Photoinduced bond dissociation in molecules is at the heart of a number of 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 various 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 talk, 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. The first part of my talk will focus on 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 &lt;100 fs lifetimes corresponding to excited state wavepacket evolution prior to dissociation of the C-I bond. I will then 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 in this molecule. 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.

Ultrafast probes of two- to multi-body molecular fragmentation

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ACS Fall 2022

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