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 U02: Imaging Ultrafast Molecule DynamicsLive
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Chair: Thomas Weinacht, Stony Brook University |
Thursday, June 3, 2021 2:00PM - 2:12PM Live |
U02.00001: Observing nuclear and electronic degrees of freedom simultaneously in a single ultrafast electron diffraction experiment Jie Yang Jie Yang1, Xiaolei Zhu2, Todd J. Martinez3 and Xijie Wang4 |
Thursday, June 3, 2021 2:12PM - 2:24PM Live |
U02.00002: Ultrafast electron diffraction of ionized toluene molecules generated by a near-infrared strong laser field Yanwei Xiong, Kurtis D Borne, Andrés Carrascosa, Sajib Kumar Saha, Kyle Wilkin, Mengqi Yang, Surjendu Bhattacharyya, Keyu Chen, Wenpeng Du, Lingyu Ma, Nathan Marshall, J. Pedro F. Nunes, Shashank Pathak, Zane D Phelps, Xuan Xu, Haiwang Yong, Kenneth Lopata, Peter M Weber, Artem Rudenko, Daniel Rolles, Martin Centurion Gas phase ultrafast electron diffraction (UED) has been a powerful method to determine the structure and structural dynamics of neutral molecules with great details, but there have not been studies of ions in the gas phase using UED. We use 90 kilo-electron-volt gas phase UED setup to investigate the ionized molecules of toluene generated by a femtosecond near-infrared laser pulse, and momentum-resolved coincidence time-of-flight ion mass spectrometry to determine the relative yield of possible ionic products. The relative yields of main fragmentation channels are determined by combining the two measurements and comparing the diffraction signal to the theory. The fitting of the diffraction signal from ions to the theory shows that electron scattering computation based on independent atom model is not a good approximation for electron scattering of ionized molecules. The diffraction signal of ions is in good agreement with ab initio computation. |
Thursday, June 3, 2021 2:24PM - 2:36PM Live |
U02.00003: The Photochemistry of cis-Stilbene Studied by Ultrafast Electron Diffraction Sajib Kumar Saha, Pedro Nunes, Hayley Weir, Bryan Moore, Monika Willams, Andrew Attar, Duan Luo, Ming-Fu Lin, Matthias Hoffmann, Jie Yang, Fuhao Ji, Matthew R Ware, Keith Jobe, Shashank Pathak, Thomas J Wolf, Todd Martinez, Martin Centurion Stilbene is a model system for studying both photoisomerization and photocyclization mechanisms, commonly pivotal to the conversion of light into chemical and mechanical energy in nature. Despite multi-decades of research, the exact details of the photoisomerization of cis-stilbene into trans-stilbene and/or its cyclization into 4a,4b-dihydrophenanthrene (DHP) have remained a topic of debate. Here we used megaelectronvolt ultrafast electron diffraction (UED) to capture and spatially resolve the photoisomerization and/or photocyclization of cis stilbene with sub-angstrom resolution and shed some light on the mechanisms mediating its rich photochemistry. At the SLAC MeV-UED beamline, cis-stilbene was optically pumped with 267 nm ultraviolet light and probed with 3.7 MeV electrons. Analyses of the difference-diffraction signals revealed features across a wide momentum transfer range, up to 8Å-1. These features were found to be in good qualitative agreement with the difference signal predicted by Ab initio multiple spawning simulations. |
Thursday, June 3, 2021 2:36PM - 2:48PM Live |
U02.00004: Extracting body frame molecular geometry from ultrafast diffraction gas-phase experiments through coherent ensemble anisotropy. Kareem Hegazy, Varun Mikhija, Ryan Coffee Ultrafast electron and x-ray diffraction from molecular gases both aim to retrieve sub-picosecond and sub-nanometer intramolecular dynamics. These measurements record time-dependent pair-correlation distributions: histograms of all atomic pairwise distances in the molecule. With unlabelled distances alone, the geometry can rarely be retrieved uniquely without the aid of complex excited state dynamics simulations. Here we show a novel method that uses coherent rotational wave packets to better retrieve the molecular frame geometry. We derive a relation between the measured ensemble laboratory frame anisotropy and the molecule’s pairwise distances and molecular frame angles which otherwise remain inaccessible without extensive simulation. With this expression we retrieve the molecular geometry with both the atomic pairwise distances and the angles in the molecular frame. We do this using a monte-carlo markov chain approach that avoids the need for complex excited-state simulations. We demonstrate this method in the asymmetric top molecule NO2. |
