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 O5: The Ultra Intense Laser Frontier (Strong Field, Intense Laser Processes) |
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Chair: Chris H. Greene, University of Colorado Room: Nittany Lion Inn Boardroom I |
Friday, May 30, 2008 8:00AM - 8:12AM |
O5.00001: Non-sequential multiple ionization: signature of single-recollision pathway Phay Ho, Joseph Eberly We present the result of inclusive classical calculations of non-sequential multiple ionization (NSMI) in 780nm laser pulses with intensities above 10$^{14}$ W/cm$^{2}$.~ Using a large ensemble of four-electron trajectories, we obtain the end-of-pulse momentum distributions of ions for the processes of double, triple and quadruple ionization.~ These calculated distributions agree very well with the observed momentum distributions of Ne$^{2+}$, Ne$^{3+}$ and Ne$^{4+}$. Our analysis shows that a double-peak structure in the end-of-pulse momentum distribution of multiply charged ions can be regarded as the signature of NSMI via a single recollision. [Preview Abstract] |
Friday, May 30, 2008 8:12AM - 8:24AM |
O5.00002: Molecular Hydrogen in Intense Ultashort Laser Pulses Alejandro Saenz, Yulian Vanne, Manohar Awasthi Even for the simplest neutral stable molecule, H$_2$, the theoretical treatment of its behavior in intense laser pulses remains an unsolved challenge. In our efforts to achieve this goal, we have now developed a full {\it ab initio} treatment of two-electron diatomic molecules exposed to intense ultrashort laser pulses within the fixed-nuclei approximation. In contrast to previous cases the approach considers all six spatial dimensions and thus allows to investigate, e.g., also a perpendicular orientation of the molecular axis with respect to the laser field. This allows for the first time a direct comparison of the excitation and ionization dynamics of molecular hydrogen in linear polarized laser pulses with either a parallel or a perpendicular orientation of the molecule. Besides total excitation and ionization yields also more detailed information like energy and angular distributions of ejected electrons will be shown. [Preview Abstract] |
Friday, May 30, 2008 8:24AM - 8:36AM |
O5.00003: On the Strong-Field Approximation Describing Atoms and Molecules in Intense Laser Fields Alejandro Saenz, Yulian Vanne A very popular theory to describe atomic (and recently also molecular) behavior in intense laser fields is the strong-field approximation (SFA), also known as Keldysh-Faisal-Reiss theory. It can be seen as the first-order term of an in principle exact time-dependent $S$-matrix theory. We have recently investigated some aspects of this theory in more detail. This includes the correctness of a recently proposed ``exact'' Keldysh theory based on the residue theorem instead of the saddle-point method. Furthermore, we have derived an analytical expression of the velocity-gauge SFA in the long-wavelength limit including the full Coulomb interaction. This revealed a break-down of the theory, but also allowed for a derivation of a new Coulomb correction factor and of a simplified quasi-static SFA (QSFA) that is of special interest for experimental studies in which the wavelength is scanned over large ranges. Finally, we will comment on a recently proposed SFA formulation in a ``gauge-invariant gauge.'' [Preview Abstract] |
Friday, May 30, 2008 8:36AM - 8:48AM |
O5.00004: Single-Active-Electron Approximation for Describing Molecules in Ultrashort Laser Pulses Alejandro Saenz, Manohar Awasthi, Yulian Vanne, Alberto Castro, Piero Decleva A numerical approach that allows for the solution of the time-dependent Schr\"odinger equation (TDSE) describing molecules exposed to intense short laser pulses was developed. The molecular response to the strong field is described within the single-active electron approximation (SAE). The method is applied to molecular hydrogen and the validity of the SAE is investigated by comparing the ionization and electronic excitation yields to full two-electron solutions of the TDSE. The present results are also used to investigate the validity of approximate SAE methods like the molecular Ammosov-Delone-Krainov and the strong-field approximation. Finally, results for larger molecules like O$_2$, N$_2$, and C$_2$H$_2$ (acetylene) are presented. [Preview Abstract] |
Friday, May 30, 2008 8:48AM - 9:00AM |
O5.00005: The effect of position within a laser focus on strong-field ionization Jay Paquette, Jan Chaloupka Atoms in an intense laser field can become doubly ionized through a direct process known as rescattering, where a single electron is liberated through tunnel ionization and is driven back to the ion core by the laser field, leading to impact ionization and release of a second electron. Not surprisingly, the trajectory of the first liberated electron in the field has a strong influence on the probability of release of a second electron. Using completely classical 3-D simulations, we have studied the effects of realistic focal conditions on the single and double ionization yields from a model two-electron atom. In particular, variations in the longitudinal electric field (E$_{z})$ throughout the focal volume can lead to interesting spatially dependent effects, due to an effective elliptical polarization even for a linearly polarized driving field. We will present our latest results from the simulation and discuss the likelihood of observing this effect experimentally. [Preview Abstract] |
