Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections III |
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Sponsoring Units: DCP Chair: Albert Stolow, NRC Canada Room: Colorado Convention Center 205 |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L26.00001: Exploring conical intersections through high resolution photofragment translational spectroscopy Invited Speaker: High resolution measurements of the kinetic energies of H atom fragments formed during UV photolysis of gas phase imidazole, [1,2] pyrrole, [3] phenol [4] and thiophenol molecules show that: (i) X-H (X = N, O, S) bond fission is an important non-radiative decay process from the $^{1}\pi \sigma $* excited states in each of these molecules, and (ii) that the respective co-fragments (imidazolyl, pyrrolyl, phenoxyl and thiophenoxyl) are formed in very limited sub-sets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photo-induced evolution from parent molecule to ultimate fragments. \newline \newline [1] M.N.R. Ashfold, B. Cronin, A.L. Devine, R.N. Dixon and M.G.D. Nix, \textit{Science }(2006), \textbf{312}, 1637. \newline [2] A.L. Devine, B. Cronin, M.G.D. Nix and M.N.R. Ashfold, \textit{J. Chem. Phys.} (in press). \newline [3] B. Cronin, M.G.D. Nix, R.H. Qadiri and M.N.R. Ashfold, \textit{Phys. Chem. Chem. Phys.} (2004), 6, 5031. \newline [4] M.G.D. Nix, A.L. Devine, B. Cronin, R.N. Dixon and M.N.R. Ashfold, \textit{J. Chem. Phys.} (2006), \textbf{125}, 133318. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:42PM |
L26.00002: Dynamic Stark Control of Photochemical Processes Invited Speaker: A technique for controlling the outcome of photochemical reactions using the dynamic Stark effect due to a strong, nonresonant infrared field is demonstrated numerically and experimentally. A precisely timed infrared laser pulse is used to reversibly modify a potential energy barrier during a chemical reaction without inducing any real electronic transitions. Dynamic Stark control (DSC) is experimentally demonstrated during the nonadiabatic photodissociation of IBr. Significant modification of reaction channel probabilities are observed. The DSC process is nonperturbative, insensitive to laser frequency, and affects all polarizable molecules, suggesting broad applicability. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L26.00003: Ab initio design of laser pulses to control molecular motion Gabriel Balint-Kurti, Qinghua Ren, Frederick Manby, Maxim Artamonov, Tak-San Ho, Herschel Rabitz, Shiyang Zou, Harjinder Singh Our recent attempts to design laser pulses entirely theoretically, in a quantitative and accurate manner, so as to fully understand the underlying mechanisms active in the control process will be outlined. We have developed a new Born-Oppenheimer like separation called the electric-nuclear Born-Oppenheimer (ENBO) approximation. In this approximation variations of both the nuclear geometry and of the external electric field are assumed to be slow compared with the speed at which the electronic degrees of freedom respond to these changes. This assumption permits the generation of a potential energy surface that depends not only on the relative geometry of the nuclei, but also on the electric field strength and on the orientation of the molecule with respect to the electric field. The range of validity of the ENBO approximation is discussed. Optimal control theory is used along with the ENBO approximation to design laser pulses for exciting vibrational and rotational motion in H$_{2}$ and CO molecules. Progress on other applications, including controlling photodissociation processes, isotope separation, stabilization of molecular Bose-Einstein condensates as well as applications to biological molecules also be presented. *Support acknowledged from EPSRC. