Session A34: Focus Session: Impact of Ultrafast Lasers I: X-rays and THz

Ultrafast X-ray Laser Studies of Chemical Dynamics

First light at the LCLS x-ray free electron laser at the SLAC National Accelerator Laboratory marked the beginning of hard x-ray laser science 2009. With pulse energies in excess of a milliJoule and pulse durations as short as 5 femtoseconds in duration, the LCLS provides a novel and potentially transformative approach for investigating chemical dynamics in complex systems. Understanding the coupled evolution of electrons and nuclei during chemical transformations remains the central and vexing challenge in the study of chemical reaction dynamics. Ultrafast optical electronic spectroscopy can monitor both the nuclear and the electronic evolution that occurs during a chemical reaction, but this joint sensitivity often impedes the robust interpretation of experimental measurement. The LCLS provides the opportunity to simultaneously measure electronic dynamics with x-ray fluorescence and nuclear dynamics with elastic x-ray scattering, providing a robust means for disentangling the coupled motions of electrons and nuclei during excited state internal conversion and intersystem crossing. These exciting new opportunities will be discussed in the context of recent studies of photo-induced spin crossover dynamics in iron(II) tris-bipyridine. [Fe(bpy)$_{3}$]$^{2+}$.   

Coherently controlled orientation and alignment of molecules in the gas phase by intense THz fields

We report the use of intense terahertz (THz) pulses to orient polar molecules in the gas phase. Short THz fields exert torques that drive coherent molecular rotational motion, and multiple interactions with strong THz fields can yield multiple-quantum rotational coherences with the prospect of high degrees of orientation (dipoles pointing in the same direction in space) and alignment (molecular axes parallel to each other regardless of dipole orientation). THz-induced molecular orientation offers new possibilities in gas-phase x-ray diffraction, molecular orbital mapping through high harmonic generation and photoelectron angular distribution imaging, and other applications. We demonstrate significantly enhanced coherent control using two THz pulses with an optimized relative time delay. We show in the case of atmospheric water that a short, strong THz field induces long-lived coherent THz emission (free induction decay) that drives significant further rotational responses in a pre-excited polar gas sample. This class of experiments enables broad new capabilities for molecular spectroscopy and control beyond those afforded through molecular alignment by intense optical fields, which do not produce any net orientation.   

Using time-resolved Ru L-edge X-ray absorption spectroscopy to capture photoinduced transient electronic structure in a solar cell dye molecule

Understanding the electronic structure of transition metal dye molecules used in dye-sensitized solar cells (DSSC) is critical for determining the functioning of these devices. Ru$^{II}$ dyes such as Ru(dcbpy)$_{2}$(NCS)$_{2}$ (termed RuN3) have been components in some of the most effective DSSCs. We use synchrotron-based picosecond Ru L-edge X-ray absorption spectroscopy (XAS) to monitor changes the changes in the electronic structure of RuN3$^{4-}$ that accompany the $^{1}$A$_{1}$ to $^{3}$MLCT conversion initiated with a 400 nm light pulse. The results are interpreted by simulating the Ru L$_{3}$ X-ray absorption spectra of the $^{1}$A$_{1}$ and $^{3}$MLCT states with time-dependent density functional theory (TD-DFT). We observe the formation of the Ru$^{III}$ oxidation state of RuN3$^{4-}$ within the 70 ps time resolution of our experiment. The TD-DFT simulation allows us to assign a spectral feature in the Ru L-edge spectrum as a probe of the electronic structure of the NCS ligands due to overlap between Ru 4d and NCS $\pi $* orbitals. A 1.2 eV blue shift in this feature between ground and $^{3}$MLCT state corresponds to depletion in charge density on the NCS ligands in the excited state. We will discuss in detail the local electronic structure around the Ru atom in the transient $^{3}$MLCT state measured by time-resolved XAS.   

