Stephen R. Leone
CSD Senior Faculty Scientist
Professor of Chemistry and Physics, UC Berkeley
Gas Phase Chemical Physics Program
Office Phone: (510) 486-4754
Office Location: Bldg. 2, Room 300C
Fax: (510) 495-2690
Professor Leone's research interests include ultrafast laser investigations and soft x-ray probing of valence and core levels, attosecond physics, state-resolved collision processes and kinetics investigations, coherent processes and wave packets, dynamics of nanoparticles, nanoscale probing with near field optical microscopy, and neutrals imaging. Current projects are grouped along several main themes: Ultrafast laser molecular dynamics, including x-ray probing and attosecond pulse production and investigations and chemical dynamics of molecules, nanoparticles, and clusters. Several examples are considered briefly and one in more detail. Ultrafast lasers are used to probe the dynamics of molecular motion on the time scales of vibrational, rotational, or electronic periods. The Leone group investigates coherent properties. 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 are produced by high fields in a rare gas and used to probe valence shell photoelectron spectra and core level spectroscopy of time-evolving systems, ranging from atoms to small molecules to metal clusters. Phase-shaping of the high order harmonics has been investigated. Transient x-ray absorption is used to probe alignment and molecular fragmentation pathways through core level spectroscopy. By using few cycle carrier-envelope phase-stabilized laser pulses, isolated attosecond pulses are generated to study electronic timescales in molecules and clusters by ejecting inner shell electrons on attosecond timescales. An apparatus to probe photoelectron angular images with time-resolved high order harmonics is used to study outgoing electron waves and phases. Research also investigates the ultralow temperature gas phase kinetics for the atmospheres of Titan and Saturn, as well as to probe combustion dynamics through radical reactions. Heterogeneous chemistry is a significant new area of investigation, with applications to fuel droplet combustion and aerosol aging in the atmosphere. Leone’s current projects in attosecond physics are described here in more detail. 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 one 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 another 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 or 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.