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 U3: Focus Session: Ultrafast Ionization |
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Chair: Thomas Rescigno, Lawrence Berkeley National Laboratory Room: Keller Building 104 |
Saturday, May 31, 2008 8:00AM - 8:36AM |
U3.00001: Imaging and Controlling Ultrafast Ionization Processes Invited Speaker: We describe how the combination of an attosecond pulse train (APT) and a synchronized infrared (IR) laser field can be used to image and control ionization dynamics in atomic systems. In two recent experiments, attosecond pulses were used to create a sequence of electron wave packets (EWPs) near the ionization threshold in helium. In the first experiment$^\dagger$, the EWPs were created just below the ionization threshold, and the ionization probability was found to vary strongly with the IR/APT delay. Calculations that reproduce the experimental results demonstrate that this ionization control results from interference between transiently bound EWPs created by different pulses in the train. In the second experiment$^{\ddagger}$, the APT was tailored to produce a sequence of identical EWPs just above the ionization threshold exactly once per laser cycle, allowing us to study a single ionization event stroboscopically. This technique has enabled us to image the coherent electron scattering that takes place when the IR field is sufficiently strong to reverse the initial direction of the electron motion causing it to re-scatter from its parent ion.\\ $^\dagger$P. Johnsson, {\it et al.}, PRL {\bf 99}, 233001 (2007).\\ $^\ddagger$J. Mauritsson, {\it et al.} PRL, to appear (2008).\\ In collaboration with A. L'Huillier, J. Mauritsson, P. Johnsson, T. Remetter, E. Mantsen, M. Swoboda, and T. Ruchon. [Preview Abstract] |
Saturday, May 31, 2008 8:36AM - 8:48AM |
U3.00002: Extracting accurate cross sections for one- and two-photon double ionization from a time-dependent wavepacket Alicia Palacios, Thomas Rescigno, C. William McCurdy A method is described for extracting double ionization amplitudes from a quantum wave packet for an atom or molecule after a short radiation pulse, but while the electrons are still interacting. In contrast to other time-dependent methods in which the propagated wavepacket is projected onto an approximate final state at the end of the pulse, we use exterior complex scaling to effectively propagate the field-free solution to infinite times. Our formulation allows us to use existing integral formulas for double ionization amplitudes for two electron atoms and molecules. These amplitudes can be used to obtain the complete angle- and energy-dependent ejection probabilities for the one- or two-photon ionization. We will present results for one-and two-photon double ionization of helium. [Preview Abstract] |
Saturday, May 31, 2008 8:48AM - 9:00AM |
U3.00003: Time-series analysis of vibrational nuclear wave packet dynamics U. Thumm, T. Niederhausen, B. Feuerstein We discuss the extent to which measured time-dependent fragment kinetic energy release (KER) spectra and calculated nuclear probability densities can reveal 1) the transition frequencies between stationary vibrational states, 2) the nodal structure of stationary vibrational states, 3) the ground-state adiabatic electronic potential curve of the molecular ion, and 4) the progression of decoherence induced by random interactions with the environment. We illustrate our discussion with numerical simulations for the time-dependent nuclear motion of vibrational wave packets in the $D_2^+$ molecular ion caused by the ionization of its neutral $D_2$ parent molecule with an intense pump laser pulse. Based on a harmonic time-series analysis, we suggest a general scheme for the full reconstruction, up to an overall phase factor, of the initial wave packets based on measured KER spectra. [Preview Abstract] |
Saturday, May 31, 2008 9:00AM - 9:12AM |