Ultrafast molecular processes mapped by femtosecond x-ray diffraction

X-ray diffraction with a femtosecond time resolution allows for mapping photoinduced structural dynamics on the length scale of a chemical bond and in the time domain of atomic and molecular motion. In a pump-probe approach, a femtosecond excitation pulse induces structural changes which are probed by diffracting a femtosecond hard x-ray pulse from the excited sample. The transient angular positions and intensities of diffraction peaks give insight into the momentary atomic or molecular positions and into the distribution of electronic charge density. The simultaneous measurement of changes on different diffraction peaks is essential for determining atom positions and charge density maps with high accuracy. Recent progress in the generation of ultrashort hard x-ray pulses (Cu K$_{\alpha}$, wavelength $\lambda=0.154$ nm) in laser-driven plasma sources has led to the implementation of the powder diffraction and the rotating crystal method with a time resolution of 100 fs. In this contribution, we report new results from powder diffraction studies of molecular materials. A first series of experiments gives evidence of a so far unknown concerted transfer of electrons and protons in ammonium sulfate [(NH$_4$)$_2$SO$_4$], a centrosymmetric structure. Charge transfer from the sulfate groups results in the sub-100 fs generation of a confined electron channel along the c-axis of the unit cell which is stabilized by transferring protons from the adjacent ammonium groups into the channel. Time-dependent charge density maps display a periodic modulation of the channel's charge density by low-frequency lattice motions with a concerted electron and proton motion between the channel and the initial proton binding site. A second study addresses atomic rearrangements and charge dislocations in the non-centrosymmetric potassium dihydrogen phosphate [KH$_2$PO$_4$, KDP]. Photoexcitation generates coherent low-frequency motions along the LO and TO phonon coordinates, leaving the average atomic positions unchanged. The time-dependent maps of electron density demonstrate a concomitant oscillatory relocation of electronic charge with a spatial amplitude of the order of a chemical bond length, two orders of magnitude larger than the vibrational amplitudes. The coherent phonon motions drive the charge relocation, similar to a soft mode driven phase transition between the ferro- and paraelectric phase of KDP.   

Time-resolved X-ray fragmentation probing of molecular isomerization

Short intense X-ray pulses, now available at FELs, are already having a strong impact on the AMO science. In addition to scattering or resonant absorption, it is possible to use X-ray radiation to probe processes on molecular timescales through non-resonant absorption by initiating a molecular fragmentation in a time-resolved manner. Core-ionized molecules composed of light elements predominantly relax through Auger decay into multiply charged molecular ions, which subsequently fragment through Coulomb repulsion. The fragmentation patterns (ion time-of-flight spectra, ion-kinetic energy release, Auger spectra, etc.) encode information about instantaneous nuclear geometry and momenta. Unlike intense IR laser field fragmentation, X-ray fragmentation occurs in the weak-field regime. In order to test the potential of time-resolved X-ray fragmentation for probing isomerization, we selected the example of ring opening of 1,3-cyclohexadiene. In a time-resolved UV-pump - X-ray fragmentation-probe experiment performed at Linac Coherent Light Source at SLAC we observed an increase in the average ion-KER and an increase in the number of lighter fragments upon photoexcitation. We discuss how the evolving fragmentation patterns reflect the structural change that the molecule is undergoing.   

A Femtosecond Time-Resolved Transient X-ray Absorption Study of Light-Induced Coupling and Transparency in Xenon

We have performed a femtosecond time-resolved transient x-ray absorption spectroscopy study to monitor the light induced coupling between bright and dark states in xenon exposed to infrared (780 nm) laser pulses with intensities up to 5$\times$10$^{13}$ W/cm$^{2}$. Significant transient variations in the inner-shell absorption spectra between $\sim $63 eV and $\sim $69 eV photon energy are observed. Near time zero, the transmission at 67 eV increases from 6(1) {\%} (field-free) to 14(3) {\%} (laser dressed). Transient absorption and transient transparency effects are interpreted within a picture of strong-field induced coupling between core-excited 4$d^{-1}(^{2}$D$_{5/2})$6$p(^{2}$P$_{3/2})$ and 4$d^{-1}(^{2}$D$_{3/2})$6$p(^{2}$P$_{1/2})$ states at 65.1 and 67 eV, respectively, and nearby dark states with \textit{ns} and \textit{nd} characters. The major features of the transient absorption spectra can be well described within a three-level (Autler-Townes) coupling scheme. Employing this model, a dipole-moment for the 4$d^{-1}(^{2}$D$_{3/2})$6$p(^{2}$P$_{1/2})$ to 4$d^{-1}(^{2}$D$_{3/2})$6$s(^{2}$S$_{1/2})$ transition of 0.4(0.1) Debye is derived.   

High Resolution THz Studies of Crystalline Channel Hydrates

In this study, we use vibrationally state-resolved THz spectroscopy to examine the phonon mode structure of several peptides and carbohydrates co-crystallized with water. Studies are performed before and after the water is removed under vacuum on a range of systems for water in hydrophobic and hydrophilic environments. Predictions from first principles calculations are used to model the observed spectra and to obtain detailed energetic information on the dehydration processes. A new phase-coherent heterodyne method based on chirped THz pulses may be discussed.