Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 66, Number 6
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S01: Focus Session: Understanding Photochemistry through Molecular ImagingFocus Live
|
Hide Abstracts |
Chair: Spiridula Matsika, Temple University |
Thursday, June 3, 2021 10:30AM - 11:00AM Live |
S01.00001: Imaging Excited State Structures and Ultrafast Dynamics with X-Ray Scattering Invited Speaker: Peter M Weber In ultrafast time-resolved x-ray scattering on free molecules in the gas phase optical laser pulses excite the molecules and ultrashort pulses from an x-ray free electron laser probe the transient dynamics. The experiments provide a wealth of information about the structures of molecules in excited states and their dynamic and kinetic time evolution. Complete, excited state structures are obtained by measuring the difference between the excited state and ground state scattering patterns and referencing them to the known ground state structures. Optical excitation leads to a change in the electron density distributions, which gives rise to a distinct signature in the x-ray scattering signal and which is in agreement with predictions from electronic structure calculations. The quantitative analysis of time-evolving signal intensities results in detailed information about chemical reaction kinetics including sequential and branching reactions. In addition to the kinetic time constants, structures of transient and unstable species are obtained. These topics are discussed using polyatomic molecules such as N-methyl morpholine, 1,3-cyclohexadiene, tri-methyl amine as examples. |
Thursday, June 3, 2021 11:00AM - 11:12AM Live |
S01.00002: Filming the molecular dynamics of acetylacetone with time-resolved x-ray scattering Matthew R Ware, Pedro Nunes, Nanna Holmgaard List, Martin Centurion, Thomas J Wolf Time-resolved x-ray scattering from UV photoexcited acetylacetone was measured at the LINAC Coherent Light Source in February 2020. Using a 266 nm, 42 fs, 5 uJ laser pulse, acetylacetone is excited to the ππ* state. Simulations suggest it subsequently undergoes hydrogen transfer dynamics and internal conversion to a spectroscopically dark nπ* state. The nπ* state further relaxes via intersystem crossing to a triplet ππ* state. Time-resolved x-ray scattering at 15 keV films the acetylacetone dynamics in reciprocal space up to a momentum transfer of 7 inverse Angstroms, which may then be used to recover the real-space electron distribution with 0.89 Angstroms of spatial resolution. The real-space electron distribution is sensitive to the pump-probe delay between the laser and x-ray pulses, and it generates a film strip of the real-space dynamics of acetylacetone with sub-50 fs time-resolution, which may then be compared to simulation. The fine time-resolution enables the generation of film strips in reciprocal-space and reciprocal-time through application of a temporal Fourier transform to the time-resolved x-ray scattering measurement, which provides a state-selective view of the molecular dynamics, for example, vibrations and dissociations may be segregated in this representation. Moreover, this measurement marks the finest temporal resolution (sub-50 fs) at 0.89 Angstroms of Fourier-transform limited spatial resolution in a gas-phase scattering experiment. This is thanks to the recent upgrade of the LCLS undulators, which now provide up to 1 mJ of pulse energy at photon energies around 20 keV. |
Thursday, June 3, 2021 11:12AM - 11:24AM Live |
S01.00003: Femtosecond dynamics and coherence of retro-Diels-Alder reactions in radical cations Marcos Dantus, Shuai Li, Bethany Jochim, James Jackson Ultrafast tunnel ionization enables femtosecond time-resolved dynamics measurements of the retro-Diels-Alder reactions of positively charged cyclohexene, norbornene, and dicyclopentadiene as they likely occur under electron-ionization mass spectrometry. Unlike the sub-200 fs reaction times that have been observed following UV excitation of the neutral species, on the ionic potential energy surfaces, these reactions occur on the picosecond timescale and in some cases exhibit vibrational coherence that is likely associated with a concerted reaction mechanism. |
