Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session M7: Time-resolved Molecular Dynamics and Femtochemistry |
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Chair: Thomas Weinacht, Stony Brook Unversity Room: 553AB |
Thursday, May 26, 2016 8:00AM - 8:12AM |
M7.00001: Strong-field induced dissociation dynamics in 1,2-dibromoethane traced by femtosecond XUV transient absorption spectroscopy A. S. Chatterley, F. Lackner, D. M. Neumark, S. R. Leone, O. Gessner Strong field induced dissociation dynamics of the small haloalkane 1,2-dibromoethane (DBE) have been explored using femtosecond XUV transient absorption spectroscopy. Dynamics are initiated by a near IR pump pulse with intensities between 75 and 220 TW cm$^{\mathrm{-2}}$, and are probed by the atomic site specific XUV absorption of the Br 3d levels. Immediately upon ionization, the spectral signatures of molecular ions appear. These molecular peaks decay in tandem with the appearance of atomic Br peaks in charge states of 0, $+$1 and $+$2, which are all monitored simultaneously. Neutral Br atoms are eliminated in 300 fs, presumably from statistical dissociation of vibrationally hot DBE$^{\mathrm{+}}$ ions, Br$^{\mathrm{+}}$ ions are eliminated in 70 fs from a more energetic dissociative ionization pathway, and Br$^{\mathrm{++}}$ ions are eliminated within the duration of the 35 fs pump pulse. The simultaneous recording of multiple parent molecule and fragment ion traces enables new insight into predominant dissociation pathways induced by strong field ionization of organic molecules. [Preview Abstract] |
Thursday, May 26, 2016 8:12AM - 8:24AM |
M7.00002: Photo-induced dynamics in heterocyclic aromatic molecules probed by femtosecond XUV transient absorption spectroscopy Florian Lackner, Adam S. Chatterley, Chaitanya D. Pemmaraju, Daniel M. Neumark, Stephen R. Leone, Oliver Gessner We report on the ring-opening and dissociation dynamics of strong-field ionized selenophene (C$_{\mathrm{4}}$H$_{\mathrm{4}}$Se), studied by transient XUV absorption spectroscopy at the Se 3d edge. The table-top experiments are facilitated by high-order harmonic generation coupled with a gas phase transient XUV absorption setup that is optimized for the study of organic compounds. Employing element-specific core-to-valence transitions, the ultrafast molecular dynamics are monitored from the perspective of the well-localized Se atoms. Spectral features are assigned based on first principles TDDFT calculations for a large manifold of electronic states. We observe signatures of rapidly (\textasciitilde 35 fs) decaying highly excited molecular cations, the formation of ring-opened products on a 100 fs time scale and, most notably, the elimination of bare Se$^{\mathrm{+}}$ ions in a very rapid multi-step process. A delayed onset of the Se$^{\mathrm{+}}$ ions provides direct evidence that both selenium-carbon bonds are broken within only \textasciitilde 130~fs and that a sequential mechanism, presumably an initial ring-opening followed by a subsequent breaking of the second bond, is required to eliminate the atomic fragments. [Preview Abstract] |
Thursday, May 26, 2016 8:24AM - 8:36AM |
M7.00003: Time-Resolved UV-Pump (4.8eV) and Vacuum-UV (8eV) Probe Experiments of Neutral Excited State Dynamics Spencer Horton, Yusong Liu, Spiridoula Matsika, Thomas Weinacht Excited state dynamics in polyatomic molecules involve a rich mixture of internal conversion, intersystem crossing, isomerization, and dissociation. Probing these dynamics with ultrafast laser pulses poses a number of challenges, in terms of both the execution of the measurements and their interpretation. We have developed an apparatus for probing excited state dynamics using a 260nm UV-pump pulse and a 156nm Vacuum-UV (VUV) probe pulse. For many systems of interest, an 8eV probe pulse can ionize the molecule from essentially any position along the excited state potential, while not having a background ionization yield from the ground state. Furthermore, given the perturbative interaction of each pulse with the molecule, it is possible interpret and model the experimental results with greater ease and confidence than more complicated probe interactions such as strong field ionization. We compare UV-IR strong-field ionization pump-probe experiments previously conducted directly with our 8eV probing and explore the differences between the two. [Preview Abstract] |
