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Absolute kinetics for reactions of C2H2 with a series of ∼60 individual carbon nanoparticles (NPs) from graphite, graphene, graphene oxide, carbon black, diamond, and nano-onion feedstocks were measured for temperatures (TNP) ranging from 1200 to 1700 K. All the NPs were observed to gain mass by carbon addition under conditions that varied with feedstock but with large variations in initial growth rate. Long reaction periods were studied to allow the evolution of growth rates over time to be observed. Diamond NPs were found to passivate against C2H2 addition if heated above ∼1400 K, and the highly variable initial reactivity for carbon nano-onions was found to depend on the presence of non-onion-structure surface carbon. For graphitic and carbon black NPs, three distinct growth modes were observed, correlated with the initial NP mass (Minitial). Smallest graphitic and carbon black NPs, with masses <∼25 MDa, initially grew rapidly but also passivated quickly after adding <4% of Minitial. NPs in the 20–50 MDa range also passivated but only after multiple waves of fast growth separated by periods of low reactivity, with up to ∼11% total mass gain before passivation. The largest carbon black and graphitic NPs, with Minitial >50 MDa, grew rapidly and continuously, adding up to ∼300% of Minitial with no sign of rate slowing as long as C2H2 was present. The efficiencies for C2H2 addition and etching by O2 are strongly correlated, but the correlation changes as the NPs passivate. Growth and passivation mechanisms are discussed. [ABSTRACT FROM AUTHOR]

An atomic layer deposition process is used to modify size-selected Pt7/alumina model catalysts by Sn addition, both before and after Pt7 cluster deposition. Surface science methods are used to probe the effects of Sn-modification on the electronic properties, reactivity, and morphology of the clusters. Sn addition, either before or after cluster deposition, is found to strongly affect the binding properties of a model alkene, ethylene, changing the number and type of binding sites, and suppressing decomposition leading to carbon deposition and poisoning of the catalyst. Density functional theory on a model system, Pt4Sn3/alumina, shows that the Sn and Pt atoms are mixed, forming alloy clusters with substantial electron transfer from Sn to Pt. The presence of Sn also makes all the thermally accessible structures closed shell, such that ethylene binds only by π-bonding to a single Pt atom. The Sn-modified catalysts are quite stable in repeated ethylene temperature programmed reaction experiments, suggesting that the presence of Sn also reduces the tendency of the sub-nano-clusters to undergo thermal sintering. [ABSTRACT FROM AUTHOR]

The nature of CO adsorption on Pdn/TiO2(110) (n = 1, 2, 7, 20) has been examined using temperature-programmed desorption (TPD), temperature-dependent helium ion scattering (TD-ISS), and X-ray photoelectron spectroscopy (XPS). All samples contain the same number of Pd atoms (0.10 ML-equivalent) deposited as different size clusters. The TPD and TD-ISS show that CO binds in two types of sites associated with the Pd clusters. The most stable sites are on top of the Pd clusters ('on-top' sites), however, there are also less stable sites, in which CO is bound in association with, but not on top of the Pd ('peripheral' sites). For saturation CO coverage over a fixed atomic concentration of Pd (present in the form of Pdn clusters of varying size), the population of CO in peripheral sites decreases with increasing cluster size, while the on-top site population is size-independent. This is consistent with what geometric considerations would predict for the density of the two types of sites, provided the clusters adsorb predominantly as 2D islands, which ISS results suggest to be the case. The XPS analysis indicates that CO-Pd binding is dominated by π-backbonding to the Pdn clusters. The results also show evidence for efficient support-mediated adsorption (reverse-spillover) of CO initially impinging on TiO2 to binding sites associated with the Pd clusters. [ABSTRACT FROM AUTHOR]

