Bulletin of the American Physical Society
50th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 64, Number 4
Monday–Friday, May 27–31, 2019; Milwaukee, Wisconsin
Session K03: Time-resolved molecular dynamics and femtochemistry |
Hide Abstracts |
Chair: Thomas Weinacht, Stony Brook University Room: Wisconsin Center 101CD |
Wednesday, May 29, 2019 2:00PM - 2:12PM |
K03.00001: Angle-dependent strong-field ionization and fragmentation of CO2 using rotational wave packets Huynh Lam, Suresh Yarlagadda, Anbu Venkatachalam, Rajesh Kushawaha, Tomthin Wangjam, Chuan Cheng, Peter Svihra, Andrei Nomerotski, Tom Weinacht, Daniel Rolles, Vinod Kumarappan The angle-dependent single ionization of carbon dioxide by linearly polarized probe pulse has been actively discussed over the past decade. In this work, the angle dependence of ionization was extracted with high resolution from time-domain measurements on an impulsively-excited rotational wave-packet. The results are in good agreement with direct angle scan measurements, and are consistent with measurement using circularly polarized probe pulse. We further expand the method to photo-ion momentum measurements, and quantify the breakdown of the axial recoil approximation for the $\mbox{CO}^{+}+\mbox{O}^{+}$ channel. These molecular frame measurements can be extended to asymmetric top molecules, and well as to photoelectron angular distributions. [Preview Abstract] |
Wednesday, May 29, 2019 2:12PM - 2:24PM |
K03.00002: Substantial impact of the orientation of transition dipole moments on the dynamics of diatomics in laser fields Andras Csehi, Peter Badanko, Gabor Halasz, Lorenz Cederbaum, Agnes Vibok The formation of light-induced conical intersections (LICIs) between electronic states of diatomic molecules has been thoroughly investigated over the past decade. In the case of running laser waves, the rotational, vibrational, and electronic motions couple via the LICI giving rise to strong nonadiabatic phenomena. In contrast to natural conical intersections (CIs) which are given by nature and hard to manipulate, the characteristics of LICIs are easily modified by the parameters of the laser field. The internuclear position of the created LICI is determined by the laser energy, while the angular position is given by the orientation of the transition dipole moment (TDM) with respect to the molecular axis. In the present work, using MgH+ as a showcase example, we exploit the strong impact of the orientation of the TDMs exerted on the light-induced nonadiabatic dynamics. Comparing the photodissociations induced by parallel or perpendicular transitions, a clear signature of the created LICIs is revealed in the angular distribution of the photofragments. [Preview Abstract] |
Wednesday, May 29, 2019 2:24PM - 2:36PM |
K03.00003: The influence of donor-acceptor groups on the excited-state dynamics of ethylenic molecules Anja Roeder, Anders Skov, Andrey E. Boguslavskiy, Katherine Herpberger, Ryan J. MacDonell, Michael S. Schuurman, Albert Stolow Conical intersections (CIs) govern molecular dynamics in many light-driven processes, such as vision or photophysics. The non-adiabatic dynamics of ethylene, the simplest double-bond containing molecule, has been extensively investigated as a model system: after initial excitation to the ππ* state, the molecule deforms on its way to the CI - it simultaneously twists along the C=C bond and pyramidalizes on one of the carbons, before returning to a vibrationally hot ground state. The pyramidalization is also associated with an increase in electron density: can we, by changing the electron density on either side of the double bond a priori of the dynamics, control and tune the photophysics of ethylene? To answer this question, we explored the excited-state dynamics of a series of donor-acceptor substituted ethylenes, namely acrylonitrile (acceptor-substituted), methylvinylether (donor-substituted) and 3-methoxyacrylonitrile (donor and acceptor substituted). This joint time-resolved photoelectron spectroscopy and ab initio molecular simulation investigation demonstrates both experimentally and theoretically the fascinating involvement of the donor-acceptor groups in the excited-state dynamics of the double bond. [Preview Abstract] |
Wednesday, May 29, 2019 2:36PM - 2:48PM |
