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We present the first scanning tunneling microscopy (STM) study of the deposition of mass-selected silver clusters (Agn,n=1, 2, 3) on a rutile TiO2(110)-1×1 surface at room temperature under hard-landing conditions. Under hard-landing conditions, only small features are observed on the surface in all cases without sintering or surface damage. This suggests that the high impact energy of the clusters mainly dissipates as thermal energy in the substrate, resulting in the recovery of any initial impact-induced surface damage and the formation of bound clusters on the surface near the impact point. STM images indicate that Ag1 binds on the bridging oxygen rows twice as often as on the Ti rows. Density-functional Theory (DFT) calculations are consistent with Ag1 binding at either bridging oxygen vacancies or with two adjacent bridging oxygen atoms in the same bridging oxygen row. STM images of Ag2 and Ag3 depositions indicate almost exclusive binding centered on the Ti-atom rows. DFT calculations suggest that the Ag2 and Ag3 clusters are bound between two bridging oxygen rows, which is consistent with the STM observations. [ABSTRACT FROM AUTHOR]

Mass-selected Agn+ (n=1,2,3) clusters with impact energy less than 2 eV per atom were deposited from the gas phase onto rutile titania (110)-(1×1) single crystal surfaces at room temperature and imaged using ultra-high vacuum scanning tunneling microscopy. Upon reaching the surface, Ag monomers sintered to form three-dimensional islands of approximately 50 atoms in size, with an average measured height of 7.5 Å and diameter of 42 Å. This suggests that the monomers are highly mobile on the titania surface at room temperature. Dimers also sintered to form large clusters upon deposition, approximately 30 atoms in size, with an average height of 6.2 Å and diameter of 33 Å. Clusters formed from monomer deposition appeared approximately three times more frequently at step edges than clusters formed from dimer deposition, indicating that the surface mobility of deposited monomers is higher than that of deposited dimers. In sharp contrast to the deposition of monomers and dimers, the deposition of trimers resulted in a high density of very small clusters on the order of a few atoms in size, indicative of intact trimers on the surface, implying that deposited trimers have very limited mobility on the surface at room temperature. [ABSTRACT FROM AUTHOR]

A mass selected ion beam of ArN+2 clusters is brought to a spatial focus and crossed with the polarized output of an argon ion laser. Photofragment ions are mass and energy analyzed using an electrostatic analyzer and detected using single ion counting methods. Photoproducts observed over the photon energy range of 2.1 to 3.5 eV are Ar+/N2 and N+2/Ar with the former favored by about a factor of 3. Analysis of the data indicate the upper state is purely repulsive leading to strongly translationally and vibrationally excited products. The absolute cross section has an onset at about 600 nm and smoothly increases to 357 nm. In order to reasonably interpret the data it is suggested the higher energy Ar+(2P3/2)/N2 asymptote diabatically correlates to the ground state of ArN+2 and the lower energy N+2(X 2∑)/Ar asymptote diabatically correlates to the repulsive state accessed by the photon. Detailed dynamics in the region where the curves cross are responsible for the observed product distribution. Application of an impulsive model indicates the ground state of ArN+2 is linear. Finally, equilibrium measurements of the reaction (N2)+2+Ar↔ArN+2+N2 indicate ΔH00=-1.0±0.3 kcal/mol and ΔS0=1.5±0.5 cal k-1 mol-1 in agreement with the result of Teng and Conway which had been disputed in the literature. [ABSTRACT FROM AUTHOR]

The formalism is presented for incorporating the charge–permanent dipole interaction into the statistical phase space theory model (as developed by Chesnavich and Bowers for polyatomic ion–nonpolar molecule systems) of kinetic energy release distributions. The theory is applied to the dissociation of H2O·H3O+ clusters for which the measured kinetic energy release distribution is in the range of a few meV. Excellent agreement between theory and experiment is obtained when the ion–dipole interaction is treated according to ADO theory. It is suggested that the success of the ADO treatment in modeling experimental results in this energy regime is an indirect indication that ADO collision rates may be accurate down to the very low temperatures of interstellar clouds (e.g., 3/2 kT at 10 K is roughly 1.3 meV). This point is discussed with regard to recent work which has led to the opposite conclusion. [ABSTRACT FROM AUTHOR]

