Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session W5: Molecules in Strong Fields |
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Chair: Bob Jones, University of Virginia Room: Clark Hall 107 |
Saturday, May 23, 2009 8:00AM - 8:12AM |
W5.00001: Resonance enhanced multiphoton ionization of benzene-like molecules in focused femtosecond radiation with three-dimensional spatial resolution Timothy Scarborough, David Foote, James Strohaber, Cornelis Uiterwaal We present experimental results of spatially resolved ion densities of benzene and other benzene-like molecules through the use of a ``photodynamical test tube' which circumvents volumetric weighting of the focus (the volume effect) [1]. Within the volume of the test tube, the laser intensity is essentially constant, resulting in measurements very near to the true ionization probability of the target molecule. Using laser radiation with a central wavelength of 800 nm and pulse duration of 50 fs focused to intensities ranging from 10$^{13}$ to 10$^{15}$ W/cm$^{2}$, we report experimental verification of REMPI processes in benzene, fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene. In addition, we present preliminary results of the ionization and fragmentation of six-ring aromatic hydrocarbons toluene (C$_{6}$H$_{5}$-CH$_{3})$, aniline (C$_{6}$H$_{5}$-NH$_{2})$, nitrobenzene (C$_{6}$H$_{5}$-NO$_{2})$, and phenol (C$_{6}$H$_{5}$-OH). Finally, we present similar preliminary results for five-ring aromatic heterocyclic compounds pyrrole (C$_{4}$H$_{4}$NH), furan (C$_{4}$H$_{4}$O), and thiophene (C$_{4}$H$_{4}$S). [1] J. Strohaber and C. J. G. J. Uiterwaal, Phys. Rev. Lett. \textbf{100} 023002 (2008). [Preview Abstract] |
Saturday, May 23, 2009 8:12AM - 8:24AM |
W5.00002: Recoil collisions as a portal to field assisted ionization in the strong field ionization of diatomic molecules Agapi Emmanouilidou We explore the dependence of the double ionization of diatomic molecules on the frequency of a strong laser field while keeping the ponderomotive energy constant. For the case of the He atom we have recently found [1] that as we increase the frequency we find that the remarkable ``finger-like'' structure for high momenta recently found for $\omega$=0.055a.u. for the double ionization of the He atom [2] persists for higher frequencies. We compare and discuss the differences between the ``finger-like'' structure in the correlated momenta of strongly driven diatomic molecules versus two electron atoms. \\[4pt] [1] A. Emmanouilidou, Phys. Rev. A 78, 023411 (2008).\\[0pt] [2] A. Staudte et al., Phys. Rev. Lett. 99, 263002 (2007);A. Rudenko et al., Phys. Rev. Lett. 99, 263003 (2007) [Preview Abstract] |
Saturday, May 23, 2009 8:24AM - 8:36AM |
W5.00003: Semiclassical propagation method for tunneling ionization Ilya Fabrikant, Gordon Gallup We apply the semiclassical propagation technique to tunneling ionization in atomic and molecular systems. Semiclassical wave functions and the tunneling flux are calculated from the solution of the classical equations of motion in the complex time plane. We illustrate this method by rederiving the known result for the decay rate of a negative ion in a weak electric field. We then obtain numerical results for atomic hydrogen, H$_2^+$, H$_2$ and Ar, and compare them with the results of the asymptotic (ADK) theory. The asymptotic theory gives surprisingly good results for the atomic and molecular ionization rates. In particular, our calculations for the simplest case of molecular suppression, ionization of H$_2$ versus Ar, confirms the ADK analysis of Tong {\it et al}, Phys. Rev. A {\bf 66}, 013409 (2002) explaining that the suppression is mainly due to the different symmetries of the ionized orbitals, $s$ in H$_2$ and $3p_z$ in Ar. [Preview Abstract] |
Saturday, May 23, 2009 8:36AM - 8:48AM |
W5.00004: Coherent control of the electron motion and localization in the dissociating H$_2^+$ by attosecond and femtosecond laser pulses Feng He, Andreas Becker, Uwe Thumm We show that the electronic dynamics in a molecule driven by a strong field is complex and potentially even counterintuitive. As a prototype example we simulate the interaction of a dissociating H$_2^+$ molecule with an intense infrared laser pulse. Depending on the laser intensity, the direction of the electron's motion between the two nuclei is found to follow or oppose the classical laser electric force. We explain the sensitive dependence of the correlated electronic-nuclear motion in terms of the diffracting electronic momentum distribution of the dissociating two-center system. The distribution is dynamically modulated by the nuclear motion and periodically shifted in the oscillating infrared electric field. [Preview Abstract] |
