M. Todd Coolbaugh

Argon mediated electron impact ionization of Arn(CH3OH)m heteroclusters

In this paper we have contrasted electron impact cluster mass spectra of a methanol/argon expansion with a methanol/helium expansion and have measured appearance potentials for several of the prominent ions in each case. We have observed a number of ionic species in the argon expansion (i.e., CHxO+ where x=0–4) which are not observed in the helium expansion. Appearance potentials for these species fall in the region 11.3–11.8 eV indicating that the threshold ionization of Arn(CH3OH)m heteroclusters is mediated by the Ar 4s excited states (3P2,0;11.55 and 11.72 eV). This represents the first time that intracluster Penning ionization has been positively identified using electron impact measurements.

Observation of ‘‘magic numbers’’ in the population distributions of the (NH3)n−1NH+2 and (NH3)nH+2 cluster ions: Implications for cluster ion structures

Abstract : We present the relative yields of the cluster ions, (NH3)n-1NH2+ and (NH3)nH2+, produced by electron impact ionization of a supersonic molecular beam of neutral ammonia clusters, as functions of cluster ion size, neutral beam stagnation temperature, and electron impact energy. Our observation of a magic number at n = 7 in the distribution of (NH3)n-1NH2+ cluster ions is interpreted in terms of an intracluster bimolecular association reaction between a nascent NH2+ daughter ion and an adjacent NH3 solvent molecule, giving rise to a protonated hydrazine product ion. Our observation of magic numbers at n = 5 and n = 8 in the (NH3)NH2+ cluster ion distribution is consistent with the production of the N2H8+ Rydberg radical cation via an intracluster ion molecule reaction. The dependence of the yields of these solvated Rydberg radical cations on the stagnation temperature of the neutral molecular beam supports the notion that extensive solvation destabilizes these cluster ions. Amines, Hydrogen, Clusters.

Ion-molecule chemistry within doubly charged ammonia clusters

Abstract We present the observation of “intramolecular” ion-molecule reactions within doubly charged clusters produced via electron impact ionization of neat beams of ammonia molecules. The fragmentation pattern strongly suggests that reactions occurring within these doubly charged clusters are similar to those observed for singly charged ammonia clusters. This behavior implies that these cluster ions consist of two separate, singly charged ammonia cations which act separately within the bulk neutral cluster. That is, either one, both, or neither of the ammonia cations may react with the neutrals solvating them within the doubly charged cluster.

Observation of a magic number in the ion distribution of ethene clusters

Abstract We present in this Letter the observation of a magic number of n = 4 in the (C 2 H 4 ) n + cluster ion distribution only under experimental conditions of low nozzle temperature or high stagnation pressure. We attribute this result not to any special thermodynamic stability of the neutral or ionic cluster, and speculate that it is due to the formation of a C 8 H 16 + molecular ion. We suggest that this product cation is formed via a series of exoergic condensation reactions which occur within the parent ethene cluster ion. Also presented are pressure dependence data which are used to elucidate other competing reactive mechanisms.

Comment on: ‘‘Electron bombardment fragmentation of size selected NH3 clusters’’

New experimental results are presented utilizing ammonia clusters, which demonstrate that the differences in ion intensity reported in Ref.1, as a function of cluster size, are in fact attributed to beam conditions.(AIP)

Electron impact ionization efficiency curves of van der Waals clusters

The study of van der Waals clusters is an area of growing interest and is being widely studied for a number of reasons. The measurement of the ionization efficiency (IE) curves have yielded a wealth of information by enabling ionization and appearance energies of ions to be determined which are essential for the calculation of thermochemical data. In the case of van der Waals clusters, the measurement ofIE curves enables one to determine the qualitative trends in the ionization potentials as a function of cluster size. In additionIE curves have also offered valuable insight into ionization related processes occurring in clusters. This paper will cover some of the more recent studies of Penning ionization, exciton induced decay and Coulomb explosion in van der Waals clusters through the use of electron impactIE curves.