Quasiclassical trajectory study of the collision-induced dissociation of CH3SH+ + Ar

Quasiclassical trajectory calculations were carried out to study the dynamics of energy transfer and collision-induced dissociation (CID) of CH(3)SH(+) + Ar at collision energies ranging from 4.34 to 34.7 eV. The relative abundances calculated for the most relevant product ions are found to be in good agreement with experiment, except for the lowest energies investigated. In general, the dissociation to form CH(3)(+) + SH is the dominant channel, even though it is not among the energetically favored reaction pathways. The results corroborate that this selective dissociation observed upon collisional activation arises from a more efficient translational to vibrational energy transfer for the low-frequency C-S stretching mode than for the high-frequency C-H stretching modes, together with weak couplings between the low- and high-frequency modes of vibration. The calculations suggest that CID takes place preferentially by a direct CH(3)(+) + SH detachment, and more efficiently when the Ar atom collides with the methyl group-side of CH(3)SH(+).     

Quasiclassical trajectory study of the collision-induced dissociation dynamics of Ar + CH3SH+ using an ab initio interpolated potential energy surface

Classical trajectory calculations have been performed to investigate the collision-induced dissociation (CID) of the CH(3)SH(+) cation with Ar atoms. A new intramolecular potential energy surface for the CH(3)SH(+) cation is evaluated by interpolation of 3000 ab initio data points calculated at the MP2/6-311G(d,p) level of theory. The new potential energy surface includes seven accessible dissociation channels of the cation. The present QCT calculations show that migration of hydrogen atoms, leading to the rearrangement CH(3)SH(+) <--> CH(2)SH(2)(+), is significant at the collision energies considered (6.5-34.7 eV) and that the formation of CH(3)(+), CH(3)S(+), and CH(2)(+) cations takes place primarily by a "shattering" mechanism in which the products are formed just after the collision. The theoretical product abundances are found to be in qualitative agreement with the experimental data. However, at high collision energies, the calculated total cross sections for the formation of CH(3)(+) and CH(2)SH(+) cations are noticeably larger than the experimental determinations. Several features of the dynamics of the CID processes are discussed.     

Quasiclassical trajectory study of energy transfer and collision-induced dissociation in hyperthermal Ar + CH4 and Ar + CF4 collisions

We present a study of energy transfer in collisions of Ar with methane and perfluoromethane at hyperthermal energies (E(coll) = 4-10 eV). Quasiclassical trajectory calculations of Ar + CX(4) (X = H, F) collisions indicate that energy transfer from reagents' translation to internal modes of the alkane molecule is greatly enhanced by fluorination. The reasons for the enhancement of energy transfer upon fluorination are shown to emerge from a decrease in the hydrocarbon vibrational frequencies of the CX(4) molecule with increasing the mass of the X atom, and to an increase of the steepness of the Ar-CX(4) intermolecular potential. At high collision energies, we find that the cross section of Ar + CF(4) collisions in which the amount of energy transfer is larger than needed to break a C-F bond is at least 1 order of magnitude larger than the cross sections of Ar + CH(4) collisions producing CH(4) with energy above the dissociation limit. In addition, collision-induced dissociation is detected in short time scales in the case of the fluorinated species at E(coll) = 10 eV. These results suggest that the cross section for degradation of fluorinated hydrocarbon polymers under the action of nonreactive hyperthermal gas-phase species might be significantly larger than that of hydrogenated hydrocarbon polymers. We also illustrate a practical way to derive intramolecular potential energy surfaces for bond-breaking collisions by improving semiempirical Hamiltonians based on grids of high-quality ab initio calculations.     

