Recent Coincidence Studies of Atoms and Small Molecules Using COLTRIMS Reaction Microscopes

Coincident many-particle momentum spectroscopy is a powerful tool to investigate dynamic properties of fundamental atomic and molecular processes. After introducing briefly the experimental technique, the talk will show several examples of recent experimental endeavors. In the first part, the single photon double ionization of helium will be revisited: The interaction of photons with atoms and molecules is dominated by electronic dipole transitions due to the photon spin. Any transfer of additional orbital angular momentum arises from the photon’s linear momentum and is consequently suppressed for low photon energies. Whenever a transition leads to the continuum, i.e., to the ejection of one or more electrons, the angular momentum becomes observable in their outgoing angular distributions. Even though, these angular distributions result from a coherent superposition of the different multipole contributions as the various angular momentum states of a free particle are energetically degenerate, the angular distributions are, due to the dominance of the dipole contribution, only slightly modified by the interference term between the quadrupole and the dipole transition. As such, the quadrupole transition amplitude alone has not been directly observed until now. In the present work we succeeded in experimentally isolating the quadrupole contribution to photo-double ionization and visualize a pure quadrupole pattern in the angular distribution of electrons emitted from a helium atom. With the implementation of the European X-ray free electron laser a unique tool for time-resolved studies of small molecules and clusters emerges. The targeted pulse durations and photon energies will enable investigations which, firstly, involve more tightly bound K-shell electrons of common small molecules as N2, CO or hydrocarbons, and secondly, address typical timescales of molecular decay process in the low fs-regime. By means of coincidence detection techniques the molecular time domain is already accessible in measurements using synchrotron radiation in some cases. The talk will present examples of such time-resolved coincidence measurements. Investigations of the temporal evolution of an electron orbital during the dissociation of a small molecule will be shown. A ‘true’ molecular movie will be presented depicting the nuclear dynamics of small clusters as these deexcite in Interatomic Coulombic Decay. A novel experimental approach, which has been termed PCI-streaking, was employed to access the molecular time domain in the latter case. It will be briefly introduced. The talk will end with most recent examples on measurements recently performed at the European X-ray free electron laser including double core hole creation in oxygen molecules and Coulomb Explosion Imaging of Iodopyridine.