Marcus P. Hertlein

Probing Impulsive Strain Propagation with X-Ray Pulses

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes.

Femtosecond isomerization dynamics in the ethylene cation measured in an EUV-pump NIR-probe configuration

FAST TRACK COMMUNICATION Femtosecond isomerization dynamics in the ethylene cation measured in an EUV-pump NIR-probe configuration J. van Tilborg 1 , T. K. Allison 1,2 , T. W. Wright 1 , M. P. Hertlein 1 , R. W. Falcone 1,2 , Y. Liu 1 , H. Merdji 3 , A. Belkacem 1 Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA University of California at Berkeley, Berkeley, California 94720, USA Service des Photons, Atomes et Mol´ cules, CEA-Saclay, 91191 Gif-sur-Yvette, e France E-mail: JvanTilborg@lbl.gov Abstract. Dynamics in the excited ethylene cation C 2 H + lead to isomerization to the ethylidene configuration (HC-CH 3 ) + , which is predicted to be a transient configuration for electronic relaxation. With an intense femtosecond EUV (extreme ultraviolet) pump pulse to populate the excited state, and an NIR (near infrared) probe pulse to produce the fragments CH + and CH + (which provides a direct signature of ethylidene), we measure optimum fragment yields at a probe delay of 80 fs. Also, an H 2 -stretch transient configuration, yielding H + upon probing, is found to succeed the ethylidene configuration. We find that a simple single- or double-decay model does not match the data, and we present a modified model (introduction of an isomerization delay of 50 ± 25 fs) that does provide agreement. PACS numbers: 33.80.-b, 33.80.Rv, 33.50.Hv, 31.50.-x

Molecular frame Auger electron energy spectrum from N2

Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K-shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum.

Sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy setup for pulsed and constant wave X-ray light sources

An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records the position and arrival time of every single electron at the exit aperture of the hemisphere with ~0.1 mm spatial resolution and ~150 ps temporal accuracy. The kinetic energies of the photoelectrons are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pump-probe time-delays are provided by the electron arrival times relative to the pump pulse timing. An average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy E(p) = 150 eV and an electron kinetic energy range KE = 503-508 eV. The time-resolution of the setup is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution within the analyzer hemisphere and within the electrostatic lens system that images the interaction volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies between ~9 ns at a pass energy of 50 eV and ~1 ns at pass energies between 200 eV and 400 eV. The correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of the entire trajectories including the electrostatic lens system. In agreement with previous studies, we find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However, both experiment and computer simulations show that the lens system indirectly affects the time resolution of the setup to a significant extent by inducing a strong dependence of the angular spread of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular spread with the retarding ratio can be well approximated by applying Liouvilles theorem of constant emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated by characterizing the laser fluence-dependent transient surface photovoltage response of a laser-excited Si(100) sample.

Separation of high order harmonics with fluoride windows

We demonstrate the use of fluoride windows to temporally separate the lower orders produced in high harmonic generation. We use this technique to conduct a VUV pump/IR probe experiment on the ethylene molecule.

Glass-like recovery of antiferromagnetic spin ordering in a photo-excited manganite Pr0.7Ca0.3MnO3

Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward undertsanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr0.7Ca0.3MnO3 following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.

Time-resolved x-ray photoelectron spectroscopy techniques for real-time studies of interfacial charge transfer dynamics

X-ray based spectroscopy techniques are particularly well suited to gain access to local oxidation states and electronic dynamics in complex systems with atomic pinpoint accuracy. Traditionally, these techniques are applied in a quasi-static fashion that usually highlights the steady-state properties of a system rather than the fast dynamics that often define the system function on a molecular level. Novel x-ray spectroscopy techniques enabled by free electron lasers (FELs) and synchrotron based pump-probe schemes provide the opportunity to monitor intramolecular and interfacial charge transfer processes in real-time and with element and chemical specificity. Two complementary time-domain xray photoelectron spectroscopy techniques are presented that are applied at the Linac Coherent Light Source (LCLS) and the Advanced Light Source (ALS) to study charge transfer processes in N3 dye-sensitized ZnO semiconductor nanocrystals, which are at the heart of emerging light-harvesting technologies.

Ultrafast X-ray-pump, laser-probe spectroscopy at LCLS

We report the first pump-probe spectra using 1 keV pulses from LCLS to excite N2 in delayed coincidence with 800 nm laser pulses. The delay between pump and probe was controlled to within 50 fsec.

Scattering bottleneck for spin dynamics in metallic helical antiferromagnetic dysprosium

We have performed time-resolved resonant x-ray scattering studies in the Lanthanide metal Dy to reveal the dynamic response of the helical order exchange coupling to injection of unpolarized spins. The observed spin dynamics are significantly slower than that exhi bited by the ferromagnetic phase in Lanthanide metals and are strongly dependent on temperature and excita tion fluence. This unique behavior results from transient changes in the shape of the conduction electron Fe rmi surface and subsequent scattering events that transfer the excitation to the core spin. PACS numbers: 71.20.Eh, 78.70.Ck

Thin Water Film Formation on Metal Oxide Crystal Surfaces

Reactions taking place at hydrated metal oxide surfaces are of considerable environmental and technological importance. Surface-sensitive X-ray methods can provide structural and chemical information on stable interfacial species, but it is challenging to perform in situ studies of reaction kinetics in the presence of water. We have implemented a new approach to creating a micrometer-scale water film on a metal oxide surface by combining liquid and gas jets on a spinning crystal. The water films are stable indefinitely and sufficiently thin to allow grazing incidence X-ray reflectivity and spectroscopy measurements. The approach will enable studies of a wide range of surface reactions and is compatible with interfacial optical-pump/X-ray-probe studies.