Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 67, Number 7
Monday–Friday, May 30–June 3 2022; Orlando, Florida
Session Z07: Femtosecond Structural DynamicsRecordings Available
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Chair: Thomas Weinacht, Stony Brook Room: Salon 5/6 |
Friday, June 3, 2022 10:30AM - 10:42AM |
Z07.00001: Strong-field-induced bond rearrangement in CH3NCS ions Shuai Li, Bethany C Jochim, Jacob Stamm, Stephen Yuwono, Piotr Piecuch, James Jackson, Marcos Dantus The methyl isothiocyanate (MITC) molecule, CH3NCS, a common agricultural pesticide, is of great interest in terms of its health and environmental impacts. We study intense ultrafast laser-induced ionization and fragmentation of this molecule, focusing on the CHnS+ (n=0-3) fragments arising due to bond rearrangement. Methyl thiocyanate (MTC), CH3SCN, is a potential impurity that could also contribute these fragments through bond cleavage. This issue is tackled by interrogating MITC and MTC separately utilizing a femtosecond pump-probe approach to record the time-resolved behavior of the fragment ions. For MITC, we find modulations in the CH3S+ yield at frequencies of about 130 cm-1 and 250 cm-1. CH3S+ from MTC exhibits a markedly different formation time from the case of MITC and also does not show the aforementioned oscillations. To shed light on our findings, we have performed high-level quantum calculations and simulations investigating the role of bending motion in the rearrangement process. |
Friday, June 3, 2022 10:42AM - 10:54AM |
Z07.00002: Imaging rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction in real-time Yusong Liu, David Sanchez, Matthew Ware, Elio Champenios, Jie Yang, Pedro Nunes, Martin Centurion, James P Cryan, Ruaridh Forbes, Ming-Fu Lin, Sajib Kumar Saha, Michael Minitti, Xijie Wang, Markus Gühr, Todd J Martinez, Thomas J Wolf We investigate structural dynamics of wavepacket relaxation along pericyclic transition states with a combination of ultrafast electron diffraction (UED) and ab-initio multiple spawning (AIMS) excited state wavepacket simulation. We focused this study on the molecular system alpha-terpinene (αTP, C10H16), which is a derivative of 1,3-Cyclohexidiene (CHD, C6H8) by the addition of two substituents, a methyl group and an isopropyl group (replacing the relevant hydrogen atoms). We took the advantage that αTP does not qualitatively alter the photochemical dynamics in comparison of CHD, but the carbons in the substituent groups act as “reporter” atoms in adding signatures of carbon-carbon bond distance to the time-dependent pair distribution functions (PDFs), which were missing in our previous study of CHD. We observed real-time signatures of the structural evolution towards the pericyclic minimum in both measurements and simulations. Detailed analyses from the simulated wavepacket revealed the signatures to be due to the hybridization change from sp3 to sp2 configurations largely happening in the excited state prior to bond dissociation to the ring-opening which takes place directly after the internal conversion to the ground state. Our combined experimental and theoretical approaches reflected structural dynamics leading to overlap of the conjugated π-system with the σ-bond prior to its dissociation. In addition, this work also questions the simple picture of the conrotatory motion predicted by the famous Woodward-Hoffmann rules. |
Friday, June 3, 2022 10:54AM - 11:06AM |
Z07.00003: UV-induced dissociation dynamics of bromoform probed by Ultrafast Electron Diffraction Lars Hoffmann, Benjamin W Toulson, Pedro Nunes, Martin Centurion, Ming-Fu Lin, Andrew Attar, Michael W Zuerch, Oliver Gessner The photochemistry of halogen-containing molecules has significant implications for polar ozone depletion, to which bromoform makes a major contribution. In previous work, using UV pump – XUV probe femtosecond transient absorption spectroscopy, it was shown that the major UV-induced dissociation channel of bromoform is the production of atomic bromine via a symmetric fragmentation pathway.1 Molecular dynamics simulations predicted that a minor channel proceeds through an intermediate iso-CHBr3 configuration. An unresolved photochemical curiosity is that the quantum yield for the C-Br fission following excitation at 266 nm is only 0.76±0.03 and the remaining relaxation channels are unknown. Here, the UV photochemistry of bromoform is investigated using mega-electron-volt ultrafast electron diffraction (MeV-UED). UED is a direct probe of the changes in nuclear degrees of freedom and complementary to the transient absorption study. Although, the excitation ratio is only ~0.3%, we can follow the C-Br dissociation in 50 fs time steps. Applying a test structure-based fit to the experimental data in k-space shows contributions of other products besides CHBr2 + Br. Individual features are associated with distinct timescales. The findings will be discussed in terms of the possible associated photofragmentation pathways. |
