Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R13: Focus Session: Ultrafast and Ultrahigh Field Chemistry III: Ultrafast Processes |
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Sponsoring Units: DCP Chair: Yaron Silberberg, Weizmann Institute Room: Baltimore Convention Center 305 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R13.00001: Attosecond electron wave packets Invited Speaker: Attosecond electron wave packets are created when a train of extreme ultraviolet attosecond pulses is used to ionize an atom or molecule via one photon absorption. These novel wave packets differ in several useful ways from the temporally confined electron wave packets that are made via tunnel ionization. They can, for instance, be tailored for different applications by controlling the light field that generates the attosecond pulse train. In this talk we will describe how attosecond electron wave packets can be used to study the one photon ionization process via interferometry, and their application in the study and control of strong field processes in such as harmonic generation and above threshold ionization. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R13.00002: Probing ultrafast electronic motions in atoms with the attosecond pump-probe Lee Collins, Suxing Hu, Barry Schneider Through full-dimensional numerical simulations with using our recently-developed efficient and accurate parallel solver for the time-dependent Schr\"odinger equation, we have demonstrated that an attosecond pulse can effectively {\it probe} the extremely fast motion of an electronic wave packet in atoms. Pumped by a broadband femtosecond UV pulse, one electron of ground-state Helium can be launched into a superposition of low-lying excited states, thus forming a wavepacket that begins to orbit the atomic core. A time-delayed attosecond EUV pulse (probe) then ionizes the atom causing three-body breakup. Measuring either the energy sharing of the ionized electrons or the total ionization probability as a function of the time delay displays the internal motion of the excited electron. Our simulation has shown that an ultrashort Kepler period of 2 $fs$ can be followed for several cylces. This opens the prospect of a wealth of similar pump-probe experiments to examine {\em{electronic}} motion. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:54PM |
R13.00003: Tomographic Imaging of Molecular Orbitals Using Femtosecond Lasers Invited Speaker: I will show how we can experimentally reconstruct a single-electron orbital wave function from a simple molecule, dinitrogen. We use a seemingly unlikely technique, high harmonic generation (HHG). HHG occurs when a gas is irradiated with an intense femtosecond laser pulse. The atoms or molecules are tunnel-ionized by the laser field, and then are driven back to the parent ion within the same optical cycle. Some of these electrons recombine with the parent, and release their kinetic energy as xuv photons. The resulting spectrum covers the photon range of 10--100 eV. The radiation results from a transition from a continuum electron wave function, described as a plane wave, and the orbital from which the electron was removed, the HOMO. Thus the experiment is a measure of the transition dipole matrix elements between the single-electron orbital and a set of plane waves. By rotating the molecule in the gas phase using a second laser pulse, we can map out the matrix elements for different projections of the molecule. This data can then be inverted using a tomographic algorithm to yield an image of the orbital wave function. We will show the recovered $3\sigma_g$ orbital of N$_2$. Beyond the single active electron approximation, there are other transitions that are allowed between the HOMO and inner orbitals. This is due to the indistinguishability of the electrons. Including these exchange terms gives even better agreement between theory and experiment. Because this measurement is made with a 25 fsec laser pulse, it is now possible to perform pump-probe experiments to observe dynamic changes in the electronic structure of molecules. It may even be possible to observe {\it electronic} wave packet motion within an atom with attosecond resolution. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:30PM |
R13.00004: Coherent excitation and control of surface phonons Invited Speaker: The excitation and control of nuclear wavepackets using tailored laser pulses have attracted a lot of interest recently and being realized mainly in gas-phase molecules. In contrast, there have been little studies on the coherent excitation and control for adsorbates particularly on metal surfaces. This is because dephasing is substantially rapid on metal surfaces due to efficient couplings between adsorbates and metals. Recently, we have demonstrated the time-domain observation of nuclear wavepacket dynamics of monolayer adsorbate by femtosecond time-resolved second harmonic generation (TRSHG). When metal surfaces covered with alkali metal atoms are irradiated by ultrafast laser pulses, coherent surface phonon modes are excited. The formation and dissipation processes of coherent surface phonons are probed by time-resolved second harmonic generation. SHG signal intensities are enhanced by alkali atoms adsorption by various resonant transitions in the adsorbate-substrate system. However, not all resonant electronic transitions lead to the generation of coherent stretching vibrations of alkali atoms. The measurements of TRSHG traces as a function of the excitation photon energy at a fixed alkali coverage indicate that resonant transitions between adsorbate-induced surface states is responsible for the coherent vibrational motions. By carefully examining the Cs coverage dependence of the TRSHG waveform, we found that TR-SHG traces show beating structures. This indicates that the oscillatory TR-SHG traces are contributed by at least two kinds of coherent surface phonon modes: the Cs-Pt stretching mode (2.3 THz) and the Rayleigh phonon mode (2.6 or 2.9 THz, depending on the Cs coverage). We used fs pulse trains with the repetition frequencies of 2.0 - 2.9 THz that are synthesized by using a spatial-light modulator as an excitation source for the coherent phonons. By tuning the pulse train frequency, we succeed in the selective excitation of a coherent phonon mode. