Joshua Williams - US grants

Electrons can be ejected from molecules when they absorb energetic photons. This can weaken the bonds and cause the molecule to dissociate into ionic fragments. Studying ionization and dissociation processes in small molecules is important for testing quantum theories and for practical reasons such as understanding radiation damage. This research studies double photoionization of water molecules in which a single photon ejects two electrons. A sophisticated measurement method is used in which the ejected electrons and the ionic fragments are detected in coincidence to determine how they share energy and in which direction they move away from each other. The details of those measurements will help theoretical physicists refine quantum mechanical models of molecules that absorb ionizing radiation. Once the basic physics is understood and included in the theoretical models, the calculations can be used to understand how radiation interacts with larger molecules and materials.

The well-known COLTRIMS technique will be used to study double photoionization of water molecules with 57 eV photons. The initial momenta of the photoelectrons and ionic fragments will be determined. In addition, the dication molecular state will be characterized prior to dissociation. The data analysis methods will also be able to project the electrons' momenta into the frame of the dissociated molecule. As a result, state-selected, body-fixed differential cross sections for the coupled photoelectrons can be determined, achieving as-yet unprecedented knowledge of the dissociating molecule.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

In this project, physicists will attempt to measure what happens in an atom or molecule after a tightly bound electron is removed by a pulse of x-ray light, leaving the atom or molecule in an excited state which eventually decays to a lower energy configuration. The work will focus on a specific mode of decay (Auger decay) in which the excited atom or molecule ejects another (loosely bound) electron to carry away the excess energy. Here the Auger decay is expected to take around 100 attoseconds (0.0000000000000001 seconds). After Auger decay, a molecule will likely break up into pieces, leaving only its constituent atoms. The experimenters will try to measure this Auger decay time and correlate it with the type of fragments that are released, along with other measured quantities. This study will help illuminate our understanding of the quantum mechanical processes that play out during molecular breakup, and in particular our fundamental understanding of the Auger decay process. Additionally, it is expected that this will serve as the basis for at least one Ph.D. student’s dissertation, several undergraduate students' senior theses, and the employment and training of a postdoctoral fellow. <br/> <br/>This proposed experiment will attempt to measure the time-resolved dynamics of Auger decay in xenon, methyl iodide (CH3I), and expand our preliminary studies of dichloroethylene (C2H2Cl2). This experiment will employ the well-established COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) technique to simultaneously determine the gas-phase molecular frame (i.e. molecular orientation in the laboratory frame), and the Auger decay time. Access to the timing information is achieved through the measurement of the electron momentum and knowledge of the time-dependent nature of the electric field. In this experiment the time-dependent electric field is supplied by the atomic or molecular system via the Auger decay. This experiment will expand our understanding of the dynamics involved in atomic and molecular systems following the ultrafast Auger decay. In the case of molecular systems, the experimenters will attempt to determine the correlation between the Auger decay timing and the Molecular Frame Angular Distribution (MFPAD). The MFPAD is highly sensitive to the molecular potential and at least in principle could be used to glean information about the molecular potential (i.e. the configuration of the molecule’s constituent atoms in space at the “moment” the photoelectron is ejected).<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.