Stephen R. Leone

Prof of Physics / Chemistry
Director, Chemical Dynamics Beamline, LBNL
John R. Thomas Endowed Chair in Physical Chemistry

cchem.berkeley.edu/leonegrp/

Research Interests & Current Projects

Professor Leone's research interests include 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.

Current projects in attosecond physics are described here in more detail and follow along several main themes: Isolated attosecond pulses in the extreme ultraviolet (XUV) region are used in time-resolved spectroscopy of atoms, molecules, and condensed matter. These isolated attosecond pulses are produced by high-order harmonic upconversion in a Ne gas target of a few-cycle near-infrared (NIR) pulse into the XUV spectral region.

A new method of ionization gating allows the generation of energy-tunable isolated attosecond pulses, as opposed to the conventional amplitude gating method, which yields pulses with limited tunability. This increased energy tunability is critical for the application of isolated attosecond pulses to time-resolved spectroscopy. In addition, we have shown that scanning of the carrier-envelope phase allows rapid determination of the contrast ratio of the isolated attosecond pulse. A variety of complementary attosecond spectroscopic techniques are being pursued and developed in the Leone group.

In the first method, initiation of ultrafast dynamics by the XUV isolated attosecond pulse is followed by streak-field detection of the photoelectrons. The dynamics of the various quantum states involved in the time-dependent evolution of the photoexcited state is encoded in the photoelectron spectrum collected as a function of time delay between the XUV pump pulse and the NIR streaking probe pulse. In addition to measuring the photoelectron kinetic energies by linear time-of-flight, photoelectron angular distributions can also be obtained via velocity map imaging. Time-of-flight mass spectrometry is also used to detect photofragments originating from dissociative ionization processes in polyatomics.

In the second method, a few-cycle NIR pulse is used to drive plasmon oscillations in metallic nanostructures, thereby mapping the temporal phase of the oscillation to a kinetic energy modulation of the photoelectrons ejected by the isolated attosecond pulse. This method allows direct observation of plasmon decoherence in real time and paves the way for the rational design of plasmonic nanomaterials. In the third method, photoinitiation of coherent electron dynamics by a few-cycle ultraviolet to NIR pulse is followed by isolated attosecond probing of atomic core level absorptions in atoms and molecules. In addition to transient absorption, transient linear dispersion of a sample can also be measured and yields changes in the real part of the refractive index upon initial photoexcitation. For opaque condensed matter samples, transient reflectivity is employed as an alternative to transient absorption, thereby allowing the study of carrier dynamics in solid state nanomaterials on the attosecond time scale.

Experiments are being explored in confocal microscopy, apertureless near field optical microscopy, and single pulse coherent anti-Stokes Raman microscopy with phase control, and studies involving quantum dot blinking and pump-probe ultrafast studies of semiconductor nanocrystals. Other projects study aerosol light scattering and spectroscopy, aerosol reactions, as well as surface probing of neutrals desorbed by scanning ion microprobes using the chemical dynamics beamline at the Advanced Light Source.

Professor Leone is also available as a research director for the Applied Sciences and Technology (AS&T) Ph.D. program.

Selected Publications

A. A. Cordones, and S. R. Leone, " Mechanisms for charge trapping in single semiconductory nanocrystals probed by fluorescence blinking," Chem. Soc. Rev. (in press) (2012).

S. Chen, M. J. Bell, A. R. Beck, H. Mashiko, M. Wu, A. N. Pfeiffer, M. B. Gaarde, D. M. Neumark, S. R. Leone, and K. J. Schafer, "Light-induced states in attosecont transient absorption spectra of laser-dressed helium, Phys. Rev. A 86, 063408 (2012)..

A. N. Pfeiffer, and S. R. Leone, "Transmission of an isolated attosecond pulse in a strong-field dressed atom," Phys. Rev. A 85, 053422 (2012).

J. S. Prell, L. J. Borja, D. M. Neumark, and S. R. Leone, "Simulation of attosecond-resolved imaging of the plasmon electric field in metallic nanoparticles," Ann. Phys. (Berlin) 1-11, (2012).