Ultrafast X-ray Science Laboratory (UXSL)

Program Summary

The UXSL's overall goal of understanding electron dynamics in response to photoexcitation, and its implications for the properties and behavior of molecules in the gas and condensed phases, demands the integration of the broad range of experimental and theoretical efforts.

The UXSL program is a multipronged attack on the fundamental problem of electron and molecular dynamics with a combination of efforts in atomic physics, chemical physics and condensed phase chemistry.  The principal tools of the effort are ultrafast ultraviolet (UV), extreme ultraviolet (XUV), and X-ray pulses together with novel detection techniques used to probe the essential time scales of combined electronic and nuclear motion. The UXSL consists of five subtasks that are interrelated and synergistic with one another, and several of the PIs participate in more than one of the subtasks.

Research Center Leader

C. William McCurdy

Principal Investigators

Ali Belkacem

Oliver Gessner

Martin Head-Gordon

Stephen R. Leone

Daniel M. Neumark

Robert W. Schoenlein

Thorsten Weber

Projects

Ultrafast Soft X-ray Chemical Dynamics – Oliver Gessner, Daniel M. Neumark and Stephen R. Leone

Atomic, Molecular and Optical Sciences – Thorsten Weber and Ali Belkacem

Ultrafast X-ray Studies of Condensed Phase Molecular Dynamics – Robert W. Schoenlein

Theory of Short Pulse Interactions, and the Determination of Electronic Excited States – C. William McCurdy and Martin Head-Gordon

Attosecond Atomic and Molecular Science – Stephen R. Leone, Daniel M. Neumark and C. William McCurdy

Ultrafast Soft X-ray Chemical Dynamics

Driven by the dream to watch Chemistry unfold as it happens, physical chemistry is traditionally one of the most prominent beneficiaries of ultrafast science. The Ultrafast X-ray Science Laboratory (UXSL) Chemical Dynamics project is aimed at unleashing the power of novel ultrafast X-ray techniques to capture the decisive moments in which macroscopically observable rates of chemical processes are defined. X-ray based methods for electronic structure determination and atomic scale probing of chemical environments are translated into the time-domain. Compared to conventional time-domain techniques, ultrafast X-ray science offers the unique possibilities to probe virtually all chemically relevant states of matter by well-controlled single photon transitions and with the inherent element specificity of inner shell excitations. The UXSL Chemical Dynamics project exploits these novel capabilities in order to gain an atomic level real-time perspective of the formation and breaking of bonds, the interplay between electronic and nuclear degrees of freedom, and the flow of charge and energy in molecules, in complex systems, and across interfaces.\



Structure (right) and simulated ad-sorption geometry of Ruthenium 535 on a TiO₂ surface.

Atomic, Molecular and Optical Sciences

The tools of the UXSL AMO experimental subtask are an intense femtosecond XUV high harmonic generation source and the femtosecond slicing source at Beamline 6 at the Advanced Light Source. These sources are combined with sophisticated momentum imaging methodologies to probe electron correlation in excited states of molecules and atoms, to study non-adiabatic coupling of electronic and nuclear degrees of freedom in small molecules, and to investigate fundamental processes involving soft X-ray interaction with inner-shell electrons of atoms and molecules in the presence of strong laser fields. The ability to have two tailored XUV photons interacting (simultaneous or delayed) with a single molecule or atom is opening up an array of possibilities that will help shed light on some of the most basic electronic and molecular processes that take place in excited states and drive photochemistry.

VUV-XUV pump and XUV probe high harmonic set up (left) using a split-mirror interferometer system (right).

Ultrafast X-ray Studies of Condensed Phase Molecular Dynamics

The goal of the Condensed Phase Molecular Dynamics subtask is to apply advanced ultrafast X-ray spectroscopy and diffraction techniques to achieve a new understanding of the atomic and electronic structural dynamics of solvated molecules. These goals are achieved through a collaborative effort involving scientists from the Chemical Sciences Division and the Advanced Light Source at the LBNL, and from international research institutions including Michigan State University, Stanford University, University of Washington, and Pusan National University, Korea. This team combines expertise in chemical dynamics, condensed matter physics, ultrafast laser and X-ray spectroscopy, X-ray beamlines, and synchrotron X-ray research and instrumentation.

The central goal of this research program is to apply ultrafast X-rays to develop new insight into condensed-phase molecular dynamics. Time-resolved measurements of XANES (X-ray Absorption Near Edge Structure) provide detailed information about dynamics of the valence charge structure, while time-dependent EXAFS (Extended X-ray Absorption Fine Structure) provides information about changes in the local atomic structure. This research program is based on two femtosecond X-ray beamlines at the Advanced Light Source (ALS) operating in the soft X-ray range (0.2–1.8 keV) and in the hard X-ray range (2–10 keV), providing ~250 fs X-ray pulses for time-resolved X-ray science.

Left: Molecular Structures of Mn₂(CO)₁₀ and Mn(CO)₅. Right: and UV-Visible spectra of Mn₂(CO)₁₀
in cyclohexane from reference⁴⁶.

Theory of Short Pulse Interactions, and the Determination of Electronic Excited States

The theory effort spans nearly the entire range of processes investigated in the UXSL, while focusing primarily on the prediction of electronic and nuclear dynamics in atoms and molecules subject to short XUV pulses, and on the determination of potential energy surfaces and properties of molecular excited states for both small and large molecules. We develop new theoretical and computational approaches and also apply them to specific systems of interest to support and stimulate the experimental efforts of the UXSL. We attack two key problems in the theory for the interpretation and design of ultrafast experiments: (1) the prediction of electronic and nuclear dynamics in atoms and molecules subject to short XUV pulses, and, (2) the determination of potential energy surfaces and properties of molecular electronic excited states.


(left) Energy levels and pulse energy profiles for two-time two-electron interference
experiment. The 69 eV pulse follows the 35 eV pulse for sequential ionization, but
the energy bandwidths overlap. (right) Two-electron interference pattern plotted as
function of the energies E₁ and E₂ of the two electrons detected in coincidence.

Attosecond Atomic and Molecular Science

The attosecond subtask uses sub-femtosecond pulses in experiments on a variety of atomic and molecular systems, focusing entirely on the timescale of electron dynamics. These studies will investigate the dynamics of Auger decay and the fate of superexcited (autoionizing, multiply excited) molecular states. New experimental techniques are also developed in this subtask including transient dispersion and attosecond photoelectron and mass spectroscopy. It is widely recognized that the field of atomic and molecular physics has opened a new frontier in ultrafast processes on the attosecond time scale, and this effort is configured to make early discoveries in some of the first experiments on molecules on that frontier.

Photoelectron streak traces produced with DOG. (a) Measured and (b) reconstructed spectrogram.(c) Retrieved temporal and (d) spectral structure of isolated attosecond pulse. Phases are also indicated.