Reactions of HOD+ with N2 have been studied for HOD+ in its ground state and with one quantum of excitation in each of its vibrational modes: (001)-predominately OH stretch, 0.396 eV, (010)-bend, 0.153 eV, and (100)-predominately OD stretch, 0.293 eV. Integral cross sections and product recoil velocities were recorded for collision energies from threshold to 4 eV. The cross sections for both H+ and D+ transfer rise slowly from threshold with increasing collision energy; however, all three vibrational modes enhance reaction much more strongly than equivalent amounts of collision energy and the enhancements remain large even at high collision energy, where the vibration contributes less than 10% of the total energy. Excitation of the OH stretch enhances H+ transfer by a factor of ∼5, but the effect on D+ transfer is only slightly larger than that from an equivalent increase in collision energy, and smaller than the effect from the much lower energy bend excitation. Similarly, OD stretch excitation strongly enhances D+ transfer, but has essentially no effect beyond that of the additional energy on H+ transfer. The effects of the two stretch vibrations are consistent with the expectation that stretching the bond that is broken in the reaction puts momentum in the correct coordinate to drive the system into the exit channel. From this perspective it is quite surprising that bend excitation also results in large (factor of 2) enhancements of both H+ and D+ transfer channels, such that its effect on the total cross section at collision energies below ∼2 eV is comparable to those from the two stretch modes, even though the bend excitation energy is much smaller. For collision energies above ∼2 eV, the vibrational effects become approximately proportional to the vibrational energy, though still much larger than the effects of equivalent addition of collision energy. Measurements of the product recoil velocity distributions show that reaction is direct at all collision energies, with roughly half the products in a sharp peak corresponding to stripping dynamics and half with a broad and approximately isotropic recoil velocity distribution. Despite the large effects of vibrational excitation on reactivity, the effects on recoil dynamics are small, indicating that vibrational excitation does not cause qualitative changes in the reaction mechanism or in the distribution of reactive impact parameters. [ABSTRACT FROM AUTHOR]

Reactions of HOD+ with CO2 have been studied for HOD+ in its ground state, and with one quantum of excitation in each of its vibrational modes: (001)-predominantly OH stretch, 0.396 eV; (010)-bend, 0.153 eV; and (100)-predominantly OD stretch, 0.293 eV. Integral cross sections and product recoil velocities were recorded for collision energies from threshold to 3 eV. The cross sections for both H+ and D+ transfer rise with increasing collision energy from threshold to ∼1 eV, then become weakly dependent of the collision energy. All three vibrational modes enhance the total reactivity, but quite mode specifically. The H+ transfer reaction is enhanced by OH stretch excitation, whereas OD stretch excitation has little effect. Conversely, the D+ transfer reaction is enhanced by OD stretch excitation, while the OH stretch has little effect. Excitation of the bend strongly enhances both channels. The effects of the stretch excitations are consistent with previous studies of neutral HOD mode-selective chemistry, and can be at least qualitatively understood in terms of a late barrier to product formation. The fact that bend excitation produces the largest overall enhancement is surprising, because this is the lowest energy excitation, and is not obviously connected with the reaction coordinates for either H+ or D+ transfer. A rationalization in terms of the effects of water distortion on the potential surface is proposed. [ABSTRACT FROM AUTHOR]

Integral cross sections and product recoil velocity distributions were measured for reaction of C2H2+ with NO2, in which the C2H2+ reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν5) and CC stretch (n·ν2, n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO2+), O- transfer (NO+), and O transfer (C2H2O+) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO+ + triplet ketene, generated predominantly by O- transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO2+ formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C2H2O+) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O- transfer, producing NO+, with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C2H2+ vibration are modest, but mode specific. For all three product channels (i.e., charge, O-, and O transfer), excitation of the CC stretch fundamental (ν2) has little effect, 2·ν2 excitation results in ∼50% reduction in reactivity, and excitation of the cis-bend overtone (2·ν5) results in ∼50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later. [ABSTRACT FROM AUTHOR]

A series of planar model catalysts have been prepared via deposition of Irn+ on thermally grown amorphous SiO2/Si(100) and ion scattering spectroscopy was used to probe surface structure as a function of cluster size, impact energy, and surface temperature. Deposition of Ir2 or Ir10 at low energies and room temperature results in stable clusters forming one- or two-dimensional single layer islands on the oxide surface. Heating the samples to 750 K leads to agglomeration, forming multilayer structures on the surface. Ir1 deposited under similar conditions sinters into large clusters at room temperature. Deposition at 110 K at least partially stabilizes the Ir atoms with respect to diffusion and sintering. At higher deposition energies, partial implantation into the surface is observed, but this appears to be insufficient to stabilize the clusters against sintering at elevated temperature. At low temperatures, substrate-mediated adsorption of CO is found to be highly efficient, leading to near saturation coverages of CO bound atop the Irn clusters. The CO can be removed by careful He+ sputtering. The deposition/binding behavior of Irn on SiO2 is quite different from Irn/TiO2(110), for which the clusters bind in three-dimensional morphology, starting at Ir5. That system also shows substrate-mediated adsorption of CO, but the CO preferentially binds at the periphery of the clusters rather than on top. [ABSTRACT FROM AUTHOR]