Saturday, May 23, 2009 8:48AM - 9:00AM |
W5.00005: Real-time dynamics of electron localization observed in dissociating N$_{2}^{3+}$ Igor Litvinyuk, Irina Bocharova, Dipanwita Ray, Sankar De, Lew Cocke When a diatomic molecule (molecular ion) dissociates, the end result is a pair of well separated non-interacting atomic (ionic) fragments. One intriguing question is: when during dissociation do the electrons choose a specific nucleus as their final location? We attempt to answer that question experimentally by measuring dependence of yields and kinetic energies for symmetric (2,2) and asymmetric (3,1) dissociation channels of quadruple-ionized N$_{2}$ on time delay between few-cycle 800 nm pump and probe pulses. The pump pulse produces a dissociating molecular tri-cation from which the time-delayed probe pulse removes the fourth electron. That may result in either (2,2) or (3,1) final dissociation channel. We observe that the asymmetric (3,1) channel is produced only for delays up to 20 fs and completely suppressed for longer delays. The symmetric (2,2) channel is efficiently produced for all studied delays. We interpret that observation as an indication that the unpaired electron in dissociating N$_{2}^{3+}$ is completely localized on one of the ions after 20 fs. After this localization completes, the probe pulse can only further ionize N$^{+}$ as the other ion (N$^{2+})$ has a much higher ionization potential. Such electron localization is an important step in the mechanism of charge-resonant enhanced ionization (CREI). [Preview Abstract] |
Saturday, May 23, 2009 9:00AM - 9:12AM |
W5.00006: Suppression in dissociation probability of H$_2^+$ by ultrashort laser pulses in a high resolution experiment Fatima Anis, J. McKenna, B. Gaire, Nora G. Johnson, M. Zohrabi, A.M. Sayler, K.D. Carnes, I. Ben-Itzhak, B.D. Esry We will present theoretical and experimental results for the kinetic energy spectrum of H$_2^+$ dissociation in short laser pulses. For Ti:Sapphire laser pulses, a surprising suppression of the dissociation probability for the 12th vibrational state of H$_2^+$ has been reported in many studies. This suppression always occurs for the same vibrational state with varying strengths over a range of laser intensities. By changing the wavelength, however, a different vibrational state shows suppression. In some past studies, this suppression has been interpreted as stabilization due to vibrational trapping. We will argue, however, that this behavior is due to the energy dependence of the bound-free vibrational dipole matrix elements. The suppression can thus be predicted with perturbation theory, and we will present experimental evidence supporting these findings. [Preview Abstract] |
Saturday, May 23, 2009 9:12AM - 9:24AM |
W5.00007: Laser Coulomb Explosion Imaging of molecular dynamics in CO$_{2}$ molecule Francois Legare, Irina Bocharova, Igor Litvinyuk, Joseph Sanderson, Reza Karimi Molecular structure dynamics and dissociation pathways of CO$_{2}$ molecule initiated by interaction with strong laser field were investigated by Laser Coulomb Explosion Imaging (LCEI) technique. Momentum imaging of ions in tree-body fragmentation break-up channels O$^{+}$+C$^{+}$+O$^{+}$ (1,1,1) and O$^{2+}$+C$^{2+}$+O$^{2+}$ (2,2,2) was used to determine full geometry of CO$_{2}$ ionic states before explosion. Varying laser pulse length from sub-7 fs to 200 fs at the same laser field intensity we were able to follow the evolution of the molecular structure and observe dramatic change in total kinetic energy of O$^{2+}$+C$^{2+}$+O$^{2+}$ channel with increasing pulse length. We observed significantly bent structure of parent ion and low kinetic energy of the (2,2,2) channel for long pulses, compared to the very close to linear geometry, and very high kinetic energy for sub-7 fs laser pulse. This observation supports the idea that a phenomenon known as enhanced ionization takes place for CO$_{2}$ molecule with the same mechanism as in hydrogen molecule. It also lets us put temporal and spatial limits on this process, and in the future, probing molecular structure within the critical distance range, establish connection between changing geometry and dissociation pathways. [Preview Abstract] |
