Imaging the photochemical dynamics of cyclobutanone with MeV ultrafast electron diffraction

TL;DR

This study uses MeV ultrafast electron diffraction to observe the real-time photochemical dynamics of cyclobutanone, revealing sub-picosecond dissociation processes and branching ratios of reaction channels, advancing understanding of Norrish Type I reactions.

Contribution

First application of MeV ultrafast electron diffraction to map the complete photochemical dynamics of cyclobutanone in real time.

Findings

Photodissociation occurs on a sub-picosecond timescale.

C3 dissociation channel dominates with a 5:3 ratio to C2.

Approximately 80% of products are from C3 and C2 channels, 20% ring-opened structures.

Abstract

We study the photoinduced chemical dynamics of cyclobutanone upon excitation at 200 nm to the 3s Rydberg state using MeV ultrafast electron diffraction (UED). We observe both the elastic scattering signal, which contains information about the structural dynamics, and the inelastic scattering signal, which encodes information about the electronic state. Our results suggest a sub-picosecond timescale for the photodissociation dynamics, and an excited state lifetime of about 230 femtoseconds. The dissociation is found to be dominated by the C3 channel where cyclopropane and CO are produced. The branching ratio of the C3 channel to the C2 channel where ethene and ketene are produced, is estimated to be approximately 5:3. Our data suggest that the C3 and C2 channels account for approximately 80% of the photoproducts, with the remaining 20% exhibiting ring-opened structures. It is found that the timescale associated with the dissociation process in the C2 channel is shorter compared to that in the C3 channel. Leveraging the enhanced temporal resolution of MeV UED, our results provide a real-time mapping of the nuclear wavepacket dynamics, capturing the complete photochemical dynamics from S2 minimum through the S1/S0 conical intersection, and finally to the dissociation. Our experimental results provide new insights into the Norrish Type I reaction and can be used to benchmark non-adiabatic dynamics simulations.