J. P. Farrell

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.

High harmonic generation for N2 and CO2 beyond the two-point model

Strong field high harmonic generation (HHG) can reveal the quantum structure of the source molecule. We calculate the effect of interference between the recombining photoelectron and the orbital from which it was field ionized in the single-active-electron standard picture of HHG in N2 and CO2. We compare our results for the highest occupied molecular orbitals (HOMOs) to the predictions of a popular two-point scattering model. For N2, we find an agreement for very large internuclear separations and no agreement for the ground-state internuclear distance. We reduce the arguments to the Fourier transform of the HOMO, which depends on the internuclear separation. For CO2, we distinguish between two geometries. For one of these, we find a perfect agreement with the two-point scattering model; however, the emitted radiation is not phase matched in this case. The experimentally accessible radiation does not agree with the simple model.

Calibration of a high harmonic spectrometer by laser induced plasma emission.

We present a method that allows for a convenient switching between high harmonic generation (HHG) and accurate calibration of the vacuum ultraviolet (VUV) spectrometer used to analyze the harmonic spectrum. The accurate calibration of HHG spectra is becoming increasingly important for the determination of electronic structures. The wavelength of the laser harmonics themselves depend on the details of the harmonic geometry and phase matching, making them unsuitable for calibration purposes. In our calibration mode, the target resides directly at the focus of the laser, thereby enhancing plasma emission and suppressing harmonic generation. In HHG mode, the source medium resides in front or after the focus, showing enhanced HHG and no plasma emission lines. We analyze the plasma emission and use it for a direct calibration of our HHG spectra.

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.

Strongly dispersive transient Bragg grating for high harmonics

We create a transient Bragg grating in a high-harmonic generation medium using two counterpropagating pulses. The Bragg grating disperses the harmonics in angle and can diffract a large bandwidth with temporal resolution limited only by the source size.

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.

Isotope Effect in the High Harmonics of Water

We show that the high harmonics of water are affected by sub-femtosecond nuclear motion launched via ionization of the inner valence 3a1 orbital. This introduces a new method to find multi-orbital contributions to high harmonics.

Observation of an intensity dependent minimum and harmonic dependent linewidths in the high harmonic spectrum of N 2

We observe an intensity dependent minimum and harmonic dependent linewidths in the high harmonic spectrum of N2. The quantum mechanical origin of these features and their relevance to molecular orbital reconstruction is discussed.

High harmonic spectral minimum and phase in Argon and nitrogen

Measurements of the high harmonic amplitude and phase in Argon and N₂ display amplitude and phase modulations which can be related to a Cooper minimum in Argon and recombination interference in N₂.