Scott L. Anderson

Collision‐induced dissociation of aluminum cluster ions: Fragmentation patterns, bond energies, and structures for Al+2–Al+7

Collision‐induced dissociation (CID) of cooled, mass selected aluminum cluster ions (Al+2–7) by xenon, has been studied over an energy range of 0–10 eV (center of mass). These experiments were carried out in a new apparatus which is described in detail. From the product branching ratios and cross section magnitudes we derive qualitative structural information about the cluster ions. The fragmentation thresholds are analyzed to yield dissociation energies, approximate ionization potentials, and further structural information about the cluster ions and their neutral counterparts. Cluster stabilities range from 0.85±0.40 eV for Al+4 to 2.25±0.70 eV for Al+7. The results provide a stringent test for recent calculations on Al2–6.

Interaction of boron cluster ions with water: Single collision dynamics and sequential etching

Reactions of mass‐selected, cooled, boron cluster ions (B+n, n=1–14) with water have been studied for collision energies from 0.1 to 6.0 eV. Most work was done with D2O, however isotope effects were examined for selected reactant cluster ions. For all size clusters there are exoergic product channels, which in most cases have no activation barriers. Cross sections are generally large, however there are fluctuations with cluster size in total reactivity, collision energy dependences, and in product distributions. For small cluster ions, there is a multitude of product channels. For clusters larger than B+6, the product distributions are dominated by a single channel: Bn−1D++DBO. Under multiple collision conditions, the primary products undergo a remarkable sequence of secondary ‘‘etching’’ reactions. As these occur, boron atoms are continuously replaced by hydrogen, and the intermediate products retain the composition: Bn−mH+m. This highly efficient chemistry appears to continue unchanged as the compositio...

Ne++C60: Collision energy and impact parameter dependence for endohedral complex formation, fragmentation, and charge transfer

We report measurements of C60−2nNe+ and C+60−2n formation in collisions of Ne+ with C60 over a range from 2–212 eV (CM). Activation energies and crude angular information are given for each channel. Endohedral complex formation and fragmentation both require low impact parameter collisions, while charge transfer can occur with larger impact parameters as well.

Vibrational mode effects, scattering dynamics, and energy disposal in reaction of C2H+2 with methane

The effects of collision energy and mode‐selective vibrational excitation on the reaction of C2H+2 with CH4 and CD4 have been measured, along with the corresponding product velocity distributions. Two distinct reaction mechanisms are active in the energy range below 5 eV. At low energies, a long‐lived C3H+6 complex forms efficiently, then decomposes primarily to C3H+5+H and C3H+4+H2. The RRKM lifetime of this complex is estimated to range between ∼10 ns and ∼10 ps over the experimental energy range, and this is sufficient time to allow substantial H‐atom scrambling. Complex formation is strongly inhibited by collision energy, weakly inhibited by CC stretching, and enhanced by bending excitation. Competing with the complex‐mediated mechanism is a direct H‐atom abstraction reaction, producing C2H+3+CH3 with little atom scrambling. This reaction is shown to have a ∼150 meV activation barrier and is strongly enhanced by collision energy, becoming the dominant channel above 0.4 eV. CC stretching provides a wea...

Collision of alkali ions with C60/C70: Insertion, thermionic emission, and fragmentation

Interactions of Li+, Na+, and K+ with C60 molecules have been studied over the collision energy range from 0 to 150 eV. For Li+ and Na+, insertion of the alkali to form the endohedral [Li@C60]+ and [Na@C60]+ species is observed, with appearance energies of ∼6 and ∼18 eV, respectively. At higher collision energies several processes are observed. For Na+, the hot [Na@C60]+ appears to relax mainly by loss of C2 units from the fullerene cage, yielding [Na@C60−2n]+. For [Li@C60]+, escape of Li+ dominates over C2 loss. For K+, very little [K@C60]+ is observed, and the main endohedral species observed are fragments [K@C60−2n]+ (n=1–7), which begin to appear at ∼40 eV. Pure fullerene fragment ions (C60−2n+) are also observed, with cross sections that increase sharply at ∼30 eV for all three projectile ions. These ions are believed to result primarily from thermionic emission of collisionally excited neutrals. For K+, we also examined collisions with C70, obtaining results similar to those for C60. Changes in the ...

