Collision-induced Dissociation Cross Sections Research Articles

Collision-induced dissociation of Fe+n (n=2–10) with Xe: Ionic and neutral iron binding energies

Cross sections for collision-induced dissociation (CID) of Fe+n with Xe, 2≤n≤10, are presented. Experiments were performed on a newly constructed guided ion beam mass spectrometer, the design and capabilities of which are described in detail. The single mechanism for dissociation of iron cluster ions is sequential loss of iron atoms with increasing collision energies. There is no evidence for fission to molecular neutral products. The cross section threshold energy dependences are analyzed to give the bond dissociation energies (BDEs), D0(Fe+n−1–Fe). Data analysis employs an empirical model that incorporates RRKM theory to account for inefficient dissociation on the time scale of the experiment. Results show that Fe+6 has the strongest BDE, D0(Fe+5–Fe) =3.44±0.18 eV, while Fe+3 is the most weakly bound, D0(Fe+2–Fe) =1.64±0.15 eV. Neutral cluster BDEs are derived from ionic binding energies and known ionization potentials. Branching ratios and other cross section features are also discussed with respect to cluster size.

Collision-induced dissociation of niobium cluster ions: transition-metal-cluster binding energies

The cross sections for collision-induced dissociation (CID) of Nb{sub n}{sup +} (n = 2-6) with Xe are presented. Experiments are conducted on a recently constructed guided ion beam mass spectrometer, which produces intense beams of thermalized, mass-selected, niobium cluster ions. Nb{sub n}{sup +} are observed to fission to all possible ionic fragments and the largest possible neutral fragments at collision energies < 10 eV. Evidence is presented for loss of multiple Nb atoms from the cluster at energies higher than 10 eV. This fragmentation pattern differs markedly from that previously observed for small iron cluster ions. CID thresholds are used to derive D{degree}(Nb{sub n{minus}1}{sup +}-Nb) for n = 2-6, along with D{degree}(Nb{sub m}) and ionization potentials (IPs) of Nb{sub m} for m = 2 and 3. By using known IPs, D{degree}(Nb{sub n}) for n = 4,5 and 6 are also obtained. Nb{sub 2}{sup +} is found to be the most strongly bound cluster ion, D{degree}(Nb{sup +}-Nb) = 6.15 {plus minus} 0.15, and Nb{sub 3}{sup +} is the most weakly bound cluster ion, D{degree} (Nb{sub 2}{sup +}-Nb) = 4.60 {plus minus} 0.15 eV.

The effect of a conical intersection on cross sections for collision-induced dissociation

The cross section for H+H2(v, j)→3H, where v and j denote selected vibrational and rotational quantum numbers, is calculated by the quasiclassical trajectory method, using trajectory surface hopping to include the effect of the first excited electronic state which has a conical intersection with the ground state. The excited electronic state allows for collision-induced dissociation by the process H+H2(X 1Σ+g)→H3(1 2A′)→H3(2 2!iA′) →H +H2(b 3Σ+u) →3H, where the various transitions all occur in the course of a single collision. This new surface hopping mechanism increases the cross sections and rate constants for production of unbound states by 2%–44% for the conditions examined.

Collision-induced dissociation and ab initio studies of boron cluster ions: determination of structures and stabilities

Bonding in small boron cluster cations (BZTl3+) is examined by measurement of appearance potentials and fragmentation patterns for collision-induced dissociation (CID) with Xe. Cluster stabilities are generally found to increase with increasing cluster size; however, there are large fluctuations from the overall trend. The lowest energy fragmentation channel for all size cluster ions is loss of a single atom. Clusters smaller than six atoms preferentially lose B', while for the larger clusters the charge remains on the BW1+ fragment. The results are used to estimate cluster ionization potentials and geometries. Comparison of measured stabilities with magic numbers in the cluster ion size distribution and with total CID cross sections shows that neither is a reliable indicator of stability. We also report on ab initio calculations for both neutral and ionic BI4. The results include cluster geometries, ionization potentials, charge distributions, dissociation energies, and bonding character. The results for IPS, geometries, and De)s are compared with experiment.

