Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session T4: Molecular Dynamics in Strong Fields |
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Chair: Timur Osipov, SLAC National Accelerator Laboratory Room: Union DE |
Friday, June 12, 2015 8:00AM - 8:12AM |
T4.00001: Excited State Molecular Dynamics in Intense Laser Fields Lucas Zipp, Adi Natan, Philip Bucksbaum We investigate the dynamics of excited electronic states in molecules created through strong field, multiphoton excitation. Several excited states can come into multiphoton resonance with the ground state during an intense laser pulse due to large AC stark shifts, and while most of the excited state population is subsequently ionized, a fraction of the population remains in these excited states. We probe this excited state population in N$_{\mathrm{2}}$ with a time delayed weak field and collect the angle-resolved photoelectron spectrum. By varying the pump intensity and the probe delay, we gain insight into bound state electron dynamics in molecules in intense laser fields, and the ensuing field free dynamics. [Preview Abstract] |
Friday, June 12, 2015 8:12AM - 8:24AM |
T4.00002: The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense few-cycle laser pulses E. Wells, A. Voznyuk, J.B. Mahowald, D.G. Schmitz, T.G. Burwitz, B. Jochim, M. Zohrabi, K.J. Betsch, T. Severt, B. Berry, N.G. Kling, U. Ablikim, K.D. Carnes, I. Ben-Itzhak, R. Siemering, M.F. Kling, R. de Vivie-Riedle Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which have relatively well separated electronic states. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C$_{2}$D$_{2}$, C$_{2}$D$_{4}$, and C$_{2}$D$_{6}$ reveals that the photofragment angular distributions can only be understood when ionization from Rydberg orbitals is considered. These commonly neglected Rydberg orbitals are readily populated for some orientations of the molecule relative to the laser polarization, leading to dissociation patterns and an intensity dependence consistent with significant Rydberg orbital influence. Our results suggest that Rydberg states should be routinely considered when studying polyatomic molecules in intense laser fields. [Preview Abstract] |
Friday, June 12, 2015 8:24AM - 8:36AM |
T4.00003: Long-lived Electronic Coherence of Rydberg States in the Strong-Field Ionization of a Polyatomic Molecule Arkaprabha Konar, Yinan Shu, Benjamin Levine, Vadim Lozovoy, Marcos Dantus Here, we report on quantum coherent control of a large (\textgreater 20 atoms) polyatomic molecule. In particular, we explore the time resolved dynamics of dicyclopentadiene when excited by a pair of phase-locked intense 800nm femtosecond pulses by monitoring changes in ion yield of the parent and fragments. Long-lived oscillations are observed for $\sim$ 500 fs in the parent ion yield indicating the presence of long lived-electronic states. We take advantage of the long-lived electronic coherence to control the yield of different fragment ions. The presence of Rydberg states is further supported by \textit{ab initio} calculations at the EOM-CCSD/6-31$+$G** level of theory which identified five low-lying electronic states of neutral DCPD in the regions between 6.4 and 7.0 eV in vertical excitation energy. States of both pure Rydberg and mixed $\pi \to \pi $*/Rydberg character are observed in this low energy region and are known to originate from ethylene. The multiphoton excitation of two or more Rydberg states, separated by the photon energy is the key to the observed long-lived electronic coherence in DCPD with a quantum beat at the difference frequency. Rydberg states are expected to have very similar potential energy surfaces and the Rydberg electron is relatively uncoupled to the nuclear dynamics, therefore supporting long electronic coherence time. [Preview Abstract] |
Friday, June 12, 2015 8:36AM - 8:48AM |
