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Stephen Leone Research

Being the chair and director of many chemical laboratories, Stephen Leone has a vast variety of research interests including ultrafast laser investigations, attosecond physics and chemistry dynamics, and quantum dots. His research dates back to the 1970s with his first publication for Raman spectroscopy scattering in inorganic crystals. Here he grew crystals in a drawer in the laboratory at Northwestern using slow evaporation of the solvent. He obtained some transparent truncated pyramid crystals, and other flawed crystals which were destroyed by the laser from the laser Raman Spectrometer. This was the time where Leone was first introduced to lasers and they became a significant point of interest in his future research.

His interest in lasers allowed for research using nanosecond pulsed laser reaction chemistry which later became femtosecond wave packet dynamics and further, attosecond dynamics. Much of his latest research focuses heavily on attosecond and femtosecond dynamics and their applications to small atoms, molecules, metals, and semiconductors. This work would not be possible if it were not for his research in time-resolved spectroscopy which is a result of ultra-fast laser spectroscopy which uses the pulses in order to study chemical dynamics.

High Harmonic Generation

Femtosecond timescales are not fast enough in order to study electron dynamics and therefore attoseconds must be used. There are different ways to create these extremely short pulses, one being high harmonic generation (HHG). In this process, an electron initially bound to an atom is ionized, quasi-classically accelerated and radiatively recombined, all within one optical cycle of an intense laser field. When the electron is ionized part of its wavefunction goes with it and once the electron is driven back to the atom it originated from that part of its wavefunction interferes with the part of its wavefunction that was left behind at the atom. This creates a high-frequency dipole moment and results in the high harmonic attosecond pulse.

Once the attosecond pulses are inside what is called the interaction region there are two techniques used in order to interpret what happened in those attoseconds. The two techniques are carrier-envelope scanning where the timing of the electrons can be identified by their extra momentum and optical streaking where a subsequent streak spectrogram compares the energy of the pulse to the photoelectron, as the two change over time. The streak spectrogram confirms the existence of individual attosecond pulses, measures their length, and determines when a secondary electron is produced. The generation of attosecond pulses and advancements in the field of time-resolved spectroscopy will help to understand molecular dynamics on electronic timescales.

Femtosecond Dynamic Research

Femtosecond Dynamics in Molecules

Leone’s work in femtosecond molecular dynamics is used in order to study and better understand photochemical reactions in real time and to examine very short lived chemical reaction intermediates which include radical species. In his October 2014 publication, Leone used femtosecond high-harmonic extreme ultraviolet transient absorption spectroscopy in order to determine the pathway and mechanism in which CH₂IBr dissociates into both CH₂Br and CH₂I radicals. It was determined that the major pathway was for the carbon-iodine bond to dissociate after undergoing excitation at a wavelength of 266 nm leaving a major product of the radical species CH₂Br. This spectroscopy can be used to better understand and learn about the mechanisms of chemical reactions.

Femtosecond Dynamics in Semiconductors

The same femtosecond high-harmonic extreme ultraviolet transient absorption spectroscopy was used in order to observe charge carrier dynamics in semiconductors. He researched what would happen if spinel Co₃O₄ were to be excited using femtosecond high-order harmonic pulses created by a titanium and sapphire laser. A 2 eV redshift was observed when comparing the absorption edge and the ground state. This led them to believe that the cobalt underwent a reduction via charge transfer between the oxygen and cobalt. A similar experiment was conducted using Fe₂O₃ which has the hexagonal close-packed structure with iron ions filling two-thirds of the octahedral interstices rather than the spinel structure like Co₃O₄ . Likewise, it was observed that charge transfer occurred after absorption from oxygen to iron. These surface electronic properties of semiconductors with or without photocatalytic center, metals, have numerous potential applications to be photocatalysts.

Attosecond Dynamics in Silicon

In 2014, attosecond extreme ultraviolet spectroscopy was used in order to see the electron transfer from the valence band to the conduction band of silicon. The electrons were moved into the conduction band by the aforementioned laser pulses. The experiment showed that the electron dynamics proceed faster than a quadrillionth of a second after laser excitation from the comparatively slower lattice motion of the silicon atomic nuclei.