Friday, June 3, 2022 11:06AM - 11:18AM |
Z07.00004: Dissociation dynamics of tribromomethane probed by time-resolved coincident ion momentum imaging Surjendu Bhattacharyya, Kurtis D Borne, Enliang Wang, Farzaneh Ziaee, Shashank Pathak, Anbu S Venkatachalam, Xiang Li, Keyu Chen, Nathan Marshall, Charles Fehrenbach, Travis Severt, Itzik Ben-Itzhak, Artem Rudenko, Daniel Rolles The dissociation dynamics of tribromomethane (CHBr3) induced by single-photon absorption in the UV and deep UV as well as multi-photon absorption in the near-infrared (NIR) are probed by Coulomb explosion imaging employing 28-fs NIR pulses at a peak intensity of 4 x 1014 Wcm-2. Time-dependent effects are observed in several multi-body breakup channels in which up to five ions are measured in coincidence. The time-resolved kinetic energy distributions and ion momentum correlations are analyzed to identify possible pathways leading to isomerization or to HBr and Br2 elimination. |
Friday, June 3, 2022 11:18AM - 11:30AM |
Z07.00005: Ultrafast molecular dynamics in ionized 1- and 2-propanol: from simple fragmentation to complex isomerization and roaming mechanisms Debadarshini Mishra, Juan Reino-González, Razib Obaid, Aaron C LaForge, Sergio Díaz-Tendero, Fernando Martin, Nora Berrah We will present time-resolved ultrafast nuclear dynamics in ionized 1- and 2-propanol, observed using a combination of pump-probe spectroscopy and coincident Coulomb explosion imaging. Using the experimental data along with quantum chemistry calculations, we identify the two- and three-body breakup channels for both isomers. These isomers exhibit a wide variety of dynamics ranging from direct C-C bond fragmentation to hydrogen migration which are supported by state-of-the-art molecular dynamics simulations. Additionally, the direct fragmentation channel in 2-propanol shows evidence of methyl roaming which manifests as a unique distribution in the kinetic energy release spectrum. We find that the electronic distribution of the roaming methyl fragment is potentially responsible for the enhanced ionization we also observe in this channel. We conclude by comparing the dynamics of 1- and 2-propanol with similar studies done on ethanol and acetonitrile, which establishes a correlation between the carbon chain length and probability of hydrogen migration. |
Friday, June 3, 2022 11:30AM - 11:42AM |
Z07.00006: Coupled electron-nuclear dynamics of molecular dissociation in few-cycle intense laser pulses by surface hopping method Phi Hung Tran, Hung V Hoang, Anh-Thu Le We present theoretical study of the surface hopping method for simulation of coupled electron-nuclear dynamics of molecular dissociation in the presence of few-cycle intense laser pulses. We provide detailed analysis of the results from different versions of the surface hopping method for several systems. Furthermore, by benchmarking with exact numerical solutions of the coupled-channel time-dependent Schrodinger equation for those systems, we propose a simple modification to significantly improve the method and show that both dissociating nuclear wave packet and its phase can be quite accurately simulated. Our results also demonstrate the importance of decoherence and velocity adjustment during the hopping process. As phase of the wave packet becomes accessible, potential applications of the surface hopping method to other intense laser processes such as high-harmonic generation (HHG) with targets evolving in different potential energy surfaces can be envisioned. So far, only a limited number of applications to HHG have been reported, and more importantly, the adequacy of the simulation is still largely unknown. |
Friday, June 3, 2022 11:42AM - 11:54AM |
Z07.00007: 3D Velocity Map Imaging of Electrons Gonenc Mogol, Chuan Cheng, Thomas Weinacht, Andrei Nomerotski, Carlos A Trallero Traditionally velocity map imaging (VMI) experiments only measure the momenta of particles transverse to the time of flight axis and recover the the momentum in the direction of time of flight axis by Abel inversion or other means. Although in previous work we measured the 3D momentum of photoions using a fast camera, electrons have a much lower spread in their time of arrival. Hence, so far it has not been possible to resolve 3D momenta of electrons. |
