Christopher Jesse Kliewer

Disulfide photochemistry impacts a wide range of chemical processes from biological UV photodamage to polymer self-healing and green photocatalysis. Despite this, the response of disulfide bonds to deep-UV light remains poorly understood. Dimethyl disulfide serves as a model system for understanding the fundamental properties of this chemical motif, including its UV photodynamics. While 267-nm excitation exclusively cleaves the S–S bond on a subpicosecond time scale, 200 nm excites electronic states of Rydberg and antibonding character, resulting in competing C–S and S–S dissociation on an unknown time scale. Using mega-electron-volt ultrafast electron diffraction and time-resolved velocity-mapped ion imaging, we find subpicosecond as well as unexpectedly long, picosecond lifetimes. Nonadiabatic mixed quantum-classical dynamics reveal mechanisms and observables. We attribute the subpicosecond dynamics to bond dissociation on singlet states and the picosecond dynamics to dissociation after intersystem crossing. Our study reveals that deep-UV excitation in disulfide-containing molecules can induce complex, multi-time scale dynamics.

Harnessing quantum coherences in chemical reactions is a driving force in the development of next-generation science and technology, from tailored material design to sustainable energy conversion. At their core, such reactions originate from collisions between species, where intermolecular interactions govern the redistribution of coherence and energy that ultimately drive chemical transformation. Because these coherences decay on the ultrafast timescale, capturing such dynamics requires a simultaneous combination of high spatial- and ultrafast temporal resolution that current measurement approaches simply cannot provide. Here we present Coherence Lifetime Imaging, a new approach that records two-dimensional, single-shot movies of coherence dynamics at 0.2 THz frame rates, enabling direct observation of ultrafast quantum coherence evolution in transient macroscopic environments. Using a tailored ultrafast pulse train, we generate a coherent rotational wavepacket and track its dynamics, providing an in-situ quantum tracer of the local collisional environment during a laser-induced plasma breakdown event. We reveal highly transient shockwave gradients with elevated chemical reactivity and that enhanced mixing persists hundreds of microseconds following breakdown. This approach provides a new route to quantify and map phase sensitive collisional dynamics in complex, transient systems, linking fundamental quantum interactions to macroscopic chemical quantities.

Platinum serves as a highly effective catalyst in oxygen-rich environments, yet its high-temperature oxide formation remains debated. Using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we investigated the oxygen surface coverage on Pt(111) at temperatures from 873 K to 1073 K in a 1 mbar O ₂ environment. Our findings reveal the formation of a high-temperature stable surface oxide (PtO _x ) at temperatures significantly lower than previously reported. We observed a single O 1s peak at 529.7 eV, with a second peak growing in at 531.2 eV at 1073 K, indicative of a unique high-temperature oxide structure formed in the absence of an α-PtO ₂ phase. These results provide new insights into the mechanisms and kinetics of platinum oxide formation under these conditions.

Transient plasma ignition (TPI) utilizes non-equilibrium plasmas, produced by nanosecond high-voltage pulses, to improve lean-fuel combustion performance and reduce emission. It is known that the relatively high reduced electric field (E/N) in TPI plays an important role in generating energetic electrons and facilitating energy-efficient radical productions, resulting in reliable ignition for lean combustion. Determining the reduced electric field in the discharge is hence important for the understanding of the TPI process and ultimately allowing for the control of the plasma chemistry. This study reports spatiotemporally resolved measurements of the electric field (E) in a 10-ns pulsed plasma that is produced by a pin-to-pin electrode configuration in extremely lean CH4/air mixtures, with the equivalent ratio (ϕ) ranging from 0.4 to 0.8, at 1 bar using the electric field induced second harmonic generation (EFISH) technique. At a relatively low voltage such as 8 kV, a streamer discharge is typically initiated between the electrodes. The peak electric field was found to be 45 kV/cm, corresponding to an E/N of 184 Td, for an 8 kV pulse in a lean (ϕ= 0.4) CH4/air mixture at 1 bar. Increasing the pulse voltage to be ≥13 kV under a single pulse operation would cause the discharge to transition to a spark, which was found necessary to achieve reliable ignition for combustion. The peak E was measured to be 66 kV/cm or an E/N of 270 Td, for a 13-kV pulse. Interestingly, the E reaches its maximum during the rising phase of the voltage pulse and drops quickly as the current increasing. The radially symmetric electric field was found highest near the high voltage pin, followed by that near the ground pin, and lowest near the midplane. The effect of the pulse repetition frequency (PRF) on E/N was studied by employing two-pulse sequences with various pulse-to-pulse delays ranging from 10 to 1000 µs. Measurements of the electric field during the 2nd voltage pulse revealed that the transient spark occurred at a lower voltage with a lower peak E (i.e., 40 – 48 kV/cm), but corresponding to a greatly enhanced E/N due to the elevation in the gas temperature of the discharge. In addition, the impact of ϕ on E was found insignificant, implying that the measurements obtained at the extreme lean conditions can be applied to stoichiometric or richer fuel-air mixtures.