Martin F. Jarrold

An exact hard-spheres scattering model for the mobilities of polyatomic ions

Abstract We describe an exact hard-spheres scattering model for calculating the gas phase mobilities of polyatomic ions. Ion mobility measurements have recently been used to deduce structural information for clusters and biomolecules in the gas phase. In virtually all of the previous ion mobility studies, mobilities were evaluated for comparison with the experimental data using a projection approximation. Comparison of the collision integrals calculated using the exact hard-spheres scattering model with those estimated using the projection approximation shows that large deviations, over 20%, occur for some geometrics with grossly concave surfaces.

Ion Mobility Measurements and their Applications to Clusters and Biomolecules

Ion mobility measurements can be used to obtain structural information for large polyatomic ions in the gas phase. The methods are flexible and can be applied to a wide range of chemical systems. This article reviews the development of these methods and discusses recent applications to complex ions such as atomic clusters and large biomolecules.

Structures of medium-sized silicon clusters

Silicon is the most important semiconducting material in the microelectronics industry. If current miniaturization trends continue, minimum device features will soon approach the size of atomic clusters. In this size regime, the structure and properties of materials often differ dramatically from those of the bulk. An enormous effort has been devoted to determining the structures of free silicon clusters. Although progress has been made for Sin with n < 8, theoretical predictions for larger clusters are contradictory and none enjoy any compelling experimental support. Here we report geometries calculated for medium-sized silicon clusters using an unbiased global search with a genetic algorithm. Ion mobilities determined for these geometries by trajectory calculations are in excellent agreement with the values that we measure experimentally. The cluster geometries that we obtain do not correspond to fragments of the bulk. For n = 12–18 they are built on a structural motif consisting of a stack of Si9 tricapped trigonal prisms. For n ⩾ 19, our calculations predict that near-spherical cage structures become the most stable. The transition to these more spherical geometries occurs in the measured mobilities for slightly larger clusters than in the calculations, possibly because of entropic effects.

Nanosurface Chemistry on Size-Selected Silicon Clusters

Studies of the chemistry that occurs on the nanosurfaces of size-selected silicon clusters reveal a number of fascinating qualitative similarities to the behavior of bulk surfaces. However, silicon clusters containing up to 70 atoms appear to be much less reactive than bulk silicon surfaces. This unexpected result suggests that these large silicon clusters are not just small crystals of bulk silicon, but have much more compact geometric structures

HIGH-RESOLUTION ION MOBILITY MEASUREMENTS

Gas phase ion mobility measurements can resolve structural isomers for polyatomic ions and provide information about their geometries. A new experimental apparatus for performing high-resolution ion mobility measurements is described. The apparatus consists of a pulsed laser vaporization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the diffusion of neutral species from the source into the drift tube. Ions travel along the drift tube under the influence of a uniform electric field. At the end of the drift tube some of the ions exit through a small aperture. They are focused into a quadrupole mass spectrometer, where they are mass analyzed, and then detected by an off-axis collision dynode and by dual microchannel plates. The apparatus is operated with a drift voltage of up to 14 000 V and a helium buffer gas pressure of around 500 Torr. The resolving power for ion mobility measurements is over an order of magnitude higher than ...

Collision induced dissociation of metal cluster ions: Bare aluminum clusters, Al+n (n=3–26)

The collision‐induced dissociation of aluminum clusters, Al+n (n=3–26), by argon, at a center of mass collision energy of 5.25 eV, has been studied using a low energy ion beam apparatus. Product branching ratios and collision induced dissociation cross sections are presented and discussed. The main product is Al+ for the smaller clusters and Al+n−1 for the larger ones. The cross sections rise to a peak at Al+6−Al+9 and then decrease with increasing cluster size. Cross sections for Al+7, Al+13, Al+14, and Al+23 are significantly smaller than their neighbors. A crude kinetic model is used to derive approximate cluster ionization potentials from the product branching ratios. The IPs initially rise with cluster size, peak at Al6 and then decrease. The IP of Al7 is particularly low and there is a sharp drop in IP at Al14 where the IP falls below that of the atom. The results suggest that the dissociation energies increase for the larger clusters and there is evidence that Al+7, Al13, Al+13, Al+14, and Al+23 ha...

Annealing c60+: synthesis of fullerenes and large carbon rings.

Laser vaporization of graphite generates C60+ cluster ions that are fullerenes and a mixture of roughly planar polycyclic polyyne ring isomers. Experimental studies of the annealing of the non-fullerene C60+ ions indicate that they can be converted (in the gas phase) into the fullerene and an isomer that appears to be a large monocyclic ring. Some fragmentation is associated with conversion to the fullerene geometry, but the majority of the non-fullerene C60+ isomers are cleanly converted into an intact fullerene. The emergence of the monocyclic ring (as the clusters are annealed) suggests that this is a relatively stable non-spheroidal form of these all carbon molecules. The estimated activation energies for the observed structural interconversions are relatively low, suggesting that these processes may play an important role in the synthesis of spheroidal fullerenes.

Mobilities of silicon cluster ions: The reactivity of silicon sausages and spheres

The mobilities of size selected silicon cluster ions, Si+n (n=10–60), have been measured using injected ion drift tube techniques. Two families of isomers have been resolved by their different mobilities. From comparison of the measured mobilities with the predictions of a simple model, it appears that clusters larger than Si+10 follow a prolate growth sequence to give sausage‐shaped geometries. A more spherical isomer appears for clusters with n>23, and this isomer completely dominates for unannealed clusters with n>35. Annealing converts the sausage‐shaped isomer into the more spherical form for n>30. Activation energies for this ‘‘sausage‐to‐sphere’’ structural transition have been estimated for several cluster sizes and are ∼1.2–1.5 eV. We have examined the chemical reactivity of the sausages and spheres towards both C2H4 and O2. With C2H4 large differences in reactivity of the isomers were found, with the spherical isomer often being more reactive than the sausage form by more than an order of magnit...

Ionization of medium-sized silicon clusters and the geometries of the cations

We have performed a systematic ground state geometry search for the singly charged Sin cations in the medium-size range (n⩽20) using density functional theory in the local density approximation (LDA) and generalized gradient approximation (GGA). The structures resulting for n⩽18 generally follow the prolate “stacked Si9 tricapped trigonal prism” pattern recently established for the lowest energy geometries of neutral silicon clusters in this size range. However, the global minima of Sin and Sin+ for n=6, 8, 11, 12, and 13 differ significantly in their details. For Si19 and Si20 neutrals and cations, GGA renders the prolate stacks practically isoenergetic with the near-spherical structures that are global minima in LDA. The mobilities in He gas evaluated for all lowest energy Sin+ geometries using the trajectory method agree with the experiment, except for n=18 where the second lowest isomer fits the measurements. The effect of gradient corrections for either the neutral or cationic clusters is subtle, but...

High-resolution ion mobility measurements for silicon cluster anions and cations

High-resolution ion mobility measurements have been performed for silicon cluster anions and cations, Sin− and Sin+, n=6–55. New isomers have been resolved for every cluster size larger than Si20. The results for the anions and the cations have the same global features. However, changing the charge often causes a shift in the isomer distribution, or causes new isomers to emerge. For example, the transition from prolate geometries to more-spherical ones is shifted to larger cluster sizes for the anions than for the cations. The mobilities of the anions are systematically smaller than those of the cations, presumably because of differences in the exterior electron densities.