A specially designed coaxial drift tube type ion source has been utilized to generate ionic hydrogen clusters. The cluster spectrum observed is dominated by odd clusters (H+5, H+7, H+9, and H+11 ) in line with past studies on hydrogen ion clusters. However, for the first time even hydrogen cluster ions are formed in a high pressure ion source, with relative intensities (H+6>H+8>H+10>H+4) . The observation of a relatively intense H+6 peak (H+6/H+5 =0.05) was unexpected. The peak at nominal mass 6 was confirmed to be H+6 (and not H4D+) by high resolution studies. A number of possible formation mechanisms for H+6 are discussed. The most likely mechanism is reaction of odd hydrogen cluster ions (H+3, H+5, H+7,... ) with metastable electronically excited odd hydrogen cluster neutrals (H[ATOTHER]@B|[/ATOTHER]3, H*5, H[ATOTHER]@B|[/ATOTHER]7,...) . The H*3 species has been experimentally characterized by other research groups (Herzberg; Kuppermann) and is probably formed by electron recombination with H+3 (v≠0), H+5 or H+7 in our source. The H+6 ion is observed to lose both H atom and H2 by metastable decomposition in a ratio of H/H2[bar_over_tilde:_approx._equal_to]4. The kinetic energy distribution for H loss is broad and bimodal with an average release of 38 meV and a maximum release of 140 meV. Collision induced decomposition studies are utilized to characterize both the even and odd clusters. A 50/50 mixture of H2 and D2 yielded a large number of isotopically mixed clusters that were identified by high resolution mass spectrometry. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of (SO2)-2 to form SO2/SO-2 products is investigated over the wavelength range 656 (1.89 eV) to 458 nm (2.71 eV). Product angular distributions are obtained. An asymmetry parameter analysis indicates the lifetime of the (SO2)-2 photoexcited state is much less than a rotational period. Product kinetic energy distributions are obtained at all wavelengths. Both the overall shape of these distributions and comparison with statistical phase space theory calculations indicate the excited state assessed by the photon is repulsive consistent with the asymmetry parameter analysis. An impulsive model analysis suggests the bond between the two SO2 moeities in (SO2)-2 is probably between one oxygen atom on each moeity and the structure is quasilinear. Structure is also observed in the kinetic energy distributions. It is suggested this is due to selective photoexcitation of vibrational states of either the SO2 or SO-2 moeity in (SO2)-2. Hodges and Vanderhoff have reported a bimodel photodestruction cross section for (SO2)-2, with major peaks near 600 and 400 nm. We argue these peaks are due to photodissociation of (SO2)-2 not photodetachment, with the first excited doublet state (repulsive) leading to the 600 nm peak and the second excited doublet state (bound) leading to the second maxima. The repulsive state must correlate to ground state SO2/SO-2 products and the bound state to electronically excited SO2/SO-2 products. Predictions regarding the dissociation dynamics of the bound state are made. [ABSTRACT FROM AUTHOR]

Reduced zero field mobilities of Cr+, Co+, and Ni+ in their ground state configuration (3dn) and their metastable excited state configuration (3dn-14s1) have been measured in He and Ne as a function of temperature. Parameters for a n-6-4 interaction potential are obtained by fitting the mobility vs temperature curve. Bond dissociation energies, vibrational frequencies, and equilibrium bond lengths are extracted from these curves and compared with data from traditional equilibrium measurements and ab initio theory. Dramatic changes in bonding are observed for the 3dn and 3dn-14s1 configurations; the 3dn configuration yielding very much stronger bonds and very much shorter equilibrium bond lengths than the 3dn-14s1 configuration. The presence of multiple potential curves in the interaction of Co+(3F,3d8) and Ni+(2D,3d9) with He/Ne make interpretation of the ground state mobility data ambiguous. The analysis of each system is discussed in some detail. [ABSTRACT FROM AUTHOR]

