave-packets and, (iii) to study correlated sub-fs few-electron dynamics in strong-field multiple ionization. For that purpose, we have developed a unique combination of a `reaction microscope' spectrometer (with an integrated Li-MOT target) imaging the complete many-particle final-state momentum space and a pump-probe setup providing two 7 fs, ca. 0.1 PW/cm**2 pulses at variable delays between 0 and 3300 fs, reaching absolute and relative precisions as good as 70 as and 1 as, respectively. (i) Via Coulomb explosion imaging we reconstruct the time-dependent probability density of the dissociating, rotating and vibrating nuclear wave-packets in the most fundamental molecular systems, the hydrogen and deuterium molecular ions. We observe the `collapse' and `revival' of their vibrational wave packets, investigate their composition via Fourier analysis, show novel routes to directly visualize field modified potential curves yielding a complete characterization of the field-induced ultra-fast molecular dynamics and, most recently, study the formation of hydrogen molecular ions in laser-induced fragmentation of methane. A one attosecond relative accuracy is demonstrated mapping the vibrational motion in the neutral deuterium molecule and the corresponding excitation mechanism is identified by determination of the absolute quantum phase of the motion. (ii) Using a Li-MOT target as an effective one-electron target we have coherently populated and observed fast electronic wave packets in low-lying states. (iii) For multiple ionization of atoms recoil-ion momentum distributions allow us to distinguish different ionization pathways and to reveal first time information on few-electron emission. For Ne we observe signatures of highly correlated recollision-induced three- and four-electron processes measured to occur on a 500 as time scale. [Preview Abstract] |
Friday, May 30, 2008 2:36PM - 3:12PM |
Q1.00002: Using high-harmonics for probing real-time dynamics of highly excited molecules Invited Speaker: Molecular processes such as inner-valence ionization and shake-up can lead to formation of highly excited states which are close to the double ionization threshold. In order to time resolve the dynamics of such states, one needs a source of light with high photon energy and femtosecond time resolution. Laser high harmonic generation provides such a source with photon energies up to 100 eV with time durations approaching the attosecond regime. We report our recent experiments where we use high-harmonics in conjunction with coincident 3-dimensional photo-fragment momentum imaging technique to time resolve the excited state molecular dynamics. High- harmonic source is used to pump the molecule to highly-excited molecular ion state. The ensuing dynamics is observed by using a time delayed infra-red pulse to further ionize the molecular ions to doubly charged state. [Preview Abstract] |
Friday, May 30, 2008 3:12PM - 3:48PM |
Q1.00003: Correlation in double ionization of He by ultrashort pulses Invited Speaker: Double ionization of helium has long been of considerable interest in atomic physics since it provides insight into the role of electronic correlation in the full three-body Coulomb break-up process, which is of fundamental importance for the understanding of the dynamics in more complex atoms. The recent availability of attosecond XUV pulses allows to directly probe and possibly control the temporal structure of the ionization process. We have implemented an ab initio simulation of the interaction of ultrashort laser pulses with a helium atom. The wave function is represented in a time-dependent close- coupling (TDCC) scheme and time integration is performed utilizing the Arnoldi-Lanczos method. The spatial discretization employs an FEDVR basis, which lends itself to effective parallelization. We will present results on two-photon double ionization of He by ultrashort pulses over a wide range of photon energies. At low energies only non-sequential double ionization is possible (where both electrons share the energy of the photons, and consequently have to be ionized within a short period). For photon energies above $54.4$\,eV (the ionization potential of the He$^+$ ground state), sequential double ionization is allowed. This process proceeds in two steps -- single ionization of He followed by ionization of the remaining He$^+$ ion. By using attosecond XUV pulses, these two separated stages of the sequential process are confined to within a short time interval of each other. We show that the angular distributions of the emitted electrons reveal the signature of a non-sequential process under the condition that sufficiently short pulses are used, while for longer pulses the sequential process completely dominates. The correlation time for double ionization can thus be directly observed using attosecond XUV pulses. This work was performed in collaboration with S.~Nagele, R.~Pazourek, E.~Persson, B.~I.~Schneider, L.~A.~Collins, and J.~Burgd\"orfer. [Preview Abstract] |
Friday, May 30, 2008 3:48PM - 4:24PM |
Q1.00004: Few-Photon Ionization Using Complex Exterior Scaling Invited Speaker: The method of exterior complex scaling allows one to compute numerically converged scattering wave functions without the detailed specification of outer boundary conditions. These wave functions together with integral amplitude extraction methods have previously been successful in treating one-photon double ionization processes. The approach also lends itself to treating few-photon ionization processes. I will illustrate the approach with a description of our recent investigation of two-photon double ionization of helium. For photon energies below 54.4 eV, the ionization energy of He$^+$, two-photon double ionization is necessarily a non-sequential process, with both photons ``cooperating'' to doubly ionize the atom. Above 54.4 eV, the sequential process, where in effect each photon ejects a single electron, is expected to dominate. I will present total, differential, and nuclear recoil cross sections over a range of energies below and above the sequential ionization threshold. One can find a clear signature of sequential ionization in the differential ionization cross sections. In fact, the signature of sequential ionization is seen even below 54.4 eV, where it is only a virtual process. [Preview Abstract] |
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