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 Z03: Ultrafast Molecular SpectroscopyLive
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Chair: Siqi Li, SLAC National Lab |
Friday, June 4, 2021 10:30AM - 10:42AM Live |
Z03.00001: Probing excited state dynamics of cis,cis-1,3-cyclooctadiene with time-resolved photoelectron spectroscopy Yusong Liu, Pratip Chakraborty, Samuel A McClung, Spiridoula C Matsika, Thomas Weinacht Photoinduced isomerization dynamics play an important role in conjugated molecular systems with double bonds. We extend previous studies from small rigid systems, such as trans-1,3-butadiene ( C4H6 ) and 1,3-cyclohexidiene ( C6H8 ), to a larger and more flexible molecule, cis,cis-1,3-cyclooctadine ( C8H12). We perform time-resolved photoelectron spectroscopy ( TRPES ) measurements on cis,cis-1,3-cyclooctadiene, and we interpret the measurement with trajectory surface hopping calculations. The measurements indicate that the molecule undergoes rapid internal conversion to the ground state, and the features in the measured photoelectron spectra are interpreted in terms of ionization to several states of the molecular cation. |
Friday, June 4, 2021 10:42AM - 10:54AM Live |
Z03.00002: Which form of the molecular Hamiltonian is the most suitable for simulating the nonadiabatic quantum dynamics at a conical intersection? Seonghoon Choi, Jiri J Vanicek Choosing a suitable representation of the molecular Hamiltonian is a challenge faced by simulations of the nonadiabatic dynamics around a conical intersection. The adiabatic, exact quasidiabatic, and strictly diabatic representations are exact, whereas the approximate quasidiabatic Hamiltonian ignores the residual nonadiabatic couplings. A rigorous numerical comparison of these four representations is difficult due to the exceptional nature of systems where these can be defined exactly and the necessity of an accurate algorithm that avoids mixing numerical errors with errors due to the different representations. We are able to perform this comparison [1,2] using the quadratic Jahn-Teller model and high-order geometric integrators [3] and find that only the rarely employed exact quasidiabatic Hamiltonian yields nearly identical results as the strictly diabatic Hamiltonian, which is unavailable in general. In this model and with the same grid, the approximate quasidiabatic Hamiltonian led to inaccurate wavepacket dynamics, while the adiabatic Hamiltonian was the least accurate due to the singular nonadiabatic couplings. |
Friday, June 4, 2021 10:54AM - 11:06AM Live |
Z03.00003: Using time-resolved photoelectron spectroscopy to benchmark excited state molecular dynamics calculations Samuel A McClung, Yusong Liu, Pratip Chakraborty, Spiridoula C Matsika, Thomas Weinacht A TRPES (time-resolved photoelectron spectroscopy) experiment was performed to compare trajectory surface hopping calculations for three different levels of theory (CASSCF, MRCIS, and XMS-CASPT2). In particular, we consider the radiationless decay following excitation to the first bright state of uracil (S2). The measurements allow us to track independently the excitation and decay of S1 and S2 and discriminate between different levels of theory. |
Friday, June 4, 2021 11:06AM - 11:18AM Live |
Z03.00004: Time-resolved monitoring of photoinduced charge transfer dynamics in wet nanoplasmonic light-harvesting systems using ambient pressure x-ray photoelectron spectroscopy Matthew Fraund, Mario Borgwardt, Felix Brausse, Friedrich Roth, Monika Blum, Oliver Gessner Photoelectrochemical (PEC) water splitting is an attractive method to produce hydrogen fuel because it is clean and renewable; however, materials for efficient PEC systems are not economically viable at industrial scales compared to non-renewable methods. One option to improve solar light harvesting and charge separation in the photocatalyst is to use plasmonic metal nanoparticles (NPs), which are particularly useful to sensitize wide-bandgap semiconductors due to their chemical stability and strong absorption at visible wavelengths. 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). The technique allows monitoring charge dynamics on the electron donor and acceptor sites separately and in real-time. Measurements are performed under two conditions: a high vacuum sample environment and a water vapor background pressure of ~9 Pa. In vacuum, a charge injection efficiency of ~2 electrons per NP (~0.1% photon-to-charge efficiency) is observed, followed by recombination over two timescales: 60 ps and ~1 ns.1 Once water vapor is introduced, charges remain separated for up to ~2 orders of magnitude longer timescales and exhibit more complex recombination dynamics compared to the high vacuum case. A simple physical model is used to interpret the effect that the water environment may have on extending charge recombination lifetimes. |
Friday, June 4, 2021 11:18AM - 11:30AM Live |
Z03.00005: Measurement of Multiple Components of the Molecular Frame Third-order Nonlinear Response Tensor in Impulsively Aligned CO2 Molecules Siddhant Pandey, Francis F Walz, Varadharajan Muruganandam, Varun S Makhija, Niranjan Shivaram We perform measurements of multiple components of the molecular frame third-order nonlinear response tensor (second hyper-polarizability) in impulsively aligned carbon dioxide (CO2) molecules. We use a moderately strong femtosecond near-infrared laser pulse to impulsively align CO2 molecules, and use a pair of femtosecond pulses to measure the non-linear response using the Optical Kerr effect. The non-linear response is measured at a rotational revival for different alignment pulse polarization angles which allows us to measure multiple molecular frame tensor components. We then extend this to a snapshot measurement where we use an azimuthally polarized vector beam for alignment and a focal volume imaging scheme to measure nonlinear response for different alignment angles simultaneously. We interpret the tensor components in terms of symmetries of the valence electronic character. These studies are an important step towards using non-linear response tensor measurements to study ultrafast dynamics on electronically excited states in molecules. |
Friday, June 4, 2021 11:30AM - 11:42AM Live |
