Prediction Challenge: Cyclobutanone Photochemistry
Leading groups in the field of photoexcited molecular dynamics were invited to take part in the Journal of Chemical Physics Prediction Challenge investigating the photochemistry of cyclobutanone. Researchers from the COSMOS project have submitted research articles attempting to predict the results of experiments performed at the SLAC Megaelectronvolt Ultrafast Electron Diffraction facility.
The papers submitted from the COSMOS Project Grant can be browsed at: Prediction Challenge Papers
The full list of publications for this challenge can be found at: Journal of Chemical Physics Prediction Challenge: Cyclobutanone Photochemistry
Motivation
The simulation of photochemical molecular dynamics has been a major challenge to theoretical chemistry because of the need to simultaneously describe quantum mechanical effects of both nuclei and electrons. Numerous advances have been made over the past decade and many would agree that excited state simulations have demonstrated their value in the interpretation of experiments . However, one can question whether these simulations have been unambiguously predictive. True predictive capabilities would pave the way to rational design of light-driven molecular systems, with revolutionary implications for renewable solar energy (directly to electricity or to fuels), bioimaging, optogenetics, and photochemical synthesis. Thankfully, new ultrafast diffraction experiments1 have come on-line which provide both spatio-temporal resolution on the atomic scale, i.e. molecular movies. This provides a novel opportunity - a double-blind test of the accuracy of excited state simulations.
Experimental Details
The following are the experimental details as given at the start of the challenge:
The experiment will be performed at the SLAC Megaelectronvolt Ultrafast Electron Diffraction facility. A gas sample of about 1 mbar of cyclobutanone will be irradiated with 200 nm light (≈80 fs cross-correlation) and electron diffraction images will be obtained with 150 fs time resolution (FWHM) and 0.6 Å spatial resolution (2π/Smax), with the scattering vector S ranging from 1-10 Å-1. The experiment will be performed at a repetition rate of 360 Hz and the gas sample will be exchanged after each optical/electron pulse pair. Note that the excitation is believed to target a Rydberg (3s) excited state (i.e., n->3s) and not the n->π* state (280 nm) which is believed to be the lowest singlet excited state. This is because the oscillator strength of the n->π* state is too low and direct excitation to this state would likely lead to multiphoton transitions. The intensity of the 200 nm excitation light will be kept as low as experimentally feasible (5 µJ) to excite ≈10% of the molecules and avoid multiphoton excitation. The experiment will collect diffraction images for time delays from -1 ps to 10’s of ps in variable step sizes. The immediate region around time zero (-200 fs to 200 fs) will be scanned with 30 fs stepsize. Longer positive delays will be scanned with step sizes up to several ps. It is very possible that one or more triplet electronic states will be involved in the dynamics.
Entry Rules
The challenge is to predict the results of the experiment and prepare manuscripts for submission to a special issue in The Journal of Chemical Physics. To be considered, submissions must be received before the experimental results are revealed and they must include direct predictions of the key experimental observables.