About

Stephen R. Leone is a Professor of Chemistry at the University of California, Berkeley, holding the John R. Thomas Endowed Chair in Physical Chemistry. His research interests encompass ultrafast laser investigations and soft x-ray probing of valence and core levels, attosecond physics and chemistry, state-resolved collision processes and kinetics investigations, nanoparticle fluorescence intermittency, aerosol chemistry and dynamics, probing with near field optical microscopy, and neutrals imaging. His projects are grouped along themes such as ultrafast laser molecular dynamics, chemical dynamics of molecules, nanoparticles, and clusters, as well as nanostructured materials investigations with scanned probe microscopies. Leone's work involves using femtosecond laser dynamics, ultrafast soft x-ray, and time-resolved x-ray photoelectron dynamics to study molecular motion on vibrational, rotational, and electronic timescales, including the analysis of molecular photodissociation via soft x-ray laser techniques. He has contributed to the development of high order harmonic generation for probing valence shell photoelectron spectra and core level spectroscopy, as well as generating isolated attosecond pulses to investigate electronic timescales in molecules and clusters. His research also extends to ultralow temperature gas phase kinetics relevant to planetary atmospheres, combustion dynamics, and heterogeneous chemistry with applications to fuel droplet combustion and atmospheric aerosol aging. Leone's work involves advanced microscopy techniques, surface probing, and studies of aerosol light scattering, with a focus on understanding molecular and nanoparticle dynamics, surface processes, and chemical reactions at ultrafast timescales.

Research topics

• Physics
• Optics
• Atomic physics
• Quantum mechanics
• Molecular physics
• Chemistry
• Materials science
• Condensed matter physics
• Mathematical optimization
• Physical chemistry
• Photochemistry
• Geometry
• Computational chemistry
• Organic chemistry
• Mathematics
• Optoelectronics

Selected publications

• Attosecond Transient Grating Spectroscopy with Near-Infrared Grating Pulses and an Extreme Ultraviolet Diffracted Probe

  ACS Photonics · 2025-03-12 · 5 citations

  articleOpen accessSenior authorCorresponding

  Transient grating spectroscopy has become a mainstay among metal and semiconductor characterization techniques. Here, we extend the technique toward the shortest achievable time scales by using tabletop high-harmonic generation of attosecond extreme ultraviolet (XUV) pulses that diffract from transient gratings generated with sub-5 fs near-infrared (NIR) pulses. We demonstrate the power of attosecond transient grating spectroscopy (ATGS) by investigating the ultrafast photoexcited dynamics in an Sb semimetal thin film. ATGS provides an element-specific, background-free signal unfettered by spectral congestion, in contrast to transient absorption spectroscopy. With ATGS measurements in Sb polycrystalline thin films, we observe the generation of coherent phonons and investigate the lattice and carrier dynamics. Among the latter processes, we extract carrier thermalization, hot carrier cooling, and electron-hole recombination, which are on the order of 20 fs, 50 fs, and 2 ps time scales, respectively. Furthermore, the simultaneous collection of transient absorption and transient grating data allows us to extract the total complex dielectric constant in the sample dynamics with a single measurement, including the real-valued refractive index, from which we are also able to investigate carrier-phonon interactions and longer-lived phonon dynamics. The outlined experimental technique expands the capabilities of transient grating spectroscopy and attosecond spectroscopies by providing a wealth of information concerning carrier and lattice dynamics with an element-selective technique at the shortest achievable time scales.

  PublisherOA PDFDOI

• Onset of coherent phonon motion in Peierls-distorted antimony by attosecond transient absorption

  Physical review. B./Physical review. B · 2025-09-09 · 3 citations

  articleSenior author

  Optical excitation of carriers can launch coherent lattice motion by changing the energy landscape of solids. But how does the relaxation of optically excited carriers affect this coherent motion? Using extreme ultraviolet transient absorption spectroscopy with few-femtosecond temporal resolution, the authors investigate here the onset of coherent phonon motion in Peierls-distorted antimony and find that the carrier relaxation during the first 100 fs following excitation leaves a lasting impact on the phase of the coherent lattice motion.

  PublisherDOI

• Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction

  The Journal of Physical Chemistry Letters · 2025-05-14 · 3 citations

  articleOpen accessSenior authorCorresponding

  Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet–triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular O–H···O hydrogen bond. Following excitation into the S2 (ππ*) state at 266 nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization, and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization, and bond elongation all within the first 700 fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the S1 potential energy surface, formed after very rapid transfer from S2 to S1, allowing AcAc to reach the conical intersection to intersystem crossing. Such time scales of nuclear motion are contrasted with the time scales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (<100 fs) and intersystem crossing (∼1.5 ps).