Friday, June 3, 2022 11:54AM - 12:06PM Withdrawn |
Z07.00008: High-resolution Fourier transform momentum spectroscopy of broad rotational wave packets in cationic states of diatomic molecules Vinod Kumarappan, Tomthin Nganba Wangjam, Huynh Van Sa Lam We use velocity map imaging and Fourier transform spectroscopy to investigate rotational wave packets in bound cationic states of diatomic molecules. The wave packets are launched by strong-field ionization of the molecules by 800 nm, ≈100 fs pulses through pre- and post-ionization excitation. The molecules are dissociated by either the fundamental at 800 nm or the third harmonic. High resolution FFT spectroscopy allows us to identify the electronic, vibrational and rotational sates involved, and provides access to many examples where the coupling to rotational angular momentum to spin and electronic angular momentum is critical in determining the rotational dynamics. We will discuss these dynamics in D2+, O2+ and N2+. |
Friday, June 3, 2022 12:06PM - 12:18PM |
Z07.00009: Free rotation of D2 molecules in superfluid helium nanodroplets Junjie Qiang, Lianrong Zhou, Peifen Lu, Kang Lin, Yongzhe Ma, Shengzhe Pan, Chenxu Lu, Wenyu Jiang, Fenghao Sun, Wenbin Zhang, Hui Li, Xiaochun Gong, Ilya Averbukh, Yehiam Prior, Constant Schouder, Henrik Stapelfeldt, Igor Cherepanov, Mikhail Lemeshko, Wolfgang Jäger Laser-induced molecular alignment was recently extended to molecules in superfluid helium nanodroplets. Studies on in-droplet OCS, CS2 and I2 molecules show that the effective rotational constant BHe is smaller, and the centrifugal distortion constant DHe is much larger than those of the gas-phase molecules. On the other hand, the IR spectroscopy studies show that the DHe and BHe values have an empirical correlation DHe = 0.038BHe1.88. For lighter molecules, the BHe/Bgas ratio tends towards 1, notably when Bgas > 1 cm-1. How do light molecules rotate inside superfluid helium nanodroplets? We answer this question by investigating the rotational dynamics of in-droplet D2 molecules induced by a moderately intense femtosecond pump pulse. By measuring the time-dependent yield of HeD+, created through strong-field dissociative ionization with a probe pulse, the rotational coherence was traced to persist for longer than 100 ps (500 rotational periods). Our results show that the BHe and DHe values of the in-droplet D2 molecules are the same as those for the isolated D2 molecules, and the empirical correlation between BHe and DHe does not apply to D2 molecules. Our experiment demonstrates that D2 molecules inside superfluid helium nanodroplets essentially rotate as free D2 molecules. |
Friday, June 3, 2022 12:18PM - 12:30PM |
Z07.00010: Monitoring photoinduced charge transfer dynamics in wet nanoplasmonic light-harvesting systems using time-resolved ambient pressure x-ray photoelectron spectroscopy Matthew Fraund, Chuying Huang, Lida Kaphar, Lars Hoffmann, Sasawat Jamnuch, Tod Pascal, Jin Qian, Friedrich Roth, Monika Blum, Oliver Gessner Improving the efficiency and economic viability of hydrogen production via photoelectrochemical (PEC) water splitting is a major hurdle for clean and renewable hydrogen production. One promising system utilizes plasmonic metal nanoparticles (NPs) to improve light harvesting and charge separation in wide-bandgap semiconductor systems, enhancing visible light absorption and photocatalytic performance. Here, a model system of 20 nm gold NPs atop a layer of TiO2 is studied with picosecond time-resolved ambient pressure x-ray photoelectron spectroscopy (TRAPXPS) to observe charge carrier dynamics in real-time with electron donor and acceptor site specificity. Measurements both in vacuum (dry) and with a background pressure of ~9 Pa of water vapor (wet) are performed and compared with previous results.1,2 Recombination dynamics under both dry and wet conditions were substantially faster in recent measurements; however, the timescale of the wet sample charge separation dynamics remained orders of magnitude faster. Differences and similarities between current and previous results will be discussed in the context of the different sample preparation conditions. |
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