William R. Peifer

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)

Unusual photofragmentation dynamics in the multiphoton ionization of Cr(CO)6/methanol van der Waals heteroclusters

Mixed van der Waals clusters containing Cr(CO)6 and methanol are generated in the free‐jet expansion of a pulsed beam of seeded helium and subjected to 248 nm multiphoton ionization (MPI) at moderate laser fluence, and the product ions are analyzed by time‐of‐flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of solvated Cr(CO)6 are strikingly different from those of the naked molecule. Two principal sequences of heterocluster ions are identified in the mass spectrum. A major sequence with the empirical formula SnCr(CO)+x (x=0,1,2), where S is a methanol molecule, first appears in the mass spectrum at n+x=6. A minor sequence with the empirical formula SnCr(CO)+x (x=5,6), first appears in the mass spectrum at n+x=7.We discuss two possible dynamical schemes for MPI of Cr(CO)6 /methanol heteroclusters: One scheme, in which initial photoionization of neutral Cr(CO)6 ‐containing clusters gives rise to solvated Cr(CO)+6 primary photoions, which subsequently photodissocia...

Doubly charged clusters of 1,1‐difluoroethylene: Observation of an anomalous critical size

We present in this Communication the first observation of a critical size (n2) of 9 for doubly charged clusters of 1,1‐difluoroethylene. This is a highly unusual result in that it is the lowest critical size measured so far (the previous record was n2=21 for SO2). In addition, this is the first example of a doubly charged cluster where its critical size is not predicted by the Echt scaling law (that the critical size of the doubly charged cluster is proportional to the inverse of the product of Tc and v1/3, where Tc is the critical temperature and v is the molecular volume). This suggests that the doubly charged CH2CF2 clusters are packed in a nonspherical arrangement (i.e., chains, rings, or stacking of the ethylene planes).

Multiphoton ionization of group VIB hexacarbonyl Van Der Waals clusters: trends in intracluster photochemistry

Abstract Van der Waals clusters of M(CO)2 (M = Cr, Mo, W) generated in the free-jet expansion of a pulsed beam of seeded helium are subjected to multiphoton ionization (MPI) and the product ions analyzed by quadrupole mass spectrometry. We previously reported the observation of efficient production of MoO+ and MoO2+ following of MPI of Mo(CO)6 van der Waals clusters, and proposed that these ions arise through novel reactions between a neutral photoproduced metal atom and the ligands of an adjacent metal carbonyl “solvent” molecule within the cluster. In order to test some of the predictions of this model, we have now examined the MPI of van der Waals clusters of the other Group VIB hexacarbonyls. We find the same novel behavior (viz. efficient production of metal oxide ions) in the W(CO)6 system as that previously observed for Mo)CO)6. However, we find no evidence of such behavior in the Cr(CO)6 system. Based on these observations, we suggest that the reactivity of first-row transition metal atoms may be fundamentally different from that of second- or third-row metals. These differences are discussed in terms of the occupancy and relative size of the metal d orbitals.

Chemistry within Van Der Waals Clusters of Unsaturated Molecules: Observation of Cationic Polymerization

We present in this paper the observation of intracluster ion-molecule polymerization reactions within clusters containing molecules which possess double or triple C-C bonds. This effect is borne out by the observation of unusual magic numbers in the cluster ion distribution only under expansion conditions which favor the production of large neutral clusters. These magic numbers are rationalized in terms of the productio of covalently bonded cyclic molecular ions which terminate the cationic polymerization process.

Chemistry Within Molecular Clusters

Abstract : A review of our gas phase cluster work with potential application to the generation of new materials (i.e, cluster assembled thin films).