Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N13: Focus Session: Ultrafast and Ultrahigh Field Chemistry I: Strong Field Phenomena |
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Sponsoring Units: DCP Chair: Robert Levis, Temple University Room: Baltimore Convention Center 305 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N13.00001: Strong-Field Physics with Coherently Prepared Molecular Targets Invited Speaker: Intense, short laser pulses can create rotational wavepackets in molecules, resulting in transient preferential molecular alignment in a field free environment. The availability of aligned rather than randomly oriented molecular samples is enabling new strong-field molecular physics experiments which offer additional insight into a variety of complex phenomena. For example, high harmonic generation (HHG) is mediated by electrons that are first tunnel-ionized and then driven back into their parent ions by an intense laser field. Both the initial ionization and recollision events can be strongly dependent on the orientation of the molecular axis with respect to the laser field. Once this dependence is well understood, information regarding the structure of the parent molecule at the instant of the electron/ion recollision might be extracted from the resulting electron and/or photon emission. I will describe methods for manipulating and probing molecular alignment, as well as our recent measurements of the dependence of intense laser ionization rates, HHG yields, and the polarization of high-order harmonics on the alignment of the molecular axis relative to the polarization direction of the intense laser field. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N13.00002: Non-adiabatic Electronic Excitation of Linear Polyenes in the Strong Field Regime Stanley Smith, Xiaosong Li, Alexei Markovich, Dmitri Romanov, Robert J. Levis, H. Bernhard Schlegel Using a newly developed unitary transform time-dependent Hartree-Fock (UT-TDHF) algorithm, the electronic response to an ultrashort strong-field laser pulse was studied on a series of molecules -- ethylene, butadiene, and hexatriene -- in which molecular size and conjugation increase systematically. The evolution of electronic subsystem of molecules exposed to 760 nm 8.75$\times $10$^{13}$ W/cm$^{2}$ of 7 fs duration was calculated using the 6-31G(d,p) basis set. Two scenarios are envisioned: in the first, the molecule interacts with the pulse immediately after ionization; in the second, sufficient time elapses for the molecular geometry to relax. The non-adiabatic behavior of the instantaneous dipole moment and the charge distribution in a molecule is more pronounced for the monocations than for the dications or neutrals. For a given charge state and geometry, the non-adiabatic effects increase with the length of the molecule. As Fourier analysis reveals, the residual (after-pulse) oscillations of the dipole moment are mainly due to non-resonant excitations of the lowest excited states with significant oscillator strength. For each molecule, the non-adiabatic coupling is greater for geometry with the lower excitation energies. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N13.00003: A new type of wavelength dependence in strong-field ionization Dmitri Romanov, Katharine Moore, Ryan Compton, Robert J. Levis It is commonly assumed that in mid-IR region the strong-field ionization approaches quasistatic limit (tunneling, or ADK regime) and ceases to depend on the laser wavelength. Contrary to this notion, ionization yields for the noble gas Xe at intensities from 10$^{13}$-10$^{15}$ W cm$^{-2}$ for wavelengths spanning from 800 to 1500nm reveal strong and counterintuitive wavelength dependence. There is an increasing ionization probability in the strong field regime as the excitation wavelength increases from 800nm to 1500 nm at fixed field intensity. The measured thresholds for the ionization event scale approximately as $\lambda ^{-2}$. We developed a simple quantitative model that extends through-the-barrier tunneling with single photon ionization from a Rydberg intermediate state and captures the observed wavelength dependence. This wavelength dependence will be reduced to some degree if the ionization occurs in a strong DC electric field that is capable to independently ionize the Rydberg states. The wavelength dependence of ionization rate in the quasitstatic regime is of considerable importance for ascertaining the correct physics for various strong field processes. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N13.00004: Orientation effects in Coulomb explosion of H$_{2}$S in intense laser fields studied by coincidence momentum imaging Akiyoshi Hishikawa, Eiji Takahashi The Coulomb explosion of H$_{2}$S in an ultrashort intense laser field (12 fs, 0.33 PW/cm$^{2})$, H$_{2}$S$^{3+} \quad \to $ H$^{+}$ + S$^{+}$ + H$^{+}$, has been studied by the coincidence momentum imaging method to study how the nuclear dynamics depends on the molecular orientation with respect to the laser polarization vector. When the molecular plane, defined as the plane spanned by the fragment momentum vectors, is perpendicular to the laser polarization vector ($\varepsilon )$, the distribution of the total kinetic energy release E shows a peak at E = 21(1) eV. On the other hand, the distribution peak is observed at a substantially smaller value, E = 15(1) eV, when the molecular plane is perpendicular to $\varepsilon $, showing that the Coulomb explosion dynamics of H$_{2}$S depends sensitively on the orientation of the molecular frame to the laser polarization vector. The difference in the peak kinetic energies indicates that the geometrical structure for the perpendicular orientation is less elongated prior to the Coulomb explosion than that for the perpendicular orientation. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:48AM |