Thursday, June 3, 2021 2:48PM - 3:00PM Live |
U02.00005: Influencing the formation of H3+ from ethane using shaped ultrafast laser pulses Eric Wells, Tiana A Townsend, Charles Schwartz, Naoki Iwamoto, J. Napierala, Selamawit Tegegn, Abel Solomon, Shitong Zhao, KANAKA PANDIRI, Bethany C Jochim, Travis Severt, Peyman Feizollah, Kevin D Carnes, Itzik Ben-Itzhak Using both closed- and open-loop control to tailor ultrafast laser pulses, we study the ratio of D2H+ to D3+ produced from the D3C-CH3 isotopologue of ethane, which selects between trihydrogen cations formed from atoms on one or both sides of ethane. The D2H+:D3+ ratio can be modified by a factor of two or more using an adaptive feedback method. In addition, two-dimensional scans of linear chirp and third-order dispersion are conducted for a few fourth-order dispersion values while the D2H+ and D3+ production are monitored. In an effort to better understand the processes leading to D3+ formation, we conduct COLTRIMS measurements of dissociative ionization of C2D6 to complement the velocity map imaging data obtained during the control experiments. |
Thursday, June 3, 2021 3:00PM - 3:12PM Live |
U02.00006: Imaging rotation of a radical co-fragment in UV-induced photodissociation of dihalomethanes Farzaneh Ziaee, Enliang Wang, Kurtis D Borne, Surjendu Bhattacharyya, Nathan Marshall, Balram Kaderiya, Daniel Rolles, Artem Rudenko Numerous studies of the A-band photodissociation of dihalomethanes suggest that more than 80% of the excess photon energy can be deposited into internal degrees of freedom, triggering rotation of the molecular reaction product. We directly map such rotation upon the C-I bond cleavage in CH2BrI and CH2ClI at 264 nm by time-resolved Coulomb explosion imaging. The dissociating molecule is multiply-ionized by the 790-nm probe pulse and breaks up into CH2+ and two halogen ions, detected in coincidence. The rotation of CH2Br or CH2Cl products shows up as an oscillatory structure (period ~270 fs) in the delay-dependent kinetic energies and relative emission angles of the detected ions. Our results agree well with the simulation based on a rigid-rotor model of the radical co-fragment rotation with increasing C-I bond length and suggest that the dissociating molecule periodically approaches a nearly linear geometry, where the fragment located in the middle acquires minimal kinetic energy. |
Thursday, June 3, 2021 3:12PM - 3:24PM Live |
U02.00007: Reaction and fragmentation dynamics of CH2I2 induced by UV photoabsorption Yijue Ding, Brett Esry, Loren Greenman Recent studies have revealed interesting dynamics including two-body breakup in CH2I2 [1, 2]. Our study is inspired by an experiment at the JRML pumping gaseous CH2I2 molecules with a 266 nm UV laser pulse and measuring the resulting dynamics using Coulomb explosion induced by an intesnse 800 nm probe pulse. Various reaction channels are found such as photodissociation and photoionization, and a transient isomerization pathway is predicted but not conclusively detected before [3]. We calculate the potential energy surfaces of CH2I2 and CH2I2+ using post Hartree-Fock methods, based on which we study the reaction dynamics of the molecules. We discuss the two-body and three-body dissociation and the photoionization processes, and compare them to the Coulomb explosion images, focusing on the direct intra-molecular isomerization mechanism. |
