Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B26: Focus Session: Non-Adiabatic Dynamics: New Insights from Experiment and Theory II |
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Sponsoring Units: DCP Chair: David R. Yarkony, Johns Hopkins University Room: 204A |
Monday, March 2, 2015 11:15AM - 11:51AM |
B26.00001: Multistate trajectory and statistical theories of spin-forbidden kinetics Invited Speaker: Ahren Jasper Rate coefficients for several spin-forbidden reactions relevant to combustion are calculated using multistate trajectory and statistical theories. The two approaches are compared, and the appropriateness of treating singlet/triplet crossing seams as ``nonadiabatic transition states'' for spin-forbidden reactions is discussed. We show that the spin-forbidden reaction coordinate is coupled to the remaining nuclear degrees of freedom, leading to multidimensional effects not typically included in statistical treatments. We identify: static multidimensional effects due to the geometry-dependence of the shape of the crossing seam and spin-orbit coupling, dynamical multidimensional effects where the electronic transition probability depends on the distribution of the total internal energy of the system, and nonlocal multidimensional effects due to the instantaneous value of the electronic phase at multiple seam crossings. A semiclassical model based on short-time full-dimensional trajectories that includes all three multidimensional effects as well as a model for electronic decoherence is presented. The results of this new multidimensional nonadiabatic statistical theory are compared with the results of one-dimensional Landau-Zener and weak coupling models for several reactions. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B26.00002: Evidence for quantum effects in laser driven photodissociation of methylamines Ilana Bar, Michael Epshtein, Alexander Portnov Non-adiabatic dynamics at conical intersections (CI) extensively affects the photostability of biomolecules by efficiently photoinducing decay routes that dissipate harmful excess ultraviolet energy. Here the photodissociation of the model test molecules, methylamine (CH$_{3}$NH$_{2})$ and its partially deuterated isotopologue (CD$_{3}$NH$_{2})$, excited to different specific vibrational modes in the electronically excited state has been investigated by H(D) photofragments detection with two-color reduced-Doppler ion imaging [1]. The H products, resulting from N-H bond cleavage via two dissociation pathways, showed anomalous distributions for some of the vibronic states, as indicated by dynamic resonances in the product branching ratio and in the anisotropy parameters. This vibronic-specific control is attributed to distinctive dynamical interferences of the initially prepared wavepackets, affecting the passage efficiency through the S$_{1}$/S$_{0}$ CIs. It is suggested that the H product distributions are extremely sensitive to the positions and energies of the CIs in the two molecules, rather than to the unique initial nuclear motion that promotes the coupling between the two electronic states. These observations reveal uniquely detailed insights into the dynamics of state-specific control of internal conversion. \\[4pt] [1] M. Epshtein, A. Portnov, R. Kupfer, S. Rosenwaks, and I. Bar, J. Chem. Phys. \textbf{139}, 184201 (2013). [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B26.00003: ABSTRACT MOVED TO Q48.00005 |
Monday, March 2, 2015 12:15PM - 12:27PM |
B26.00004: Real-Time Subsystem TD-DFT and its Ehrenfest Dynamics: Applications to solvation and exciton transfer Michele Pavanello The subsystem formulation of DFT known as Frozen Density Embedding (FDE) provides a divide-and-conquer approach to Kohn--Sham DFT for a collection of weakly bound subsystems. We present theory and computer code development of the time-dependent extension of FDE. The code is now part of the Quantum--ESPRESSO suite of softwares. We also present the associated Ehrenfest dynamics, in which nuclei and electrons of selected subsystems are propagated simultaneously. Application of the code to exciton transfer phenomena as well as to solvatochromic shifts are discussed. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 1:03PM |
B26.00005: Ultrafast dynamics in DNA base pairs following ultraviolet excitation. Invited Speaker: Andrew Orr-Ewing Photo-protective mechanisms in DNA are essential to maintain the integrity of the genetic code by preventing damage from absorption of solar ultraviolet (UV) radiation. We have used time-resolved infra-red (TRIR) spectroscopy to observe the dynamics of Watson-Crick nucleobase pairs following absorption of femtosecond UV laser pulses. The base pairs are prepared as nucleosides in solution, and photo-induced dynamics are probed in the carbonyl and N-H bond stretching regions using broadband IR pulses with picosecond time resolution. Results will be presented for the guanine-cytosine (G-C) base pair, contrasting the rapid recovery of ground-state products (the photo-protection pathway) with formation of other photoproducts which might represent photo-damage mechanisms. This work is a collaboration with the group of Prof F. Temps (Christian-Albrechts-Universitat zu Kiel). [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B26.00006: Femtosecond Heterodyne Transient Grating Studies of Nonradiative Decay in $\beta$-Carotene and Peridinin: Contributions of Dark Intermediates and Double Quantum Coherences Soumen Ghosh, Jerome Roscioli, Harry Frank, Warren Beck Femtosecond transient grating spectroscopy with optical heterodyne detection was employed to characterize the ultrafast events in the nonradiative decay of $\beta$-carotene and peridinin in solution from the $S_2$ state. The contribution of double-quantum coherences to the time evolution of the third order signal was probed by scanning the interpulse delay between the first two pulses in the transient grating or stimulated photon-echo sequence. The results show that the double-quantum coherence pathways contribute significantly to the transient grating signal only at negative population delays, which is consistent with the requirements determined from double-sided Feynman diagrams when the third order signal is detected in the $-k_1 + k_2 + k_3$ direction. Response function calculations support the conclusion that the ultrafast ($<$20 fs) decay that contributes to the third order signal at positive population delays arises from an kinetic intermediate, which has been previously assigned to the $S_x$ state. We suggest that this intermediate arises not from a discrete electronic state but rather from a twisted conformation of the conjugated polyene. This proposal has significant implications with respect to the energy transfer function of carotenoids in photosynthesis. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B26.00007: Vibrational Conical Intersections: Implications for Ultrafast Vibrational Dynamics Mahesh Dawadi, Bishnu Prasad Thapaliya, Ram Bhatta, David Perry The presence of conical intersections (CIs) between electronic potential energy surfaces is known to play a key role in ultrafast electronic relaxation in diverse circumstances. Recent reports have documented the existence of vibrational CIs connecting vibrationally adiabatic surfaces. Just as electronic CIs are now appreciated to be ubiquitous, controlling the rates of many photochemical processes, the present work on methanol and methyl mercaptan suggests that vibrational CIs may also be widespread, possibly controlling the outcome of some high-energy processes where vibrationally excited species are present. Other examples of vibrational CIs include the vibrational Jahn-Teller effect in C$_{\mathrm{3V}}$ organic molecules and transition metal complexes. While the present work addresses only the couplings within bound molecules, the concept of vibrational CIs providing pathways for ultrafast relaxation also applies to molecular collisions. [Preview Abstract] |
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