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L26.00004: Probing NO$_{2}$ close to the $\tilde {A}{ }^2B_2 /\tilde {X}{ }^2A_1 $ conical intersection by time-resolved imaging spectroscopy Benjamin Whitaker, Nick Form, Valerie Blanchet, Beatrice Chatel, Bertrand Girard Time-resolved imaging spectroscopy (TRIS) is emerging as a versatile technique with which to study the non-adiabatic coupling of vibrational and electronic degrees of freedom in molecules. The electronic predissociation of NO$_{2}$ in the near UV proceeds by internal conversion between the $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states and is a benchmark example of such barrierless reactions. We have applied time-resolved imaging to measure the time-evolution, angular and kinetic energy distributions of NO$^{+}$, NO$_{2}^{+}$ and photo electrons produced in pump-probe experiments using harmonics from a regeneratively amplified self-mode locked Ti:sapphire laser. Oscillations in the slow NO$^{+}$ and photoelectron signals are observed and are interpreted as measuring the energy level density of the coupled $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states close to the conical intersection. By using an optical pulse shaper we are able to manipulate the spectrum of the $\sim $400 nm excitation to create pulse sequences with which we can exert partial control over the coupling between the $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L26.00005: The Jahn Teller and pseudo-Jahn Teller effect in the dark \~{A} state of the nitrate radical NO$_{3}$ Kana Takematsu, David Robichaud, Mitchio Okumura, John Stanton Despite its apparent simple molecular structure, the lowest electronic states of the nitrate radical NO$_{3}$ remain poorly understood. In particular, the three lowest states of the radical provide a benchmark for testing models of the Jahn-Teller (JT) and pseudo-JT effects. The dark \~{A} state of NO$_{3}$ undergoes strong JT distortion, suggesting that models with only linear and quadratic vibronic couplings are inadequate. We present cavity ringdown (CRD) and integrated cavity output (ICOS) spectra of the forbidden $\mathop A\limits^\sim { }^2E"\leftarrow \mathop X\limits^\sim { }^2A_2 '$ transition (preliminary report in Deev, et. al. \textit{J.Chem. Phys}., 2005. 122:224305) and compare them to a simulation based on a model Hamiltonian developed by Koppel, Domcke and Cederbaum that incorporates both JT and PJT couplings. New insights into the pseudo-JT effect among the lowest states are gained by examination of intensity-borrowing mechanisms for the observed vibronic bands. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L26.00006: Nonadiabatic Electronic and Rotational Energy Partitioning in F + H$_{2}$O $\to$ HF + OH Michael Ziemkiewicz, Alex Zolot, David Nesbitt OH product state distributions from F + H$_{2}$O $\to $ HF + OH have been carried out at a COM collision energy of 6(2) kcal/mol. These measurements complement earlier work on the dueterated version of the system (F + D$_{2}$O $\to $ DF + OD) where extensive non-adiabatic interactions led to a population of spin-orbit excited OD products despite energetically inaccessible barriers on all but the ground electronic surface. In the present F + H$_{2}$O measurements, the branching ratio is, within error bars, the same as in the deuterated case: 69(1){\%} of the molecules are found in the ground spin-orbit state, and 31(1){\%} are found in the excited (nonadiabatic) state. In contrast to this isotopomer-independent electronic branching ratio, the rotational distributions for this system are distinctly different from the deuterated case. A detailed analysis of the rotational distributions for the title reaction leads to an estimate of the vibrational distribution of the unobserved HF fragment (v=2:v=1) of 3:1. The fact that isotoperization dramatically changes the rotational distributions while leaving electronic distributions unchanged sheds light on the important question of how and where nonadiabatic transitions take place in this four-atom system. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L26.00007: Quantum State Resolved Reactive Scattering Near Conical Intersections: $F(^2P)+HCl\to HF(v,J)+Cl(^2P)$ and $F(^2P)+H_2 O\to HF(v,J)+OH(^2\Pi )$ via High Resolution IR Spectroscopy on Nascent HF Product Alexander Zolot, Michael Ziemkiewicz, Michael Deskevich, David Nesbitt State resolved reaction dynamics of the reactions$F({ }^2P)+HCl\to HF(v,J)+Cl({ }^2P)$and $F({ }^2P)+H_2 O\to HF(v,J)+OH({ }^2\Pi )$have been studied under rigorous single collision conditions in crossed molecular jets via IR absorbance of the HF product. Supersonic jet collision energies exceed the ground electronic state barrier height predicted by ab initio (DW-MCSCF) calculations, but can not overcome the larger barriers on excited state surfaces. The experimental results reveal highly vibrationally inverted nascent HF populations containing significant population above the average energy available to products for both of the title reactions. Such excited products may be formed by the tail of the collision energy distribution, but may also be favored by the extra $\sim $1.1 kcal/mol available for reaction with spin-orbit excited fluorine, previously observed in other systems. F+HCl product rotational distributions are found to be particularly non-statistical and are poorly modeled by single surface QCT. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L26.00008: Optical control of dynamics in a simple chemical reaction: The cyclohexadiene ring-opening reaction Roseanne Sension UV excitation of 1,3-cyclohexadiene (CHD) results in an optically induced ring-opening reaction to form 1,3,5-cis-hexatriene (ZHT). The initial excited state wave packet accelerates away from the Frank-Condon region and is funneled through a conical intersection onto the 2A excited state where the nuclear ring-opening reaction occurs. Return to the ground state proceeds within a few hundred femtoseconds through two or more conical intersections between the 1A and 2A potential energy surfaces. Recent studies have demonstrated that multiphoton excitation of CHD can be used to influence the photochemical yield of ZHT. The multidimensional search of the control space for optimal pulses identified both the quadratic and cubic phase parameters of the pulse as important control parameters. These results are discussed in terms of potential physical models. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L26.00009: Matching-pursuit/split-operator-Fourier-transform simulations of excited-state nonadiabatic quantum dynamics in pyrazine. Victor Batista, Xin Chen A simple approach for numerically exact simulations of nonadiabatic quantum dynamics in multidimensional systems is introduced and applied to the description of the photoabsorption spectroscopy of pyrazine. The propagation scheme generalizes the recently developed matching-pursuit/split-operator-Fourier-transform (MP/SOFT) method [Y. Wu and V. S. Batista, J. Chem. Phys. 121, 1676 (2004)]. The time-evolution operator is applied, as defined by the Trotter expansion to second order accuracy, in dynamically adaptive coherent-state expansions. These representations are obtained by combining the matching-pursuit algorithm with a gradient-based optimization method. The accuracy and efficiency of the resulting computational approach are demonstrated in calculations of time-dependent survival amplitudes and photoabsorption cross sections, using a model Hamiltonian that allows for direct comparisons with benchmark calculations. Simulations in full-dimensional potential energy surfaces involve the propagation of a 24-dimensional wave packet to describe the S1 /S2 interconversion of pyrazine after after S0-S2 photoexcitation. The reported results show that the generalized MP/SOFT method is a practical and accurate approach to model nonadiabatic reaction dynamics in polyatomic systems. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L26.00010: Femtosecond pump -- shaped-dump -- probe quantum control Patrick Nuernberger, Philipp Marquetand, Gerhard Vogt, Tobias Brixner, Volker Engel, Gustav Gerber We present a three pulse pump--shaped-dump--probe scheme for femtosecond spectroscopy. The objective is a reversion of regular control schemes for optimal excitation in which the pump pulse is shaped. Instead, we seek optimal de-excitation with a shaped dump pulse. Besides variation of the time delay between pump and dump pulses, the versatility of a femtosecond pulse shaper furthermore allows to record systematic fitness landscapes as a function of selected pulse parameters, providing additional information on wave-packet evolution and the potential energy surfaces of the system under study. Since the dump pulse is independent from the pump pulse, the pump-- shaped-dump--probe scheme facilitates control of molecular systems away from the initial Franck-Condon window in regions of the potential-energy landscape where the decisive reaction step occurs, e.g. near conical intersections. Experimental results on the retinal photoisomerization reaction in bacteriorhodopsin and exemplary model calculations demonstrate the potential of this new scheme. [Preview Abstract] |
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