T4.00004: Comparison of strong-field dissociation dynamics in Li$_2^+$ and H$_2^+$ G.S.J. Armstrong, B.D. Esry Building on previous studies for H$_2^+$, we investigate the dissociation dynamics of Li$_2^+$ in intense laser fields. As the simplest alkali dimer ion, Li$_2^+$ provides a natural prototype system for study after H$_2^+$. Moreover, the more complex electronic structure of Li$_2^+$ allows numerous excited electronic states to play a role in the dynamics, marking a departure from the typical two-state approach used for H$_2^+$. In this study, we solve the time-dependent Schr\"{o}dinger equation in full dimensionality. We identify the dominant dissociation channels over a range of laser parameters, and compare how short-pulse carrier-envelope phase effects are manifest in each molecule. \\[4pt] This work is supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy. [Preview Abstract] |
Friday, June 12, 2015 8:48AM - 9:00AM |
T4.00005: Fragmentation of CD$^{+}$ induced by intense ultrashort laser pulses M. Zohrabi, B. Gaire, U. Ablikim, Bethany Jochim, B. Berry, T. Severt, K.J. Betsch, A.M. Summers, K.D. Carnes, B.D. Esry, I. Ben-Itzhak, I.D. Williams, L. Graham, U. Lev, O. Heber, D. Zajfman The fragmentation of CD$^{+}$ in intense ultrashort laser pulses was investigated using a coincidence three-dimensional momentum imaging technique improved by employing both transverse and longitudinal electric fields. The most probable dissociation pathways for the two lowest dissociation limits, C$^{+} + $ D and C $+$ D$^{+}$, were identified for 22 fs, 798 nm and 50 fs, 392 nm pulses. Curiously, the charge-asymmetric dissociation of CD$^{2+}$, which was measured using 798 nm photons, was not observed with 392 nm photons at the same peak intensity. [Preview Abstract] |
Friday, June 12, 2015 9:00AM - 9:12AM |
T4.00006: Channel competition in strong-field dissociation of CS$^+$ Bethany Jochim, M. Zohrabi, K.J. Betsch, U. Ablikim, Ben Berry, T. Severt, A.M. Summers, K.D. Carnes, B.D. Esry, I. Ben-Itzhak We study intense ultrafast laser-induced dissociation of a CS$^+$ ion beam, utilizing a coincidence 3-D momentum imaging technique. Over a laser intensity range of 10$^{10}$--10$^{16}$ W/cm$^2$, we find clear intensity-dependent behavior of the C$^+$+S and C+S$^+$ branching ratios. Specifically, we observe that the branching ratios are nearly equal at low intensities ($\sim$10$^{10}$--10$^{12}$ W/cm$^2$) and deviate from each other at higher intensities ($>$10$^{13}$ W/cm$^2$), where C+S$^+$ dominates. We propose that the low-intensity branching ratio behavior is due to strong mixing of states corresponding to the relevant dissociation limits mediated by the non-adiabatic couplings, and we identify possible dissociation pathways involving these couplings. Another aspect of channel competition, closing and opening of the two dissociation channels as a function of total energy, is distinctly observed, and this behavior is characterized using the well-known Wigner law for near-threshold behavior [1,2]. \\[4pt] [1] E. P. Wigner, Phys. Rev. \textbf{73}, 1002 (1948).\\[0pt] [2] H. R. Sadeghpour \emph{et al.}, J. Phys. B \textbf{33}, R93 (2000). [Preview Abstract] |
Friday, June 12, 2015 9:12AM - 9:24AM |
T4.00007: Using vibrational Cooper minima to determine strong-field molecular-dissociation pathways T. Severt, M. Zohrabi, G.S.J. Armstrong, J. McKenna, B. Gaire, Nora G. Kling, U. Ablikim, K.D. Carnes, B.D Esry, I. Ben-Itzhak We explore the possibility of using vibrational ``Cooper minima'' (VCM) locations as a method to determine dissociation pathways of molecules in a strong laser field. As a test case, we study the laser-induced dissociation of an O$_2^+$ ion beam by several wavelengths ($\lambda = 800$, 400, and 266 nm) using a coincidence three-dimensional momentum imaging technique. Vibrational structure is observed in the kinetic energy release spectra, revealing a suppression of the dissociation of certain vibrational levels, which is a manifestation of the VCM effect. Previously, it has been shown in H$_2^+$ that first-order time-dependent perturbation theory can be used to predict the locations of the VCM [1]. We explore if the VCM locations predicted by perturbation theory can help uniquely identify dissociation pathways in O$_2^+$ and consider its utility for other systems. \\[4pt] [1] J. McKenna et. al., Phys. Rev. Lett. 103, 103006 (2009). [Preview Abstract] |