C~6F~6^+→C~6F~5^++F气相单分子分解及碰撞诱导分解 反应研究

用质量分析离子动能谱(MIKES)研究了C~6F~6^+→C~6F~5^++F的气相单分子分解及其与Ar和He的碰撞诱导分解(CID)反应。实验结果表明,C~6F~6^+在电离室中获得足够能量而被激发到某一长效激发态,而CID是诱导其分解的必要步骤,且该分解过程有两条能量不同的反应途径。当碰撞气体为Ar时,两条途径所对应的能量变化分别为0eV和+9.8eV(将多余能量转化为动能),而当碰撞气体为He时,则分别为0eV和-17eV(将多余能量转化为内能)。CID/Ar诱导该长效激发态在C-F键断裂之前将多余能量转化为动能,而在CID/He中则将多余能量转化为内能。     

The bond-forming reactions of atomic dications with neutral molecules: formation of ArNH+ and ArN+ from collisions of Ar2+ with NH3

An experimental and computational study has been performed to investigate the bond-forming reactivity between Ar(2+) and NH(3). Experimentally, we detect two previously unobserved bond-forming reactions between Ar(2+) and NH(3) forming ArN(+) and ArNH(+). This is the first experimental observation of a triatomic product ion (ArNH(+)) following a chemical reaction of a rare gas dication with a neutral. The intensity of ArNH(+) was found to decrease with increasing collision energy, with a corresponding increase in the intensity of ArN(+), indicating that ArN(+) is formed by the dissociation of ArNH(+). Key features on the potential energy surface for the reaction were calculated quantum chemically using CASSCF and MRCI methods. The calculated reaction mechanism, which takes place on a singlet surface, involves the initial formation of an Ar-N bond to give Ar-NH(3)(2+). This complexation is followed by proton loss via a transition state, and then loss of the two remaining hydrogen atoms in two subsequent activationless steps to give the products (3)ArN(+) + H(+) + 2H. This calculated pathway supports the sequential formation of ArN(+) from ArNH(+), as suggested by the experimental data. The calculations also indicate that no bond-forming pathway exists on the ground triplet surface for this system.     

Differences between the internal energy depositions induced by collisional activation and by electron transfer of W(CO)6(2+) ions on collision with Ar and K targets

Doubly charged tungsten hexacarbonyl W(CO)(6) (2+) ions were made to collide with Ar and K targets to give singly and doubly charged positive ions by collision-induced dissociation (CID). The resulting ions were analyzed and detected by using a spherical electrostatic analyzer. Whereas the doubly charged fragment ions resulting from collisional activation (CA) were dominant with the Ar target, singly charged fragment ions resulting from electron transfer were dominant with the K target. The internal energy deposition in collisionally activated dissociation (CAD) evaluated with the Ar target was broad and decreased with increasing internal energy. The predominant peaks observed with the K target were associated with singly charged W(CO)(2) (+) and W(CO)(3) (+) ions: these ions were not the result of CA, but arose from dissociation induced by electron transfer (DIET). The internal energy deposition resulting from the electron transfer was very narrow and centered at a particular energy, 7.8 eV below the energy level of the W(CO)(6) (2+) ion. This narrow internal energy distribution was explained in terms of electron transfer by Landau-Zener potential crossing at a separation of 5.9 x 10(-8) cm between a W(CO)(6) (2+) ion and a K atom, and the coulombic repulsion between singly charged ions in the exit channel. A large cross section of 1.1 x 10(-14) cm(2) was estimated for electron capture of the doubly charged W(CO)(6) (2+) ion from the alkali metal target, whose ionization energy is very low. The term "collision-induced dissociation," taken literally, includes all dissociation processes induced by collision, and therefore encompasses both CAD and DIET processes in the present work. Although the terms CID and CAD have been defined similarly, we would like to propose that they should not be used interchangeably, on the basis that there are differences in the observed ions and in their intensities with Ar and K targets.     

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+-rare gas collisions: an experimental and trajectory study

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 approximately 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.     