NO2+ in six different vibrational states was reacted with C2H2 over the center-of-mass energy range from 0.03 to 3.3 eV. The reaction, forming NO++C2H2O and NO+C2H2O+, shows a bimodal dependence on collision energy (Ecol). At low Ecol, the reaction is quite inefficient (<2%) despite this being a barrierless, exoergic reaction, and is strongly inhibited by Ecol. For Ecol>∼0.5 eV, a second mechanism turns on, with an efficiency reaching ∼27% for Ecol>3 eV. The two reaction channels have nearly identical dependence on Ecol and NO2+ vibrational state, and identical recoil dynamics, leading to the conclusion that they represent a single reaction path throughout most of the collision. All modes of NO2+ vibrational excitation enhance both channels at all Ecol, however, the effects of bend (010) and bend overtone (0200) excitation are particularly strong (factor of 4). In contrast, the asymmetric stretch (001), which intuition suggests should be coupled to the reaction coordinate, leads to only a factor of ∼2 enhancement, as does the symmetric stretch (100). Perhaps the most surprising effect is that of the bending angular momentum, which strongly suppress reaction, even though both the energy and angular momentum involved are tiny compared to the collision energy and angular momentum. The results are interpreted in light of ab initio and Rice-Ramsperger-Kassel-Marcus calculations. [ABSTRACT FROM AUTHOR]

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by ∼4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2++Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+–NO2 charge state. [ABSTRACT FROM AUTHOR]

The effects of collision energy (Ecol) and six different H2CO+ vibrational states on the title reaction have been studied over the center-of-mass Ecol range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H2+ transfer, hydrogen transfer, and proton transfer channels is strongly affected by Ecol and H2CO+ vibrational excitation, providing insight into the factors that control competition and charge state “unmixing” during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion. [ABSTRACT FROM AUTHOR]

Model catalysts were prepared by deposition of size-selected Aun (n=1–7) on rutile TiO2(110), and characterized by a combination of electron spectroscopy, ion scattering, temperature-programmed desorption, and pulse-dosing mass spectrometry. CO oxidation activity was found to vary strongly with deposited cluster size, with significant activity appearing at Au3. Activity is not obviously correlated with affinity for CO, or with cluster morphology, but is strongly correlated with the clusters’ ability to bind oxygen (during O2 exposure) on top of the gold. The temporal dependence of CO2 evolution in reaction of O2 pre-exposed samples with CO pulses shows an interesting cluster size dependence. For Au5 and Au6, the peak CO2 production is coincident with the peak CO flux, but for Au3, Au4, and Au7, there are significant induction periods for CO2 evolution. In addition, it is observed that some of the most active cluster sizes have the slowest CO2 evolution rates. Several mechanistic scenarios capable of accounting for the observations are laid out. [ABSTRACT FROM AUTHOR]

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass Ecol range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effects on the title reaction of collision energy (Ecol) and five H2CO+ vibrational modes have been studied over a center-of-mass Ecol range from 0.1 to 2.3 eV. Electronic structure and Rice–Ramsperger–Kassel–Marcus calculations were used to examine properties of various complexes and transition states that might be important. Only the hydrogen abstraction (HA) product channel is observed, and despite being exoergic, HA has an appearance energy of ∼0.4 eV, consistent with a transition state found in the electronic structure calculations. A precursor complex-mediated mechanism might possibly be involved at very low Ecol, but the dominant mechanism is direct over the entire Ecol range. The magnitude of the HA cross section is strongly, and mode specifically affected by H2CO+ vibrational excitation, however, vibrational energy has no effect on the appearance energy. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quasiclassical, direct dynamics trajectories, calculated at the B3LYP/6-31G** level of the theory, have been used to study the energy transfer dynamics and collision-induced dissociation (CID) of formaldehyde cation in collisions with Ne. Effects of varying collision energy were probed for ground state H[SUB2]CO[SUP+], and H[SUB2]CO[SUP+] with excitation in three different vibrational modes: ν[SUP+,SUB5] (in-plane CH[SUB2] rock), ν[SUP+,SUB4] (out-of-plane bend), and ν[SUP+,SUB5] (asym. CH stretch). The trajectories are in excellent agreement with an earlier detailed experimental study of state-selected H[SUB2]CO[SUP+] CID. Energy disposal branching between product recoil, vibrational energy, and rotational excitation is found to vary with collision energy, vibrational state, impact parameter, and orientation. An analysis of the trajectories illustrates how mode-specific vibrational excitation effects the efficiency of translational-to-vibrational (T→V) energy transfer to promote CID. [ABSTRACT FROM AUTHOR]