Thursday, June 3, 2021 3:24PM - 3:36PM Live |
U02.00008: Single-shot visualization of the optical Kerr effect, ionization, and rotational Raman effect during laser matter interactions via frequency-domain holography Dennis Dempsey, GARIMA C NAGAR, Jack Agnes, Russell Berger, Bonggu Shim We visualize the ultrafast dynamics caused by intense femtosecond laser pulses in both thin flexible glass as well as gaseous atoms and molecules using single-shot Frequency Domain Holography (FDH) [1-3]. FDH is a robust, single-shot, time-resolved visualization technique that employs chirped pulses. Femtosecond laser micromachining of glass materials relies critically on the Kerr effect and ionization, thus direct observation of their dynamics can help produce optical devices such as waveguides. For gases, single-shot visualization of laser-matter interactions will allow for a better understanding of nonlinear optical phenomena such as filamentation [4] and Raman-induced extreme spectral broadening [5]. Using FDH, we have previously observed the ionization dynamics of thin, flexible glass and measured its nonlinear index [3], and are currently investigating the ultrafast dynamics of gases under intense laser fields. [1] S. P. Le Blanc et al., Opt. Lett. 56, 764-766 (2000). [2] K. Y. Kim et al., APL, 88 4124-4126 (2002). [3] S. Huang et. al., OFC 1-3 (2014). [4] A. Couairon et al., Phys. Rep. 441, 47 (2007). [5] D. Dempsey et al. Opt. Lett. 45, 1252-1255 (2020). |
Thursday, June 3, 2021 3:36PM - 3:48PM Live |
U02.00009: Characterization of light-induced potentials in the strong-field dissociation of O2+ Paul Abanador, Uwe Thumm We examine the imprints of light-induced potentials (LIPs) on the dissociation dynamics of O2+ molecular ions, as observed in the angle-resolved fragment kinetic-energy-release (KER) spectra. By numerically solving the time-dependent Schrödinger equation within the Born-Oppenheimer approximation, we follow the vibrational and rotational motion of O2+ molecular ions exposed to 800-nm, 40-fs laser pulses. For infrared (IR) peak intensities between 1013 and 1014 W/cm2, we calculate angle-resolved KER spectra which reveal characteristic energy- and angle-dependent fringe structures. In general, we find that these fringes shift downward in energy as the molecular alignment angle θ relative to the IR-pulse polarization direction increases from 0 to π/2. The angle-dependent shifts in the KER fringes increase for higher IR-pulse peak intensities and follow the angle and peak-intensity dependence of the vibrational spectrum in the associated Floquet bond-hardening well, which is a manifestation of the transient O2+ nuclear-probability trapping in the LIP during the dissociation process. Based on the analysis of the rovibrational dynamics of the dissociating molecular cation near the light-induced conical intersection (LICI) in the LIP surface at θ = π/2, we furthermore identify pertinent angle-dependent, LICI-related features in the KER spectra. |
Thursday, June 3, 2021 3:48PM - 4:00PM Live |
U02.00010: Accurate theoretical simulation of attosecond transient absorption spectroscopy in N2 Hung Hoang, Anh-Thu Le We report theoretical investigation of attosecond transient absorption spectroscopy (ATAS) of molecular nitrogen. Our calculations are based on the numerical solution of the couple-channel time-dependent Schrodinger equation with the account for the accurate potential energy surfaces together with the non-adiabatic and dipole couplings. Furthermore, the effect of molecular alignment with respect to the polarization directions of the extreme ultraviolet (XUV) pump and near-infrared probe lasers are also fully considered. We find improved agreements with all the main features of the earlier experimental measurements by Warrick et al, Chem. Phys. Lett. 683, 408 (2017). Our results indicate that ATAS measurements are capable of probing potential energy surfaces of the target dark states. |
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