Fall 2022 Physics Colloquium: Ultrafast Probes of Two- to Multi-Body Molecular Fragmentation


3 p.m., Oct. 28, 2022


Krupa Ramasesha, Staff Scientist, Combustion Research Facility, Sandia National Laboratories

Abstract: Photoinduced bond dissociation in molecules is at the heart of numerous photochemical phenomena ranging from photocatalysis to atmospheric chemistry. Developing a detailed understanding of the mechanisms of photodissociation and the underlying excited state dynamics are therefore crucial and have been the subject of intense study for many years. By probing different aspects of the dynamics – electronic, vibrational, and structural – that are responsible for the outcomes of molecular photofragmentation, my group is trying to develop a comprehensive picture of complex photodissociation mechanisms on ultrafast timescales. In this seminar, I will present recent results on two- to many-body photofragmentation dynamics in three gas-phase systems: (1) acetyl iodide, which is a photolytic precursor for the acetyl radical, an important intermediate in atmospheric chemistry, (2) dimethyl disulfide, which is the smallest molecule containing the disulfide bond present in certain structural and regulatory proteins, and (3) transition metal carbonyls, which are model photocatalysts. To provide insights into the excited state dynamics in these systems, we have applied ultrafast core-level spectroscopy, ultrafast electron diffraction, and ultrafast infrared spectroscopy, coupled with quantum chemical calculations using equation-of-motion coupled cluster methods and time-dependent density functional theory. In this seminar, I will present results from core-to-valence transient absorption spectroscopy following 266 nm excitation of acetyl iodide to its lowest 1np* state, which has revealed transient features with <100 fs lifetimes corresponding to excited state wavepacket evolution prior to dissociation of the C-I bond. Further, I will discuss the application of ultrafast electron diffraction for following the 200 nm photodissociation of dimethyl disulfide, which has yielded structural information on the competition between C-S and S-S bond dissociation channels. Finally, I will present results from ultrafast infrared and core-level spectroscopies of Fe(CO)5 and Ni(CO)4 following ultraviolet excitation of metal-to-ligand charge transfer states. These studies have uncovered complex excited state dynamics on ca. 100 fs to picosecond timescales governing the loss of multiple CO groups.

Bio: Krupa Ramasesha is a staff scientist at the Combustion Research Facility, Sandia National Laboratories, Livermore, where she is leading a research program investigating ultrafast dynamics in gas phase and condensed phase systems, funded by the DOE/BES Gas Phase Chemical Physics program and the Laboratory Directed Research and Development program. Krupa’s research interests lie in exploring photoinduced energy transfer and excited state dynamics governing bond dissociation, conformational changes, and charge transfer. She received her undergraduate degree from the University of Wisconsin-Madison, where she studied solution-phase photoisomerization and bimolecular reaction dynamics using transient UV/Vis absorption spectroscopy. She then moved to the Massachusetts Institute of Technology for her Ph.D., where she investigated hydrogen bonding and proton transfer dynamics using ultrafast broadband multidimensional infrared spectroscopy. She performed her postdoctoral work at the University of California, Berkeley studying electron dynamics in semiconductors via attosecond soft X-ray spectroscopy.

*Refreshments served at 2:45PM in PAS 218

Note: Fall 2022 Colloquium in-person only