Friday, June 12, 2015 9:24AM - 9:36AM |
T4.00008: Enhanced harmonic generation in double-well potentials George Gibson Recently, a new 3-level structure, called the $\Gamma$-system, has been identified and studied, and shows strong multiphoton coupling and harmonic generation. Under certain circumstances, the $\Gamma$-system appears in 1D double well potentials, suggesting that strong multiphoton excitation and harmonic generation may be seen in diatomic molecules at favorable internuclear separations. This talk presents numerical solutions to the TDSE of two electrons in a double well potential approximating a dication (A$_2^{2+}$) at various internuclear separations. These calculations are compared to 2- and 3-level approximations of the full energy level structure. I show: 1) the presence of the $\Gamma$-system in the 1D double-well potential; 2) that harmonic generation is enhanced at certain R; 3) the enhancement is due to the $\Gamma$-system. In addition, I investigate a neutral double well potential (A$_2$). While the level structure is not as clear as the dication, the enhanced harmonic generation is still present. Therefore, neutral diatomic molecules at intermediate R should be a strong source of harmonic generation when exposed to intense laser fields. [Preview Abstract] |
Friday, June 12, 2015 9:36AM - 9:48AM |
T4.00009: Strongly-Driven Triatomic Molecules: Pathways to Formation of Highly Excited Neutral Atoms Agapi Emmanouilidou We present theoretical calculations for the formation of highly excited neutral atoms in molecules. We do so using a 3-d sophisticated quasiclassical technique that fully accounts for the Coulomb singularity. We account for nuclear and electronic motion at the same time. We show that two pathways underlie the formation of highly excited neutral fragments. One resembles ``frustrated'' enhanced ionization while the other resembles ``frustrated'' non-sequential double ionization. That is two-electron effects prevail in one of the two pathways. We discuss how elliptical laser fields can be used to control the contribution of each one of these two pathways.\\[4pt] [1] H. Price, C. Lazarou and A. Emmanouilidou, ``Toolkit for semiclassical computations for strongly-driven molecules: `Frustrated' ionization of H$_{2}$ driven by elliptical laser fields,'' Phys. Rev. A 90, 053419 (2014).\\[0pt] [2] Emmanouilidou, C. Lazarou, A. Staudte and U. Eichmann, ``Routes to formation of highly excited neutral atoms in the breakup of strongly driven hydrogen molecule,'' Phys. Rev. A (Rapid Communication) 85, 011402 (R) (2012). [Preview Abstract] |
Friday, June 12, 2015 9:48AM - 10:00AM |
T4.00010: Ionization and fragmentation of endohedral fullerenes Ho$_{3}$N@C$_{80}$ in an intense femtosecond laser field Hui Xiong, Li Fang, Timur Osipov, Emily Sistrunk, Thomas Wolf, Markus Guehr, Nora Berrah Gas phase endohedral fullerenes Ho$_{3}$N@C$_{80}$ have been studied by excitation with intense femtosecond laser pulses at 800nm. Multiple charge states were found after the laser excitation. The highest charge state of the mother molecules was found to be Ho$_{3}$N@C$_{80}^{4+}$. The molecules absorb multiple photons during the excitation, and may release extra energy by evaporating multiple C$_{2}$ dimers from the carbon cage. Ho$_{3}$N@C$_{70}^{\mathrm{n+}}$ and Ho$_{3}$N@C$_{50}^{\mathrm{n+}}$ were found to be more stable than other fullerene productions. The molecules may also fragment into Ho$^{+}$, HoC$_{2}^{+}$ and HoCN$^{+}$ fragments. The yields of different fragments dependence on laser intensity will be compared to understand the fragmentation pathways. [Preview Abstract] |
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