Imaging ultrafast dissociation dynamics: roaming in formaldehyde
Bibliographic record
Abstract
As a photoexcited molecule breaks apart, we can see fragments following the direct dissociation path, as well as fragments deviating from this minimum energy path. So-called roaming fragments explore the potential energy landscape in a statistical manner and are directly captured using Coulomb explosion imaging (CEI). Individual pathways are distinguished based on state-of-the-art theory analysis. How to image statistical dynamics of individual reaction pathways hidden in an overwhelming background? We made use of the capability of CEI to detect the ion-breakup of single-molecules in real time. We investigated the uni-molecular dissociation reactions in the deuterated formaldehyde molecule (D₂CO). In addition to the conventional dissociation channels – molecular and radical dissociation – a third channel termed roaming has been discovered in 2004, which bypasses the transition state. Identified through spectroscopic footprints using high-resolution spectroscopy and found in a variety of molecules, roaming fragments had however never been observed directly and in a time-resolved way. We succeeded in doing so by using the highly sensitive CEI method in combination with high-level first-principles simulations of all critical experimental steps. Distinctive roaming signatures were identified in the experimental data (see Fig. 1). These were rendered observable by extracting rare stochastic events out of an overwhelming background. Additionally, this first identification of transient roaming features allowed to generalize the definition of roaming. While it was previously assumed that roaming results in molecular dissociation only, we observed that roaming might also be followed by radical dissociation. We further investigated how the overall dissociation depends on the employed UV pump wavelength.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".