New light on acetone: a master equation model for gas phase photophysics and photochemistry

Photochemistry of Photoinduced-Reaction Generated Bubbles

UV light can create and grow bubbles (herein referred to as PIRGBs for photoinduced-reaction generated bubbles) at liquid/solid interfaces through photoinduced reactions that produce gases. Unlike the simple experience of blowing water bubbles through a straw, in which the bubbles quickly move away from their nucleation sites, not only can a deep UV laser beam create PIRGBs in liquid acetone, but also can hold and grow them. Free bubbles could be attracted to the excitation region from millimeters away, indicating that the reactions cause radial inward flow on the liquid surface. The radial flow can be due to imbalanced surface tensions at the interfaces. Raman measurements reveal that the gases in the PIRGBs include C2H6, CO, and H2, and in liquid acetone, sp2-carbon species are detected upon the UV excitation. Time series Raman measurement discloses a photocarbonization process in which small acyclic carbon species gradually form small clusters with carbon rings and eventually produce a large piece of amorphous carbon at the top of a PIRGB in pure liquid acetone. The photocarbonization may open new avenues for development of carbonaceous materials. Using PIRGB, miniature or microscale gas production reactors can be developed for producing gases.

The dynamics of CO production from the photolysis of acetone across the whole S1 ← S0 absorption spectrum: Roaming and triple fragmentation pathways

The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2–305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C–C bond cleavage on the lowest excited triplet state, T1. At higher energy ( λ < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol−1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band.