N13.00005: Ultrafast hydogen atom dynamics of small hydrocarbon molecules in intense laser fields - Ejection of H3+ and hydrogen migration Invited Speaker: In ultrashort intense laser fields, molecules are decomposed into fragments via a variety of dissociation pathways. Among them, ultrafast migration of hydrogen atoms within molecules as well as efficient ejection of H3+ molecular ions are noteworthy [1]. By referring to our recent studies on small hydrocarbon molecules in intense laser fields [2] by the coincidence momentum imaging method [3], I will show how ultrafast dynamics of hydrogen atoms are induced within duration of ultrashort intense laser pulses. \newline \newline [1] Y. Furukawa, K. Hoshina, K. Yamanouchi, H. Nakano, Chem. Phys. Lett. 414, 117 (2005). \newline [2] T. Okino, Y. Furukawa, P. Liu, T. Ichikawa, R. Itakura, K. Hoshina, K. Yamanouchi, and H. Nakano, Chem. Phys. Lett. 419, 223 (2005). \newline [3] H. Hasegawa, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 349, 57 (2001). [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N13.00006: Femtosecond Laser Ionization of Organic Amines with Very Low Ionization Potential. Tomoyuki Yatsuhashi, Takashi Obayashi, Michinori Tanaka, Masanao Murakami, Nobuaki Nakashima The interaction between high intensity femtosecond laser and molecules is one of the most attractive areas in laser chemistry and ionization is the most fundamental subject. Theoretical consideration successfully reproduced the ionization behavior of rare gases. However, the understanding of ionization mechanisms of large molecules is difficult more than those of rare gases due to their complexity. Generally speaking, molecules are harder to ionize than rare gases even if they have the same ionization potential. The suppressed ionization phenomena are one of the important features of molecular ionization. Hankin \textit{et al}. examined 23 organic molecules with ionization potentials between 8.25 and 11.52 eV. We have examined ionization and/ or fragmentation of many organic molecules, including aromatic compounds, halogenated compounds, methane derivatives etc. at various wavelengths below 10$^{16}$ Wcm$^{-2}$. In order to investigate the nature of molecular ionization, it is interesting to examine a variety of molecule in a wide range of ionization potential. In this study, we examined several organic amines because we can explore the uninvestigated ionization potential range down to 5.95 eV. In addition to the significant suppression of the ionization rates, stepwise ionization behavior, which was not observed in rare gases, was observed. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N13.00007: All-solid-state, ultraviolet, high power laser system using Ce:LiCAF as a gain medium Nobuhiko Sarukura, Shingo Ono, Hidetoshi Murakami, Alex Quema, Elmer Estacio, Tsuguo Fukuda High peak-power, femtosecond, ultraviolet (UV) lasers have attracted new interest. Chirped pulse amplification (CPA) in the UV region has been demonstrated using Ce:LiCaAlF6 (Ce:LiCAF) crystal as the gain medium. The peak power of the amplified and compressed pulse (115 fs) reached 30 GW at 290 nm. To increase the peak power to the terawatt (TW) level, further pulse compression is desired. Since Ce:LiCAF has a tunability of 281nm to 315nm, it holds promise for 3-fs pulse generation which are required for seeding TW-class Ce:LiCAF lasers. The pulse-width of the frequency-tripled Ti:sapphire regenerative amplifier was measured to be 210 fs. The seed pulses were then focused into a hollow fiber filled with argon to spectrally broaden the pulses due to self-phase-modulation. The pulses were then compressed to 25 fs by dispersion-compensation. The fourth harmonics of a Nd:YAG laser (266 nm) is an ideal pump source as it falls within the absorption band Ce:LiCAF. We have generated 430 mJ fourth harmonics with a total conversion efficiency of 30.5{\%} using Li2B4O7$_{ }$(LB4) crystals. A Ce:LiCAF$_{ }$double-pass power-amplifier was then designed with a peak energy of 98 mJ for a 13 mJ seed pulse and an extraction efficiency of 25{\%}. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N13.00008: Influence of linear chirp on non-vertical transitions in a dye solution Elizabeth Carroll, Andrei Florean, Philip Bucksbaum, Roseanne Sension, Ken Spears Chirped laser pulses can manipulate vibrational coherences in dye solutions. We show that the effect of linear chirp also depends strongly on the power spectrum of the ultrafast pulse. We use a programmable phase mask to control the spectral phase of 25-fs visible pulses generated in a noncollinear optical parametric amplifier (NOPA). Following chirped pulse excitation of the oxazine laser dye LD690, coherent oscillations are observed in the time-resolved transient absorption. When a vertical transition is excited, negative chirp leads to strong ground state vibrational coherences as resonant stimulated Raman processes are enhanced by the frequency sweep of the laser pulse. When the chirped pulse excitation is higher frequency than the Franck-Condon transition, the optical response of the dye is significantly affected by excited state absorption. In this case, coherences are established in excited states but no chirp-enhancement of the ground state wavepacket is observed. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N13.00009: Terahertz absorption spectrum of water vapor at different humidity at room temperature Xuying Xin, Hakan Altan, David Matten, Angelamaria Saint, Robert Alfano We measured the absorption spectrum of water vapor in 0.2-2.4THz range at different humidity from 17{\%} to 98{\%} at room temperature using Er: doped fiber laser (IMRA America Inc.) based terahertz time-domain spectroscopy. The experiments were performed in a nitrogen-purged cage at atmosphere environment to obtain the reference and water absorption information. The seventeen absorption lines were observed due to water molecular rotations in the ground vibration state. The first three absorption lines at low frequencies increase with humidity, following the Beer-Lambert Law, while some of high frequency lines were found to decrease with humidity. These effects will be discussed. The observed line broadening is due to collisions occurring among water and nitrogen molecules. [Preview Abstract] |
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