Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session M6: Strong Field Processes II |
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Chair: Andreas Becker, University of Colorado Room: 302 |
Thursday, June 6, 2013 8:00AM - 8:12AM |
M6.00001: Direct observation of strong-field enhanced ionization in CO and N$_{2}$ Wei Lai, Chunlei Guo Enhanced ionization (EI) of molecules has been predicted as a common process in molecular dissociative ionization in strong fields over two decades ago. However, direct evidence for EI has only been found in I$_{2}$ and H$_{2}$. In this work, we perform the first direct study of EI in CO and N$_{2}$. We find a new dissociation channel in each of these two molecules following double ionization that has not been previously resolved. Interestingly, EI occurs only in the newly discovered channels with a lower kinetic energy release but, surprisingly, does not happen in the commonly-seen dissociation channels that were previously assigned undergoing EI. [Preview Abstract] |
Thursday, June 6, 2013 8:12AM - 8:24AM |
M6.00002: R-dependent ionization of the valence orbitals of I2 by strong laser fields Hui Chen, Vincent Tagliamonti, George Gibson Using a pump-dump-probe technique and Fourier-transform spectroscopy, we study the internuclear separation $R$ dependence and relative strength of the ionization rates of the $\pi$ and $\sigma$ electrons of I$_2$, whose valence orbitals are $\sigma_g^2\pi_u^4\pi_g^4\sigma_u^0$. We find that ionization of the HOMO-2 ($\sigma_g$) has a strong dependence on $R$ while the HOMO and HOMO-1 do not. Surprisingly, the ionization rate of the HOMO-2 exceeds the combined ionization rate of the less bound orbitals and this branching ratio increases with $R$. Since our technique produces target molecules that are highly aligned with the laser polarization, the $\sigma$ orbitals will be preferentially ionized and undergo enhanced ionization at larger $R$ compared to the $\pi$ orbitals. Nevertheless, it is highly unusual that an inner orbital provides the dominant strong field ionization pathway in a small molecule. [Preview Abstract] |
Thursday, June 6, 2013 8:24AM - 8:36AM |
M6.00003: Enhanced ionization of an inner orbital of I$_2$ by strong laser fields George Gibson, Hui Chen, Vincent Tagliamonti Using pump-probe spectroscopy, strong-field enhanced-ionization is found in an inner orbital of I$_2$. A wavepacket is launched in the B state of I$_2$, whose valence orbitals are $\sigma^2_g$$\pi^4_u$$\pi^3_g$$\sigma^1_u$, and singly ionized to the I + I$^+$ dissociation channel. The ionization signal peaks at two different internuclear separations: 7.3 and 8.7 a.u. The latter shows enhanced ionization of the $\sigma_u$ state, which has been studied before with the I$_2^+$ signal. However, the peak at smaller R corresponds to enhanced ionization of the $\sigma_g$ state. The peak at 8.7 a.u. in the dissociating channel reveals that there could be strong mixing of different molecular orbitals when the two iodine atoms are pulled apart. [Preview Abstract] |
Thursday, June 6, 2013 8:36AM - 8:48AM |
M6.00004: A look at Strong Field Molecular Ionization via photoelectron-photoion coincidence imaging P\'{e}ter S\'{a}ndor, Arthur Zhao, William Lunden, Thomas Weinacht Strong Field Ionization (SFI) is an important tool for initiating and probing electronic dynamics in molecules. It is the first step of high-harmonic generation, which is currently the main method for producing attosecond pulses. We study strong field ionization of small polyatomic molecules (with several closely spaced low lying ionic states) using velocity map imaging of the photoelectrons in coincidence with the photoions. Our measurements reveal different features in the photoelectron spectrum associated with different fragment ions, and illustrate the production of multi-hole electron wave packets (superpositions of ionic states) via direct ionization from multiple molecular orbitals. [Preview Abstract] |
Thursday, June 6, 2013 8:48AM - 9:00AM |
