eV Collision Energy Research Articles

Observation of collision-energy, product-state, and angular-scattering specificity in the charge-transfer reaction of Ar +( 2P [formula omitted]) with N 2(X 1Σ g, υ = 0)

The reaction Ar +( 2P 3 2 ) + N 2(X 1Σ g, υ = 0) → Ar + N 2 +(X 2Σ g, υ″) was studied at 1.1 eV collision energy using a crossed-beam method. In contrast with results at higher and lower collision energies, the scattering diagram obtained at this energy exhibits clearly resolved structure which suggests that product ions are produced in vibrational levels υ = 1, 2, 3, and possibly 4. Further, the population of each of these vibrational levels is correlated with scattering at specific center-of-mass angles.

Absolute triple differential cross sections for electron impact ionization of helium: Comparison between experimental and theoretical results at 256 eV collision energy

Triple differential cross sections have been measured for electron impact ionization of helium at 256 eV collision energy, 3 eV energy of the slow outgoing electrons and scattering angles of the fast outgoing electrons of 4°, 6°, 8°, and 10°. The data have been put on absolute scale by extrapolating the generalized oscillator strength to zero momentum transfer. In this optical limit the triple differential cross sections can be normalized by using the well-known cross sections for photoionization.

Dynamics of the H+D2→HD+D reaction: Dependence of the product quantum state distributions on collision energy from 0.98 to 1.3 eV

Measurement of the nascent HD product rotational and vibrational state distributions for the H+D2→HD+D reaction is reported. Ultraviolet photolysis of HI using a pulsed laser at 291 or 280 nm is used to create H atoms in an HI/D2 gas mixture, giving H+D2 collisions with relative energies of 0.98 or 1.1 eV. Pulsed-laser coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to record rotationally and vibrationally resolved Raman spectra of the HD product of the photolytically initiated H+D2 reaction, under effectively single-collision conditions. The HD product quantum state distributions are extracted from these CARS spectra. The present data are combined with results we obtained previously at 1.3 eV collision energy to reveal the collision energy dependence of the product quantum state distributions. We find that at all three collision energies the product distributions can be quite accurately represented by a linear rotational and vibrational surprisal function. The rotational surprisal parameter is large, positive, and nearly constant (θR=3.0–3.5) over this energy range, indicating a strong and consistent dynamical bias against product rotational excitation. The vibrational surprisal parameter, in contrast, varies much more, and is not even a monotonic function of collision energy (λV=2.2, 3.3, and 2.6 at 0.98, 1.1, and 1.3 eV, respectively). This behavior indicates enhancement of the cross section for the vibrationally nonadiabatic reaction channel at certain energies. At all collision energies 69%–73% of the available energy appears as translational energy of the products, with 22%–24% in rotation, and only 4%–7% in vibration. Product quantum state distributions derived from quasiclassical trajectory calculations at 0.98 and 1.3 eV collision energies agree very well with our measurements, indicating that classical mechanics provides an adequate description of the dynamics at these energies, and that the ab initio H3 potential surface used in these calculations must be at least reasonably accurate.

Energy transfer in collisional activation. Energy dependence of the fragmentation of n-alkylbenzene molecular ions

n-Alkylbenzene molecular ions, C 6H 5CH 2R +·, show two major fragmentation routes, reactions (1) and (2). C 6H 5CH 2R +· → C 7H 7 ++R · → C 7H 8 + +(R-H) Using charge exchange mass spectrometry, the C 7H 7 +/C 7H 8 +· ratio was found to increase from 0 at ∼ 2 eV internal energy to values ranging from ∼ 8 (R = C 3H 7) to ∼ 2 (R = C 7H 15) at ∼ 7 eV internal energy of the fragmenting C 6H 5CH 2R +· ions. The low energy (tandem quadrupole) collision-induced dissociation (CID) of C 6H 5CH 2R +· ions show increasing C 7H 7 +/C 7H 8 +· ratios with increasing ion kinetic energy but reach plateau values, dependent on the identity of R, at ∼ 60 eV collision energy using N 2 and Ar collision gases. From a comparison of the CID and charge exchange results, the average kinetic energy transformed into internal energy on collision is evaluated. The energy transfer efficiency is high (33–87%) and increases with increasing degrees of vibrational freedom in the ion. These results are consistent with the formation of a collision complex between the ion and the neutral species in which the relative translational energy is converted into vibrational energy of the complex. The high energy (8 keV) CID of the alkylbenzene molecular ions shows that the average energy transfer is in the range 4.5–6.7 eV, again increasing with increasing size of the alkylbenzene.

