Ultrafast Internal Conversion in Ethylene. II. Mechanisms and Pathways for Quenching and Hydrogen Elimination

TL;DR

This study combines experimental and theoretical methods to investigate ultrafast internal conversion, quenching, and hydrogen elimination pathways in excited ethylene molecules, revealing detailed mechanisms and confirming theoretical predictions.

Contribution

It provides new insights into ethylene's nonadiabatic dynamics, experimentally confirming the existence of two transition states and their branching ratios through time-resolved photoionization spectroscopy.

Findings

Enhanced CH$_2^+$ and CH$_3^+$ yields within 100 fs post-excitation

Confirmation of two distinct transition states and their branching ratios

Observation of rapid H$_2$ and H atom elimination

Abstract

Through a combined experimental and theoretical approach, we study the nonadiabatic dynamics of the prototypical ethylene (C$_2$H$_4$) molecule upon $\pi \rightarrow \pi^*$ excitation with 161 nm light. Using a novel experimental apparatus, we combine femtosecond pulses of vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation with variable delay to perform time resolved photo-ion fragment spectroscopy. In this second part of a two part series, the extreme ultraviolet (17 eV$ < h \nu < 23$ eV) probe pulses are sufficiently energetic to break the C-C bond in photoionization, or photoionize the dissociation products of the vibrationally hot ground state. The experimental data is directly compared to ab initio molecular dynamics simulations accounting for both the pump and probe steps. Enhancements of the CH$_2^+$ and CH$_3^+$ photoion fragment yields, corresponding to molecules photoionized in ethylene (CH$_2$CH$_2$) and ethylidene (CH$_3$CH) like geometries are observed within 100 fs after $\pi \rightarrow \pi^*$ excitation. Quantitative agreement between theory and experiment on the relative CH$_2^+$ and CH$_3^+$ yields provides experimental confirmation of the theoretical prediction of two distinct transition states and their branching ratio (Tao, et al. J. Phys. Chem. A. 113, 13656 (2009)). Fast, non-statistical, elimination of H$_2$ molecules and H atoms is observed in the time resolved H$_2^+$ and H$^+$ signals.