The effects on the title reaction of collision energy (E[SUBcol]), five H[SUB2]CO[SUP+] vibrational modes, and deformation vibrations of methane have been studied, including the measurement of product integral and differential cross sections over a center-of-mass E[SUBcol] range from 0.09-3.3 eV. Electronic structure and RRKM calculations are reported, providing an additional mechanistic insight. The total reaction efficiency is well below unity, despite there being two exoergic reaction pathways with no activation barriers. The energetically more favorable channel corresponds to H elimination (HE) from an intermediate complex, however, this channel accounts for only ∼15% of the total reaction cross section at low E[SUBcol] and is negligible at high energies. The dominant channel, hydrogen abstraction (HA) by H[SUB2]CO[SUP+] from methane, is dominated by a complex-mediated mechanism at low E[SUBcol], switching over to a direct hydrogen-stripping mechanism at high E[SUBcol]. Both HA and HE are inhibited in a strongly mode-specific fashion by H[SUB2]CO[SUP+] vibrational excitations, and greatly enhanced by excitation of methane deformation vibrations. The strong mode specificity indicates that the reaction-limiting step occurs early in the collisions. [ABSTRACT FROM AUTHOR]

The effects of collision energy (E[SUBcol]) and five different H[SUB2]CO[SUP+] vibrational modes on the title reaction have been studied, including measurements of product ion recoil velocity distributions. A series of ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important. Four product channels are observed. Proton transfer (PT) dominates at low E[SUBcol], and is suppressed by E[SUBcol] but mildly enhanced by H[SUB2]CO[SUP+] vibrational excitation. PT occurs by a direct mechanism at high energies, but appears to be mediated by reactantlike complexes at low energies. The other major low energy channel corresponds to H[SUP+,SUB2] transfer, and the majority of these product ions go on to eliminate CO, producing H[SUB2]S[SUP+]. Both H[SUP+,SUB2] transfer and H[SUB2]S[SUP+] channels are strongly inhibited by E[SUBcol] and vibrational excitation, which is interpreted in terms of competition with other channels. Charge transfer occurs in short time scale collisions at all energies, and is strongly enhanced by E[SUBcol] and by vibrational excitation. The vibrational effects for all channels are mode specific. [ABSTRACT FROM AUTHOR]

We report a study of Ni[sub n]/TiO[sub 2] samples prepared by size-selected deposition of Ni[sup +]°[sub n] (n = 1,2,5,10,15) on rutile TiO[sub 2](110). The effects of deposition energy and support preparation conditions on the oxidation state of the clusters are examined by x-ray photoelectron spectroscopy (XPS). On the stoichiometric surface, Ni[sub n] is stable, but oxidation can be driven by increased impact energy. For TiO[sub 2] surfaces with chemisorbed oxygen, deposited Ni[sub n] are oxidized even at low impact energies. Low energy ion scattering spectroscopy was used to characterize the dispersion of Ni on the support, and provide some insight into binding morphology. Small clusters bind preferentially to oxygen sites. Large clusters bind in compact geometries and appear to retain some three dimensional character on the surface. The data suggest that the clusters neither fragment, nor agglomerate, in room temperature deposition. Temperature programmed desorption (TPD) of CO was used to characterize deposited clusters. For these small clusters, no strong desorption features are observed in the temperature range above 140 K, where CO desorbs from TiO[sub 2]. The lack of CO binding is discussed in terms of strong Ni-TiO[sub 2] binding. The ion scattering data indicate that there is significant sintering, and possibly partial encapsulation, of the Ni clusters during the TPD experiments. XPS reveals little change in oxidation state. This is the first study where the oxidation state and morphology of size-selected deposited clusters has been studied, before and after TPD. [ABSTRACT FROM AUTHOR]

We report a study of collision-induced dissociation (CID) of H[sub 2]CO[sup +], including measurement of the effects of collision energy (E[sub co1)] and five different H[sub 2]CO[sup +] vibrational modes on the CID integral and differential cross sections. CID was studied for collision with both Xe and Ne, and the Ne results provide a very detailed probe of energy transfer collisions leading to CID. The CID appearance threshold is found to depend only on total energy, but for all energies above threshold, vibrational energy is far more effective at driving CID than E[sub co1], with some mode-specificity. Results are fit with an impact parameter-based mechanism, and considerable insight is obtained into the origins of the E[sub co1] and vibrational effects. A series of ab initio and RRKM calculations were also performed to help interpret the results. [ABSTRACT FROM AUTHOR]

Investigates the reactions of vibrationally state-selected ND[sub +][sup 3] with phenol in a guided beam arrangement. Effects of reactant vibrational excitation and collision energy; Reaction of charge transfer mediated by hydrogen-bonded complexes; Causes of isomerization of the ring bound to hydrogen-bonded complexes.