The charge transfer and collision-induced dissociation cross sections of state-selected H +2 and D +2 ions

The collision-induced dissociation (CID) and charge transfer (CT) cross sections for H + 2 (υ = O to υ = 10) reactants with H 2 have been measured by threshold photoelectron-photoion coincidence (TPEPICO)_at center-of-mass translational energies of 4, 8, 12 and 16 eV. The relative cross sections, normalized by using the known absolute charge transfer cross sections for H + 2 (υ = 0 and 1), exhibit a marked dependence on the H + 2 vibrational energy. The CID cross sections increase from less than 1 Å 2 for υ = 0 to more than 6 Å 2 for υ = 10, while the CT cross section decreases from about 11 to 3 Å 2. One observes a much weaker variation with translational energy. The analysis of the H + product ion time of flight distribution shows that the H + ions are formed in a distribution of kinetic energies which peaks at both low and high laboratory kinetic energies. A direct dissociation model in which the projectile ion is excited to the repulsive 2Σ + u surface is consistent with the observation of fast H + ions. If the CID is preceded by a charge transfer step, then the same mechanism will give rise to the slow H + product ions. The ratio of fast to slow H + ions is unity for low υ but increases as υ increases. This is consistent with the observed decreasing charge transfer cross section with increasing υ.

Collision induced dissociation of aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2): Influence of electronic structure on stability

The collision induced dissociation of size selected aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2) by argon has been studied using a low energy ion beam apparatus. The main product from AlnO+ is Al+n−2. Aln−2O+ and Al+n−4 are observed from AlnO+2. These products can be accounted for by loss of Al2O (which is very strongly bound) from the clusters. The experimental results were used to bracket the Aln/O interaction energies at 8.0±1.0 eV for Al+6 and 7.5±1.0 eV for Al+19. These large interaction energies indicate that oxygen is multiply bonded to the clusters. The collision induced dissociation cross sections show a strong cluster size dependence. Minima in the cross sections as a function of cluster size are ascribed to particularly stable clusters. The number of aluminum atoms in the particularly stable AlnO+m cluster increases with the addition of oxygen atoms. The stability of these clusters can be accounted for by the electronic shell model if it is assumed that binding oxygen atoms to the clusters removes electrons from the cluster orbitals so that more aluminum atoms are required to close the electronic shells.

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

The collision-induced dissociation of aluminum clusters, Al+n (n=3–26), by argon, at a center of mass collision energy of 5.25 eV, has been studied using a low energy ion beam apparatus. Product branching ratios and collision induced dissociation cross sections are presented and discussed. The main product is Al+ for the smaller clusters and Al+n−1 for the larger ones. The cross sections rise to a peak at Al+6−Al+9 and then decrease with increasing cluster size. Cross sections for Al+7, Al+13, Al+14, and Al+23 are significantly smaller than their neighbors. A crude kinetic model is used to derive approximate cluster ionization potentials from the product branching ratios. The IPs initially rise with cluster size, peak at Al6 and then decrease. The IP of Al7 is particularly low and there is a sharp drop in IP at Al14 where the IP falls below that of the atom. The results suggest that the dissociation energies increase for the larger clusters and there is evidence that Al+7, Al13, Al+13, Al+14, and Al+23 have enhanced stability. The results are compared to the predictions of the electronic shell model which can account for some of the results but predicts additional features which are not observed.

Collision-induced dissociation of keV methane molecular ions. Analysis of the pressure dependence using probability theory

Collision-induced dissociation (CID) of methane molecular ions with 2 ∼ 7 keV kinetic energy has been investigated as a function of collision gas (He and N 2) pressure using mass-analyzed ion kinetic energy spectrometry. The experimentally observed pressure dependence of the parent and daughter ion intensities has been analyzed using probability theory. Cross-sections for various collision-induced processes have been obtained. Even though the CID cross-section for CH 4 +· ions was larger with N 2 collision gas than with He, the CID yield was higher with He than with N 2 at a given value of the total attenuation of the parent ion intensity. This is explained in terms of the relative magnitudes of the cross-sections for various collision-induced processes. The shape of the collisional energy-deposition function has been estimated from the experimental data. Average collisional energy transfer has been found to be proportional to the square-root of the parent-ion kinetic energy.

