Exploiting Non-equilibrium Charge Dynamics in Polyatomic Molecules to Steer Chemical Reactions

The main objectives of this proposal are to achieve two major research goals of ultrafast science:
1. Creating and probing photo-induced charge migration dynamics of pure electronic origin on the attosecond to few femtoseconds time scales and studying the role of non-equilibrium charge distributions in inducing selectivity of chemical reactions in polyatomic molecules.
2. Achieving mode-selective chemistry in polyatomic molecules using intense ultrashort mid-infrared pulses.

Scientific Discovery through Ultrafast Materials and Chemical Sciences, Department of Energy, United States

For the first goal, coherent electronic wave packets due to the superposition of a few electronic states in molecular cations will be created with strong field photoionization by ultrashort optical pulses and the subsequent charge migration along the molecular backbone will be monitored by various advanced methods including a photoionization probe implemented with intense attosecond pulse trains (APT) coupled with ion-electron coincidence measurements and attosecond transient absorption with high energy attosecond pulses. In the APT ionization experiments, the time-resolved asymmetry parameter of the photoelectron momentum distributions will reveal the charge oscillation dynamics. The central energy and the temporal structure of the APTs will be carefully tuned to yield good sensitivity to the ultrafast electronic dynamics. Because our theoretical models simulate both the pump and the probe steps with minimum approximations, they will provide realistic experimental parameters and are also well suited to extract electronic dynamics from experimental observations. In the transient absorption experiments, the time-resolved spectra reflect the instantaneous electron density with element and site specificity and thus charge migration dynamics will be captured in real time. Employing both probing methods, the nuclear dynamics following coherent excitation of electronic states will be monitored on longer time scales to study the influence of transient non-equilibrium charge distributions on the outcome of a chemical reaction. Control schemes will be designed to steer chemical reactions. Systems of interest include the polyatomic molecules lazabicyclo[ 3.3.3]undecane (ABCU,C10H19N) and 2-phenylethyl-N,N-dimethylamine (PENNA), that have already been explored by our team in preliminary work. For the second goal, we have recently shown in models that mode-selective chemistry can be achieved using intense mid-IR laser excitation through charge polarization on the ground electronic state. When the intensity of the excitation laser is much higher than those employed in conventional infrared multiphoton dissociation (IRMPD), nuclear kinetic energy can be efficiently pumped into specific vibrational mode(s) as the result of the very large charge polarization. Molecular axial alignment/orientation is essential to direct the input kinetic energy. Because the large kinetic energy forces reactions to proceed in a prompt fashion, a selected reaction pathway can be complete well within one picosecond, defeating the detrimental effect of intramolecular vibrational redistribution (IVR). The study will be carried out in molecular ions to minimize complications from competing strong field ionization. Mid-IR laser pulses at 3-4 μm will be focused to reach 1013-1014 W/cm2 to achieve modeselectivity in aligned polyatomic molecules. Systems of interest include iodobenzene dication and formyl chloride cation. Additional systems will be designed using computation and simulations.
By exploiting the state-of-the-art in ultrafast laser technologies and attosecond pulse production techniques as well as the theoretical methodologies that we already have available in our laboratories, the proposed research aims to resolve a long-standing challenge in ultrafast molecular science and promises significant advances in understanding, predicting and controlling chemical reactions.

2015-2016 Progress Report for DE-SC0012628: UCLA and ULg

Partners
Wen Li, Wayne State University (Lead PI)
Raphael D. Levine, University of California-Los Angeles (PI)
Henry C. Kapteyn, University of Colorado at Boulder (PI)
H. Bernhard Schlegel, Wayne State University (PI)
Françoise Remacle, University of Liège, Belgium (Co-PI)
Margaret M. Murnane, University of Colorado at Boulder (Co-PI)