Z03.00006: Simulating Nonlinear Optical Signals from Ultrafast Transient Polarization Spectroscopy in Excited State Molecules: An Experimentalist's Approach Richard Thurston, Matthew Brister, Liang Tan, Thorsten Weber, Niranjan Shivaram, Daniel Slaughter Ultrafast Transient Polarization Spectroscopy is a nonlinear spectroscopic method that is able to measure excited state nonlinear optical signals as a function of time delay between the incident pulses. In this technique, an excitation pulse first excites a population of target molecules to an excited state and a pair of probing pulses use the optical Kerr effect to sample the transient nonlinear optical susceptability of the excited system. These signals are often difficult to interpret due to the nonlinear nature of signal generation and a lack of theoretical treatment for these experiments. Here we present a flexible approach to simulating experimentally measurable nonlinear excited state signals by treating the case of an N-level molecular system using a Liouville Space framework in the time domain. Applications of this method to aid in the analysis of recently collected data on optically excited nitrobenzene are also explored. |
Friday, June 4, 2021 11:42AM - 11:54AM Live |
Z03.00007: Finite-temperature, anharmonicity, and Duschinsky effects on the two-dimensional electronic spectra from ab initio thermo-field Gaussian wavepacket dynamics Tomislav Begusic, Jiří Vaníček Accurate description of finite-temperature vibrational dynamics is indispensable in the computation of two-dimensional electronic spectra. Such simulations are often based on the density matrix evolution, statistical averaging of initial vibrational states, or approximate classical or semiclassical limits. While many practical approaches exist, they are often of limited accuracy and difficult to interpret. Here, we use the concept of thermo-field dynamics to derive an exact finite-temperature expression that lends itself to an intuitive wavepacket-based interpretation [1]. Furthermore, an efficient method for computing finite-temperature two-dimensional spectra is obtained by combining the exact thermo-field dynamics approach with the thawed Gaussian approximation for the wavepacket dynamics [2-4], which is exact for any displaced, distorted, and Duschinsky-rotated harmonic potential but also accounts partially for anharmonicity effects in general potentials. Using this new method, we directly relate a symmetry breaking of the two-dimensional signal to the deviation from the conventional Brownian oscillator picture. |
Friday, June 4, 2021 11:54AM - 12:06PM Live |
Z03.00008: Studies of vibrational relaxation of photoexcited electrons in endofullerene molecules Esam A Ali, Mohamed Madjet Using a density functional theory-based scheme of non-adiabatic molecular dynamics simulations [1,2], the electron relaxation dynamics in a prototypical endofullerene Mg@C60 after an initial localized photoexcitation in Mg is investigated. Two approaches to the electronic structure of the excited electronic states are used [3]: (i) an independent particle approximation based on a density functional theory description of molecular orbitals and (ii) a configuration-interaction description of the many-body effects. Results of electronic properties, optical excitations and ultrafast population decay dynamics will be presented. The effect of different exchange-correlation (XC) functionals will be discussed. The advantages and drawbacks of the two methods will be outlined. |
Friday, June 4, 2021 12:06PM - 12:18PM Live |
Z03.00009: Effects of decoherence and propagation in remote detection of molecules using CARS Jabir Chathanathil, Gengyuan Liu, Svetlana A Malinovskaya The backscattered signal of Coherent Anti-Stokes Raman Spectroscopy (CARS) is theoretically analyzed for remote detection of molecules in the atmosphere. An optimization of the CARS signal is done by maximizing vibrational coherence using chirped control pulses. The factors affecting the intensity of the output signal are investigated such as decoherence and multiple collisions with the target molecules upon propagation. The Liouville von Neumann equations with the relaxation terms of spontaneous decay and collisional dephasing are used. A numerical analysis of the dependence of the molecular state populations and coherence on the peak Rabi frequency and other field parameters is performed demonstrating adiabatic regime of light-matter interaction and mitigation of decoherence. For 100fs incoming pulses, a significant amplification of the anti-Stokes signal is demonstrated, up to 3 orders of magnitude after multiple scattering events. A machine learning technique is implemented to extract an analytical phase of scattered fields from their numerical values. |
Friday, June 4, 2021 12:18PM - 12:30PM Live |
Z03.00010: Understanding the Photochemistry Of Iron Complexes Using Time-Resolved X-ray Spectroscopy Christopher Otolski, Zhu-Lin Xie, Gilles Doumy, Karen Mulfort, Linda Young, Anne M March Iron is an earth abundant transition metal found in numerous natural and synthetic chemical processes making it an archetype for exploring and understanding new areas of chemistry and physics. Iron has been shown to be a viable substitute as a far less expensive and toxic alternative to ruthenium and osmium metal complexes, but with the risk of altering the chemical reaction dynamics. Therefore, it is paramount to understand and characterize the reaction dynamics and possible intermediate species in order to control the iron based chemical reactions. In this work we explore the interesting photochemistry of a bistable spin-crossover iron complex, which can be reversibly switched by light irradiation. The attractive use of light to change the spin-state of the system for potential applications in molecular electronics and spintronics. The system uses a photochromic ligand that isomerizes upon light absorption, and in turn induces a modification in the ligand field resulting in a change of the spin-state that is stable at room temperatures. Due to their novelty, these systems have been scarcely studied, and the mechanism behind the ligand field modification is still unclear. Therefore, to elucidate the ligand-driven light-induced spin change mechanism we utilize hard x-ray absorption and x-ray emission spectroscopy techniques at the liquid-jet endstation of the Advanced Photon Source (APS), where we can take advantage of the high-repetition-rate, ultra-stable, and widely tunable x-ray pulses. |
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