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 M4: Ultrafast Molecular Dynamics |
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Chair: Li Fang, University of Texas-Austin Room: Union DE |
Thursday, June 11, 2015 8:00AM - 8:12AM |
M4.00001: Imaging CS2 Molecules in Intense Laser Fields with Ultrafast Electron Diffraction Jie Yang, Martin Centurion It has been previously demonstrated that ultrafast electron diffraction from aligned molecules (UEDAM) can resolve ground state molecular structures with atomic resolution. Here we use UEDAM to investigate the dynamics in carbon disulfide (CS2) following the interaction with an intense femtosecond IR laser pulse. The atomic positions in transient states are imaged with 0.03{\AA} precision and 1ps temporal resolution. Alignment, structural deformation and dissociation are observed in molecular images under different laser intensities. This proof-of-principle work demonstrates the capability of studying molecular dynamics with high spatial and temporal resolution using UEDAM. [Preview Abstract] |
Thursday, June 11, 2015 8:12AM - 8:24AM |
M4.00002: Probing ultrafast molecular dynamics in O$_{2}$ using XUV/IR pump-probe studies D. Ray, F.P. Sturm, T.W. Wright, P. Ranitovic, N.H. Shivaram, I. Bocharova, A. Belkacem, Th. Weber We investigate the molecular dynamics via different dissociative and autoionizing pathways in molecular oxygen using a pump-probe scheme with ultrashort extreme ultraviolet (XUV) laser pulses. Our primary focus is to study the molecular dynamics in the superexcited Rydberg states in a time-resolved manner. The O$_{2}$ molecules are pumped by 20.2 eV and 23.1 eV XUV pulses (13$^{\mathrm{th}}$ and 15$^{\mathrm{th}}$ harmonics). Probing the relaxation dynamics with an infrared (IR) pulse at very long delays (100s of fs) enables us to measure the lifetimes of these Rydberg states. We also observe an enhancement and suppression of vibrational levels of the O$_{2}^{+}$ ion due to the presence of IR. The high flux XUV pulses used for this experiment are generated in an Ar gas by IR pulses from our state-of-the-art 30 mJ, 50 Hz laser system. The pulses are overlapped with the supersonic jet in our Momentum Imaging for TimE Resolved Studies (MISTERS) setup. The cold target in our setup, combined with a very tight focussing geometry and a 3D momentum detection capability gives a high kinetic energy resolution. Molecular dynamics in other polyatomic molecules are also under investigation. [Preview Abstract] |
Thursday, June 11, 2015 8:24AM - 8:36AM |
M4.00003: UV Pump -- VUV Probe Studies of Ultrafast Dynamics in Simple Aromatic Molecules Niranjan Shivaram, Elio Champenois, James Cryan, Travis Wright, Ali Belkacem Aromatic molecules like nitro-phenols play an important role in atmospheric chemistry. They have a high absorption cross section in the ultraviolet (UV) where excitations lead to different fragmentation pathways involving internal relaxation processes. These pathways lead to elimination of the hydroxyl and nitro groups, internal re-arrangement of these groups and even formation of bonds between them. We use a high pulse energy, high repetition rate femtosecond laser system (30 mJ, 1 kHz, 780 nm, 25 fs) to generate high flux vacuum ultraviolet (VUV)/extreme ultraviolet (XUV) high order harmonics in a gas such as argon. These harmonics are then used to study femtosecond time resolved dynamics in neutral 2-Nitrophenol excited to a manifold of states around 4.75 eV and probed with higher harmonics. A velocity map imaging spectrometer is used to obtain energy/angle resolved photo-ion and photoelectron spectra as a function of pump-probe delay. [Preview Abstract] |
Thursday, June 11, 2015 8:36AM - 8:48AM |
M4.00004: Charge Migration in Phenylalanine Initiated by Attosecond Pulses Jason Greenwood, Andrea Trabattoni, David Ayuso, Louise Belshaw, Simone De Camillis, Sunil Anumula, Fabio Frassetto, Luca Poletto, Alicia Palacios, Piero Declava, Fernando Martin, Francesca Calegari, Mauro Nisoli In the past few years attosecond techniques have been implemented for the investigation of ultrafast dynamics in molecules. The generation of isolated attosecond pulses characterized by a relatively high photon flux has opened up new possibilities in the study of molecular dynamics. We report on experimental and theoretical results of ultrafast charge dynamics in a biochemically relevant molecule, namely, the amino acid phenylalanine [1-2]. The data represent the first experimental demonstration of the generation and observation of a charge migration process in a complex molecule, where electron dynamics precede nuclear motion. The application of attosecond technology to the investigation of electron dynamics in biologically relevant molecules represents a multidisciplinary work, which can open new research frontiers: those in which few femtosecond and even sub-femtosecond electron processes determine the fate of biomolecules. [1] L. Belshaw et al., J. Phys. Chem. Lett., 3, 3751--3754 (2012). [2] F. Calegari et al., Science 346, 336-339 (2014). [Preview Abstract] |