Photodissociation dynamics of CO3−

The dynamics of CO3− photodissociation is studied with a new photodissociation spectrometer that allows kinetic energy-resolved detection of parent ions and photofragments. Kinetic energy release distributions, photodissociation spectra, and the dependence of the photofragment intensity on the laser power and background pressure are presented. Photodissociation of CO3− in the energy range 1.95–2.2 eV leads to CO2+O− fragments, and is found to occur by two distinct mechanisms. These mechanisms involve three electronic states that correlate with CO2+O−—the 2B1 ground state, a 2A1 weakly bound state, and a repulsive 2B2 state. The first mechanism begins with a low cross section 2A1 ← 2B1 transition that gives structure to the spectra. From this intermediate state, a second photon carries the ion to the 2B2 state. Dissociation to the observed photofragments occurs rapidly on the repulsive surface. In this two photon mechanism, at least 20% of the available energy is disposed of in relative translation of photofragments. The second mechanism is also initiated by the 2A1 ← 2B1 transition. Deexcitation of the 2A1 bound state by internal conversion, however, leads to high lying vibrational levels of the ground 2B1 state. These vibrational levels are found to have an enhanced collision-induced dissociation cross section.

Collision-induced dissociation of the cesium halides

Total cross sections vs the relative collision energy are reported for collision-induced dissociation (CID) of CsF and CsBr to ion pairs on collision with beams of accelerated projectiles M, where M includes Ar, Kr, and Xe atoms as well as SF6 molecules. Measurements of the CID of CsCl with Ar and SF6 are also presented. Partial cross sections for both three body dissociation and for the formation of the weakly bound molecular ions CsM+ are determined. The cross sections for CID of CsF are found to be considerably smaller than those for the other halides for all projectiles but particularly for SF6. For Xe colliding with CsF the CID cross sections are determined to be only weakly dependent on the CsF internal energy, in contrast to earlier studies on CsI where a strong dependence was obtained. Two-dimensional in-plane trajectory calculations for Xe colliding with the four cesium halides are performed to identify the important dynamical effects governing the efficiency of CID. For CsCl, CsBr, and CsI the dissociation efficiency is found to depend mainly on the effects of relative mass, whereas for CsF, the relative size of the projectile vs the F− ion strongly affects the dissociation efficiency. A surprisingly strong influence of the weak attractive forces between the projectile and the cesium or halogen product ion is found which affects an increase in the energy transfer, and is related to the formation of either bound or transient molecular ions. A general discussion of the experimental results for the four cesium halides, as well as the dynamical effects involved in both three body dissociation and molecular ion formation, is presented.

Third body efficiencies for collision-induced dissociation of diatomics. Rate coefficients for H+H2→3H

Fixed-energy cross sections, thermal-translation rate constants, energies of activation, opacity functions, and probabilities of reaction as functions of initial relative orientation angle are presented for the reaction H+H2(v, j)→3H on the lowest-energy potential surface for selected initial vibrational-rotational states (v, j) of H2. In agreement with the experimental result for steady dissociation of a thermal mixture of initial states, the collision-induced dissociation process for these state-selected processes is found to be considerably enhanced for H as collision partner as compared to Ar as collision partner. We find that the cross sections for collision-induced dissociation are strongly increasing functions of initial internal energy and initial vibrational quantum number. The general trend at 4500 K is that the rate constants increase more rapidly than the equilibrium populations decreases as we increase v. The activation energies for complete dissociation are larger than the fixed-state binding energies by 0.2–1.1 eV.

High-energy collision-induced dissociation of H2 by H

New calculations are reported for the H + H2 (v,j) to 3H transition, where v and j are vibrational and rotational quantum numbers, at relative translational speeds 45-93 km/s for v = 0 and j = 6 and at a relative translation temperature of 4500 K for v = 0,1 and j = 0-30. The calculations are performed by the standard quasi-classical trajectory method, and at the end of each trajectory, the outcome is analyzed as complete dissociation, quasi-bound state, or bound state. The initial collision conditions are representative of those in molecular clouds behind high-speed shock fronts. The collision-induced dissociation cross section appears to have a classical dynamic threshold about 1 eV above the energetic threshold and to plateau at about 10 to the -20 sq m. Some state-selected rate constants for a relative translational temperature of 4500 K are also presented.

Fully converged three-dimensional collision-induced dissociation calculations with Faddeev—AGS theory

The quantum-mechanical calculation of the collision-induced dissociation cross section in a three-dimensional model system is reported. The obtained results resemble those from some collinear models but indicate clearly the futility of multiple-scattering approximations except at hyperthermal energies.

