Research overview

X-Ray Diffraction from Isolated and Strongly Aligned Gas-Phase Molecules

Controlled-molecule diffraction sketch

Artists impression to the controlled-isolated molecule x-ray-diffraction experiment

How do you watch some of the smallest structures on earth, comprised of just a handful of atoms? You need the most powerful microscope, built using an x-ray laser of unprecedented power and brightness as well as molecules precisely fixed in space. At the Linac Coherent Light Source, an x-ray laser in California, it is possible to generate x-ray flashes that are as powerful as focusing all the sunlight hitting the earth into a micrometer spot, but just for a tenth of a millionth of a millionth of a second. This x-ray light is used to illuminate a group of isolated single molecules, and the exposure time to the flash of x-rays is so short that molecular motions are frozen. The molecules have been sorted and are aligned along an axis, using yet another powerful laser beam. The resulting sharp image, generated by x-ray diffraction, allows us to peer at the atoms in the molecules.

Disentangling the structure-function relationship through conformer-specific reactivity studies

We have investigated the structure-function relationship of conformer-selected complex molecules. In collaboration with the Willitsch group in Basel we have performed a pioneering benchmark experiment to investigate structure-dependent chemical reactivities. We have determined the conformer-specific rate constants for the reactions of cis- and trans-3-aminophenol with Ca+ ions localized in a so called Coulomb Crystal.

Pure quantum state samples of complex molecules

Exploiting the distinct effective dipole moments of molecular eigenstates we have created samples of molecules in individual quantum states. Using the electric deflector, we have created a pure sample of OCS molecules all in the absolute ground state. Moreover, in our second generation alternating gradient focuser we have created almost pure ground-state samples of benzonitrile (C6H5N). Some contamination from a J=2 state resulted from the accidental degeneracy of the effective dipole moment of this specific state.

Imaging the electronic structure of complex molecules

The quantum-state-selected polar ensembles produces by our methods allow unprecedented degrees of alignment and orientation. Such state- and conformer selected oriented ensembles are ideal samples for the recording of pictures and movies of molecules in action. We prepare and perform experiments along these lines, e. g., for diffractive imaging studies of gas-phase molecules, for high-harmonic generation and tomographic imaging of the molecules electronic structure, for molecular-frame photoelectron angular-distribution studies, for reactive scattering, and many other experiments in chemistry and physics.

Fixing molecules in space

Quantum-state selected cold samples enables strong laser alignment and mixed-field orientation. We have demonstrated the one- and three-dimensional mixed-field orientation of prototypical large molecules. Subsequently, we have disentangled the non-adiabatic effects in the long-pulse mixed-field orientation of OCS.

Conformer separation

We apply electric fields to manipulate the translations and rotations of large molecules. This ranges from dc electric fields in the electric equivalent of the Stern Gerlach deflector over switched electric fields in AC guides to linear decelerators for neutral molecules. These are the demonstrations of the the neutral molecule equivalents of the charged particle bender, focuser and linear accelerator (LINAC), respectively. We have independently demonstrated conformer (structural isomer) selection of cis- and trans-3-aminophenol using the focuser and the deflector.