Ian Tenney

Ultrafast X-ray Auger probing of photoexcited molecular dynamics

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation--X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.

Experimental strategies for optical pump - soft x-ray probe experiments at the LCLS

Free electron laser (FEL) based x-ray sources show great promise for use in ultrafast molecular studies due to the short pulse durations and site/element sensitivity in this spectral range. However, the self amplified spontaneous emission (SASE) process mostly used in FELs is intrinsically noisy resulting in highly fluctuating beam parameters. Additionally timing synchronization of optical and FEL sources adds delay jitter in pump-probe experiments. We show how we mitigate the effects of source noise for the case of ultrafast molecular spectroscopy of the nucleobase thymine. Using binning and resorting techniques allows us to increase time and spectral resolution. In addition, choosing observables independent of noisy beam parameters enhances the signal fidelity.

Ultrafast X-ray probe of nucleobase photoprotection

We will present first results of a UV-pump X-ray-probe study of the photoprotection mechanism of thymine. The experiment used element specific Auger spectroscopy and was carried out at the LCLS.

Structure Dependence of Kinetic Energy Released in X-ray-Induced Fragmentation

We have analyzed transient structures in deuterated acetylene isomerized by 10 fs 400 eV X-rays, and probed by coulomb explosion with a second delayed X-ray pulse. Structural changes are revealed through fragment kinetic energy release.

Delayed ultrafast X-ray induced auger probing

We present a new method for probing photoexcited molecular dynamics based on the site selective ionization via ultrafast x-rays followed by Auger decay.