We report cross sections for reaction of mass-selected carbon cluster ions (Cn+, n=2–14) with O2 over a collision energy range of 0.1–10 eV. The results give insight into the oxidation mechanism and how it is affected by the isomeric structure of the reactant cluster ion. Small clusters, which are linear, react with no activation barrier, producing primarily CnO+ at low energies. Larger clusters, which are monocyclic, have activation barriers for reaction, and the product distribution is quite different. Reactions were also studied under multiple collision conditions, and addition of up to two O atoms is observed. Possible reaction mechanisms are discussed and comparison is made with the thermal kinetics results of McElvany and co-workers [J. Chem Phys. 86, 715 (1987)]. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report a study of the absolute cross sections, reaction threshold energies, and product branching ratios for interactions of aluminum cluster ions (Al+n, n=1–9) with O2 and N2O. Most of the reactions are observed to have collision energy thresholds which are attributed to activation barriers or bottlenecks. At collision energies above threshold, exoergic oxidation of the cluster ions occurs, and the resulting energy release causes extensive fragmentation to yield a variety of product ions. Comparison of threshold behavior for different product channels suggests that in addition to the dominant oxidation reactions, the clusters undergo Al abstraction, O2–Al+ exchange, and collision induced dissociation. [ABSTRACT FROM AUTHOR]

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 1[uppercase_phi_synonym]u (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. [ABSTRACT FROM AUTHOR]

Studies the collisions of rare gas ions with fullerenes by means of a guided-beam tandem mass spectrometer. Charge transfer (CT) as dominant channel over the collision energy range; Dissociative CT and endohedral complex formation at high collision energies; Threshold energies for endohedral penetration.

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 system. [ABSTRACT FROM AUTHOR]

We report the effects of vibrational excitation and collision energy on the cross sections and branching ratios for reaction of acetylene cations with D2, using two different guided-ion-beam instruments. Two major reaction channels are observed, both of which are nearly thermoneutral. Hydrogen atom exchange is slightly exoergic due to zero point energy, and is inhibited by both collision and vibrational energy. Formation of the two isotopic ‘‘C2H+3 ’’ products is enhanced by collision energy and C–C stretching vibration, but not by bending vibration. The branching ratios at low collision energies are consistent with reaction via an intermediate complex, and Rice–Ramsberger–Kassel–Marcus (RRKM) analysis is used to extract further information. At collision energies above 1 eV, D-atom transfer by a direct mechanism is also observed as a route to C2H2D+ production. Comparison of our results using both the Stony Brook and Freiburg instruments is made with the state-selected experiments of Honma, Kato, Tanaka, and Koyano [J. Chem. Phys. 81, 5666 (1984)], who previously studied both the C2H+2+D2 and C2D+2+H2 isotopic reactants. Our results for C2H+2+D2 are consistent with their C2D+2+H2 data for all collision energies and with their C2H+2+D2 data at 0.2 eV collision energy. We do not reproduce the anomalous vibrational effects they reported for 1 and 2 eV collision energies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report the first study of the effects of mode specific vibrational excitation on collision-induced dissociation of a polyatomic cation. Resonance-enhanced multiphoton ionization was used to produce OCS+ ions with variable excitation in each of the three vibrational modes, in combination with either spin–orbit electronic state. Cross sections for fragmentation of the state-selected ions in collision with Xe and Ar atoms were measured over the collision energy range from 0.5 to 9.0 eV, using a guided ion beam tandem mass spectrometer. Both spin-allowed and spin-forbidden channels are observed for both collision partners, and the cross sections in both cases are enhanced by collision energy and all modes of vibrational excitation. Vibration is more effective than collision energy, but the effect is nonmode specific. The absolute magnitude of the dissociation cross sections and the relative vibrational effects are bigger for collisions with Xe than for Ar. The spin–orbit state of the primary ion has no effect on dissociation of ground vibrational state OCS+, but there appears to be some synergy between vibrational and spin–orbit excitation. The results are discussed in view of the electronic structure of OCS+ and possible scattering mechanisms. [ABSTRACT FROM AUTHOR]