Isomerization dynamics and thermodynamics of ionic argon clusters

The dynamics and thermodynamics of small Ar(n) (+) clusters, n=3, 6, and 9, are investigated using molecular dynamics (MD) and exchange Monte Carlo (MC) simulations. A diatomic-in-molecule Hamiltonian provides an accurate model for the electronic ground state potential energy surface. The microcanonical caloric curves calculated from MD and MC methods are shown to agree with each other, provided that the rigorous conservation of angular momentum is accounted for in the phase space density of the MC simulations. The previously proposed projective partition of the kinetic energy is used to assist MD simulations in interpreting the cluster dynamics in terms of inertial, internal, and external modes. The thermal behavior is correlated with the nature of the charged core in the cluster by computing a dedicated charge localization order parameter. We also perform systematic quenches to establish a connection with the various isomers. We find that the Ar(3) (+) cluster is very stable in its linear ground state geometry up to about 300 K, and then isomerizes to a T-shaped isomer in which a quasineutral atom lies around a charged dimer. In Ar(6) (+) and Ar(9) (+), the covalent trimer core is solvated by neutral atoms, and the weakly bound solvent shell melts at much lower energies, occasionally leading to a tetramer or pentamer core with weakly charged extremities. At high energies the core itself becomes metastable and the cluster transforms into Ar(2) (+) solvated by a fluid of neutral argon atoms.     

Threshold collision-induced dissociation of diatomic molecules: a case study of the energetics and dynamics of O2- collisions with Ar and Xe

The energetics and dynamics of collision-induced dissociation of O2- with Ar and Xe targets are studied experimentally using guided ion-beam tandem mass spectrometry. The cross sections and the collision dynamics are modeled theoretically by classical trajectory calculations. Experimental apparent threshold energies are 2.1 and 1.1 eV in excess of the thermochemical O2- bond dissociation energy for argon and xenon, respectively. Classical trajectory calculations confirm the observed threshold behavior and the dependence of cross sections on the relative kinetic energy. Representative trajectories reveal that the bond dissociation takes place on a short time scale of about 50 fs in strong direct collisions. Collision-induced dissociation is found to be remarkably restricted to the perpendicular approach of ArXe to the molecular axis of O2-, while collinear collisions do not result in dissociation. The higher collisional energy-transfer efficiency of xenon compared with argon is attributed to both mass and polarizability effects.     

On the doubly ionized states of Ar2 and their intra- and interatomic decay to Ar2 3+

Potential energy curves of the Auger state Ar+(2p(-1))-Ar, the different one- and two-site dicationic states Ar2 ++ (with energies in the range of 32-77 eV), and the lowest two-site tricationic states Ar++ - Ar+ (with energies in the range of 64-76 eV) computed using elaborated ab initio methods are reported. The accessible relaxation channels of the electronic states of Ar++ - Ar populated by Auger decay are studied. In particular, we study in detail the interatomic Coulombic decay following the population of one-site satellite states of Ar++(3s(-1)3p(-1))-Ar recently observed experimentally. Other relaxation pathways of Ar++ - Ar, including radiative charge transfer, nuclear dynamics through curve crossing, and intra-atomic decay processes are also investigated.     

Ar solvation shells in K(+)-HFBz: from cluster rearrangement to solvation dynamics

The effect of some leading intermolecular interaction components on specific features of weakly bound clusters involving an aromatic molecule, a closed shell ion, and Ar atoms is analyzed by performing molecular dynamics simulations on potential energy surfaces properly formulated in a consistent way. In particular, our investigation focuses on the three-dimensional Ar distributions around the K(+)-hexafluorobenzene (K(+)-HFBz) dimer, in K(+)-HFBz-Ar(n) aggregates (n ≤ 15), and on the gradual evolution from cluster rearrangement to solvation dynamics when ensembles of 50, 100, 200, and 500 Ar atoms are taken into account. Results indicate that the Ar atoms compete to be placed in such a way to favor an attractive interaction with both K(+) and HFBz, occupying positions above and below the aromatic plane but close to the cation. When these positions are already occupied, the Ar atoms tend to be placed behind the cation, at larger distances from the center of mass of HFBz. Accordingly, three different groups of Ar atoms are observed when increasing n, with two of them surrounding K(+), thus, disrupting the K(+)-HFBz equilibrium geometry and favoring the dissociation of the solvated cation when the temperature increases. The selective role of the leading intermolecular interaction components directly depending on the ion size repulsion is discussed in detail by analyzing similarities and differences on the behavior of the Ar-solvated K(+)-HFBz and Cl(-)-Bz aggregates.     