Thursday, June 3, 2021 11:24AM - 11:36AM Live |
S01.00004: Imaging CDn+ fragmentation induced by intense ultrashort laser pulses Itzik Ben-Itzhak, Naoki Iwamoto, Kevin D Carnes, Travis Severt The fragmentation of CDn+ (n=2–3) in intense ultrashort laser pulses‡ was investigated using an upgraded version of coincidence three-dimensional momentum imaging of fast (keV) molecular ion beams. One of the main goals of this study is to determine the relative importance of hydrogen elimination, CDn+ → CDn-1+ + D, versus deprotonation, CDn+ → CDn-1 + D+. Curiously, hydrogen elimination is also observed in the dissociation of the dication following ionization of CD3+, namely CD32+ → CD22+ + D, though it is much smaller than the competing deprotonation process leading to CD2+ + D+. Other two- and three-body breakup channels will be discussed also. |
Thursday, June 3, 2021 11:36AM - 11:48AM Live |
S01.00005: Substituent and conformer effects on photochemistry investigated by ultrafast electron diffraction and excited state wavepacket simulations Invited Speaker: Thomas J Wolf Ultrafast photochemistry plays an important role in processes like human vision or the photosynthesis of vitamin D. A more thorough understanding of the underlying non-adiabatic dynamics through conical intersections between electronic states could help in tailoring the molecular structure of a reactant towards a desired photochemical outcome, e.g. by chemical substitution in specific positions. Chemical substitution, e.g. through hydrocarbon side chains, can also lead to the occurrence of conformers. The latter are often close enough in energy that several conformer geometries are populated in a room temperature gas phase sample.Not much is known so far about the influence of conformer geometries on photochemical reaction dynamics. |
Thursday, June 3, 2021 11:48AM - 12:18PM Live |
S01.00006: Ultrafast electronic and nuclear imaging of ammonia with electron diffraction Elio G Champenois, Nanna List, Thomas J Wolf We demonstrate the simultaneous and direct imaging of electron and hydrogen dynamics during a photochemical reaction. In ammonia (NH3), the substantial role of the valence electronic structure leads to the breakdown of the independent atom model for molecular scattering. Using an ultrafast electron diffraction (UED) probe, we make use of this to follow concerted dynamics involving the electronic and nuclear degrees of freedom following photoexcitation. |
Thursday, June 3, 2021 12:18PM - 12:30PM Live |
S01.00007: UV-induced dissociation dynamics of bromoform probed by Ultrafast Electron Diffraction Lars Hoffmann, Benjamin W Toulson, J. Pedro F. Nunes, Martin Centurion, Ming-Fu Lin, Andrew Attar, Michael Zuerch, Oliver Gessner The UV photochemistry of bromoform is investigated by using mega-electron-volt ultrafast electron diffraction (MeV-UED) to study excited-state dissociation dynamics on femtosecond to picosecond timescales. In previous work, using UV pump – XUV probe transient absorption spectroscopy, it was shown that two timescales dominate the photodissociation dynamics of bromoform, each related to a characteristic timescale for either Br or C/H atomic motion.1 Molecular dynamics simulations identified that two pathways to dissociation may exist: (1) C-Br fission along an approximately CS symmetrical reaction pathway and (2) a minor pathway that forms a transient species containing a carbon-halogen-halogen bond, iso-CHBr3, on a timescale of several hundred femtoseconds. During the first 40 fs of the dominant pathway distortion from C3V to a quasi-planar CHBr2 causes significant changes to the excited state valence structure. The displacement of the heavy Br atoms requires up to 300 fs to form separate Br + CHBr2 products. UED enables a direct probe of the changes in the nuclear degrees of freedom and is complementary to the transient absorption study. Here, we present first results from a recent MeV-UED experiment at SLAC National Accelerator Laboratory, which further elucidate the 266 nm induced photodissociation of bromoform. 1B. W. Toulson et al., Struct. Dyn. 6,054304 (2019) |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700