Friday, May 30, 2008 9:00AM - 9:12AM |
O5.00006: Testing the Plane Wave Approximation in Electron-Molecule Scattering Zachary B. Walters, Chris H. Greene Laser driven electron recollisions with a parent molecule, in which a strong driving laser first liberates then accelerates and rescatters an electron from a parent molecule, lie at the heart of many current and anticipated efforts to probe the structure of molecules. Techniques such as molecular tomography attempt to reconstruct molecular properties by monitoring scattering output channels such as the generation of high harmonic light. Such techniques are necessarily sensitive to the nature of the electron wavefunction in the region where the electron interacts strongly with the parent ion. We investigate the origins and validity of the commonly used plane wave approximation to the scattering wavefunction, showing how it arises from the semiclassical Gutzwiller propagator and giving the next-order correction. We also compare photoionization (the time reverse of recombination) spectra calculated with the plane-wave approximation to spectra calculated using electron-molecule scattering states for continuum wavefunctions. [Preview Abstract] |
Friday, May 30, 2008 9:12AM - 9:24AM |
O5.00007: Correlated Electron Pairs and Recollision Excitation in Double Ionization Stan Haan, Zach Smith, John VanDyke In strong-field (10$^{14}$-10$^{15}$ W/cm$^{2})$ double ionization of atoms, the production of correlated electron pairs is widely associated with recollision ionization. In this talk we will consider recollision excitation with ionization of the excited electron during the next field maximum. We employ 3d classical ensembles and 1d quantum models to show the important role this process plays in producing correlated electron pairs. We also consider how this process, when combined with backscattering off the nucleus, can lead to electrons with longitudinal momentum exceeding 2$\sqrt {U_p } $, where $U_{p}$ is the ponderomotive energy, such as were reported in [1]. [1] J.S. Parker, \textit{et al.}, Phys. Rev. Lett. \textbf{96}, 133001 (2006); A. Staudte, \textit{et al.}, \textit{ibid.} \textbf{99}, 263002 (2007); A. Rudenko, \textit{et al}, \textit{ibid.} \textbf{99}, 263003 (2007). [Preview Abstract] |
Friday, May 30, 2008 9:24AM - 9:36AM |
O5.00008: Relativistic,$>$1MeV, photoelectrons from the single atom response of Ar and Xe to a 10$^{19}$W.cm$^{-2}$ laser field Isaac Ghebregziabher, Anthony D. Dichiara, Rob Sauer, Jane Waesche, Sasi Palaniyappan, Bruce Wen, Barry C. Walker We measured photoelectron products, with a dynamic range of four orders of magnitude, from the single atom photoionization of argon and xenon exposed to relativistic laser intensity, 10$^{19}$W/cm$^{-2}$. The highest detected photoelectron energies from ionization in a circularly polarized laser field are 1.2MeV and 1.3MeV for Ar and Xe atoms respectively. The measurements show different photoelectron energy spectra for Ar and Xe. The spectrum for Ar drops rapidly by three orders of magnitude over the energy range from 0 to 250KeV. Beyond this range, the spectrum exhibits a broad local maximum at about 550KeV and extends out to 1.2MeV.The spectrum for Xe doesn't show local maximum, instead the spectrum drops by only three orders of magnitude over the entire range from 0 to 1.3MeV. Intensity dependent measurements show that photoelectrons with energies out to 5U$_{p}$, where U$_{p}$ is the nonrelativistic ponderomotive energy, are observable. Azimuthal distribution measurements from photoionization in a linearly polarized light show directional high energy electrons and isotropic low energy electrons. [Preview Abstract] |
Friday, May 30, 2008 9:36AM - 9:48AM |
O5.00009: Alignment-free Molecular Frame Tunneling Probabilities A. Staudte, H. Akagi, D. Pavicic, A. Shiner, F. Turner, R. Doerner, D.M. Villeneuve, M.Yu. Ivanov, P.B. Corkum Tunneling ionization from a molecule in a strong laser field depends on the angle between the molecule and the electric field. This fact reflects the structure of the molecular orbital the electron tunnels from. We introduce a new method to measure the angular ionization probability of a small molecule in a strong field without actively aligning the molecule. Using COLTRIMS we have measured the angular ionization probability in the molecular frame of H$_2$ molecules. We find a considerable deviation from predictions made in established theories. We have also applied this technique to a small heteronuclear molecule. There we find evidence for tunneling from a lower lying molecular orbital. [Preview Abstract] |
Friday, May 30, 2008 9:48AM - 10:00AM |
O5.00010: Triple dissociative ionization of diatomic molecules in strong laser fields J. Wu, Chunlei Guo We study triple dissociative ionization of diatomic molecules in strong laser fields with different laser polarizations. We demonstrate that we can control the molecules to reach their final states either vertically or nonvertically. Kinetic energy release is used to characterize different pathways. The results shown here allow us to gain a better understanding on controlling the pathway of molecular dissociative ionization. [Preview Abstract] |
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