The mechanism of proton exchange: guided ion beam studies of the reactions, H(H2O)n+ (n=1-4)+D2O and D(D2O)n+ (n=1-4)+H2O

Reactions of protonated water clusters, H(H(2)O)(n) (+) (n=1-4) with D(2)O and their "mirror" reactions, D(D(2)O)(n) (+) (n=1-4) with H(2)O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H(2)O and D(2)O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.     

Isomer-selective detection of microsolvated oxonium and carbenium ions of protonated phenol: infrared spectra of C6H7O+-Ln clusters (L=Ar/N2, n</=6)

Infrared photodissociation (IRPD) spectra of clusters composed of protonated phenol (C(6)H(7)O(+)) and several ligands L are recorded in the O-H and C-H stretch ranges using a tandem mass spectrometer coupled to a cluster ion source. The C(6)H(7)O(+)-L(n) complexes (L=Ar/N(2), n=1-6) are generated by chemical ionization of a supersonic expansion. The IRPD spectra of mass selected C(6)H(7)O(+)-L(n) clusters obtained in various C(6)H(7)O(+)-L(m) fragment channels (m相似文献   

Direct dynamics classical trajectory simulations of the O+ + CH4 reaction at hyperthermal energies

A Born-Oppenheimer direct dynamics simulation of the O(+) + CH(4) reaction dynamics at hyperthermal energies has been carried out with the PM3 (ground quartet state) Hamiltonian. Calculations were performed at various collision energies ranging from 0.5 to 10 eV with emphasis on high energy collisions where this reaction is relevant to materials erosion studies in low Earth orbit and geosynchronous Earth orbit. Charge transfer to give CH(4)(+) is the dominant channel arising from O(+) + CH(4) collisions in this energy range, but most of the emphasis in our study is on collisions that lead to reaction. All energetically accessible reaction channels were found, including products containing carbon-oxygen bonds, which is in agreement with the results of recent experiments. After correcting for compensating errors in competing reaction channels, our excitation functions show quantitative agreement with experiment (for which absolute magnitudes of cross sections are available) at high collision energies (several eV). More detailed properties, such as translational and angular distributions, show qualitative agreement. The opacity function reveals a high selectivity for producing OH(+) at high impact parameters, CH(3)(+)/CH(2)(+)/H(2)O(+) at intermediate impact parameters, and H(2)CO(+)/HCO(+)/CO(+) at small impact parameters. Angular distributions for CH(3)(+)/CH(2)(+)/OH(+) are forward scattered at high collision energies which implies the importance of direct reaction mechanisms, while reaction complexes play an important role at lower energies, especially for the H(2)O(+) product. Finally, we find that the nominally spin-forbidden product CH(3)(+) + OH can be produced by a spin-allowed pathway that involves the formation of the triplet excited product CH(3)(+)(?(3)E). This explains why CH(3)(+) can have a high cross section, even at very low collision energies. The results of this work suggest that the PM3 method may be applied directly to the study of O(+) reactions with small alkane molecules and polymer surfaces.     

Velocity map imaging of ion-molecule reactive scattering: the Ar(+)+ N(2) charge transfer reaction

We present a velocity map imaging spectrometer for the study of crossed-beam reactive collisions between ions and neutrals at (sub-)electronvolt collision energies. The charge transfer reaction of Ar(+) with N(2) is studied at 0.6, 0.8 and 2.5 eV relative collision energies. Energy and angular distributions are measured for the reaction product N. The differential cross section, as analyzed with a Monte Carlo reconstruction algorithm, shows significant large angle scattering for lower collision energies in qualitative agreement with previous experiments. Significant vibrational excitation of N(+)(2) is also observed. This theoretically still unexplained feature indicates the presence of a low energy scattering resonance.     