  PublisherDOI

• Polarization-resolved core exciton dynamics in LiF using attosecond transient absorption spectroscopy

  Physical review. B./Physical review. B · 2025-04-28 · 3 citations

  articleOpen accessSenior author

  The authors introduce here linear dichroism into attosecond transient absorption spectroscopy measurements to explore the orbital character of core exciton excited states in LiF thin films during their few-femtosecond exciton coherence lifetimes. The dichroic effect is well-described by atomic alignment angular momentum rules, which allow or forbid transitions from an aligned $p$-like state to a resonant $s$-like state, depending on the beam polarizations. A few-level model simulation and density functional theory calculations are used to corroborate the orbital character assignments of the core exciton states in the solid.

  PublisherOA PDFDOI

• Tracing Long-Lived Atomic Coherences Generated via Molecular Conical Intersections

  Physical Review Letters · 2025-10-16 · 1 citations

  articleOpen accessSenior author

  Accessing coherences is key to fully understand and control ultrafast dynamics of complex quantum systems like molecules. Most photochemical processes are mediated by conical intersections, which generate coherences between electronic states in molecules. We show with accurate calculations performed on gas-phase methyl iodide that electronic coherences of spin-orbit-split states persist in atomic iodine after dissociation. Our simulation predicts a maximum magnitude of vibronic coherence in the molecular regime of 0.75% of the initially photoexcited state population. Upon dissociation, one-third of this coherence magnitude is transferred to a long-lived atomic coherence where vibrational decoherence can no longer occur. To trace these dynamics, we propose a tabletop experimental approach-heterodyned attosecond four-wave-mixing spectroscopy. This technique can temporally resolve small electronic coherence magnitudes and reconstruct the full complex coherence function via phase cycling. Hence, heterodyned attosecond four-wave-mixing spectroscopy leads the way to a complete understanding and optimal control of spin-orbit-coupled electronic states in photochemistry.

  PublisherDOI

• Attosecond Optical Orientation

  Physical Review Letters · 2025-09-16 · 1 citations

  articleSenior author

  Circularly polarized light offers opportunities to probe symmetry-dependent properties of matter such as chirality and spin. Circular-dichroic measurements typically require further intrinsic or extrinsic breaking of symmetry by, e.g., enantiomeric excess, orientation, magnetic fields, or direction-sensitive detectors. Here we introduce circular-dichroic attosecond transient absorption spectroscopy by leveraging the angular momentum of two circularly polarized pulses, both pump and probe, in an isotropic medium, optically orienting the angular momentum of excited states on an attosecond timescale. We investigate a circular-dichroic measurement of the attosecond transient absorption of He Rydberg states. By limiting the allowed pathways via dipole selection rules for co- and counterrotating circularly polarized near-infrared and extreme ultraviolet (XUV) pulses, different spectral reshapings of the XUV transient absorption due to the AC Stark effect are observed. Paired with time-dependent Schrödinger equation calculations, the results show the role of selection and propensity rules and open up new opportunities to study coupling pathways of excited states as well as spin-dependent dynamics in atoms and beyond via attosecond optical orientation.

  PublisherDOI

• Probing soft X-ray induced photoreduction of a model Mn-complex at cryogenic conditions

  Journal of Synchrotron Radiation · 2025-02-03

  articleOpen access

  Soft X-ray absorption spectroscopy of first row transition elements at their respective L -edges provides important information about the oxidation and spin states of the metal centers. However, the associated sample damage in radiation-sensitive samples substantially alters the electronic and chemical structures of redox-active metal centers. Here, we measure the soft X-ray spectrum of the model Mn III (acac) 3 complex containing a redox-active Mn III metal center in an octahedral environment with a superconducting transition-edge sensor detector. To reduce the secondary damage resulting primarily from the diffusion of radicals and electrons, the spectra are collected at 30 K and 80 K on solid samples. Starting from the first scan, we detect the contribution of X-ray induced sample damage leading to a change in the Mn II intensity. However, at low temperatures, particularly at 30 K, we do not observe a gradual increase in the radiation damage with successive scans with the X-ray beam at the same spot. At our estimated dose of 90 kGy, we find 62% of Mn III (acac) 3 is still intact at 30 K. However, at room temperature, we see a gradual increase in radiation damage with increasing numbers of scans at the same spot, which is consistent with the possibility of increased diffusion rates of secondary radicals and electrons as noted in other studies.