Saturday, May 23, 2009 9:24AM - 9:36AM |
W5.00008: Fullerenes in intense laser fields Andreas Becker, Agnieszka Jaron-Becker, Farhad Faisal, Marcelo Ciappina We have investigated the response of icosahedral fullerenes to an intense laser pulse using the strong-field $S$-matrix theory. First, we have studied the ionization of C$_{60}$ and other fullerenes up to high charge states and found that the theoretical results are in excellent agreement with experimental observations. The related phenomenon of suppressed ionization of the complex molecules is found to be due to the finite cage size and a multislit interference effect between partial waves emitted from different nuclei. Next, a comparison of the $S$-matrix predictions with recent experimental observations of ionization of C$_{60}$ in elliptically polarized fields will be presented. Finally, we have shown that the multislit interference effects leave their footprints on the high-order harmonic spectra generated in the fullerenes. Characteristic modulations are found in the plateau region of the spectra at mid-infrared wavelengths but not at near-infrared wavelengths. Using a simple spherical model the radius of the fullerene can be retrieved from the positions of the interference minima. [Preview Abstract] |
Saturday, May 23, 2009 9:36AM - 9:48AM |
W5.00009: Absence of permanent dipole transitions in HD$^{+}$ strong-field dissociation B. Gaire, J. McKenna, A.M. Sayler, Nora G. Johnson, M. Zohrabi, K.D. Carnes, B.D. Esry, I. Ben-Itzhak One of the important questions in strong-field molecular physics is: \textit{what is the role of the permanent electric dipole moment of heteronuclear molecules in dissociation?} Recently Kiess \textit{et al}. [Phys. Rev. A \textbf{77}, 053401 (2008)] reported the first exciting evidence for direct two-photon dissociation of an HD$^{+}$ beam involving its permanent dipole moment, using 790 nm, 100 fs laser pulses. However, the measurement was hampered by the fact that the H$^{+ }$(H) and D$^{+ }$(D) fragments could not be well resolved. Using coincidence 3D-momentum imaging we clearly separate and distinguish all fragments in our measurement. Thus, it allows us to determine that the small peak observed and assigned to two-photon dissociation by Kiess \textit{et al}. is instead due to strong bond-softening in the one-photon dissociation. We find no evidence to support the elusive direct two-photon dissociation at 790 nm in the intensity range 5$\times $10$^{12}$ -- 2$\times $10$^{15}$ W/cm$^{2}$. [Preview Abstract] |
Saturday, May 23, 2009 9:48AM - 10:00AM |
W5.00010: Asymmetric ion ejection from hydrogen molecules using two-color laser pulses D. Ray, S. De, F. He, H. Mashiko, U. Thumm, I.V. Litvinyuk, C.L. Cocke, I. Znakovskaya, M.F. Kling, G.G. Paulus It is known that few-cycle phase-stabilized laser pulses can be used to control electron localization in the dissociating hydrogen molecular ion. Here we report experiments which demonstrate a similar control achieved by scanning the relative phase between two-color (800 and 400nm) many-cycle pulses. This approach generates, in an easily reproducible and robust manner, the required asymmetric light-field. The D$^{+ }$(or H$^{+})$ ions from the dissociation of D$_{2}^{+}$ (or H$_{2}^{+})$ are detected using both a velocity-map-imaging system and a stereo-phasemeter. The yield of the fragments, measured as a function of their kinetic energies, shows a clear left-right asymmetry oscillation with the fundamental optical period in the bond-softening and above-threshold-dissociation channels. A similar asymmetry, but out-of-phase, is observed in the rescattering channel. We study the asymmetry dynamics in the different fragmentation channels as a function of the two-color field intensity. Our results are compared with theoretical calculations based on solutions to Schroedinger's equation. [Preview Abstract] |
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