A new product channel, O-·H2O+CO2 was discovered in the photodissociation of CO-3·H2O. Experiments were conducted by crossing a mass selected 8 kV ion beam with a linearly polarized laser beam and measuring the kinetic energy distributions of the charged photodissociation products. Distributions were collected at photon energies of 2.41, 2.54, and 2.71 eV. By varying the angle between the ion and the laser polarization vector, product angular distributions were obtained. Relative photodissociation cross sections of this process were determined by comparisons with known processes: a) CO-3·H2O+hν→CO-3+H2O and b) CO-3+hν→O-+CO2. Ab initio calculations focusing on geometric structure and vibrational frequencies of the ground state of O-·H2O were performed using GAUSSIAN 82 and GAUSSIAN 88 programs. With both MP2/6-31G** and MP2/6-31+G* basis sets, the lowest energy structure obtained was quasilinear with an O-···H–O angle of 3°. Vibrational frequencies and rotational constants obtained from these calculations were employed in statistical phase space modeling. Phase space theory indicated a photoinduced ‘‘intracluster’’ reaction mechanism was responsible for this novel product channel. [ABSTRACT FROM AUTHOR]

The photodissociation process CO-3 +hν→O-+CO2 has been investigated at photon energies of 2.41, 2.50, 2.54, 2.60, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 keV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and laser polarization, angular distributions were obtained at photon energies of 2.41 and 2.54 eV. The photon energy dependence of the average photofragment kinetic energies shows conclusively that photodissociation at these photon energies does not proceed by a direct dissociation process on a repulsive potential surface, or by a statistical vibrational predissociation process on a bound surface. The photofragment angular distributions are isotropic, providing further evidence that precludes direct photodissociation on a repulsive potential surface. Ab initio calculations were performed using the gaussian86 programs. These calculations indicate that ground state CO-3 has a planar D3h geometry, and 2A’2 electronic symmetry. This ground state correlates adiabatically to the CO-2 +O dissociation asymptote, not the lower energy O-+CO2 asymptote. Taken together, these new experimental and theoretical results suggest that the photodissociation of CO-3 at these energies occurs via the interaction of bound and repulsive excited state potential surfaces. A new model of the potential surfaces of CO-3 is proposed. [ABSTRACT FROM AUTHOR]

The spin-forbidden radiative decays of the lowest energy metastable electronic states of O+2 and NO+ have been studied using a Fourier transform ion cyclotron resonance mass spectrometer. The metastable species are stored in the mass spectrometer for several hundreds of milliseconds and then selectively detected by pulsing CO2 into the ion trap and monitoring the formation of CO+2. By varying the delay time between formation of the metastable ions and injection of CO2, apparent radiative lifetimes of 101 ms for O+2(a 4Πu) and 530 ms for NO+(a 3Σ+) are measured. Our results are compared with literature values and with a theoretical calculation. The theoretical model, demonstrated for O+2, is based on spin–orbit coupling between the a 4Πu and A2Πu states and upon spin–rotation mixing of the different spin components within the a 4Πu state. The results allow quantitative interpretation of our results and those available in the literature. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CO-3·H2O and CO-3·CO2 have been investigated at photon energies of 2.13, 2.41, 2.54, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 kV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and the laser polarization vector, product angular distributions were obtained. The only ionic product observed from both systems was CO-3. The average energy partitioned into relative translation of the photofragments was determined to be ∼0.1 eV for CO-3·H2O and ∼0.07 eV for CO-3·CO2. In both cases, these kinetic energy release values were found to be nearly independent of photon energy. The small fraction of the available energy partitioned into kinetic energy of the photofragments indicates that the upper states of the transitions leading to photodissociation are bound, and that a substantial fraction of the available energy must be channeled into internal energy of the dissociating fragments. The angular distributions of CO-3 photoproducts from both CO-3·H2O and CO-3·CO2 were found to be extremely isotropic. Modeling the experimental data using statistical phase space theory shows that dissociation occurs prior to completeenergy randomization, and provides a measure of the extent of energy randomization prior to dissociation. Comparison of theory and experiment indicates the photodissociation processes proceed by the following mechanism: (1) Photon absorption occurs via a transition localized on CO-3 moiety: CO-3 (2B1)·X+hν→CO-3 (2A1)·X, where X=CO2 or H2O. (2) CO-3(2A1)·X internally converts to CO-3(2B1)·X, with a high degree of vibrational excitation being localized on the CO-3 moiety. (3) The vibrational excitation localized on the CO-3 moiety begins to slowly randomize throughout the cluster... [ABSTRACT FROM AUTHOR]

