Research Expertise and Interest
attosecond physics, attosecond chemistry, ultrafast spectroscopy, ultrafast laser, atomic and molecular dynamics, semiconductors, quantum materials, high harmonic generation, soft x-ray, charge transfer, charge migration, nonadiabatic dynamics, conical intersection, carrier dynamics, phonon dynamics, spin dynamics, core exciton dynamics
Stephen R. Leone is a professor in the Department of Chemistry and the Department of Physics. He is the John R. Thomas Endowed Chair in Physical Chemistry and a faculty investigator of Lawrence Berkeley National Laboratory.
Professor Leone's research interests include ultrafast laser investigations in the soft X-ray and extreme ultraviolet (XUV) to probe valence and core levels, attosecond physics and chemistry, attosecond wave mixing and multidimensional spectroscopy, state-resolved decay processes and time dynamics investigations, in atoms, molecules, and solids
Current projects are grouped along a few main themes:
(1) ultrafast laser-induced molecular dynamics, including molecular geometry relaxation and dissociation reactions, electronic-nuclear dynamics across curve crossings and conical intersections, and formation and propagation of coherent superpositions;
(2) dynamics of photoexcited solids, including semiconductors, ferromagnetic metals, layered heterostructures, two-dimensional van der Waals materials, and nanoparticles, such as ultrafast charge and spin dynamics in thin films and nanostructures, and electron-lattice coupling in solar relevant semiconductors;
(3) development of new methods such as four wave mixing and multidimensional XUV spectroscopy.
Examples are: Ultrafast lasers are used to probe the coherent dynamics of molecular motion on the timescales of vibrational, rotational, or electronic periods. The study of molecular photodissociation by soft X-ray laser techniques has opened the way to analyze the simple breaking of a molecular bond in greater detail. High order harmonics in the XUV and soft X-ray are produced by strong laser fields in a rare gas, and used to probe transitions to valence orbitals by core level spectroscopy of time-evolving systems ranging from atoms to small molecules.
In particular, X-ray and XUV transient absorption spectroscopy is used to probe electronic superpositions, curve crossings, passage through conical intersections and molecular fragmentation pathways. By using few-cycle carrier-envelope phase-stabilized laser pulses, isolated attosecond pulses are generated to study electronic timescales in solids, thin films, molecules and nanoparticles.
A new, XUV magnetic circular dichroism (XMCD) apparatus is built to study element-specific, spin-resolved coherent dynamics in solid state systems, measurements that are valuable for quantum information systems that rely on spin transfer to exchange information. Core-level transient absorption spectroscopy of thin semiconductor and metal films reveals the few-femtosecond dynamics of electronic thermalization, and the simultaneous sub-picosecond electronic cooling and lattice heating via electron-phonon scattering, in a single measurement.
Attosecond wave mixing and multidimensional spectroscopy experiments are designed to elucidate ultrashort time processes of associated electronic excited states and core level transitions by extending highly selective nonlinear wave-mixing techniques developed in infrared, optical and radiofrequency spectroscopies to the XUV regime. These have extended the spectral window to the carbon K-edge in several transient absorption setups. A four-wave mixing setup aiming at C K-edge is also under development. The ability to directly probe carbon atoms is opening new avenues of ultrafast dynamics in various organic and bio-relevant molecules.
1. “Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy” Science 380, 713-717 (2023)
By establishing abrupt ionization using ultrashort visible-infrared pulses, the ultrafast geometrical relaxation of methane cations was probed via soft X-ray transient absorption spectroscopy at the C K-edge. The measurements reveal how methane cations undergo Jahn-Teller distortion in less than 10 fs and follow with an internal vibrational energy redistribution within 58 fs. Theory calculations back up the experimental results and link the observables to the change in the smallest H-C-H angle.
2. Nonlinear & noncollinear wave mixing spectroscopy experiments in the solid state
Attosecond noncollinear wave mixing spectroscopy experiments in the solid state were undertaken to understand the formation, behavior, interaction and decay on an ultrafast scale of various charge carriers and quasiparticles in insulators, semiconductors and metals. Attosecond XUV-NIR four-wave mixing (4WM) spectroscopy investigations in insulators such as NaCl obtained new results on the coherence decays of core excitons ["Solid state core-exciton dynamics in NaCl observed by tabletop attosecond four-wave mixing spectroscopy," Phys. Rev. B 103, 245140 (2021)]. Core excitons are short lived quasi-particles based on the attraction of electrons for inner shell holes. A similar methodology was recently successful to establish a transient grating in tellurium and to scatter XUV photons from the carrier grating.