Sizes of pure and doped helium droplets from single shot x-ray imaging.

Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques.

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 photodissociation dynamics and nonadiabatic coupling between excited electronic states of methanol probed by time-resolved photoelectron spectroscopy.

The electronic and nuclear dynamics in methanol, following 156 nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260 nm photoionization probe pulse to produce photoelectrons that are analyzed by velocity map imaging. The yields of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features.

Involvement of a low-lying Rydberg state in the ultrafast relaxation dynamics of ethylene.

We present a measurement of the time-resolved photoelectron kinetic energy spectrum of ethylene using 156 nm and 260 nm laser pulses. The 156 nm pulse first excites ethylene to the (1)B1u (ππ(∗)) electronic state where 260 nm light photoionizes the system to probe the relaxation dynamics with sub-30 fs resolution. Recent ab initio calculations by Mori et al.

Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface.

Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.

Helium superfluidity. Shapes and vorticities of superfluid helium nanodroplets.

Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~10(8) to 10(11) atoms.