Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session J5: Ultrafast Dynamics Explored with XUV and Xray Pulses |
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Chair: Luca Argenti, University of Central Florida Room: 310 |
Wednesday, June 7, 2017 2:00PM - 2:12PM |
J5.00001: Neutral Excited State Dynamics Studied with Time-Resolved UV-VUV Pump-Probe Experiments Yusong Liu, Spencer Horton, Pratip Chakraborty, Spiridoula Matsika, Thomas Weinacht We have conducted UV/VUV pump-probe experiments to study excited state dynamics in polyatomic molecules. We are particularly interested in the competition between internal conversion and dissociation or population trapping of prototypical molecular systems. Here we present measurements of dynamics in pyrrole and uracil. Our measurements for pyrrole, in conjunction with electronic structure calculations, indicate that pyrrole undergoes rapid internal conversion to the ground state in less than 300fs. We find that internal conversion to the ground state dominates over dissociation. In uracil, our measurements indicate that there is substantial population trapping in the excited state in addition to rapid internal conversion back to the ground state. [Preview Abstract] |
Wednesday, June 7, 2017 2:12PM - 2:24PM |
J5.00002: Vibrationally Assisted Below Threshold Ionization Spencer Horton, Yusong Liu, Pratip Chakraborty, Spiridoula Matsika, Thomas Weinacht We performed time-resolved UV pump (4.8 eV) and VUV probe (8 eV) measurements of internal conversion of 1,3-cyclohexadiene (CHD). Our measurements reveal a substantial ionization of the hot ground state, following internal conversion, despite the fact that our probe photon energy is below the ionization potential (8.25 eV). With the aid of electronic structure calculations, we interpret our results in terms of vibrationally assisted below threshold ionization, where vibrational energy is converted to electronic energy. The effect relies both on having vibrational modes which can lead to this conversion, and exciting these modes during the internal conversion. We contrast our measurements in CHD with another similar molecule, 1,3-cyclooctadiene (COD), for which we don't see the effect. [Preview Abstract] |
Wednesday, June 7, 2017 2:24PM - 2:36PM |
J5.00003: Multidimensional electron-nuclear wavepacket dynamics via Time-, Energy- and Angle-resolved Photoelectron Spectroscopy. K. Veyrinas, V. Makhija, A. E. Boguslavskiy, R. Forbes, I. Wilkinson, D. Moffatt, R. Lausten, A. Stolow Generating and probing a coherent superposition of coupled vibrational-electronic (vibronic) states -- a multidimensional wavepacket -- remains a challenging problem in molecular dynamics. Here, we present recent results using time-resolved photoelectron velocity map imaging (VMI) of complex vibronic wavepacket dynamics in the NO molecule following femtosecond single photon excitation in the vacuum ultraviolet (VUV) range ($\lambda_{\mathrm{pump}}=$ 160 nm, 80 fs). The induced ultrafast dynamics, involving highly excited valence and Rydberg states, is probed by single photon ionization ($\lambda_{\mathrm{probe}}=$ 400 nm, 40 fs). Varying the pump-probe time delay, the emitted photoelectrons are detected in a VMI spectrometer for time-, energy- and angle-resolved photoelectron spectroscopy. We observe that the different final vibrational states of the NO$^{\mathrm{+}}$ (X $^{\mathrm{1}}\Sigma^{\mathrm{+}})$ cation, onto which this evolving vibronic wavepacket is projected, reveal different time dependences for the kinetic energy distribution and the laboratory frame photoelectron angular distribution (LFPAD). In particular, we observe unusually strong oscillations in the $\beta_{\mathrm{4}}$ asymmetry parameter, indicating sensitivity to the higher angular momentum components of the electronic aspect of this complex vibronic wavepacket. [Preview Abstract] |
Wednesday, June 7, 2017 2:36PM - 2:48PM |
J5.00004: Molecular Wavepacket Dynamics at a Jahn-Teller Conical Intersection Varun Makhija, Kevin Veyrinas, Andrey E. Boguslavskiy, Ruaridh Forbes, Iain Wilkinson, Doug Moffatt, Simon Neville, Michael Schuurman, Rune Lausten, Albert Stolow According to the Jahn-Teller theorem, any symmetric configuration of atoms in an electronically degenerate molecular state is unstable and distorts to a configuration of lower symmetry, hence lifting the degeneracy. Jahn-Teller dynamics feature in numerous, highly symmetric systems, such as fullerenes and in doped rare-earth magnetites where they underly the phenomenon of ‘colossal magneto-resistance’. Here, we use ultrafast time-resolved photoelectron velocity-map imaging (VMI) to study the fundamental Jahn-Teller dynamics in an excited state of isolated ammonia (NH$_3$) molecules. Supersonically cooled NH$_3$ is resonantly excited by a 160 nm, 80 fs pump pulse. A time delayed 400 nm, 40 fs probe pulse photoionizes the molecule and the kinetic energy and angular distribution of the ejected photoelectron is measured as a function of time. Dramatic changes in the time-dependent angular distributions are observed, which reveal details of non-adiabatic wavepacket propagation on the coupled Jahn-Teller potential surfaces. [Preview Abstract] |
