The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.

We perform an accurate polarimetry measurement of high-order harmonic emission from aligned molecules. We find that harmonic emission from N2 can be strongly elliptically polarized even when driven by linearly polarized laser fields. These data have broad implications for understanding molecules in strong fields because they cannot be explained by simple theories based on the strong field approximation and single active electron models. Finally, this work also shows that it is possible to engineer the polarization properties of harmonic emission by carefully preparing a molecular medium.

We present an ion imaging approach employing a real-time ion counting method with standard video. This method employs a center-of-mass calculation of each ion spot (more than 3×3pixels spread) prior to integration. The results of this algorithm are subpixel precision position data of the corresponding ion spots. These addresses are then converted to the final image with user selected resolution, which can be up to ten times higher than the standard video camera resolution (640×480). This method removes the limiting factor imposed by the resolution of standard video cameras and does so at very low cost. The technique is used in conjunction with dc slice imaging, replacing the local maximum searching algorithm developed by Houston and co-workers [B. Y. Chang, R. C. Hoetzlein, J. A. Mueller, J. D. Geiser, and P. L. Houston, Rev. Sci. Instrum. 69, 1665 (1998)]. The performance is demonstrated using HBr and DBr photodissociation at 193nm with 3+1 resonance enhanced multiphoton ionization detection of hydrogen and deuterium atom products. The measured velocity resolution for DBr dissociation is 0.50% (δv∕v), mainly limited in this case by the bandwidth of the photolysis laser. Issues affecting slice imaging resolution and performance are also discussed.

A push-pull copolymer is presented which can be used in bulk heterojunction (BHJ) solar cells with active layers greater than 200 nm and fill factors above 60%. The efficiencies of most BHJ solar cells are limited by the fact that they have active layers which are between 60 and 110 nm. While this thickness regime enables peak quantum efficiencies (EQE) of 60%–70%, the ability to fabricate thicker devices would increase average EQE values and thus device efficiencies. Discovery of materials which can maintain high performance at large thicknesses will enable higher performance in BHJ hero cells and increase the commercial viability of this technology.

We report the first experimental observation of the dependence of strong-field ionization rate on the sign of the magnetic quantum number. We measure the strong-field sequential double ionization yield of argon by two time-delayed near-circularly polarized laser pulses. It is found that double-ionization yield is enhanced more than 3 times if two lasers have the opposite helicities. Analysis shows that the single ionization of both the neutral and ion prefer the same sign of the magnetic quantum number. A qualitative and intuitive model is proposed to help understand this phenomenon.

We use extreme-ultraviolet interferometry to measure the phase of high-order harmonic generation from transiently aligned CO(2) molecules. We unambiguously observe a reversal in phase of the high-order harmonic emission for higher harmonic orders with a sufficient degree of alignment. This results from molecular-scale quantum interferences between the molecular electronic wave function and the recolliding electron as it recombines with the molecule, and is consistent with a two-center model. Furthermore, using the combined harmonic intensity and phase information, we extract accurate information on the dispersion relation of the returning electron wave packet as a function of harmonic order. This analysis shows evidence of the effect of the molecular potential on the recolliding electron wave.

Divalent europium is used catalytically in visible-light-promoted photoredox bond-forming reactions.

Synthetic methods have been developed for the preparation of new 2,3-dihalothieno[3,4-b]pyrazines, from which a variety of new 2,3-difunctionalized thieno[3,4-b]pyrazines have been produced as precursors to conjugated materials. Structural, electronic, and optical characterization of these new analogues illustrate the extent to which the electronic nature of the functional groups can be used to tune the electronic properties of the thieno[3,4-b]pyrazine unit.