Cross sections for reaction of mass-selected boron cluster ions (B+n, n=2–24) with N2O are reported for collision energies from 0.1 to 10 eV. The major product channels are addition of a single nitrogen or oxygen atom to the intact cluster ion. For small clusters, there are no activation barriers and cross sections are large, however, as cluster size increases, bottlenecks and activation barriers reduce reactivity substantially. Significant size effects are observed in the product branching distributions. The dominant reaction mechanism at low collision energies is proposed to be complex formation, where only one bond in N2O is broken, followed by desorption of the stable leaving group (N2 or NO). Reactions with boron cluster ions larger than 16 atoms in size have been studied for the first time, and in addition, supporting evidence is given for our previous suggestion that B3 has an anomalously high ionization potential. Comparisons are made with other oxidation reactions of boron cluster ions, and with aluminum and silicon cluster ion reactions with N2O. [ABSTRACT FROM AUTHOR]

Resonance-enhanced multiphoton ionization has been used to create state-selected OCS+ ions, which are then reacted with C2H2 in a guided-beam tandem mass spectrometer. OCS+ can be produced with excitation in all three of its vibrational modes, in either the upper or lower fine structure electronic state. Absolute cross sections for all product channels (C2H+2, C2HnS+(n=1, 2), and S+) are reported as a function of collision energy and vibrational state in the range from 0.06–4.5 eV. Different modes of nuclear motion have markedly different effects on reactivity and branching ratios. Production of C2H2S+, is the major chemical reaction channel, and its formation is strongly inhibited by collision energy, but only weakly affected by vibrational and fine structure state. The cross section for charge transfer (CT) shows vibrational effects that change with collision energy. For collision energies below 0.3 eV, CT is enhanced by all forms of nuclear motion, while at higher energies CT is weakly enhanced by C–S stretching, strongly enhanced by C–O stretching, and inhibited by bending. Both C2HS+ and S+ are minor channels, which turn on at higher collision energies. They are weakly affected by vibrational energy and fine structure state. These results are compared with those from our complementary study [T. M. Orlando, B. Yang, Y. Chiu, and S. L. Anderson, J. Chem. Phys. 92 7356 (1990)] of the other charge state of the [C2H2+OCS]+ system: reactions of C2H+2 with OCS. This allows comparison of the effects of 12 different reactant internal energy states on the same product channels. [ABSTRACT FROM AUTHOR]

Cross sections for oxidation reactions of boron cluster ions (B+1–14) with CO2 are reported as a function of collision energy. The results give insight into the oxidation mechanism and are a sensitive probe of the B+n–O bond energy. In addition, collision-induced dissociation (CID) was used to measure the stabilities of boron monoxide cluster ions (BnO+) and energetic and structural information is obtained that indicates that at least, in some cases, oxidation causes structural rearrangement of the clusters. Sequential oxidation of boron cluster ions by CO2 was also studied and the results give further insight into the reaction mechanism and energetics. [ABSTRACT FROM AUTHOR]

Resonance-enhanced multiphoton ionization has been used to produce beams of vibrationally state-selected acetylene cations. The ions are formed with excitation in either the symmetric C–C stretch (ν2) or in a bending mode (ν5). Reactions with OCS have been studied in a guided ion-beam mass spectrometer. Absolute cross sections for the production of OCS+ and C2HnS+ (n=1,2) are reported for collision energies ranging from 80 meV to 5 eV. Charge transfer is observed to be enhanced by bending excitation, suppressed by C–C stretching vibration, and only weakly affected by collision energy. In contrast, the C2HnS+ channels are strongly collision energy dependent, with vibrational effects that vary with collision energy. The effects of bending and stretching excitation are qualitatively similar; however, the size of the effects are different and are not what would be expected on energetic grounds. These results are contrasted to the situation for reaction of mode selectively excited C2H+2 with methane. [ABSTRACT FROM AUTHOR]

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 composition changes from pure boron to mostly hydrogen. Comparison of these results is made with boron cluster ion reactions with O2 and D2, as well as reactions with water of aluminum and silicon cluster ions. Some discussion is given of the thermochemistry for these reactions, and a possible problem with the thermochemical data in the BOD/DBO system is discussed. [ABSTRACT FROM AUTHOR]