K03.00004: Probing the UV Photodissociation Dynamics of Bromoform with Femtosecond Inner-Shell Transient Absorption Spectroscopy B.W. Toulson, M. Borgwardt, H. Wang, F. Lackner, A.S. Chatterley, D.M. Neumark, S.R. Leone, D. Prendergast, O. Gessner UV pump -- XUV probe femtosecond transient absorption spectroscopy has been used to study the photodissociation dynamics of bromoform (CHBr$_{\mathrm{3}})$. Element-specific core-to-valence transitions provide an atomic scale perspective, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. Complementary excited state molecular dynamics (MD) calculations with surface hopping explore the nuclear dynamics from which XUV spectra are calculated using time-dependent density functional theory. The XUV spectra track how the singly occupied molecular orbitals (SOMO) of transient electronic states develop throughout the C--Br bond fission, eventually forming radical Br and CHBr$_{\mathrm{2}}$ products. The combination of ultrafast transient inner-shell spectroscopy and \textit{ab initio} calculations provides detailed insight into the dynamics, indicating that a continuous change of electronic orbital characters and atomic arrangements occurs during the photochemical reaction. While a proposed sub-100~fs roaming-like isomerization pathway cannot be reproduced, the MD simulations hint that transient isomers may form on multi-100~fs timescales, albeit with a much smaller likelihood than direct CHBr$_{\mathrm{2}}$~--~Br scission. [Preview Abstract] |
Wednesday, May 29, 2019 2:48PM - 3:00PM |
K03.00005: Thionated Uracils under UV Irradiation: Intramolecular Effects on the Intersystem Crossing Dynamics Susanne Ullrich, Abed Mohammadzade The canonical nucleobases, which form the building blocks of our genetic coding material, are known to protect themselves against photodamage through ultrafast internal conversion processes that dissipate potentially harmful UV energy into heat. However, seemingly minor changes such as single atom substitutions can profoundly alter the photophysics of the nucleobases. In thiobases, where an oxygen is replaced by sulfur, these internal conversion pathways are inaccessible and crossing onto the triplet manifold becomes highly efficient. While long-lived, reactive triplet states, as observed in some of the thiobases, negate their photoprotection, these properties are highly desirable for pharmacological applications, e.g. as photosensitizers in cancer treatments. Using time-resolved photoelectron spectroscopy the response of a series of thiouracils to UV irradiation has been investigated to unravel the mechanistic details governing their unique ultrafast intersystem crossing dynamics. Remarkable differences are observed for 2-thiouracil, 4-thiouracil and 2,4-dithiouracil when the degree of thionation and position of sulfur atom is altered. [Preview Abstract] |
Wednesday, May 29, 2019 3:00PM - 3:12PM |
K03.00006: Time-dependent structural dynamics of cyclohexadiene probed by strong-field ionization Kurtis Borne, Farzaneh Ziaee, Peyman Feizollah, Joseph Harrington, Balram Kaderiya, Yubaraj Malakar, Kanaka Raju P., Travis Severt, Itzik Ben-Itzhak, Artem Rudenko, Daniel Rolles We present the results of a time-resolved UV-pump strong-field-ionization-NIR-probe experiment on a prototypical carbon ring molecule, cyclohexadiene (CHD). UV-photoabsorption propels CHD to an electronically excited state, which is coupled to the ground state by a series of conical intersections. Upon de-excitation, the molecule can either return to the initial geometry or isomerize into an open-ring hexatriene (HT) molecule. We discuss possible signatures of the electronic de-excitation and ring-opening in our experimental observables, which include coincident fragment ion yields, kinetic energies, and angular distributions obtained by a coincident ion momentum imaging measurement. [Preview Abstract] |
Wednesday, May 29, 2019 3:12PM - 3:24PM |
K03.00007: Time-Resolved Photoelectron Spectroscopy with a Tuneable Femtosecond VUV Source Ruaridh Forbes, Simon P. Neville, Martin A. B. Larsen, Andrey E. Boguslavskiy, Rune Lausten, Michael S. Schuurman, Albert Stolow Time-resolved photoelectron spectroscopy (TRPES) has emerged as the gold standard technique to interrogate non-adiabatic excited state dynamics in isolated molecules but, to date, has been mostly limited by a wavelength coverage limited the UV to IR. We report the construction of a tuneable vacuum-ultraviolet (VUV) femtosecond laser source based on four-wave mixing in rare gases, capable of producing pulses from 145-180nm. The source has been utilized in a VUV/UV TRPES investigation into Rydberg-valence coupling in the polyatomic molecule acetone. The results highlight the complex interplay between internal conversion occurring within the Rydberg manifold and population of a $\pi\pi^*$ valence state, the latter of which has not been previously experimentally observed. The results were compared to Multi-Configurational Time-Dependent Hartree (MCDTH) calculations, which verified our spectroscopic assignments. As acetone is the simplest ketene, we expect these results will have implications in the excited state dynamics for a large set of molecular systems. [Preview Abstract] |