Collision-induced dissociation of UO + and UO +2

Collision-induced dissociation (CID) of uranium monoxide ion and uranium dioxide ions impacting on argon has been studied. The CID cross section for production of U + and UO +, respectively, has been measured at relative kinetic energies ranging from threshold to 70 eV. No appreciable amounts of other products were observed. The thresholds observed in these studies agree within experimental error with the bond dissociation energies of uranium monoxide ion, D 0 0(OU +O) = 8.0 eV, and uranium dioxide ion, D 0 0(OU +O) = 7.7 eV. The CID cross section for UO + reaches a maximum at about 20 eV and falls off as E −4 at higher energies, where E is the relative kinetic energy. In the UO + 2 system, the maximum cross section occurs at about 14 eV beyond which it decreases as E −2.8.

Surface-hopping trajectory calculations of collision-induced dissociation processes with and without charge transfer

Reaction cross sections for collision-induced dissociation (CID) Ne + He 2 + → Ne + He + He +, collision-induced predissociation (CIP) Ne + He 2 + → Ne + + He + He, and ion-transfer (IT) Ne + He 2 + → NeHe + + He were computed from surface-hopping trajectories on a model diatomics-in-molecules potential surface for He 2 + vibrational energies of 0.2, 0.8 and 1.7 eV and total collision energies of 2.7, 6.0 and 10.7 eV. In agreement with experiment, CIP is the dominant process at all energies; the calculations predict that its cross section has a maximum with respect to variation of the He 2 + vibrational energy. CID is a small-impact parameter process giving rise to wider scattering angles than CIP. The IT reaction has a very small cross section at all energies. These results agree well with existing experiments and suggest new experiments with state-selected He 2 + vibrational energy. The dynamics of reaction are interpreted in terms of a charge-transfer spectator model.

Collision-induced formation of positive and negative ions from diatomic and triatomic negative hydrogen ions

The collision-induced dissociation of diatomic (H2 −, HD−, D2 −) and triatomic (H3 −, HD2 −, D3 −) negative hydrogen ions to produce negative atomic ions and positive atomic and molecular ions upon collision with target gas has been investigated in the energy range 5–15 keV. The cross sections for collision-induced dissociation and double electron detachment have been measured by monitoring the growth of the fragment ions with target gas pressure. The total collisional attenuation cross sections of the parent negative molecular ions have also been determined.

Metastable Dissociation of CH5 + Ions in the Methane Mass Spectrum

Abstract The main dissociation channels of the CH5 + ion, i. e. have been investigated by a mass spectrometric method. The pressure dependences of the ratios I14,90/I17 and I13,26/I17, in the range from 4,4·10-4 to 1·10-4 Torr, were plotted. The two ratios depend linearly on the pressure in the mass spectrometer analyzer. Since the two straight lines do not pass through the origine the conclusion is drawn that, for both dissociation channels of the CH5 + ion, besides the collision-induced transitions, spontaneous transitions, i. e. pure metastable transitions, take place. The spontaneous dissociations constants (λ1 = 7,5·105 sec-1 , λ2 = 5,2·104 sec-1) and collision-induced dissociation cross-sections (σ1 = 5,25 · 10-16 cm2 , σ2 = 3,2·10-16 cm2) have been estimated. These results are discussed and compared with those reported by other authors.

Collision-Induced Dissociation of D2+ Ions by Argon and Nitrogen

The reactions D2++X→D++D+X, where X is Ar or N2, have been studied with an angular ion-scattering apparatus at ion laboratory energies in the range 5–120 eV. Kinetic-energy distributions as well as angular distributions of the product D+ ions have been determined. The experimental method consists of directing a well-collimated, mass-analyzed, and energy-selected ion beam into a scattering region and analyzing the product ions with a 127° electrostatic velocity selector and a radio-frequency mass filter. The product ion analysis and detection system rotates about the center of the scattering region. The results are discussed in terms of a simple two-step collision model, where the D2+ ion is first excited electronically to the antibonding 2Σu state, and then dissociates. The possibility of direct excitation of the D2+ ion into the vibrational continuum is also discussed. If either model is assumed for the reaction, the experimental results indicate that the most probable reaction channel for the collision-induced dissociation reaction is one in which primary D2+ ions with large vibrational energies give rise to D and D+ products which separate with small relative velocities. These results are consistent with some existing opinion that the collision-induced dissociation cross section increases rapidly as the vibrational excitation of the primary ion is increased.

Energy Dependence of Cross Sections for Collision-Induced Dissociation and Endothermic Ion—Molecule Reactions

Energy Dependence of Cross Sections for Collision-Induced Dissociation and Endothermic Ion—Molecule Reactions