Stephen

Research Expertise and Interest

Ultrafast atomic, molecular, and solid-state dynamics, attosecond physics and chemistry, soft x-ray and extreme ultraviolet sources, high harmonic generation, ultrafast lasers and optical physics, extreme ultraviolet four wave mixing and multidimensional spectroscopy, ultrafast processes in quantum matter, including electron and spin dynamics and phase transitions, semiconductor electronic and structural physics, carrier transport in junctions, coherent electronic, vibrational and phonon superpositions, nonadiabatic dynamics at curve crossings and conical intersections, charge switching and charge migration, photophysics of nanoparticles, nanostructures, and materials with energy applications.

Research Description

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.

Recent highlights:

(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

What happens when you explode a chemical bond?

UC Berkeley scientists are probing the fleeting steps in rapid photochemical reactions with some of the shortest laser pulses possible today. In this case, a femtosecond pulse of visible light (green) triggers the breakup of iodine monobromide molecules (center), while attosecond XUV laser pulses (blue) take snapshots of the molecules. This allows them to make a movie of the evolution of electronic states (yellow lights around molecules) before the molecules blow apart.

Making molecular movies with X-rays

Leone and coworkers have developed a bench top laser based soft X-ray, and used it to follow photoexcited ring opening in cyclohexadiene (CHD). With their new ultrafast X-ray source, the researchers are able to characterize and distinguish between the structure of the electron clouds and atomic arrangement in the critically important intermediate of photoexcited CHD that eventually leads to ring opening.

Scientists measure speedy electrons in silicon

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. 

Featured in the Media

Please note: The views and opinions expressed in these articles are those of the authors and do not necessarily reflect the official policy or positions of UC Berkeley.
September 28, 2018

Multidimensional spectroscopy with attosecond extreme ultraviolet and shaped near-infrared pulses

ScienceAdvances

Dynamics following excitation with attosecond extreme ultraviolet (XUV) pulses arise from enormous numbers of accessible excited states, complicating the retrieval of state-specific time evolutions. We develop attosecond XUV multidimensional spectroscopy here to separate interfering pathways on a near-infrared (NIR) energy axis, retrieving single state dynamics in argon atoms in a two-dimensional (2D) XUV-NIR spectrum. In this experiment, we measure four-wave mixing signal arising from the interaction of XUV attosecond pulses centered around 15 eV with two few-cycle NIR pulses. The 2D spectrum is created by measuring the emitted XUV signal field spectrum while applying narrowband amplitude and phase modulations to one of the NIR pulses. Application of such a technique to systems of high dimensionality will provide for the observation of state-resolved pure electronic dynamics, in direct analogy to phenomena unraveled by multidimensional spectroscopies at optical frequencies.

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July 16, 2019

Disentangling conical intersection and coherent molecular dynamics in methyl bromide with attosecond transient absorption spectroscopy

Nature Communications

Attosecond transient absorption spectroscopy is employed to follow the valence dynamics of strong-field initiated processes in methyl bromide. By probing the 3d core-to-valence transition, we resolve the strong field excitation and ensuing fragmentation of the neutral σ* excited states of methyl bromide. The results provide a clear signature of the non-adiabatic passage of the excited state wave packet through a conical intersection. We additionally observe competing, strong field initiated processes arising in both the ground state and ionized molecule corresponding to vibrational and spin-orbit motion, respectively. The demonstrated ability to resolve simultaneous dynamics with few-femtosecond resolution presents a clear path forward in the implementation of attosecond XUV spectroscopy as a general tool for probing competing and complex molecular phenomena with unmatched temporal resolution.

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April 3, 2020

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO2-Si photoanode

ScienceAdvances

Charge transport and recombination kinetics in each layer of a Ni-TiO2-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO2, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously

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August 12, 2020

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

Nature Communications

The fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C3H7I and t-C4H9I) through a conical intersection between 3Q0/1Q1 spin–orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the 3Q0 state into the 1Q1 state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C3H7I and t-C4H9I directly lead to differences in the photoproduct branching ratio of the two systems.

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