  PublisherOA PDFDOI

• Different Rise Times of Atomic Br M _4,5 3d _3/2,5/2 Core Level Absorptions during Br ₂ C ¹ Π _u 1 _u State Dissociation via Extreme Ultraviolet Transient Absorption Spectroscopy

  The Journal of Physical Chemistry A · 2025-12-05

  articleSenior authorCorresponding

  The reported “dissociation times” for the Br2 C (1Πu 1u) state by various measurement methods differ widely across the literature (30 to 340 fs). We consider this issue by leveraging attosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the Br M4,5 3d3/2,5/2 edges (66 to 80 eV), tracking core-to-valence (3d → 4p) and core-to-Rydberg (3d → ns, np, n ≥ 5) transitions from the molecular to the atomic limit. The progress of dissociation can be ascertained by the buildup of the atomic absorption in time. Notably, the measured rise times of the 3d5/2,3/2 → 4p transitions depend on the probed core level final state, 38 ± 1 and 20 ± 5 fs for 2D5/2 and 2D3/2 at 64.31 and 65.34 eV, respectively. Simulations by the nuclear time-dependent Schrödinger equation reproduce the rise-time difference of the 3d → 4p transitions, and the theory suggests several important factors. One is the transition dipole moments of each probe transition have different molecular and atomic values for 2D5/2 versus 2D3/2 that depend on the bond length. The other is the merger of multiple molecular absorptions into the same atomic absorption, creating multiple time scales even for a single probe transition. On the other hand, the core-to-Rydberg absorptions did not allow accurate atomic Br buildup times to be extracted due to spectral overlaps with ground state bleaching, otherwise an even more comprehensive picture of the role of the probe state transition would be possible. This work shows that the measured XUV probe signals accurately contain the dissociative wavepacket dynamics but also reveal how the specific core-to-valence transition affects the apparent progress toward dissociation with bond length. These results highlight the potential probe-transition-dependent effects that need to be considered when interpreting measured signals and their time scales.

  PublisherDOI

• Coherently coupled carrier and phonon dynamics in elemental tellurium probed by extreme ultraviolet transient absorption

  Physical review. B./Physical review. B · 2025-05-20 · 4 citations

  articleOpen accessSenior author

  The authors utilize here a broadband extreme ultraviolet probe and a near-infrared pump to study coherent phonon and carrier dynamics in the elemental semiconductor tellurium. By measuring transitions from the tellurium 4$d$ semicore levels to the conduction band, they find that coherent phonon motion couples coherently with the hot holes and electrons. This results in an anticorrelation between the phonon-induced displacement and the electronic temperature. These findings provide insight into the coupling between the lattice and electronic structure in tellurium.

  PublisherOA PDFDOI

• Versatile, open-source program for simulating high-harmonic generation

  ArXiv.org · 2025-09-02

  preprintOpen accessSenior author

  Light sources based on high-harmonic generation (HHG) underpin ultrafast spectroscopy experiments across a large range of photon energies, spanning from the extreme-ultraviolet to the soft x-ray. To this day their design, implementation and improvement presents uniqe challenges, but can be aided by numerical tools. Here we present a new simulation program designed for this purpose, which accurately takes both macroscopic and microscopic aspects of high-harmonic generation into account and is therefore applicable across the broad range of parameters that HHG based light sources are today utilized in. The program is validated by calculating harmonic emission in four common experimental configurations.

  PublisherOA PDFDOI

Recent grants

• ITR: Molecular Qubit Transformations with Phase-Shaped Femtosecond Pulses

  NSF · $500k · 2002–2007

• Attosecond Electron Dynamics

  NSF · $540k · 2017–2021

• MRI: Development of an Isolated Attosecond Pulse Spectrometer at the Carbon K Edge

  NSF · $987k · 2016–2019

• Attosecond Electron Dynamics

  NSF · $540k · 2014–2017

• Attosecond Electron Dynamics

  NSF · $600k · 2020–2023

Frequent coauthors

• Daniel M. Neumark

  University of California, Berkeley

  580 shared

• Veronica M. Bierbaum

  302 shared

• Musahid Ahmed

  Lawrence Berkeley National Laboratory

  134 shared

• Craig A. Taatjes

  Sandia National Laboratories California

  132 shared

• Valeriu Scutelnic

  115 shared

• David L. Osborn

  Sandia National Laboratories California

  115 shared

• Thomas Pfeifer

  106 shared

• Mark J. Abel

  Fritz Haber Institute of the Max Planck Society

  99 shared

Labs

• Hildebrand LabPI

Awards & honors

• Department of Commerce Silver Medal Award (1980)
• American Chemical Society Pure Chemistry Award (1982)
• American Chemical Society Nobel Laureate Signature Award for…
• Coblentz Award for Spectroscopy (1984)
• Department of Commerce Gold Medal Award (1984)