M6.00005: Dissociation dynamics of Ar2+ in two-color intense laser fields M. Magrakvelidze, J. Wu, R. Doerner, U. Thumm We demonstrate that the dissociation of singly ionized argon dimers can be controlled with two laser pulses of different wavelength. We used 790 nm pump and 1400 nm probe pulses with intensities of 10$^{14}$ W/cm$^{2}$ to study the dissociation dynamics by analyzing kinetic-energy release spectra as a function of the pump-probe delay. The kinetic energy-release spectra are recorded using a COLTRIMS [1-2] setup and compared with model calculations based on the numerical propagations of the time-dependent Schr\"{o}dinger equation [2-3]. We find that the measured spectra are best reproduced by the calculations that include the adiabatic electronic states I(1/2)$_{\mathrm{u}}$ and II(1/2)$_{\mathrm{g}}$ of Ar$_{2}^{+}$. The comparison of the measured and calculated spectra allows us to identify striking frustrated dissociation mechanism.\\[4pt] [1] J. Wu, A. Vredenborg, B. Ulrich, L. Ph. H. Schmidt, M. Meckel, S. Voss, H. Sann, H. Kim, T. Jahnke, and R. D\"orner, PRA \textbf{83}, 061403(R) (2011)\\[0pt] [2] J. Wu, M. Magrakvelidze, A. Vredenborg, L. Ph. H. Schmidt, T. Jahnke, A. Czasch, R. D\"{o}rner, and U. Thumm, Phys. Rev. Lett. \textbf{110}, 033004 (2013)\\[0pt] [3] M. Magrakvelidze, F. He, Th. Niederhausen, I. V. Litvinyuk, and U. Thumm, PRA \textbf{79}, 033410 (2009) [Preview Abstract] |
Thursday, June 6, 2013 9:00AM - 9:12AM |
M6.00006: Two-photon double-ionization of the H$_2$ molecule: effects of pulse duration Xiaoxu Guan, Klaus Bartschat, Lars Koesterke, Barry Schneider In previous work~[1,2], we solved the time-dependent Schr\"odinger equation to calculate the two-photon double ionization of the hydrogen molecule induced by non-sequential absorption of photons with a central energy of 30~eV in a short laser pulse lasting for about 1.6~femtoseconds. At the equilibrium internuclear separation, however, several doubly excited $^1\Sigma_{g,u}$ states lie about 30~eV above the ground $X\,^1\Sigma_g$ state. There is significant disagreement among various results published to date on this problem already for the angle-integrated cross section, and hence for the angular distribution as well. In the present work we address and clarify the fundamental role of those doubly excited states, which are accessible through photon absorption, on the two-photon breakup process. This can only be achieved by allowing for much longer laser pulses.\\[4pt] [1] X.~Guan, K.~Bartschat, and B.~Schneider, Phys. Rev. A {\bf 82}, 041404 (2010).\\[0pt] [2] X.~Guan, K.~Bartschat, and B.~Schneider, Phys. Rev. A {\bf 84}, 033403 (2011). [Preview Abstract] |
Thursday, June 6, 2013 9:12AM - 9:24AM |
M6.00007: Electron localization and nonadiabatic dynamics in laser driven $H_3^{2+}$ Daniel Weflen, Antonio Picon, Agnieszka Jaron-Becker, Andreas Becker We study the laser driven dynamics of molecules with pairs of tightly coupled, nearly degenerate energy eigenstates using $H_3^{2+}$ and other single-active-electron molecules as model systems. In $H_2^+$, with nuclei fixed 7 a.u. apart, the dynamics of two nearly-degenerate energy eigenstates (the ground and first excited state) cause transient localization of the electron on each nucleus, which in turn results in multiple ionization bursts per cycle [1-2]. We present the results of similar calculations. For example, the first two excited states of $H_3^{2+}$ are also nearly-degenerate and closely coupled. Specifically, we reproduce the transient localization of the electron on the outer nuclei found in $H_2^{+}$, and study the effect of these states on charge resonance enhanced ionization (CREI). \\[4pt] [1] N. Takemoto and A. Becker, Phys. Rev. Lett \textbf{105}, 203004 (2010) \\[0pt] [2] N. Takemoto and A. Becker, Phys. Rev. A \textbf{84}, 023401 (2011) [Preview Abstract] |
Thursday, June 6, 2013 9:24AM - 9:36AM |
