Michael P. Minitti

Direct current slice imaging

We report a new variation of the velocity map ion imaging method that allows the central section of the photofragment ion cloud to be recorded exclusively. The relevant speed and angular distributions for a molecular photodissociation or scattering event may therefore be obtained without need to utilize inversion methods such as the inverse Abel transform. In contrast to the recently reported “slicing” technique of Kitsopoulos and co-workers [C. R. Gebhardt et al., Rev. Sci. Instrum. 72, 3848 (2001)], our method makes no use of grids or pulsed electric fields which can distort the photofragment cloud and therefore compromise the resolution of velocity mapping. We find that by operating a multilens velocity mapping assembly at low voltages, the ion cloud stretches in the acceleration region owing to the kinetic energy release in the fragments. Furthermore, this inherent stretching is sufficient to allow the central section of the distribution to be recorded exclusively by application of a narrow time gate ...

Large-Amplitude Spin Dynamics Driven by a THz Pulse in Resonance with an Electromagnon

Ultrafast Manipulation Multiferroic materials commonly show both magnetism and ferroelectricity, such that the electric field can be used to manipulate the magnetic order, and vice versa. Kubacka et al. (p. 1333, published online 6 March) used a strong terahertz electromagnetic pulse in resonance with an electromagnon—an excitation based on both electric and magnetic ordering—to control the spin dynamics of the multiferroic TbMnO3 on a sub-picosecond time scale and induce the rotation of the spin-cycloid plane of the material. The electric field of an electromagnetic pulse exerts ultrafast control on the spin dynamics of the multiferroic TbMnO3. Multiferroics have attracted strong interest for potential applications where electric fields control magnetic order. The ultimate speed of control via magnetoelectric coupling, however, remains largely unexplored. Here, we report an experiment in which we drove spin dynamics in multiferroic TbMnO3 with an intense few-cycle terahertz (THz) light pulse tuned to resonance with an electromagnon, an electric-dipole active spin excitation. We observed the resulting spin motion using time-resolved resonant soft x-ray diffraction. Our results show that it is possible to directly manipulate atomic-scale magnetic structures with the electric field of light on a sub-picosecond time scale.

Probing the transition state region in catalytic CO oxidation on Ru

Catching CO oxidation Details of the transition state that forms as carbon monoxide (CO) adsorbed on a ruthenium surface is oxidized to CO2 have been revealed by ultrafast excitation and probe methods. Öström et al. initiated the reaction between CO and adsorbed oxygen atoms with laser pulses that rapidly heated the surface and then probed the changes in electronic structure with oxygen x-ray absorption spectroscopy. They observed transition-state configurations that are consistent with density functional theory and a quantum oscillator model. Science, this issue p. 978 Ultrafast x-ray spectroscopy reveals electronic changes that occur during the oxidation of carbon monoxide on a ruthenium surface. Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the O K-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC–O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.

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 (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

Static strain in complex oxide heterostructures has been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically. Here, by exciting large-amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.

Melting of Charge Stripes in Vibrationally Driven La1.875Ba0.125CuO4 : Assessing the Respective Roles of Electronic and Lattice Order in Frustrated Superconductors

We report femtosecond resonant soft x-ray diffraction measurements of the dynamics of the charge order and of the crystal lattice in nonsuperconducting, stripe-ordered La1.875Ba0.125CuO4. Excitation of the in-plane Cu-O stretching phonon with a midinfrared pulse has been previously shown to induce a transient superconducting state in the closely related compound La1.675Eu0.2Sr0.125CuO4. In La1.875Ba0.125CuO4, we find that the charge stripe order melts promptly on a subpicosecond time scale. Surprisingly, the low temperature tetragonal (LTT) distortion is only weakly reduced, reacting on significantly longer time scales that do not correlate with light-induced superconductivity. This experiment suggests that charge modulations alone, and not the LTT distortion, prevent superconductivity in equilibrium.

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.

Optical laser systems at the Linac Coherent Light Source

This manuscript serves as a reference to describe the optical laser sources and capabilities at the Linac Coherent Light Source.

The Atomic, Molecular and Optical Science instrument at the Linac Coherent Light Source

A description of the Atomic, Molecular and Optical Sciences (AMO) instrument at the Linac Coherent Light Source is presented. Recent scientific highlights illustrate the imaging, time-resolved spectroscopy and high-power density capabilities of the AMO instrument.

Ultrafast structural dynamics in Rydberg excited N,N,N′,N′-tetramethylethylenediamine: conformation dependent electron lone pair interaction and charge delocalization

Two nitrogen atoms and a flexible carbon skeleton make N,N,N′,N′-tetramethylethylenediamine (TMEDA) an important model system to study the interplay of conformeric motions and charge delocalization. Ionization of one of the nitrogen atoms generates a localized charge that may (partially) transfer to the other nitrogen. The structural motions, conformation dependent electron lone pair interaction and charge transfer in Rydberg-excited TMEDA, where the molecular ion core closely resembles the ion, were probed by time-resolved Rydberg fingerprint spectroscopy. Excitation to the 3p Rydberg level with a 209 nm laser pulse initially created a charge-localized ion core. Rapid internal conversion to the 3s Rydberg state yielded a multitude of conformational structures, in particular structures that are close to the folded GG′G+ and GGG′+ (see text for label definitions) core structures (235 fs), and structures that are close to the extended TTT+ core structure (557 fs). The initial excitation and the internal conversion deposit about 1.89 eV of energy into the vibrational manifold, enabling a fast equilibrium between the folded and the extended structures. The forward and backward time constants were determined to be 490 fs and 621 fs, respectively. With the molecule highly vibrationally excited, the decay to 3s proceeds with a 6.77 ps time constant. Density functional theory (DFT) and ab initio calculations show evidence of strong lone pair interaction and charge delocalization in the equilibrium conformers. Importantly, DFT with self-interaction correction properly describes the binding energy of the Rydberg electron and provides excellent agreement with the experimental results.