Oxidation of small boron cluster ions (B+1–13) by oxygen

Absolute cross sections for all ionic products formed in reactions of B+1–13 with oxygen have been measured under single collision conditions, at collision energies from 0.25 to 10 eV. Three main reaction mechanisms appear to be important: oxidative fragmentation, collision induced dissociation, and boron atom abstraction. The dominant oxidation process are exoergic for all cluster sizes, but appear to have bottlenecks or activation barriers for the larger cluster ions. Clusters smaller than B+6 have similar chemistry, then there is a sharp transition in chemistry for clusters larger than B+6 . Correlations are explored between cluster reactivity and cluster stability, and the oxidation chemistry is compared to the similar results found for aluminum cluster ion oxidation.

Fragmentation of nitrous oxide by monochromatic soft x rays

Studies of the fragmentation of nitrous oxide using time‐of‐flight mass spectroscopy following excitation by monochromatic synchrotron radiation are reported. The consequences of core electron excitation of all three atoms to the lowest unoccupied molecular orbital are described. Clear changes in the mass spectra with the atomic site of excitation are observed. These changes can be understood qualitatively in terms of the atomic populations, which govern the Auger decay channels, and the overlap populations, which determine the bond structure, of the valence molecular orbitals. Mass spectra correlated with Auger electrons are very similar to mass spectra correlated with events producing low energy electrons. This result may indicate that the selectivity of the primary Auger decay process is not affected by additional excitation events.

MPI photoelectron spectroscopy of ungerade excited states of acetylene: Intermediate state mixing and ion state selection

Three photon resonant, four photon (3+1) ionization spectroscopy and photoelectron spectroscopy have been used to study the ungerade excited states of acetylene in the energy range from 74 500 to 90 000 cm−1. Sharp bands from the nR (π3u nsσg) and 1Φu (π3u ndδg) Rydberg series dominate the MPI spectrum. A large number of Rydberg and valence states which are prominent in VUV absorption spectra are absent or weak in MPI studies. These weak bands are only observable under high power conditions, which suggests that nonradiative decay is rapid enough to depopulate these states before ionization occurs. The photoelectron results provide further insight into the nature of the excited states. Ionization through the sharp bands occurs via Δν=0 Franck–Condon transitions, resulting in ions in a single vibrational state. Ionization through bands which are mixed results in complicated ion vibrational distributions including excitation of both cis and trans bends.

Multiphoton ionization photoelectron spectroscopy of phenol: Vibrational frequencies and harmonic force field for the 2B1 cation

A molecular beam of phenol, cooled by a supersonic expansion, is crossed at right angles by the output of a pulsed frequency‐doubled dye laser, causing 1+1 resonance enhanced multiphoton ionization. The kinetic energy of the resulting photoelectrons is determined as a function of laser wavelength with time‐of‐flight analysis, permitting the assignment of 11 vibrational frequencies for the 2B1 phenol‐h6 cation and ten vibrational frequencies for phenol‐d5. Of these, all but the lowest frequency one in each case are in‐plane vibrations of which phenol has a total of 19. An approximate harmonic force field for the in‐plane modes of the phenol cation is derived along with its associated frequencies and mode forms. This in turn facilitates the vibrational analysis. Analogous force field calculations have been carried out on the ground (1A1) and first excited (1B2) states of the neutral parent, permitting conclusions to be reached concerning bonding changes upon removal of an electron from the phenol electron s...

Unimolecular dissociation rates of the chlorobenzene cation prepared by multiphoton ionization

We have studied the unimolecular reaction C6H5Cl+→C6H+5 +Cl as a function of internal energy by using multiphoton ionization (MPI) on a supersonically cooled jet of chlorobenzene to prepare the excited parent ion. Specific rate constants k(E) were obtained from an analysis of the distorted C6H+5 peak shape in a time‐of‐flight (TOF) mass spectrometer. Decomposition in the wavelength region 265–270 nm is shown to proceed by two‐photon ionization followed by one‐photon absorption. The internal energy of the excited parent ion is characterized in a separate experiment which measures the photoelectron kinetic energy distribution, obtained over the same wavelength range. The specific rate constants found in this study compare well with previous literature values, thus validating this MPI TOF technique for the determination of ion decomposition rates.