Product cross sections for reaction of size-selected boron cluster ions with deuterium are reported for collision energies ranging from 0.05 to 10 eV. The major reactions are those in which one or two deuterium atoms add to the intact cluster. Other, higher energy channels involve fragmentation of the clusters, with and without addition of deuterium atoms. Cluster size dependent thresholds are observed for many reactions, and we have used these to estimate the cluster–D atom binding energy as a function of cluster size. Metastable adducts of the form BnD+2 are observed for reagent clusters as small as the tetramer. For small clusters, adducts form without activation energy, but starting at B+8 activation barriers are observed. Unimolecular decay rates of the adducts have been estimated, and the binding energy of D2 to various size cluster ions was extracted using RRKM modeling. Comparison is made with related results for boron surfaces and for aluminum cluster ions. [ABSTRACT FROM AUTHOR]

Multiphoton ionization (MPI) has been used to create C2H+2 in selected vibrational states, with excitation in either ν2 (C–C stretch) or a bending vibration (ν5-cis bend). Reactions of the state selected ions with CH4 and CD4 have been studied in a guided beam mass spectrometer. Absolute cross sections for formation of C3H+n (n=3,4,5) and C2H+3 products are reported for collision energies ranging from 60 meV to 5 eV. Evidence for both complex formation and direct reaction mechanisms is found. Vibrational excitation has different effects on the various product cross sections in different collision energy regimes. Bending excitation has also been found to cause somewhat different effects than C–C stretching vibration. [ABSTRACT FROM AUTHOR]

Multiphoton ionization (MPI) in conjunction with photoelectron spectroscopy (PES) has been used to examine Rydberg states of OCS in the energy range from 70 547 to 75 472 cm-1. The combination of techniques allows us to correct previous assignments for several vibronic bands, and to probe vibronic mixing in the Rydberg levels. In the course of this study we have found routes for preparing OCS+ with control over both vibrational mode and fine structure state. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Charge transfer and S-atom transfer have been studied for reaction of both charge states of the [C2H2:OCS]+ system. Reactions have been studied as a function of six different modes of reactant nuclear motion, including relative motion and nine levels of five vibrational modes for the two reactant ions. Integral cross section measurements provide information on how total reactivity and product branching are affected by different forms of reactant excitation. Detailed insight into the reaction mechanism is obtained from differential scattering measurements—the first ever for mode-selectively excited reactants. The S-transfer reaction is found to be nearly identical for the two reactant charge states, which appear to be coupled by charge transfer in the collision entrance channel. In both charge states S transfer is dominated by two distinct direct scattering mechanisms, rebound and glancing/stripping, each of which are affected by collision energy and vibrational state. At collision energies below 0.5 eV, complex-mediated scattering becomes an important mechanism as well. It appears that the cis-trans symmetry of the C2H+2 bending vibration strongly affects formation of [C2H2:OCS]+ complexes. Charge transfer goes by very different mechanisms in the two charge states. For C2H+2+OCS, long-range electron hopping is the dominant mechanism, while for OCS++C2H2 the dominant mechanism requires impulsive collisions. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the effects of collision energy and OCS+ vibrational state (ν1, ν2, and ν3) on the reaction of OCS+ with OCS. Production of CS+2+CO2 is exoergic and the cross section shows no evidence of an activation barrier. Nonetheless, the cross section is only ∼0.1% of the collision cross section, even at low collision energies where formation of an intermediate complex is facile. There appears to be a severe phase-space (steric) bottleneck for this rearrangement reaction. CS+2 production is weakly inhibited by collision energy, and enhanced by all three modes of OCS+ reactant vibrational excitation. Production of S+2 is endoergic and is enhanced by collision energy and by ν2 (bend) and ν3 (CS stretch) excitation. Excitation of ν1 (CO stretch) does not enhance this channel, even though it is the highest energy mode. At high collision energies, S+2 production becomes relatively efficient, suggesting that the reaction mechanism for this channel is direct with no significant bottleneck. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 weaker enhancement than collision energy, while bending enhances the reaction ∼10 times more efficiently. As collision energies increase, the C2H+3 product is increasingly forward scattered with an increasing fraction of the available energy going into recoil. Energy put into reactant vibration mostly is retained as internal energy of the products. Over the collision energy range from 0.4 to 2.8 eV, the collision time in the direct reaction varies from ≥1.3 ps to ≤70 fs. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical study of the reaction of small carbon cluster cations with N2O aimed at understanding the reaction mechanism and how it is affected by the electronic and geometric structure of the C+n reactants. Cross sections for reaction of C+n (n=3–12) with N2O were measured over a collision energy range from 0.1–10 eV, using a guided ion beam tandem mass spectrometer. Ab initio calculations were used to examine the structure and energetics of reactant and product species. Small clusters, which are linear, react with no activation barrier, resulting in either oxide or nitride formation. The branching between oxide and nitride channels shows a strong even–odd alternation, with even clusters preferentially forming nitrides. This appears to be correlated with an even/odd alternation in the ionization potential of the CnN. The larger, monocyclic C+n have activation barriers for reaction, and a completely different product distribution. Secondary reactions of the primary oxide and nitride products were studied at high N2O pressures. Products containing two O or two N atoms are not observed, but it is possible to add one of each. Possible reaction mechanisms are discussed and supported by thermochemistry derived from spin restricted ab initio calculations. [ABSTRACT FROM AUTHOR]

