Ultrafast internal conversion in ethylene. II. Mechanisms and pathways for quenching and hydrogen elimination

Abstract

Through a combined experimental and theoretical approach, we study the nonadiabatic dynamics of the prototypical ethylene (C2H4) molecule upon π → π* excitation with 161 nm light. Using a novel experimental apparatus, we combine femtosecond pulses of vacuum ultraviolet 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 XUV (17 eV < hν < 23 eV) probe pulses are sufficiently energetic to break the C–C bond in photoionization, or to photoionize the dissociation products of the vibrationally hot ground state. The experimental data is directly compared to excited state ab initio molecular dynamics simulations explicitly accounting for the probe step. Enhancements of the CH2+ and CH3+ photo-ion fragment yields, corresponding to molecules photoionized in ethylene (CH2CH2) and ethylidene (CH3CH) like geometries are observed within 100 fs after π → π* excitation. Quantitative agreement between theory and experiment on the relative CH2+ and CH3+ yields provides experimental confirmation of the theoretical prediction of two distinct conical intersections and their branching ratio [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A. 113, 13656 (2009)]. Evidence for fast, non-statistical, elimination of H2 molecules and H atoms is observed in the time resolved H2+ and H+ signals.

Ultrafast Dynamics of Neutral Superexcited Oxygen: A Direct Measurement of the Competition between Autoionization and Predissociation

“…Thus, the table-top HHG based XUV time-domain technique used here represents a significant step forward in the understanding of non-adiabatic relaxation mechanisms. In general, the coupling between electronic and nuclear degrees of freedom forms an interesting problem where direct time domain techniques could be very illuminating [30,31]. An important facet of the time-domain experimentation is the possibility of implementing real-time control, wherein a light pulse can be used to modify or change the course of the photo-chemical reaction in the transient phase.…”

Incorporating real time velocity map image reconstruction into closed-loop coherent control

“…For example, in a polyatomic molecule such as ethylene, the initial pulse might be used to stimulate an electronic excitation and the shaped secondary pulse might manipulate the nuclear wavepacket through an available conical intersection, 71 thus exerting rapid and efficient control of the electronic to nuclear energy conversion. Three-dimensional image-based feedback should make identifying the exit pathways 72,73 from the conical intersection plausible. Continuing advances in VMI and related techniques 74,75 might also suggest future possibilities for more refined feedback for adaptive control.…”

Dual color x rays from Thomson or Compton sources

“…The time-domain spectroscopy is indeed based on the interplay between the conjugate variables of frequency and time [6,7]. The time scale for such dynamics can range from 10 fs in ultrafast processes as the dissociative ionization [8], to hundred femtoseconds for less energetic chemical mechanisms [9,10]. Another important issue is the future color x-ray technology.…”