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 E10: Focus Session: Excited-State Dynamics in MoleculesFocus Live Streamed
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Chair: Jan-Michael Rost, Max Planck Complex Systems Room: Grand Ballroom D |
Tuesday, May 31, 2022 2:30PM - 3:00PM |
E10.00001: Ab Initio Time-Resolved X-ray Spectroscopy: Mechanistic Insights into Conical Intersection Dynamics Invited Speaker: Michael S Schuurman Developments in ultrafast X-ray sources offer powerful new means of probing the complex nonadiabatic dynamics in photoexcited molecules. Of particular interest are the electronic and nuclear structures associated with conical intersections, which are predicted to have unique spectral signatures when probed using X-ray spectroscopies. Using high level ab initio quantum dynamics simulations and newly developed electronic structure techniques based on the DFT/MRCI formalism, we have simulated time-resolved X-ray spectroscopy (TRXAS and TRXPS) of a number of prototypical unsaturated organic chromophores. These spectroscopies are sensitive to all aspects of the ensuing dynamics and can distinguish between the different nuclear dynamical pathways that result in electronic relaxation. The ability of femtosecond X-ray spectroscopy to provide a clear picture of the wave packet dynamics near conical intersections is related to transient charge localization at specific atomic sites that is driven by the nuclear dynamics. These dramatic changes in the local charge environment are then 'reported' using the core-electrons localized on, or proximate to, the atoms involved in the formation of these transient electronic structures. Given the complexity of the measured signals, and the difficulty in empirically (and uniquely) mapping transient spectra to molecular structures, the role of accurate simulations will be essential for the interpretation of these new time-resolved spectroscopies. |
Tuesday, May 31, 2022 3:00PM - 3:12PM |
E10.00002: Ring Opening and Rigidity: A Comparison of Time Resolved Photoelectron Spectroscopy Measurements and Calculations for 1,3-Cyclohexadiene and cis,cis-1,3-Cyclooctadiene Samuel A McClung, Yusong Liu, Pratip Chakraborty, Spiridoula C Matsika, Thomas Weinacht Time resolved photoelectron spectroscopy (TRPES) measurements can be directly compared with the same observable calculated using trajectory surface hopping calculations. In particular, 1,3-cyclohexadiene (CHD) and cis,cis-1,3-cyclooctadiene (COD) were compared to examine how size and rigidity can influence the dynamics, with the goal of developing systematic interpretations of how structural features can manifest differences in the dynamics. Though experimental data qualitatively agreed with theory for both molecules, CHD was found to have closer quantitative agreement, suggesting that some improvement in the theory is necessary to describe the dynamics accurately in general. |
Tuesday, May 31, 2022 3:12PM - 3:24PM |
E10.00003: Jahn-Teller Distortion Driven by Electronic Coherences Drake Richmond, Varun S Makhija The role of electronic coherences in molecular dynamics has recently become a subject of intense investigation given the possibility of measuring their effects using sub-femtosecond laser pulses. Their effect on dynamics occurring at conical intersections has been of been of particular interest. Jahn-Teller (JT) distortion is a canonical example of such non-adiabatic dynamics. We investigate the role of electronic coherences in the JT distortion of Ammonia in its degenerate B 1E'' state. Using a recently developed formalism and a spectroscopic Hamiltonian we compute the full rovibronic density matrix as a function of time after excitation out of the symmetric ground state. Surprisingly, we find that no population transfer occurs between the JT split electronic states. The JT distortion is driven entirely by coherence between the electronic states. |
Tuesday, May 31, 2022 3:24PM - 3:54PM |
E10.00004: Nuclear-driven Electronic Coherences in Molecules via Ultrafast Angle-Resolved Scattering Invited Speaker: Albert Stolow Electronic coherences in molecules are ultrafast charge oscillations on the Molecular Frame (MF). Their direct observation and separation from electronic population dynamics is a long-standing goal. We discuss a valence shell Lab Frame (LF) scattering method for probing electronic coherences in isolated targets. MF electronic coherences lead to LF electronic anisotropies directly observable by an ultrafast angle-resolved scattering technique. The scattering probe may be based on ultrafast electron or X-ray scattering, or angle-resolved photoemission. Moment analysis of the measured LF anisotropy completely separates electronic coherences from population dynamics. Our proof-of-concept demonstration is based on the technique of ultrafast Time-Resolved Photoelectron Angular Distributions (TRPADs). We emphasize that this general time-angle-resolved scattering anisotropy approach applies equally to attosecond/femtosecond electronic coherences in isolated systems. |