Collision-induced dissociation of transition metal-oxide ions: dynamics of VO+ collision with Xe

The collision-induced dissociation of VO(+) by Xe has been studied by the use of classical dynamics procedures on London-Eyring-Polanyi-Sato potential-energy surfaces in the collision energy range of 5.0-30 eV. The dissociation threshold behavior and the dependence of reaction cross sections on the collision energy closely follow the observed data with the threshold energy of 6.00 eV. The principal reaction pathway is VO(+) + Xe --> V(+)+ O + Xe and the minor pathway is VO(+) + Xe--> VXe(+) + O. At higher collision energies (E > 8.0 eV), the former reaction preferentially occurs near the O-V(+)...Xe collinear and perpendicular alignments, but the latter only occurs near the perpendicular alignment. At lower energies close to the threshold, the reactions are found to occur near the collinear configuration. No reaction occurs in the collinear alignment V(+)-O...Xe. The high and low energy-transfer efficiencies of the collinear alignments O-V(+)...Xe and V(+)-O...Xe are attributed to the effects of mass distribution. The activation of the VO(+) bond toward the dissociation threshold occurs through a translation-to-vibration energy transfer in a strong collision on a time scale of about 50 fs.     

Trajectory dynamics study of collision-induced dissociation of the Ar + CH4 reaction at hyperthermal conditions: vibrational excitation and isotope substitution

We investigate the role of vibrational energy excitation of methane and two deuterated species (CD(4) and CH(2)D(2)) in the collision-induced dissociation (CID) process with argon at hyperthermal energies. The quasi-classical trajectory method has been applied, and the reactive Ar + CH(4) system has been modeled by using a modified version of the CH(4) potential energy surface of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339) and the Ar-CH(4) intermolecular potential function obtained by Troya (J. Phys. Chem. A 2005, 109, 5814). This study clearly shows that CID is markedly enhanced with vibrational excitation and, to a lesser degree, with collision energy. In general, CID increases by exciting stretch vibrational modes of the reactant molecule. For the direct dissociation of CH(4), however, the CID cross sections appear to be essentially independent of which vibrational mode is initially excited. In all situations studied, the CID cross sections are always greater for the Ar + CD(4) reaction than for the Ar + CH(4) one, the Ar + CH(2)D(2) being an intermediate situation. A detailed analysis of the energy transfer processes, including their relation with CID, is also presented.     

Copper doping of small gold cluster cations: influence on geometric and electronic structure

The effect of Cu doping on the properties of small gold cluster cations is investigated in a joint experimental and theoretical study. Temperature-dependent Ar tagging of the clusters serves as a structural probe and indicates no significant alteration of the geometry of Au(n) (+) (n = 1-16) upon Cu doping. Experimental cluster-argon bond dissociation energies are derived as a function of cluster size from equilibrium mass spectra and are in the 0.10-0.25 eV range. Near-UV and visible light photodissociation spectroscopy is employed in conjunction with time-dependent density functional theory calculations to study the electronic absorption spectra of Au(4-m)Cu(m) (+) (m = 0, 1, 2) and their Ar complexes in the 2.00-3.30 eV range and to assign their fragmentation pathways. The tetramers Au(4) (+), Au(4) (+)[middle dot]Ar, Au(3)Cu(+), and Au(3)Cu(+)[middle dot]Ar exhibit distinct optical absorption features revealing a pronounced shift of electronic excitations to larger photon energies upon substitution of Au by Cu atoms. The calculated electronic excitation spectra and an analysis of the character of the optical transitions provide detailed insight into the composition-dependent evolution of the electronic structure of the clusters.     

H+ versus D+) transfer from HOD+ to N2: mode- and bond-selective effects

Reactions of HOD(+) with N(2) 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.