Dissociation and isomerization following ionization of ethylene: insights from non-adiabatic dynamics simulations

TL;DR

This study uses non-adiabatic dynamics simulations to investigate the mechanisms and time scales of dissociation and isomerization in ionized ethylene, providing detailed insights into its photochemical behavior.

Contribution

It is the first to simulate coupled electron-nuclear dynamics of ethylene's cationic states, elucidating dissociation and isomerization mechanisms and their relation to conical intersections.

Findings

Dissociation and isomerization mainly occur on the cationic ground state.

Supports the hypothesis that H2-loss can originate from ethylidene.

Predicted isomerization times are longer than previous experimental estimates.

Abstract

Photoionized and electronically excited ethylene \ce{C2H4+} can undergo \ce{H}-loss, \ce{H2}-loss, and ethylene-ethylidene isomerization, where the latter entails a hydrogen migration. Recent pioneering experiments with few-femtosecond extreme ultraviolet pulses and complementary theoretical studies have shed light on the photodynamics of this prototypical organic cation. However, no theoretical investigation based on dynamics simulations reported to date has described the mechanisms and time scales of dissociation and isomerization. Herein, we simulate the coupled electron-nuclear dynamics of ethylene following vertical ionization and electronic excitation to its four lowest-lying cationic states. The electronic structure is treated at the CASSCF level, with an active space large enough to describe bond breaking and formation. The simulations indicate that dissociation and isomerization take place mainly on the cationic ground state and allow the probing of previous hypotheses concerning the correlation between the photochemical outcome and the traversed conical intersections. The results, moreover, support the long-standing view that \ce{H2}-loss may occur from the ethylidene form. However, the ethylene-ethylidene isomerization time predicted by the simulations is considerably longer than those previously inferred from indirect experimental measurements.