Tuesday, May 31, 2022 3:54PM - 4:06PM |
E10.00005: The effect of methylation on the excited state dynamics of 2-thiouracil Susanne Ullrich, Abed Mohamadzade, Sarita Shrestha, Yingqi Qu Thiouracils, where an oxygen atom in uracil is replaced by sulfur, have been observed to undergo fast and efficient intersystem crossing (ISC) from the lowest singlet excited state, S1(nπ*), to the triplet manifold along a sulfur-out-of-plane (op-S) coordinate. Our recent time-resolved photoelectron spectroscopy (TRPES) studies on a series of thiouracils have shown that minor changes such as the position and degree of thionation profoundly alter the subsequent dynamics on the lowest triplet state, T1. These differences are attributed to intricate details of the potential energy topography, specifically the presence and relative energies of two minima with boat-like (T1 boat) and sulfur-out-of-plane (T1 op-S) geometries. In the case of 2-thiouracil, the excited state population equilibrates between both minima. ISC back to the ground state (GS) depends on spin-orbit couplings (SOC) and accessibility of the T1/GS crossing points at boat-like and op-S geometries with T1 op-S/GS clearly favored. The TRPES study presented here interrogates the equilibration and ISC dynamics of the lowest triplet state of 2-thiothymine. Methylation provides a sensitive probe of barriers along a pathway that involves displacements of the methyl substituent and will manifest itself in slower triplet dynamics. As such, it provides insight into the role of the boat-like minimum in the excited state deactivation mechanism. |
Tuesday, May 31, 2022 4:06PM - 4:18PM |
E10.00006: X-ray Induced Ultrafast Electron and Fragmentation Dynamics in Heavy-element Containing Molecules Phay J Ho, Robert W Dunford, Adam E Fouda, Gilles Doumy, Donald A Walko, Linda Young, Stephen Southworth Characterization of the inner-shell decay processes in heavy element containing molecules is key to understand x-ray damage of molecules and materials and realize medical applications with Auger-emitting radionuclides. The Advanced Photon Source provides intense, tunable, narrow bandwidth x-ray beams that are well suited to studies of x-ray absorption and core-hole decay processes in systems containing heavy elements. The 1s hole states in the heavy elements largely decay by x-ray emission and are followed by additional Auger decays that increase the ion's charge state until the final state is reached. Using the x-ray/ion coincidence method, we recorded ion time-of-flight data with a multi-hit, position-sensitive detector for Br2, IBr, CH2IBr, and CF3Br following x-ray absorption at the Br and I K-edges. We present the charge states and kinetic energies of two or more correlated fragment ions associated with core-excited states produced during the various steps of the cascades. To understand the dynamics leading to the ion data, we develop a computational model that combines Monte-Carlo/Molecular Dynamics simulations with a classical over-the-barrier model to track inner-shell cascades and redistribution of electrons in valence orbitals and nuclear motion of fragments. |
Tuesday, May 31, 2022 4:18PM - 4:30PM |
E10.00007: Excited State Dynamics of Sub-Nanometer Metal Oxide Clusters Scott G Sayres I will present our recent work1-3 on the ultrafast dynamics of sub-nanometer neutral metal oxide clusters investigated with femtosecond pump-probe spectroscopy and supported by theoretical calculations. Absorption of a UV (400 nm) photon initiates several relaxation processes, with excited state lifetimes that are strongly dependent on the nature of the electronic transition. The atomic precision and tunability of gas phase clusters highlights how the simple picture of sequential oxidation of the cluster reveals a linear tunability to the contributions of each relaxation component to the total transient signal. In chromium oxides, a ~30 fs transient signal fraction grows linearly with oxidation, matching the amount of O to Cr charge transfer character of the photoexcitation and highlighting a gradual transition between semiconducting and metallic behavior at the molecular level. The lifetimes of nickel oxide clusters exhibit a unique reliance on the nature of the atomic orbital contributions, providing new insights to the analogous band edge excitation dynamics of strongly correlated bulk material. Short lived dynamics in stoichiometric (NiO)n clusters are attributed to excitation between Ni-3d and Ni-4s orbitals, where their strong exchange coupling produces metallic-like electron-electron scattering. Oxygen vacancies introduce 3d→4p transitions, which increases the lifetimes of the sub-picosecond component by 20-60% and enables the formation of long-lived (lifetimes > 2.5 ps) states. |
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