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Overview
A general goal of Atomic, Molecular, and Optical Physics is to interpret fundamental, quantum mechanical principles from light-matter interactions in isolated systems. My research is focused on probing and controlling quantum dynamics, specifically studying the evolution of photo-induced chemical processes in atomic, molecular, and nano systems as a function of time.
In atoms, one can directly measure correlations between multiple electrons and a single ionic core. For molecules, the dynamics of the individual atoms in the process must be considered, hence providing a means to test the limitations of the standard Born-Oppenheimer model for nonadiabatic transitions. Furthermore, weakly-bound nano systems add a new layer of complexity where individual atoms and molecules can interact with neighboring particles, bridging the gap between isolated and condensed systems.
With the development of ultrafast light sources, such as lab-based tabletop lasers and facility-based free-electron lasers (FELs), we can investigate inter- and intramolecular processes in the time domain, thus mapping out their dynamic evolution. In that regard, it is possible to “make a molecular movie” of the ultrafast reactions in cold, controlled quantum systems.
Overall, this type of research plays an integral role in understanding how charge and energy can be transferred in systems of varying sizes. In particular, a complete understanding of the mechanisms and timescales of a particular process can reveal how efficient it is in nature, which can have broad applications from donor-acceptor systems in organic semiconductors to radiation damage and radical formation in biological systems.
Research Highlights
Journal cover
Ultrafast molecular dynamics–isomerization, roaming, and ring-opening: Understanding the temporal evolution of radiation-induced processes is fundamentally important to the fields of Physics, Chemistry, and Biology. I am currently pursuing this goal for a few different types of photochemical processes at the University of Connecticut. With the combination of femtosecond pump-probe spectroscopy and coincident Coulomb explosion imaging, such photo-induced dynamics can be mapped out with the utmost temporal precision.
We recently published results on how a linear molecule, acetonitrile, can transform into a ring molecule upon ionization. The work was chosen as the journal cover. I am also examining roaming reactions in small molecules. Roaming is a unique molecular process where a neutral fragment stays weakly-bound to the remaining molecule upon photoionization which can lead to secondary processes such as electron or proton transfer.
Furthermore, I recently constructed a tunable UV source, using an optical parametric amplifier and nonlinear optics, to perform two-color experiments. Here, I want to explore ring-opening dynamics in small, cyclic molecules initiated through UV excitation, where we specifically study the efficiency of ring-opening with respect to other dissociative channels. I was also recently the Principal Investigator on a complementary experiment at Stanford University using ultrafast electron diffraction (UED) to directly image ring-opening. This approach gives a complete picture of the dynamics, both structurally and kinematically.
Quantum fluid dynamics in helium nanodroplets: Bubble formation is an important process in liquids where the dynamics between bubbles is responsible for a wide variety of processes such as the assembly of proteins into functional complexes in biological systems and the formation of impurities in metals with potential applications in materials science.
At FERMI FEL, I recently performed a series of experiments in helium nanodroplets to study the timescales and efficiencies with which bubbles are formed. I specifically examined the relaxation dynamics from single-photon excitation where a bubble is formed, followed by the repulsion of the neighboring atoms, which drives the bubble to the surface of the nanodroplet. Furthermore, I studied how mutual attraction of two bubbles can lead to drastically enhanced intermolecular decay.
Simulation of the merging of two excited bubble states in condensed helium.
Schematic representation of double ICD
Intermolecular decay processes in weakly-bound nanosystems: Intermolecular decay, such as intermolecular Coulombic decay (ICD) and electron transfer mediated decay (ETMD), are unique electronically-correlated processes, which offer a means for localized energy or charge exchange in weakly-bound nanosystems.
Recently, I discovered a new type of decay process, termed double ICD, which leads to highly efficient double ionization of alkali dimers doped in helium nanodroplets. Such a strong ionization enhancement is relevant for large-scale systems where it can create additional radicals and slow electrons, which are known to cause radiation damage.
Additionally, I found an efficient means by which charge and energy can be exchanged between the helium nanodroplet and an embedded magnesium complex. This was a unique case for ETMD since it occurred from the ground state of the helium ion, which opened up additional channels for this type of intermolecular decay process.
Electron solvation in hydrogen-bonded systems: The solvation of electrons in aqueous solutions plays a crucial role in many biological and chemical processes. Moreover, it is also the simplest quantum solute, so the understanding of the process is rather fundamental.
Recently, I headed a large-scale collaboration to perform the first experiment to directly trigger electron solvation in pure water clusters with ultrashort FEL pulses. Using time-resolved photoelectron spectroscopy, we were able to completely map out the solvation process in time, from the initial scattering of the electron, to the trapping in the water conduction band, to the local molecular rearrangement that forms the solvated states.
Furthermore, we have performed a complementary experiment at ETH Zurich where we used a tabletop, high harmonic generated light source to initiate electron solvation in pure and deuterated water clusters.
Experimental Schematic
Experimental Methods
Ultrafast Lasers:
X-ray Science:
Spectroscopic Techniques:
Molecular Beams:
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