Study of Unimolecular Reactions by Coulomb Explosion Imaging: The Nondecaying Vinylidene

Abstract

The general situation of a bound system interacting with a system of much higher density of states appears in different fields of physics. Some well known examples are doorway state reactions, giant resonances [1], radiationless transitions [2], and isomerization. Often the production of “doorway states” is enabled by transitions which obey selection rules to proceed through a single resonant channel. In the particular case of molecular isomerization, the process is sometimes described as follows. A single state of an unstable (or metastable) isomer is created due to Franck-Condon overlap with an initial state. Within a short time this state interacts with the vibrationally excited levels of the stable isomer, and the molecule undergoes internal radiationless rearrangement into the stable isomeric states. This description is exact only at the limit of infinitely dense manifold of levels of the final states such as the dissociation into a continuum. For many practical cases, the high density of the stable isomer states is approximated as such a continuum. It is the purpose of this Letter to show how the Coulomb explosion imaging (CEI) method can be used to study such molecular isomerization by measuring the full nuclear density function of a molecule at a doorway energy state. The structure and relative population of the isomers provides information on the nature of the isomeric interaction. The equilibrium of the acetylene in the 1 S 1 ground electronic state has a linear (H—C — C—H) geometry and a rigid structure [3]. Numerous theoretical studies of the potential energy surface [4‐ 9] predicted a local minimum at the “vinylidene” conformation ( :C— CH2). The energy of the vinylidene isomer is about 2 eV higher than the acetylene ground state, and the estimations of the isomerization barrier range from 0 to 0.26 eV. Recent experimental evidence including this Letter support the existence of the vinylidene isomer. Yet, the time dependent picture found in the literature is of an initially prepared vinylidene isomer which decays within a few picoseconds through a low barrier to acetylenic configurations. Carrington et al. [6] studied the vinylidene-acetylene isomerization dynamics by a semiclassical reaction path and analyzed it in terms of the vinylidene decay lifetime. Their calculation yields a lifetime of 0.24 to 4.6 ps. The vinylidene isomer can be accessed directly by photodetachment from the ground state of the negative ion which also possesses the vinylidene structure [10,11]. This process has been investigated experimentally by Ervin and co-workers [12] who measured the corresponding photoelectron spectrum. Pronounced peaks were resolved and assigned to the vinylidene normal mode vibrations. Chen et al. [13] measured a high resolution spectrum of acetylene in the energy region that corresponds to the predicted isomerization. They were not able to assign the measured spectrum levels and used statistical analysis to attribute certain perturbations of the spectrum to the presence of the vinylidene isomer. The experimental apparatus at the Weizmann Institute in Rehovot is designed for CEI measurements of neutral molecules. The reader is referred to Ref. [14] (and references therein) for a detailed description of the experimental setup and the principle of the CEI technique. In the experiment reported here, C2H 2 anions were produced by