Isomerization of a highly excited vibrational state of acetylene was studied using the Coulomb explosion imaging technique. A vinylidene isomer was prepared by electron photodetachment of the negative molecular ion and the corresponding distribution function of the nuclear configurations of the molecule was sampled after a time period of 3.5 ms. The population of the vinylidene isomer was found to be significantly high (ϳ50%), in contrast to the commonly accepted notion of vinylidene as a short-lived isomer. [S0031-9007(98)07417-1]

Abstract. The Coulomb Explosion Imaging (CEI) method was used to determine the structure and photochemistry of carbon clusters. The cyclic structure of C4 was found using a combination of laser photodetachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 f 0.1 eV, more than 1 eV lower than the electron affinity of the linear isomer. INTRODUffIONThe structure of carbon clusters has been investigated recently both theoretically' and e~perirnentally.~.~ Although the theory predicted that clusters containing only four carbon atoms have a stable cyclic form, in past experiments it was found that only when the number of atoms exceeds nine could a cyclic form be detected. In the present work, the Coulomb Explosion Imaging (CEI) method4 has been used to probe the structure of C.,. Its photochemistry was also investigated by applying a laser inside the accelerator. Here we present results which indicate the existence of a cyclic C4. Data are also presented on the photodissociation processes in these clusters.The CEI method applied in this study can be described by a three-stage process:1) The first stage is the preparation of C4 molecules at 12 MeV (about 2% of the speed of light).2) The second stage occurs when this molecular beam impinges upon a very thin foil. Because of the large cross-sections for electron loss (1 0-l6 cm2) at such velocities, all the electrons that bind the fast projectile molecule (as well as several core electrons) are stripped off within the first few angstroms of penetration ( = lo-'' s) into the foil. This stripping causes a mutual Coulomb repulsion between the bare (or nearly bare) nuclei of the projectile molecule. The typical ion-target forces are comparable to the molecular forces, yet the stripping time is orders of magnitude shorter than the typical vibrational s) and rotational (lo-'* s) time scales. Hence one can consider the nuclei as stationary during the electron stripping process. There- Israel Journal of Chemistryfore, it is reasonable to assume that the final trajectories of the dissociating fragments are determined solely by the geometrical configuration of the constituent nuclei at the instant the explosion begins. Subsequently, the trajectories of the fragmented nuclei are governed simply by the Coulomb force between them.3) The third and final stage is the measurement of all the fragments in a multiparticle position and timesensitive detector which yields the final velocities of all these ions. The final velocities for each molecule can be transformed by "Coulomb trajectory" calculations into the initial molecular structure (R-space), i.e., the configuration at the moment the molecule enters the stripping film. Small carbon clusters are well suited for CEI studies because of their low mass' and their efficient production in the accelerator ion source.In recent years, new information has been accumulated on carbon clusters in general and on C4 in particular. Yang et aL3 applied the laser sputtering source to study the photoelectron spectros...

Absolute cross sections for photodetachment of negative carbon clusters are reported for Cn (n = 3, ..., 8). The results indicate that various neutral isomers exist, some with electron affinities as low as 1 electron volt. The method of production plays an important role in the characteristics of carbon clusters.

Energy loss in the MeV range of simple clusters impinging on thin carbon targets has been measured using a time-of-flight method. Stopping-power ratios defined as the ratio of the stopping power of the cluster to the sum of the stopping powers of the constituent atoms moving at the same velocity were investigated. Stoppingpower ratios close to unity were observed for O 2 and B 3 clusters, while deenhancement effect is observed in the stopping-power ratios of C 3 and C 4. The experimental results are compared both with an existing theoretical model, which takes into account the spatial correlation of the fragments, and with a simple united-atom model, which also includes the charge state evolution of the fragment ions inside the target.

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