M6.00008: Below threshold dissociation in the photodissociation of $\rm{H}_2^+$ using ultrashort intense pulse Shuo Zeng, Brett Esry With the increasing availability of few-cycle laser pulses, many studies are showing the short pulse effects that are due to the carrier-envelope phase, but short pulses drive other effects as well. We will focus on one such effect known as below threshold dissociation (BTD), {\em i.e.} the dissociation of a state with fewer {\em net} photons than simple energy conservation would seem to require. Below threshold dissociation has been known as a strong-field mechanism for some time, but its origins in non-adiabatic time evolution mean that it is now showing surprisingly large effects in the very short pulses now used. We thus revisit BTD through our ability to solve the time-dependent Schr\"odinger equation essentially exactly --- neglecting only ionization --- for the benchmark H$_2^+$ system. Our study thus not only takes BTD into a new laser parameter regime, but it also allows investigating the effects of including nuclear rotation in the calculations. Since these had been neglected in previous studies of BTD and we know that rotation effectively wipes out other non-adiabatic effects like vibrational trapping [1], we anticipate that rotation will play an important role.\\[4pt] [1] F. Anis and B. D. Esry, Phys. Rev. A 77, 033416 (2008) [Preview Abstract] |
Thursday, June 6, 2013 9:36AM - 9:48AM |
M6.00009: Ionization of Polar Atoms by Intense, Single-Cycle Fields Sha Li, R.R. Jones We have employed intense, single-cycle THz pulses to explore the ionization of atomic Stark states as a function of the magnitude and direction of their permanent dipole moments. The presence of a permanent dipole moment can substantially influence strong field ionization dynamics in atoms and molecules and lead to directional asymmetries in electron emission. In our experiments, tunable dye lasers are used to excite Na atoms to Rydberg Stark states (n $\approx $ 10) with well-defined permanent dipole moments in the presence of a static electric field. The atoms are then exposed to a single-cycle, THz pulse produced via tilted-pulse-front optical rectification of a 150fs, 780nm laser pulse in LiNbO$_{\mathrm{3}}$. The ionization yield is recorded as a function of the THz field strength. The peak THz field strength required for ionization shows a pronounced variation among states with different dipole moments but with nearly identical binding energies. This orientation dependence can be attributed to diabatic population transfer between ``uphill'' and ``downhill'' oriented states and an asymmetry in the peak THz field strength in the forward and backward directions. [Preview Abstract] |
Thursday, June 6, 2013 9:48AM - 10:00AM |
M6.00010: Electron Shell Ionization with Classical, Relativistic Scattering by Ultrastrong Fields: the Single Atom Response Nagitha Ekanayake, Sui Luo, Patrick Grugan, Willow Crosby, Arielle Camilo, Caitlin McCowan, Rosie Scalzi, Anthony Tramontozzi, Lauren Howard, Sarah Wells, Chris Mancuso, Teddy Stanev, Matthew Decamp, Barry Walker We investigate the forward scattering of ionization from neon, argon, and xenon in ultrahigh intensities of 2 $\times$ 10$^{\mathrm{19}}$ W/cm$^{\mathrm{2}}$. Comparisons between the gases reveal the energy of the outgoing photoelectron determines its momentum, which can be scattered as far forward as 45 degrees from the laser wavevector for energies greater than 1 MeV. The shell structure in the atom manifests itself as modulations in the photoelectron yield and the width of the angular distributions. We arrive at an agreeable comparison with theory using an independent electron model for the atom, dipole approximation for the bound state interaction and a relativistic, three-dimensional, classical radiation field including the laser magnetic field for continuum dynamics. The studies presented here are at the highest intensity yet observed in single atom studies. This work is supported by the Army Research Office under Award No. W911NF-09-1-0390 and National Science Foundation under Award No. 0757953. MFD acknowledges support from the DOE-EPSCoR grant DE-FG02-11ER46816. [Preview Abstract] |
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