Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session B3: Ultrafast Collision Processes |
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Chair: H. Schmidt-Boecking, University of Frankfurt Room: TELUS Convention Centre Glen 201-203 |
Wednesday, June 6, 2007 10:30AM - 11:06AM |
B3.00001: Recollision revisited: How far can we push the classical picture? Invited Speaker: The double ionization probability of nobel gases in strong laser fields at intermediate intensities exceeds the probability that can be expected on grounds of an independent electron picture by \emph{several orders of magnitude}. Electron-electron correlation is the well-known origin for this dramatic effect. We have revisited this so called nonsequential double ionization in the simplest 2-electron system, the Helium atom, and, using very high resolution coincidence techniques, we observe a surprising structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process, taking the analogy to the classical (e,2e) processes one step further. This interpretation is supported by inspecting the solution of the 2-body 3-dimensional time-dependent Schr\"{o}dinger equation. [Preview Abstract] |
Wednesday, June 6, 2007 11:06AM - 11:42AM |
B3.00002: Attosecond control of electron dynamics Invited Speaker: The availability of laser pulses with a duration down to about a hundred attoseconds opened up the possibility to study the motion of electrons on the timescales where this motion occurs in nature. Control of chemical reactions or photo-biology has been achieved by using laser fields as photonic reagents, which interact with a medium in a manner that is determined by their duration, intensity, frequency, chirp, and polarization. The introduction of phase-stabilized laser pulses now adds new functionality to photonic reagents to control electronic motion. An experiment will be presented on the dissociation of D$_{2}^{+}$ into D$^{+}$ + D by intense few-cycle laser pulses with controlled field evolution, where a pronounced dependence of the direction of the D$^{+}$ ejection (and hence of the localization of the electron in the system) on the waveform driving the reaction was observed. Quantum-classical computations reveal that light-field control of molecular electron dynamics is responsible for the observed phenomenon. The possibility to steer electron localization in a molecule and control its dissociation, comprises a completely new way of coherent control that takes place on a sub-femtosecond time scale. [Preview Abstract] |
Wednesday, June 6, 2007 11:42AM - 12:18PM |
B3.00003: Ionization and dissociation of molecular ion beams by intense ultrafast laser pulses Invited Speaker: Laser-induced dissociation and ionization of a diatomic molecular-ion beam were simultaneously measured using coincidence 3D momentum imaging, with direct separation of the two processes even where the fragment kinetic energy is the same for both processes. We mainly focus on the fundamental H$_{2}^{+}$ molecule in 7-135 fs laser pulses having 10$^{13}$-10$^{15}$ W/cm$^{2}$ peak intensity. At high intensities the kinetic energy release (KER) distribution following ionization of H$_{2}^{+}$ was measured to be broad and structureless. Its centroid shifts toward higher energies as the laser intensity is increased indicating that ionization shifts to smaller internuclear distances. In contrast, a surprising structure is observed near the ionization threshold, which we call above threshold Coulomb explosion (ATCE) [1]. The angular distributions of the two H$^{+}$ fragments are strongly peaked along the laser polarization, and the angular distribution is described well by [cos$^{2}$\textit{$\theta $}]$^{n}$, where $n$ is the number of photons predicted by our ATCE model [1]. Our data indicates that $n$ varies with the laser wavelength as predicted by the model. The KER and angular distributions of H$_{2}^{+}$ dissociation change dramatically with decreasing pulse width over the 7-135 fs range in contrast to the reported trend for longer pulses. Others contributing to this work: \textit{A.M. Sayler, P.Q. Wang, J. McKenna, B. Gaire, Nora G. Johnson, E. Parke, K.D. Carnes, and B.D. Esry}. Thank are due to Professor Zenghu Chang for providing the intense laser beams and Dr. Charles Fehrenbach for his help with the ion beams. \newline \newline [1] B.D. Esry, A.M. Sayler, P.Q. Wang, K.D. Carnes, and I. Ben-Itzhak, Phys. Rev. Lett. \textbf{97}, 013003 (2006). [Preview Abstract] |
Wednesday, June 6, 2007 12:18PM - 12:54PM |
B3.00004: Laser-induced nonsequential double and multiple ionization of atoms: what can be learned from models Invited Speaker: The significance of a nonsequential channel to double and multiple ionization in some parameter regimes has been long since established. More recently, at least for near-infrared frequencies, consensus has developed that the mechanism is related to recollision of a tunnel-ionized electron with its parent ion. With ab-initio calculations being extremely time consuming, $S$-matrix theory allows for comparatively straightforward computation, once the responsible diagrams have been identified. \\ A crucial element of such a description is the electron-electron interaction that is responsible for the ionization of the second (or more) electron(s) by the first. In this talk, I discuss different choices for this interaction and their consequences for the ion and electron momentum distributions that have been recorded in experiments. I also discuss various methods of how to compute the $S$-matrix element, including saddle-point methods that lead to the concept of quantum orbits and a certain limit that is classical but for the initial tunneling of the first electron. If the electron-electron interaction is of contact type, the latter model becomes a statistical model, which only depends on the tunneling rate, the rescattering kinematics, and the volume of phase space for given final momenta. This statistical model can also be applied for an elliptically polarized laser field. For ellipticities exceeding $\xi \approx 0.3$, interesting effects begin to develop in the momentum distributions.\\ An additional parameter that reflects the joint action of the electron-electron, electron-ion, and electron-field dynamics, can be introduced by assuming a delay between the time of recollision and the later time when a subset of electrons has thermalized with the returning electron and leaves the immediate vicinity of the ion. The existence of such a delay is supported by classical-trajectory calculations. Comparing model calculations with reality one can infer a value of this delay time. For triple and quadruple ionization of neon, a thermalization time below 500 attoseconds gives good agreement with the existing data. In collaboration with P.B. Corkum, C.F.M. Faria, S.P. Goreslavski, P.J. Ho, X. Liu, S.V. Popruzhenko, H. Schomerus, and N. Shvetsov-Shilovski. [Preview Abstract] |
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