Wednesday, May 29, 2019 3:24PM - 3:36PM |
K03.00008: Ultrafast Electronic Relaxation Dynamics of Ionized Liquid Water Zhi-Heng Loh, Pei Jiang Low, Zhaogang Nie The ionization of liquid water serves as the principal trigger for a myriad of phenomena that are relevant to radiation chemistry and radiation biology. The earliest events that follow the ionization of liquid water, however, remain relatively unknown. We have embarked on a series of studies to investigate the ultrafast dynamics of intense laser-ionized liquid water. Optical pump-probe spectroscopy employing few-cycle pulses in the visible (500--700 nm) and short-wave infrared (0.9--1.7 $\mu$m) is used to reveal the fate of the electron that is initially injected into the conduction band by ionization. These experiments yield the lifetime of the conduction-band electron and the timescale for vibrational cooling of the {\it s} electron. Remarkably, our results suggest that the relaxation of the conduction band electron to the hydrated {\it s} electron proceeds via an intermediate state --- possibly the elusive {\it p} state electron --- whose lifetime is found to be 63 $\pm$ 3 fs (94 $\pm$ 8 fs) in H$_{2}$O (D$_{2}$O). These results provide a comprehensive view of the electronic relaxation dynamics of ionized liquid water. [Preview Abstract] |
Wednesday, May 29, 2019 3:36PM - 3:48PM |
K03.00009: Femtosecond Time-Resolved Energy Transfer Dynamics in Excited Doped Helium Nanodroplets Catherine Saladrigas, Stephen R. Leone, Daniel M. Neumark, Oliver Gessner Helium nanodroplets are often utilized as a spectroscopic matrix because they are optically transparent and weakly interact with atomic and molecular dopants. In contrast, when exposed to XUV radiation, the droplet is electronically excited and subsequently undergoes a variety of relaxation mechanisms. We are interested in studying energy and charge transfer between the excited droplet environment and embedded dopants, as well as the competition with internal droplet relaxation mechanisms. Previously, energy transfer to a noble gas dopant in an electronically excited droplet was observed in a steady-state experiment. Indirect ionization of the dopant is particularly pronounced for energies at which the droplet exhibits strong absorption. In a complimentary femtosecond time-resolved experiment, we want to probe the underlying energy transfer mechanisms in the time domain. Using a high harmonic generated femtosecond XUV pulse to electronically excite the droplet and a UV probe pulse to modulate the photoelectron signal produced from the energy transfer, we monitor the energy-transfer yield as a function of pump-probe time delay. [Preview Abstract] |
Wednesday, May 29, 2019 3:48PM - 4:00PM |
K03.00010: Dissecting quantum coherence in photosynthesis: lessons from porphyrin dimers Austin Spencer, Pyosang Kim, Waleed Helweh, Lin Chen Many highly efficient photosynthetic protein complexes (PPCs) exhibit strong and persistent coherent spectral signatures that coincide with the early steps of the energy transport process. However, the inherent complexity and scale of proteins conceal the source of these coherences and their role in photosynthesis. To overcome these challenges, we apply broadband two-dimensional electronic spectroscopy (2DES) to porphyrins and their dimers as a bottom-up approach to studying energy transfer in PPCs. Using porphyrin dimers with moderate electronic coupling as a model for PPCs, we test the hypothesis that resonance between vibrational and electronic energy level gaps leads to strong nonadiabatic coupling. In a cofacial zinc porphyrin dimer, 2DES reveals a coherent spectral signature at 350 cm$^{\mathrm{-1}}$ that is absent in the monomer and which may be indicative of a Raman-activated vibration that is enhanced by resonance with the excitonic energy gap. Such a conclusion is consistent with the nonadiabatic coherence model developed for PPCs and supports the associated hypothesis that strong electronic--vibrational coupling drives both the observed coherent spectral signatures as well as rapid energy transfer. [Preview Abstract] |
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