Thursday, June 11, 2015 8:48AM - 9:00AM |
M4.00005: Femtosecond XUV transient absorption spectroscopy of small organic molecules Florian Lackner, Adam S. Chatterley, Daniel M. Neumark, Stephen R. Leone, Oliver Gessner High-order harmonic generation has evolved as a powerful method for the generation of femtosecond XUV pulses with table-top laser systems. Femtosecond XUV transient absorption spectroscopy is an emerging application of these novel light sources for the investigation of molecular dynamics. Recording time-dependent XUV induced core-to-valence transitions traces a molecular response to an initial perturbation with IR, VIS or UV laser pulses from the perspective of distinct atomic sites. Preliminary results for sulfur and selenium containing organic molecules, such as thiophene (C$_{\mathrm{4}}$H$_{\mathrm{4}}$S) and selenophene (C$_{\mathrm{4}}$H$_{\mathrm{4}}$Se), are presented. While molecular orbital dynamics in thiophene will be monitored at the sulfur 2p edge around 165 eV, experiments at the Se 3d (57 eV) and Se 3p (163 eV) edges of selenophene will provide insight about the impact of specific inner-shell transitions within the same atom on the spectroscopic fingerprint of similar dynamics. The method's element-specificity and sensitivity to local valance electronic structures will be exploited to monitor the photo-induced opening of the aromatic rings at the S-C and Se-C bonds, thereby shining new light on the primary steps of photochemical reaction pathways in organic compounds. [Preview Abstract] |
Thursday, June 11, 2015 9:00AM - 9:12AM |
M4.00006: XUV Transient Absorption of Strong-Field Ionized Ferrocene Adam S. Chatterley, Florian Lackner, Daniel M. Neumark, Stephen R. Leone, Oliver Gessner Femtosecond extreme ultraviolet (XUV) transient absorption experiments are underway to study the dynamics of ferrocene following strong field ionization. Ferrocene is a textbook organometallic compound, composed of an iron atom sandwiched between two aromatic organic rings. An intense infrared (IR, 790 nm) pump pulse is used to ionize the ferrocene molecules. Femtosecond XUV pulses, created by high harmonic generation (HHG) are used to probe the induced dynamics. Iron 3p inner-shell to valence transitions (M edge, 50 eV \textless h$\nu $ \textless 70 eV) are expected to be sensitive to the electronic structure in the vicinity of the iron atom. Hence, transient XUV absorption spectra will probe the strong-field induced molecular dynamics from the perspective of the metal center. We will induce dissociation dynamics at high field intensities and use lower IR intensities to study dynamics of electronically and/or vibrationally excited ferrocene cations. Preliminary results will be presented, demonstrating current progress of XUV transient absorption experiments on moderately large molecular systems. [Preview Abstract] |
Thursday, June 11, 2015 9:12AM - 9:24AM |
M4.00007: Observation of intracluster Coulombic decay of Rydberg-like states triggered by intense near-infrared pulses Bernd Sch\"{u}tte, Mathias Arbeiter, Thomas Fennel, Ghazal Jabbari, Kirill Gokhberg, Alexander I. Kuleff, Marc J.J. Vrakking, Arnaud Rouz\'{e}e Interatomic Coulombic decay (ICD) describes a process, where an excited atom relaxes by transferring its energy to an atom in the environment that gets ionized. So far, ICD has been observed following XUV ionization or excitation of clusters. Here we present novel results of an intracluster Coulombic decay mechanism induced by intense NIR pulses and following Rydberg atom formation in the generated nanoplasma. When a highly-excited Rydberg atom relaxes to its ground state by transferring its excess energy to a weakly bound electron in the environment, electrons with kinetic energies close to the atomic ionization potential are emitted. We show evidence for such an intracluster Coulombic decay process that leaves clear signatures in the electron kinetic energy spectra. ICD is time-resolved in a pump-probe experiment, where a weak probe pulse depopulates the excited states, leading to a quenching of the ICD signal. We find a decay time of 87 ps, which is siginificantly longer than for previous ICD observations, where inner-shell holes were created by XUV pulses. Intracluster Coulombic decay is found to be a generic process that takes places in atomic and molecular clusters and at different wavelengths. It may play an important role in biological systems and in astronomical plasmas. [Preview Abstract] |