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R13.00005: Femtosecond microscopy of surface plasmon propagation on a silver film. Atsushi Kubo, Niko Pontius, Hrvoje Petek By using interferometric time-resolved photoelectron emission microscopy (ITR-PEEM), we investigate the dynamics of surface plasmon polariton (SPP) propagation with 0.33-fs per frame time and 40-nm spatial resolution. 10-fs phase-locked pump-probe pulse pairs with 400-nm center wavelength irradiate a silver film at 65 degree angle from the surface normal to launch a SPP wave from a line defect in the film. We image the propagation of the SPP wave through its interference with the external light field. The interference periodically modulates the total amplitude of the polarization field in silver, and thereby the two-photon photoemission current from the surface. Two-dimensional microscopic maps of the photoemission intensity at pump-probe delay $\tau _d $ are recorded by the PEEM. Sequential PEEM images taken with a delay increment step of 0.33-fs record the dynamics of SPP wave packet propagation and dissipation as a movement of the oscillatory interference pattern. The progression speed and the attenuation of the oscillatory pattern are reproduced by a simulation with the known dispersion of the complex SPP wave vector. The SPP imaging experiments demonstrate the possibility of coherent control of plasmon field in metallic nanostructures. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R13.00006: Adaptive Control of the Spatial Position of White Light Filaments in an Aqueous Solution Robert Levis, George Heck, Joseph Sloss White light filamentation produced from intense laser beams represents a method to produce highly nonlinear energy deposition both spatially and temporally in the gas, liquid, or solid phase. We have demonstrated control over the spatial coordinates (position and extent) of white light filaments (supercontinuum generation) in an aqueous solution using shaped ultrafast, strong field laser pulses. These are the first experiments to achieve control of filament position through the manipulation of the spectral phase of an ultra-fast (50 fs) 800nm excitation laser pulse. A closed feedback loop employing a spatial light modulator and a genetic algorithm was used to manipulate the spectral phase of the pulses to achieve a specified filament position and length. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R13.00007: Relaxation dynamics of dissolved molecule probed by a phase locked pulse pair:Br$_{2}$ in Ar Mizuho Fushitani, Heide Ibrahim, Markus Guehr, Nikolaus Schwentner A phase locked pulse pair(PLPP) is a strong tool to investigate the electronic coherence of molecules. Recently, we have shown that the PLPP experiment is also applicable to dissolved molecules[1]. Here, we apply this method to the Br$_{2}$/Ar system and investigate relaxation dynamics in the electronically excited $B$ state of Br$_{2}$. We observed not only laser induced fluorescence(LIF) from the $B$ state but also LIF from the $A$ and $A$' states which are energetically lower than the $B$ state. Tuning the relative phase between PLPPs provides the various LIF ratio among those states, indicating relaxation channels which can be coherently controlled. The PLPP results will be compared with those obtained by frequency resolved excitations which give the LIF ratio resulting from all possible relaxation. [1] M. Fushitani, \textit{et al}. PCCP 7 (2005) 3143. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R13.00008: Vibrational Relaxation of Anions in Nonaqueous Reverse Micelles Gerald Sando, Jeffrey Owrutsky Static and ultrafast infrared spectroscopy have been used to measure vibrational frequencies and vibrational energy relaxation (VER) times for high frequency bands of small ions in nonaqueous reverse micelles (RMs). Formamide RMs are stable with anionic (AOT) and nonionic surfactants. In AOT RMs, the vibrational frequencies of azide (N$_{3}^{-})$ are blue shifted, while the VER times are slower. Bulk behavior is approached as the RM size increases. The frequency-rate correlation is opposite of what is seen in bulk solvents and aqueous RMs. The effects are small in nonionic RMs because azide near surfactant headgroups resembles that in formamide, as shown by results in TGE, a model of the polar portion of the surfactant. The dynamics for Fe(CN)$_{5}$NO$^{2-}$ depend on the probe frequency because of the inhomogeneous distribution of solvation sites throughout the poorly defined interface. Ionic liquid containing reverse micelles are formed with nonionic surfactants. The VER rates of the ionic liquid anions are faster than in the bulk ionic liquids, but slower than in TGE, consistent with a mixed solvation environment of ionic liquid and surfactant. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R13.00009: Molecular quantum computation by using ultrashort intense laser pulses under the influence of non-Markovian dissipation Yukiyoshi Ohtsuki We numerically study molecular quantum computation by combining an ensemble of molecular states and shaped ultrashort intense laser pulses through a case study of Grover's quantum search algorithm, in which qubits are implemented in the vibrational states of I2. In the simulation, the Grover iteration is divided into two basis operations, the so-called oracle and the inversion about mean operation, which are realized by laser pulses designed by an optimal control method within the density matrix formalism. These pulses perform Grover iteration with high accuracy although the lack of extreme precision leads to a slight reduction in the population associated with the solution to the search problem. The accuracy of the Grover iteration is shown to be improved by the normalization with respect to the ``qubit population.'' The relaxation effects on the accuracy of the computation are systematically examined by using the non-Markovian master equation with phenomenological relaxation parameters. The gate pulses are designed under the influence of non-Markovian dissipation, in which the pulse design equation is solved by a newly proposed iteration algorithm. [Preview Abstract] |
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