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[ATOTHER]@xa[/ATOTHER]C60]+ and [Na[ATOTHER]@xa[/ATOTHER]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[ATOTHER]@xa[/ATOTHER]C60]+ appears to relax mainly by loss of C2 units from the fullerene cage, yielding [Na[ATOTHER]@xa[/ATOTHER]C60-2n]+. For [Li[ATOTHER]@xa[/ATOTHER]C60]+, escape of Li+ dominates over C2 loss. For K+, very little [K[ATOTHER]@xa[/ATOTHER]C60]+ is observed, and the main endohedral species observed are fragments [K[ATOTHER]@xa[/ATOTHER]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 endohedral insertion mechanism with projectile size are discussed, as is the competition between C2 evaporation, thermionic emission, and IR emission. [ABSTRACT FROM AUTHOR]

Interaction of Ne+ with C60 has been studied for collision energies ranging up to 100 eV. The dominant process is charge transfer, with C60+ accounting for 85% to 100% of the total product signal in this energy range. At 25 eV collision energy, C60-2n+ dissociative charge transfer products appear, accounting for ∼13% of the total product signal at high energies. At ∼25 eV, NeC60+, NeC58+, and NeC56+ all appear together, followed by a series of NeC60-2n+ products where n increases with increasing collision energy. These products are believed to be endohedral complexes, Ne[ATOTHER]@xa[/ATOTHER]C60-2n+, where neon is physically trapped inside the fullerene cage. The production efficiency of the endocomplexes reaches a maximum of ∼1.2% at 60 eV, and decreases to 0.5% at 100 eV. Modeling the results leads to the following picture of Ne++C60 collision dynamics: Charge transfer occurs in nearly all collisions, and is the only significant process for high impact parameters. At collision energies below ∼25 eV, translational-to-internal energy transfer is inefficient, even for low impact parameter collisions. At higher energies, the inelasticity jumps to near 100%, leading to fragmentation of the nascent products. Both endohedral penetration and efficient energy transfer appear to require bond rupture in the C60. [ABSTRACT FROM AUTHOR]

The reaction of C2H+2 with CH4 is strongly enhanced by excitation of a C2H+2 bending vibration, with little effect observed for the CC stretch. The enhancement results from efficient use of bending motion in surmounting an activation barrier. Energy in the CC stretch has little effect on the threshold. [ABSTRACT FROM AUTHOR]

This letter reports a collision-induced dissociation study of the stabilities and fragmentation patterns for carbon cluster ions C+n (n = 2–15). For clusters of six or more atoms, the primary fragmentation channel is loss of C3 neutral. Fragmentation thresholds are analyzed to yield dissociation energies, which are found to be substantially higher than the upper limits estimated from photodissociation experiments of Geusic et al. [Z. Phys. D 3, 309 (1986)], but in good agreement with theoretical estimates. The stabilities oscillate strongly, with some evidence of a change in behavior as size increases above 9 atoms. This is the size where the most stable structure is believed to change from linear to cyclic. Photodissociation measurements were also done, and we find that the clusters dissociate efficiently at photon energies far below the CID thresholds. Possible reasons for the anomalous photodissociation results are discussed. [ABSTRACT FROM AUTHOR]

The first observations of dichroic effects in photoelectron angular distributions are reported for photoionization of aligned molecular excited states with circularly polarized light. Photoelectron angular distributions resulting from the two-color, (2+1) REMPI of NO via the A 2∑+, v=0, J=3/2,5/2 excited states exhibit significant left–right asymmetry. The experimental CD angular distributions are found to be well described by the general theoretical framework recently developed by Dubs, Dixit, and McKoy and are in good qualitative agreement with their calculated REMPI–CD distributions. [ABSTRACT FROM AUTHOR]