Thursday, June 11, 2015 9:24AM - 9:36AM |
M4.00008: Toward Femtosecond Time-Resolved Studies of Solvent-Solute Energy Transfer in Doped Helium Nanodroplets C. Bacellar, M.P. Ziemkiewicz, S.R. Leone, D.M. Neumark, O. Gessner Superfluid helium nanodroplets provide a unique cryogenic matrix for high resolution spectroscopy and ultracold chemistry applications. With increasing photon energy and, in particular, in the increasingly important Extreme Ultraviolet (EUV) regime, the droplets become optically dense and, therefore, participate in the EUV-induced dynamics. Energy- and charge-transfer mechanisms between the host droplets and dopant atoms, however, are poorly understood. Static energy domain measurements of helium droplets doped with noble gas atoms (Xe, Kr) indicate that Penning ionization due to energy transfer from the excited droplet to dopant atoms may be a significant relaxation channel. We have set up a femtosecond time-resolved photoelectron imaging experiment to probe these dynamics directly in the time-domain. Droplets containing 10$^{\mathrm{4}}$ to 10$^{\mathrm{6\thinspace }}$helium atoms and a small percentage (\textless 10$^{\mathrm{-4}})$ of dopant atoms (Xe, Kr, Ne) are excited to the 1s2p Rydberg band by 21.6 eV photons produced by high harmonic generation (HHG). Transiently populated states are probed by 1.6 eV photons, generating time-dependent photoelectron kinetic energy distributions, which are monitored by velocity map imaging (VMI). The results will provide new information about the dynamic timescales and the different relaxation channels, giving access to a more complete physical picture of solvent-solute interactions in the superfluid environment. Prospects and challenges of the novel experiment as well as preliminary experimental results will be discussed. [Preview Abstract] |
Thursday, June 11, 2015 9:36AM - 9:48AM |
M4.00009: Computing alignment and orientation of non-linear molecules at room temperatures using random phase wave functions Shimshon Kallush, Sharly Fleischer Quantum simulation of large open systems is a hard task that demands huge computation and memory costs. The rotational dynamics of non-linear molecules at high-temperature under external fields is such an example. At room temperature, the initial density matrix populates $\sim$ 10$^{4}$ rotational states, and the whole coupled Hilbert space can reach $\sim$ 10$^{6}$ states. Simulation by neither the direct density matrix nor the full basis set of populated wavefunctions is impossible. We employ the random phase wave function method to represent the initial state and compute several time dependent and independent observables such as the orientation and the alignment of the molecules. The error of the method was found to scale as N$^{-1/2}$, where N is the number of wave function realizations employed. Scaling vs. the temperature was computed for weak and strong fields. As expected, the convergence of the method increase rapidly with the temperature and the field intensity. [Preview Abstract] |
Thursday, June 11, 2015 9:48AM - 10:00AM |
M4.00010: Sign inversion of nonlinear optical response of nitrogen upon ionization Maryam Tarazkar, Dmitri Romanov, Robert Levis We report hyperpolarizability calculations for the nitrogen radical cation at neutral and relaxed geometries in the static and dynamic non-resonant regime, using multi-configurational self-consistent field (MCSCF) response theory. The results were compared with those computed using density functional theory (DFT). The open-shell electronic system of nitrogen radical cation was found to exhibit negative second-order optical nonlinearity. The drastic change in the magnitude and sign of the hyperpolarizability coefficient $\gamma^{(2)}$ from the neutral nitrogen molecule to radical cation indicates an enhanced role of excitations in the polarization response of ion as compared with the neutral molecule. The second-order optical properties of nitrogen radical cation have been also calculated as a function of bond length starting with the neutral molecular geometry (S$_{\mathrm{0}}$ minimum) and stretching the N-N triple bond, reaching the ionic D$_{\mathrm{0}}$ relaxed geometry, and all the way toward dissociation limit. The results obtained provide the potential for controlling optical properties of laser filament wake channels. [Preview Abstract] |
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