A new method for studying the generation and structure of carbon cluster ions is described. A rotating, translating carbon rod is subjected to the focused output of an excimer laser operating at 308 nm. The resulting plume is directly sampled by a high resolution reverse geometry mass spectrometer operating between 2 and 8 kV. In this paper we report initial results for the positive cluster ions up to n=60. The mass spectra are bimodal with a minimum occurring between C+26 and C+31 in agreement with findings of other workers using related techniques. Metastable neutral loss is observed for all cluster ions with n≥5. Below C+30 the most intense neutral loss is C3 but intense C1 and C5 loss is observed for a significant number of systems and C10 and C14 for a few systems. Above C+30 the metastable loss spectra dramatically change and only C2, and a minor amount of C4 loss, is observed. The C+60 ion is slightly enhanced in the mass spectrum but its metastable reactivity is no different than any of the other C+n ions above n=30. The detailed results are interpreted in terms of stable structures of both the parent ions and neutral fragments that have been predicted theoretically. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of the CO2·O+2 cluster are studied in a crossed ion beam–laser beam apparatus from λ=590 nm to λ=357 nm. At all wavelengths only CO+2/O2 products are observed even though O+2/CO2 products are lower in energy by 1.71 eV. The absolute cross section for photodestruction of CO2·O+2 is measured and increases smoothly from less than 1×10-18 cm2 at 590 nm to approximately 7×10-18 cm2 at 357 nm. In the range 590–458 nm ground electronic state CO+2(X 2Π)/O2(X 3∑) products are formed. Arguments are made that indicate that the O2 product is selectively vibrationally excited to the highest level energetically allowed (ν=0,1, or 2 depending on the wavelength). Asymmetry parameter analysis indicates that the photoexcited state of CO2·O+2 accessed is repulsive and the cluster dissociates in times short compared to a rotational period. At 357 nm the mechanism changes. Arguments are made that unambiguously indicate the O2 product is electronically excited at this wavelength and the product states are CO+2(X 2Π)/O2(a 1Δ). In addition, 10%–25% of the photoexcited clusters at 357 nm may initially be formed in a bound state that subsequently vibrationally predissociates. The large majority of the products at this wavelength are formed by direct dissociation from a repulsive state, however, similar to the longer wavelength data. [ABSTRACT FROM AUTHOR]

A photodissociation study of SO2 · O+2 is presented. The experiments were carried out on mass selected ion beams that were crossed with a polarized laser beam and then photoproducts were mass and energy analyzed. The SO2 · O+2 ions were formed by three body association reactions in a pressure and temperature dependent ion source. Studies were carried out at wavelengths of 582, 514, 488, 458, and 357±7 nm using an argon ion laser/dye laser system. Both O+2/SO2 and SO+2/O2 photoproducts were observed and the branching ratio measured as a function of λ. In addition, product kinetic energy distributions and angular distributions (asymmetry parameters) were measured and statistical phase space theory calculations were carried out. The results indicate the O+2/SO2 products are formed from photon absorption to a bound excited state at all wavelengths followed by internal conversion to the ground state and statistical vibrational predissociation. The SO+2/O2 products are formed by two mechanisms. At long wavelengths (582 nm) the products are formed exclusively by photon absorption to a bound state followed by internal conversion and statistical predissociation from the ground state. At short wavelengths (357 nm) direct dissociation from a repulsive upper state dominates. Both mechanisms are involved at intermediate wavelengths. Arguments are made that both the bound and repulsive upper states correlate to SO+2(X 1A1)/O2(b 1∑+g) products and that the bound upper state corresponds to the low energy band in the photodissociation cross section measurements of Hodges and Vanderhoff and the repulsive state to the high energy band. [ABSTRACT FROM AUTHOR]

The results of a study of the visible wavelength photodissociation dynamics of the (CO2)+3 cluster ion are presented. The experiments were performed using a crossed laser–ion beam apparatus. The photodissociation spectrum, product branching ratios, product angular distributions, and product energy distributions are reported. The main product from visible wavelength photodissociation of (CO2)+3 is CO+2. The results suggest photodissociation of (CO2)+3 is a two-step process: photon absorption by a (CO2)+2 chromophore, instantaneous ejection of a CO2 monomer unit derived from the chromophore, and formation of a transient dimer species; this is followed by dissociation of the transient dimer species which may survive for several rotational periods. [ABSTRACT FROM AUTHOR]

