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Unbound electrons can experience resonant scattering and transient trapping in a molecular potential. In molecular photoemission, these shape resonances manifest as peaks in the cross section with a width of several electron volts. They depend on the details of the molecular potential and the wavelength of the photoelectron. We report experimental results on photo double-ionization which show the phenomenon of a collective two-electron resonance which occurs when the de Broglie wavelength of the dielectron quasiparticle is similar to that of a single electron exhibiting a single-electron shape resonance.

The differential photoelectron circular dichroism (PECD) of O 1s photoelectrons of R-trifluoromethyloxirane enantiomers as a function of the photoelectron emission direction in the molecular frame of reference and the direction from which circularly polarized light hits the molecule, is studied experimentally and theoretically for different photoelectron kinetic energies. A coincident detection of the photoelectrons and two ionic molecular fragments, performed with cold target recoil ion momentum spectroscopy, allows us to determine the orientation of the molecule in the laboratory frame and to obtain in addition the molecular-frame photoelectron diffraction patterns. From these we deduce the differential PECD. For given molecular orientations and photoelectron emission directions, we observe a normalized PECD strength clearly beyond 50%. These observations are in agreement with respective relaxed-core Hartree-Fock calculations, performed by employing the single center method. The present results support our recent observation of a huge differential PECD in O 1s photoemission of the methyloxirane molecule.

Ultrashort x-ray pulses from free-electron lasers can efficiently charge up and trigger the full fragmentation of molecules. By coincident detection of up to five ions resulting from rapid Coulomb explosion of highly charged iodomethane, we show that the full three-dimensional equilibrium geometry of this prototypical polyatomic system can be determined from the measured ion momenta with the help of a charge buildup model. Supported by simulations of how the ion momenta would reflect specific changes in molecular bond lengths and angles, we demonstrate that Coulomb-explosion imaging with ultrashort x-ray pulses is a promising technique for recording movies of multidimensional nuclear wave packets, including hydrogen motions.

The ultrafast structural dynamics of water following inner-shell ionization is a crucial issue in high-energy radiation chemistry. We have exposed isolated water molecules to a short x-ray pulse from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we can image dissociation dynamics of individual molecules in unprecedented detail. We reveal significant molecular structural dynamics in H_{2}O^{2+}, such as asymmetric deformation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. We thus reconstruct several snapshots of structural dynamics at different time intervals, which highlight dynamical patterns that are relevant as initiating steps of subsequent radiation-damage processes.

We present a study on molecular-frame photoelectron angular distributions (MFPADs) of small molecules using circularly polarized synchrotron light. We find that the main forward-scattering peaks of the MFPADs are slightly tilted with respect to the molecular axis. This tilt angle is directly connected to the molecular bond length by a simple, universal formula. We apply the derived formula to several examples of MFPADs of C 1s and O 1s photoelectrons of CO, which have been measured experimentally or obtained by means of ab initio modeling. In addition, we discuss the influence of the back-scattering contribution that is superimposed over the analyzed forward-scattering peak in the case of homo-nuclear diatomic molecules such as N 2 .