Strongly coupled intermediate electronic states in one-color two-photon single valence ionization of O.

We present an experimental and theoretical energy- and angle-resolved investigation on the non-dissociative photoionization dynamics of near-resonant, one-color, two-photon, single valence ionization of neutral O molecules. Using 9.3 eV femtosecond pulses produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and O cations produced from one-color, two-photon ionization in coincidence.

Electron and Positron Scattering Cross Sections from CO: A Comparative Study over a Broad Energy Range (0.1-5000 eV).

In this Review, we present a comparative study between electron and positron scattering cross sections from CO molecules over a broad impact energy range (0.1-5000 eV). For electron scattering, new total electron scattering cross sections (e-TCS) have been measured with a high resolution magnetically confined electron beam transmission system from 1 to 200 eV.

Step-by-step state-selective tracking of fragmentation dynamics of water dications by momentum imaging.

The double photoionization of a molecule by one photon ejects two electrons and typically creates an unstable dication. Observing the subsequent fragmentation products in coincidence can reveal a surprisingly detailed picture of the dynamics. Determining the time evolution and quantum mechanical states involved leads to deeper understanding of molecular dynamics.

Double Core-Hole Generation in O_{2} Molecules Using an X-Ray Free-Electron Laser: Molecular-Frame Photoelectron Angular Distributions.

We report on a multiparticle coincidence experiment performed at the European X-ray Free-Electron Laser at the Small Quantum Systems instrument using a COLTRIMS reaction microscope. By measuring two ions and two electrons in coincidence, we investigate double core-hole generation in O_{2} molecules in the gas phase. Single-site and two-site double core holes have been identified and their molecular-frame electron angular distributions have been obtained for a breakup of the oxygen molecule into two doubly charged ions.

Distinguishing resonance symmetries with energy-resolved photoion angular distributions from ion-pair formation in O following two-photon absorption of a 9.3 eV femtosecond pulse.

We present a combined experimental and theoretical study on the photodissociation dynamics of ion-pair formation in O following resonant two-photon absorption of a 9.3 eV femtosecond pulse, where the resulting O ions are detected using 3D momentum imaging. Ion-pair formation states of Σg-3 and Π symmetry are accessed through predissociation of optically dark continuum Rydberg states converging to the B Σg-2 ionic state, which are resonantly populated via a mixture of both parallel-parallel and parallel-perpendicular two-photon transitions.

Time-resolved ultrafast transient polarization spectroscopy to investigate nonlinear processes and dynamics in electronically excited molecules on the femtosecond time scale.

We report a novel experimental technique to investigate ultrafast dynamics in photoexcited molecules by probing the 3rd-order nonlinear optical susceptibility. A non-collinear 3-pulse scheme is developed to probe the ultrafast dynamics of excited electronic states using the optical Kerr effect. Optical homodyne and optical heterodyne detections are demonstrated to measure the 3rd-order nonlinear optical response for the S excited state of liquid nitrobenzene, which is populated by 2-photon absorption of a 780 nm 40 fs excitation pulse.

Ultrafast Dynamics of Excited Electronic States in Nitrobenzene Measured by Ultrafast Transient Polarization Spectroscopy.

We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using ultrafast transient polarization spectroscopy, extending the well-known technique of optical Kerr effect spectroscopy to excited electronic states. The third-order nonlinear response of the excited molecular ensemble is measured using a pair of femtosecond pulses following a third femtosecond pulse that populates the S excited state. By measuring this response, which is highly sensitive to details of the excited state character and structure, as a function of time delays between the three pulses involved, we extract the dephasing time of the wave packet on the excited state.

Ultrafast isomerization initiated by X-ray core ionization.

Rapid proton migration is a key process in hydrocarbon photochemistry. Charge migration and subsequent proton motion can mitigate radiation damage when heavier atoms absorb X-rays. If rapid enough, this can improve the fidelity of diffract-before-destroy measurements of biomolecular structure at X-ray-free electron lasers.