CHO+ ions are made by electron impact on CD3OH in the source (ICR1) of a tandem ion cyclotron resonance spectrometer. These ions are injected into a differentially pumped analysis cell (ICR2) where they are reacted with a number of small molecules. The internal energy distribution in the CHO+ ions is obtained using total reactivity studies with neutral molecules of varying proton affinities. About 40% of the CHO+ ions react with D2 either by proton transfer to form D2H+ or isotopic exchange to form CDO+ ions. A series of experiments are performed that conclusively show these ions are the HOC+ isomer and the exchange is due to the catalytic isomerization reaction HOC++D2→DCO++HD which is approximately 37 kcal/mol exothermic. The product DCO+ ions are vibrationally cool indicating the reaction releases most of its energy as kinetic energy. Absolute rate constants for reactions of CHO+ ions with the neutrals 13CO, 15N2, CO2, O2, D2, and Ar are reported. HOC+ reacts with D2 at about 30% of the collision rate. This rate decreases with increasing HOC+ kinetic energy in the range 0.025 to 0.44 eV. The two products of the reaction are D2H+/CO and DCO+/HD with DCO+/HD comprising ∼42% of the products for the HOC+ ions formed from CD3OH. Phase space theory calculations are performed to determine the barrier to catalytic isomerization. The data are best fit with a barrier between 0.0 and -0.05 eV relative to the HOC+/D2 asymptotic energy. A barrier of this magnitude yields reaction rate constants at interstellar temperatures of at least 10% of the collision rate constant (i.e., k>1×10-10 cm3/s), and could explain why so little HOC+ is observed in interstellar clouds. [ABSTRACT FROM AUTHOR]

The rate constants of the charge exchange reactions, CO2++ O2→O2++CO2 and O2++O2→O2+O2+ have been studied as a function of both the internal and kinetic energies of the reactant ion in a tandem ICR spectrometer. Primary ions with known internal energies are formed in the source of the tandem by charge transfer reactions whose energy partitioning has already been determined. The rate constant of the CO2++O2 reaction is found to be 4.4×10-11 cm3 s-1 for ground state primary ions and increases by a factor of 2.9 when CO2+ ions have 1.4 eV of internal energy; this reaction is also found to be much less sensitive to the kinetic energy of the parent ion than to its internal energy. The rate constant for the symmetric charge exchange reaction O2++O2 has been determined for internal energies corresponding approximately to the v=0, 3, and 9 vibrational levels of the O2+ ground electronic state and is found to increase with internal energy, at least at low collision energy. [ABSTRACT FROM AUTHOR]

The results of an investigation of the photodissociation of the SO2·SO+2 cluster ion in the visible wavelength range (458–590 nm) are presented. The studies were performed using a crossed fast ion beam/laser beam experiment. Product relative kinetic energy distributions and product center of mass angular distributions are reported. Methods used to extract the angular distributions from the experimental measurements are described. The only ionic product observed from the photodissociation of SO2·SO+2 was SO+2. The product relative kinetic energy distributions are characteristic of statistical energy disposal. However, statistical phase space theory substantially overestimates the fraction of available energy partitioned into relative kinetic energy. The product angular distributions are discussed in terms of the asymmetry parameter β. β increases with the products relative velocity. A simple phase space theory model for β is developed for statistical dissociation with complete rotational averaging. The model accounts for the variation of β with the products relative velocity. β also varies with wavelength. The variation of β with wavelength is ascribed to changes in the lifetime of the cluster. At 458 nm the values of β indicate that complete rotational averaging has not occurred. At 590 nm the data are consistent with complete rotational averaging, implying a lifetime>5 average rotational periods. The results are interpreted in terms of a transition to a low lying bound excited state of the SO2·SO+2 cluster which arises from one of the low lying excited states of SO+2 and the ground state of SO2. [ABSTRACT FROM AUTHOR]

Studies the formation of Ti[sup +](H[sub 2])[sub n] cluster ions by temperature-dependent equilibrium measurements and density functional theory. Equilibrium experiment; Potential sources of errors in the experiment; Discussion of the experimental and theoretical results; Quantitative effects of M[sup +] electronic occupation.

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29≤n≤45 indicating three or four different structures. Fullerenes first appear at C+30 and begin to dominate above C+45. From small to large the isomeric progression is from linear to rings to fullerenes. [ABSTRACT FROM AUTHOR]

Comments on a paper on entropy bottlenecks in ion-molecule reactions. Information on the transition switching model.