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.
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 ring-opening 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. Methods include: ultrafast time-resolved soft x-ray and XUV spectroscopy via photoelectron emission and transient absorption/reflectivity dynamics on the attosecond to picosecond time scale.
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 extreme ultraviolet (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, transient x-ray and XUV 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. They have extended our 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 construction. The ability to directly probe carbon atoms is expected to open new avenues of ultrafast dynamics in various organic and bio-relevant molecules.
(1) “Coherent energy exchange between carriers and phonons in Peierls-distorted bismuth unveiled by broadband XUV pulses” Phys. Rev. Research 3, 033210 (2021)
Using XUV transient reflectivity spectroscopy, the Peierls distortion of bismuth is monitored in real time. Dynamics of electrons, holes, and phonons are resolved simultaneously, allowing a comprehensive and direct view of the electron-phonon interaction in bismuth. This technique can be applied to explore and reveal the interplay of electrons and phonons in numerous systems at unprecedented time resolution.
(2) “Theoretical analysis of the role of complex transition dipole phase in XUV transient-absorption probing of charge migration” Opt. Express 30, 5673 (2022)
They calculationally simulate the attosecond transient absorption spectrum and provide panoramic movies of charge migration in a polyatomic molecule, ICCBr+. Analytical expressions of the core-level absorption are derived and used to explain the important role of the complex phase of the transition-dipole moments in determining the oscillation phases of quantum beats. This study provides a basis for future experimental work in analyzing and interpreting quantum-beat signals arising from the complex electronic and structural coherent dynamics in molecules.
In the News
In semiconductors like silicon, electrons attached to atoms in the crystal lattice can be mobilized into the conduction band by light or voltage. Berkeley scientists have taken snapshots of this very brief band-gap jump and timed it at 450 attoseconds.
A team of German and UC Berkeley scientists, including physicist Stephen Leone, have used ultrashort flashes of laser light to directly observe for the first time the movement of an atom’s outer electrons. The technique could be valuable in the study of photosynthesis.