Wednesday, June 7, 2017 2:48PM - 3:00PM |
J5.00005: Ultrafast dynamics in photoionized CO$_{\mathrm{2}}$ studied by XUV-IR pump-probe experiments Seyyed Javad Robatjazi, Shashank Pathak, Pearson Wright Lee, Kanaka Raju Pandiri, Jeff Powell, Xiang Li, Balram Kaderiya, Itzik Ben-Itzhak, Daniel Rolles, Artem Rudenko $^{\mathrm{1}}$James R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS, USA We present the results of a pump-probe experiment studying ultrafast dynamics of photo-ionized CO$_{\mathrm{2}}$ using a combination of broadband XUV pulses and near-infrared (NIR) pulses, \textasciitilde 25 fs duration. The multi-harmonics pulse, which contains the 11$^{\mathrm{th}}$ to 45$^{\mathrm{th}}$ harmonics of a 790 nm NIR laser, is used to excite molecular wave packets in CO$_{\mathrm{2}}^{\mathrm{+}}$, which are then probed by near-infrared-induced dissociation. We use a velocity map imaging setup to measure the yields and angle-resolved kinetic energy distributions of all charged fragments as a function of XUV-NIR delay. The delay dependence of O$^{\mathrm{+}}$ and CO$^{\mathrm{+}}$ ion production for parallel and perpendicular NIR and XUV polarizations, are compared to the data reported in [1] for a shorter harmonics train, and to data obtained using narrow-band isolated harmonic (11$^{\mathrm{th}}$ or 13$^{\mathrm{th}})$ as a pump. [1] H. Timmers et al, Phys. Rev. Lett. 113, 113003 (2014). [Preview Abstract] |
Wednesday, June 7, 2017 3:00PM - 3:12PM |
J5.00006: Observing the non-adiabatic photodissociation of neutral excited CH$_{\mathrm{3}}$OH with few-femtosecond sensitivity Elio Champenois, Loren Greenman, Niranjan Shivaram, Kirk Larsen, Ali Belkacem The ultrafast dissociation dynamics of neutral excited methanol (CH$_{\mathrm{3}}$OH) have been explored using time-resolved photoelectron imaging and excited state potential energy surface calculations. Nuclear motions on the initially populated 2 $^{\mathrm{1}}$A" (S2) state, through a region of strong non-adiabatic coupling, and on a dissociative state are resolved within the first 15 femtoseconds following excitation. A CH$_{\mathrm{3}}$ hydrogen-loss channel is also observed and found to depend more strongly on OH rather than CH$_{\mathrm{3}}$ deuteration. The measurements and calculations indicate that the previously ignored 1 $^{\mathrm{1}}$A' (S3) state, rather than the 1$^{\mathrm{\thinspace 1}}$A" (S1) state, plays an important role in these dynamics. [Preview Abstract] |
Wednesday, June 7, 2017 3:12PM - 3:24PM |
J5.00007: Probing molecular dynamics in solution with x-ray valence-to-core spectroscopy. Gilles Doumy, Anne Marie March, Ming-Feng Tu, Andre Al Haddad, Stephen Southworth, Linda Young, Donald Walko, Christoph Bostedt Hard X-ray spectroscopies are powerful tools for probing the electronic and geometric structure of molecules in complex or disordered systems and have been particularly useful for studying molecules in the solution phase. They are element specific, sensitive to the electronic structure and the local arrangements of surrounding atoms of the element being selectively probed. When combined in a pump--probe scheme with ultrafast lasers, X-ray spectroscopies can be used to track the evolution of structural changes that occur after photoexcitation. Efficient use of hard x-ray radiation coming from high brilliance synchrotrons and upcoming high repetition rate X-ray Free Electron Lasers requires MHz repetition rate lasers and data acquisition systems. High information content Valence-to-Core x-ray emission is directly sensitive to the molecular orbitals involved in photochemistry. We report on recent progress towards fully enabling this photon-hungry technique for the study of time-resolved molecular dynamics, including efficient detection and use of polychromatic x-ray micro-probe at the Advanced Photon Source. [Preview Abstract] |
Wednesday, June 7, 2017 3:24PM - 3:36PM |