Thursday, May 26, 2016 8:36AM - 8:48AM |
M7.00004: Time-resolved imaging of laser-induced vibrational wave packets in neutral and ionic states of iodomethane Y. Malakar, B. Kaderiya, M. Zohrabi, W.L. Pearson, F. Ziaee, Kananka Raju P., I. Ben-Itzhak, D. Rolles, A. Rudenko Light-driven vibrational wave packets play an important role in molecular imaging and coherent control applications. Here we present the results of a pump-probe experiment characterizing laser-induced vibrational wave packets in both, neutral and ionic states of CH$_{3}$I (iodomethane), one of the prototypical polyatomic systems. Measuring yields and kinetic energies of all ionic fragments as a function of the time delay between two 25 fs, 800 nm pump and probe pulses, we map vibrational motion of the molecule, and identify the states involved by channel-resolved Fourier spectroscopy. In the Coulomb explosion channels we observe features with \textasciitilde 130 fs periodicity resulting from C-I symmetric stretch ($\nu_{3}$ mode) of the electronically excited cationic state. However the Fourier transform of the low-energy I$^{+}$ ion yield produced by the dissociative ionization of CH$_{3}$I reveals the signatures of the same vibrational mode in the ground electronic states of both, neutral and cation, reflected in 65-70 fs oscillations. We observe the degeneration of the oscillatory structures from the cationic states within \textasciitilde 2 ps and discuss most likely reasons for this behavior. [Preview Abstract] |
Thursday, May 26, 2016 8:48AM - 9:00AM |
M7.00005: Signatures of different vibrational modes in strong-field fragmentation of diiodomethane B. Kaderiya, Y. Malakar, Kanaka Raju P., W.L. pearson, F. Ziaee, K. Jensen, J. Rajput, I. Ben-Itzhak, D. Roles, A. Rudenko The diiodomethane molecule (CH$_{2}$I$_{2})$ has served as a model system for time-domain studies of large-scale bending vibrations and concerted elimination of I$_{2}^{+}$ [1]. Here we present the results of a time-resolved 3D Coulomb explosion imaging experiment on diiodomethane that maps ultrafast dynamics of both, bound and dissociating nuclear wave packets driven by a strong laser field. Measuring yields, kinetic energies and emission angles of coincident ionic fragments as a function of time delay between two 25 fs, 800 nm pump and probe pulses, we track the propagation of different dissociation pathways and visualize vibrational motion of the molecule. Analyzing channel-selective Fourier spectra, we observe signatures of both, bending and stretching vibrations of diiodomethane, and reveal correlation between bending motion (the I-C-I ``scissors'' mode) and different fragmentation pathways, including I$_{2}^{+}$ elimination. [1] D. Gei{\ss}ler \textit{et al}., J. Chem. Phys. \textbf{127}, 204305 (2007). [Preview Abstract] |
Thursday, May 26, 2016 9:00AM - 9:12AM |
M7.00006: ABSTRACT MOVED TO K1.196 |
Thursday, May 26, 2016 9:12AM - 9:24AM |
M7.00007: Investigation of Ultrafast Dynamics in O-nitrophenol using Femtosecond UV/VUV/IR Pulses Niranjan Shivaram, Elio Champenois, Travis Wright, James Cryan, Taylor Wingard, Kirk Larsen, Daniel Slaughter, Ali Belkacem Ortho-nitrophenol (C6H4OHNO2) has a high absorption cross section in the ultraviolet (UV) where excitations lead to different fragmentation pathways involving internal relaxation processes. Some of these pathways lead to elimination of the hydroxyl and nitro groups, internal re-arrangement of these neighboring groups and even formation of bonds between them. We use a 25 mJ, 1 kHz, 780 nm, 25 fs laser system to generate high flux vacuum ultraviolet (VUV)/extreme ultraviolet (XUV) high order harmonics in a gas such as argon or krypton. These harmonics are used to study ultrafast dynamics in neutral O-nitrophenol excited to states around 4.75 eV and probed with either higher harmonics or the IR pulse. In particular, we are interested in the mechanism of elimination of the NO molecule from the dissociation of O-Nitrophenol. A velocity map imaging spectrometer is used to obtain energy/angle resolved photo-fragment spectra as a function of pump-probe delay. [Preview Abstract] |