U3.00004: Two-dimensional electron angular distributions from aligned molecules Vinod Kumarappan, Lotte Holmegaard, Christian Martiny, Christian Madsen, Thomas Kjeldsen, Simon Viftrup, Lars Madsen, Henrik Stapelfeldt We report the measurement of 2D electron angular distributions from multiphoton single ionization of aligned CS$_{2}$ molecules. CS$_{2}$ molecules were cooled in a supersonic expansion and aligned non-adiabatically using a Ti:Sapphire laser pulse (800 nm, 0.5 ps, 2.9$\times $10$^{12}$ W/cm$^{2})$ without significant ionization, and a time-delayed second pulse (25 fs, 7.7$\times $10$^{13}$ W/cm$^{2}$ ) was used to singly ionize the molecules at best alignment at the first half-revival. 2D electron momentum spectra were measured using velocity map imaging as a function of angle between the alignment axis and the polarization vector of the ionizing pulse. The angular distribution was found to depend significantly on this angle. Calculations using molecular strong-field approximation were found to account for some, but not all, of the features seen in the experiment. The measurement of molecular-frame photoelectron angular distributions provides new challenges and more stringent tests for theoretical description of molecular ionization by intense lasers. [Preview Abstract] |
Saturday, May 31, 2008 9:12AM - 9:48AM |
U3.00005: Fragmentation of Small Molecules by Photo-Double Ionization Invited Speaker: Molecular structure, formation, breakup pathways and recombination formed the subject of many theoretical and experimental studies. Among molecular species like H$_{2}$, CO, N$_{2}$, O$_{2}$ recently great attention has been paid to the dynamics of the fragmentations and rearrangements of C$_{2}$H$_{2}$ molecule. Nature's smallest stable hydrocarbon, the symmetric linear acetylene molecule, C$_{2}$H$_{2}$, is an important polyatomic system for the study of photo initiated processes. Important features of the intramolecular dynamics in neutral acetylene have been revealed over many years through numerous spectroscopic studies. More recently, the availability of synchrotron radiation and intense laser sources has lead to intriguing studies of the ionization, isomerization and breakup dynamics of acetylene ions. Of particular interest are the yields into the symmetric (CH$^{+}$/CH$^{+})$, deprotonation (HCC$^{+}$/H$^{+})$ and quasi-symmetric (HHC$^{+}$/C$^{+})$ channels, the latter involving isomerization from the neutral acetylene structure into the vinylidene configuration prior to breakup. One expects that the products of dissociation, their kinetic energy releases (KER) and the isomerization times will depend on the particular initial electronic states of the dication involved, but such detailed information has heretofore not been available. We will present the results of the experiment where the dication of acetylene is prepared by Auger decay following core-level X-ray photoionization. Cold Target Recoil Ion Momentum Spectroscopy technique was used to measure the corresponding 3d momentum vectors of Auger electrons and recoil ions in coincidence. We will show that this experimental approach, in combination with \textit{ab initio} quantum mechanical calculations, can yield a comprehensive map of the two-body dissociation pathways including transition through different electronic energy surfaces, barriers to direct dissociation and the associated rearrangement channels. Work done in collaboration with T. Rescigno, T. Weber, S. Miyabe, M. Hertlein, B. Feinberg, M. Prior, and A. Belkacem, Lawrence Berkeley National Laboratory; T. Jahnke, O. Jagutzki, L. Schmidt, M. Sch\"offler, L. Foucar, S. Sch\"ossler, T. Havermeier, S. Voss, and R. D\"orner, Institut fur Kernphysik, J. W. Goethe-Universitat Frankfurt am Main; A. Landers, Auburn University, Alabama; A. Alnaser, Kansas State University; and L. Cocke, Kansas State University. [Preview Abstract] |
Saturday, May 31, 2008 9:48AM - 10:00AM |
U3.00006: Excitation and Ionization of Argon by Few-Cycle Intense Laser Pulses Xiaoxu Guan, Oleg Zatsarinny, Klaus Bartschat, Barry Schneider We have applied our recently developed general {\it ab-initio}, non-perturbative method to solve the time-dependent Schr\"odinger equation (TDSE) for the interaction of a strong laser field with a general atom~[1] to investigate excitation and ionization of Ar by an intense attosecond laser pulse. The field-free Hamiltonian and the dipole matrices are generated using a flexible $B$-spline $R$-matrix (close-coupling) method~[2]. This numerical implementation enables us to construct term-dependent, non-orthogonal sets of one-electron orbitals for the bound and continuum electrons. The solution of the TDSE is propagated in time using the Arnoldi-Lanczos method, which does not require the diagonalization of any large matrices. Results are presented for rearrangements in the outer (3p) and inner (3s) subshells and, when possible, compared with predictions from $R$-matrix Floquet calculations. [1]~X.~Guan {\it et al.}, Phys. Rev. A~{\bf 76}, 053411 (2006). [2]~O.~Zatsarinny, Comp. Phys. Commun. {\bf 174}, 273 (2006). [Preview Abstract] |
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