J5.00008: Time-Resolved Two-Color X-ray Pump/ X-ray Probe Photoelectron Spectroscopy at LCLS. Andre Al Haddad, Gilles Doumy, Antonio Picon, Maximilian Bucher, Tais Gorkhover, Ryan Coffee, Michael Holmes, Jacek krzywinski, Alberto Lutman, Agostino Marinelli, Stefan Moeller, Timur Osipov, Peter Walter, Dan Ratner, Dipanwita Ray, Stephen Pratt, Linda Young, Stephen Southworth, Christoph Bostedt Recently, X-ray Free Electron Lasers (XFELs) proved the ability to produce two intense femtosecond x-ray pulses with controlled time delay and color. Combining these unique capabilities with X-ray photoelectron spectroscopy (XPS), a powerful tool for extracting chemical information of a specific site by measuring the binding energy of core electrons, enables femtosecond time-resolved XPS experiments with chemical and site specificity. We will present our X-ray pump/X-ray probe XPS experiment aimed at studying energy flow and relaxation dynamics in CO, i.e. small hetero-nuclear molecules, in the gas phase. The data gives a glimpse of electronic and nuclear relaxation pathways upon resonant oxygen K-edge excitation proceeding on time scales \textless 40 fs. The experimental efforts are accompanied by theoretical work describing the time-resolved core-level photoemission with a time-dependent Schrodinger equation. This work lays the groundwork for further TR-XPS experiments following energy and charge transfer processes upon photo excitation in more complex molecules. [Preview Abstract] |
Wednesday, June 7, 2017 3:36PM - 3:48PM |
J5.00009: Photoinduced charge carrier dynamics at a dye-semiconductor interface probed by picosecond time-resolved X-ray photoelectron spectroscopy Johannes Mahl, Stefan Neppl, Hendrik Bluhm, Oliver Gessner We investigate laser-induced charge carrier dynamics at the interface between N3 dye molecules and a film of nanocrystalline ZnO using picosecond time-resolved laser pump-synchrotron probe X-ray photoelectron spectroscopy. Pumping the sample with 532nm light pulses induces HOMO -- LUMO excitations in the dye followed by electron injection into the ZnO conduction band. The subsequent electronic dynamics are marked by an interplay between transient charge carrier densities, corresponding interfacial potentials, and electron-hole recombination rates. Monitoring the picosecond dynamics of a dye-associated photo-line (C1s) and a substrate-associated photo-line (Zn3d), we find that both exhibit transient rigid shifts, but with different amplitudes and different dynamic trends. The difference between these trends can be described by a bi-exponential decay with time constants of 400ps and around 20ns while the absolute shifts of both individual photo-lines decay on partly much longer timescales (tens to hundreds of nanoseconds). These results demonstrate how the element specificity of inner-shell transitions may be employed to gain a local perspective of photo-induced charge carrier dynamics from both sides of the interface. The findings will be discussed within the framework of charge-injection-induced band dynamics and transient interfacial dipoles. [Preview Abstract] |
Wednesday, June 7, 2017 3:48PM - 4:00PM |
J5.00010: Visualizing the femtosecond emergence and picosecond evolution of an anisotropic nanoplasma. C Bacellar, A Chatterley, F Lackner, S Pemmaraju, R Tanyag, C Bernando, D Verma, S O’Connell, M Bucher, K Ferguson, T Gorkhover, R Coffee, G Coslovich, D Ray, T Osipov, D Neumark, C Bostedt, A Vilesov, O Gessner The dynamics of strong-field induced nanoplasmas are studied using femtosecond time-resolved X-ray coherent diffractive imaging (CDI) at the Linac Coherent Light Source (LCLS). Intense 800nm laser pulses ($\approx $10$^{\mathrm{15}}$W/cm$^{\mathrm{2}}$, 50fs) are employed to initiate nanoplasma formation in sub-micron sized helium droplets. Plasma formation and evolution dynamics are probed by femtosecond x-rays pulses ($\approx $100fs, 600eV) across timescales ranging from femtoseconds to hundreds of picoseconds. Anisotropic surface softening is observed within tens of femtoseconds after exposure to the NIR pulse. The softening continues over 300fs, after which the anisotropic surface profile stabilizes with $\approx $30{\%} larger extension along the laser polarization axis compared to the perpendicular direction. The saturation of the surface width is contrasted by an increase in anisotropic material loss that is twice as pronounced along the laser polarization axis, resulting in significantly distorted shapes with aspect ratios of $\approx $1.5 and beyond. The results will be discussed within the framework of an anisotropic plasma expansion model that provides new insight into strong-field induced nanoplasma formation and relaxation dynamics. [Preview Abstract] |
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