Thursday, May 26, 2016 9:24AM - 9:36AM |
M7.00008: Pump-probe studies of fragmentation of a fast HD$^+$ beam M. Zohrabi, Ben Berry, T. Severt, Bethany Jochim, Peyman Feizollah, Kanaka Raju P., Jyoti Rajput, K. D. Carnes, B. D. Esry, I. Ben-Itzhak Studies of fast molecular ion beams in strong-field ultrafast lasers are of particular interest for benchmark molecules, like H$_2^+$, H$_3^+$ and HeH$^+$. However, the low target density of a typical ion beam puts severe limits on studies of the time evolution through the implementation of the pump-probe technique. We have recently conducted a first-of-its-kind NIR-pump\,--\,NIR-probe measurement on a few-keV HD$^+$ beam target. Specifically, the first pulse initiates the dissociation of the HD$^+$, while the second ionizes the molecule during its dissociation. We have observed enhancement in the ionization yield of the dissociating wave packet at about 24 and 200 fs, corresponding to internuclear distances estimated classically to be about 15 and 85 a.u., respectively. The unexpected enhancement at very large internuclear separation has not been previously observed in pump-probe studies of neutral hydrogen molecules. \\ \\ \textsuperscript{*}Supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy. BJ was also supported in part by DOE-SCGF (DE-AC05-06OR23100). [Preview Abstract] |
Thursday, May 26, 2016 9:36AM - 9:48AM |
M7.00009: Two-Dimensional Fourier Transform Electronic Spectroscopy of Peridinin and Peridinin Analogs Soroush Khosravi, Michael Bishop, Razib Obaid, Hope whitelock, Ann Marie Carroll, Amy LaFountain, Harry Frank, Warren Beck, George Gibson, Nora Berrah The peridinin chlorophyll-$a$ protein (PCP) is a light harvesting complex in dinoflagellates that exhibits a carotenoid-to-chlorophyll (Chl) $a$ excitation energy transfer (EET) efficiency of 85-95{\%}. Unlike most light harvesting complexes, where the number of carotenoids is less than Chl, each subunit of PCP contains eight tightly-packed peridinins surrounding two Chl $a$ molecules. The unusual solvent polarity dependence of the lowest excited S$_{\mathrm{1}}$ state of peridinin suggests the presence of an intramolecular charge-transfer (ICT) state. The nature of the ICT state, its coupling to the S$_{\mathrm{1}}$ of peridinin, and whether it enables the high EET efficiency is still unclear. Two-dimensional electronic Fourier transform spectroscopy (2DES) is a powerful method capable of examining these issues. The present work examines the ICT state of peridinin and peridinin analogs that have diminished ICT character. 2DES data adding new insight into the spectral signatures and nature of the ICT state in peridinin will be presented. [Preview Abstract] |
Thursday, May 26, 2016 9:48AM - 10:00AM |
M7.00010: Ultrafast Electron and Ion Dynamics in Strong-Field-Ionized Liquid Water Zhi-Heng Loh, Jialin Li, Zhaogang Nie, Yi Ying Zheng, Shuo Dong, Pei Jiang Low The ionization of liquid water functions as the principal trigger for a myriad of phenomena that are relevant to radiation chemistry and biology. The earliest events that follow the ionization of water, however, remain relatively unknown. Here, femtosecond coherence spectroscopy is combined with polarization anisotropy measurements to elucidate the ultrafast electron and ion dynamics in ionized water. The results show that strong-field ionization of liquid water produces an aligned $\textit{p}$ electron distribution. Furthermore oscillations observed in the polarization anisotropy are suggestive of valence electron motion in the highly reactive H$_{2}$O$^{+}$ radical cation, whose lifetime with respect to proton transfer is found to be 196 $\pm$ 5 fs. Coherent intermolecular motions that signal initial solvent reorganization and subsequent long-lived ballistic proton transport that involves the H$_{3}$O$^{+}$ end-product are also detected in the time domain. These results offer new insight into the elementary dynamics of ionized liquid water. [Preview Abstract] |
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