Predominance of CF-stretch vibrational excitation in state resolved inelastic scattering of protons from 11 small fluorohydrocarbon molecules

Time-of-flight spectra of H+ scattered from all ten different fluorinated hydrocarbon molecules of type CHmFn and C2HmFn (m+n=4) as well as C2F6 have been measured for small scattering angles (θlab≤15°) at 9.8 eV collision energy. For the pure hydrocarbons the spectra show a complicated structure suggesting unresolved excitation of several modes and a significant effect of charge transfer collisions. In contrast all the spectra for fluorine containing molecules exhibit more or less sharp maxima indicating high overtone excitation of predominantly one vibrational mode with ωv≂1200 cm−1 corresponding to a CF-stretch vibration. At some scattering angles the overtone transition probabilities deviate somewhat from a Poisson distribution suggesting small unresolved contributions from additional vibrational modes. Over- all, however, there is good agreement for the low symmetry molecules, some of which have sizable dipole moments, with the predictions of the simple forced oscillator model previously tested only for the spherically symmetric molecules CF4 and SF6. The relative magnitudes of the observed energy transfers are consistent with absolute infrared intensities where these are available from the literature.

The effect of collision energy and vibrational excitation on the reaction of C2H+2+H2

The relative efficiencies between the excitation of C–C stretching vibration and collision energy on the promotion of the H atom transfer reaction of C2H+2 +H2→C2H+3 +H have been examined. At low collision energies the reaction is strongly vibrationally enhanced in agreement with previous thermal energy studies. Translational energy was found to be not as effective as exciting the C–C stretching vibration in promoting the reaction. The results indicate that the H atom transfer from H2 to the C atom should have a late barrier, but the C–C stretching vibration of C2H+2 is apparently coupled to the reaction coordinate. Because the C–C bond in C2H+3 is expected to be longer than that of C2H+2, vibrational energy added to the C–C bond might reduce the barrier for H atom transfer from H2 to a C atom in C2H+2.

Collision energy effects in tanden mass spectrometry as revealed by a proton‐bound dimer ion

AbstractMass spectra resulting from collision‐induced decomposition of the proton‐bound dimer of iso‐propylamine and sec‐butylamine have been obtained as a function of laboratory collision energy over the range 10‐6000 eV. The ratio of the two principal fragment ions from the dimer ion measured as a function of collision energy is compared with the ratio expected as a function of internal energy as calcualted based on the statistical theory of mass spectra. This comparison indicates that the average energy deposited into the dimer ion upon collision reaches a maximum at a collision energy of ∼70 eV. The average internal energy of the ions at this collision energy is ∼4.3 eV. Other fragment ions which arise from higher energy decompositions are also observed in the spectra at much lower intensities. The relative intensities of these fragments indicate that the probability for large energy transfers are highest at ke V collision energies. These observations are interpreted on the basis of differences in the postcollision internal energy distributions resulting from keV and eV collisions.

Vibrational energy disposal in reactive and inelastic collisions of H(D)+HCl(DCl) at 1–3 eV

Vibrational energy disposal due to reactive exchange and unreactive translational-to-vibrational excitation in hyperthermal collisions of H+HCl and deuterated analogs is investigated by the excimer laser photolysis/infrared fluorescence technique. In the H+DCl system the excitation of DCl vibration increases from an energy efficiency of 4% to 9% as the initial H atom energy is increased from 1.4 to 3.1 eV. Only about 2% of the available energy appears as HCl vibration via H atom exchange, independent of the initial energy. The exchange reaction barrier is determined to be less than 0.95 eV (22 kcal/mol). At 2.3 eV collision energy the vibrational distribution of the HCl exchange product is relatively hot, with about 86% of the vibrational energy in molecules with v≥2. In the H+HCl system, where only the sum of inelastic and reactive collisions can be observed, the HCl vibrational distribution is markedly cooler, with only 60% of the vibrational energy in molecules with v≥2. The overall vibrational excitation efficiency of H+HCl is also a sensitive function of energy, increasing from 3% to 18% over the range 0.95 to 3.1 eV. This is consistent with an interpretation that the inelastic T–V excitation pathway is predominant. No information is obtained on the H atom abstraction channel.

The effects of collision energy and vibrational excitation on H+2, HD++He reactions

An experimental study of proton and deuteron transfer in H+2 + He and HD+ + He has been carried out as a function of kinetic and vibrational energy. The data gives evidence that at lower kinetic energies, the spectator stripping mechanism indeed plays an important role when H+2 or HD+ is vibrationally excited. The H+2 (v=0) reaction has a much smaller cross section than the v=1–4 reactions and seems to go through intimate, small impact parameter collisions involving all the atoms. Investigation of the competition between both the proton and deuteron transfer channels for the HD+ case, shows that vibrational enhancement towards forming the HeD+ product falls off sooner with increasing kinetic energy than does the HeH+ product again in accordance with the spectator stripping model. The higher yield for HeH+ production at both higher vibrational levels of HD+ and at lower kinetic energy and the behavior of translational energy dependence of HeH+ seems to indicate the importance of the induced orientation of HD+ during the collision with He. Because of the displacement of the center of mass from the center of charge in HD+, the charge induced dipole interaction between He and HD+ tends to swing the H atom more towards He during the approach of He and HD+.

Dissociative attachment in e--HCl, DCl collisions

Dissociative attachment of electrons on the various rovibrational levels of the HCl and DCl molecules is studied in the 0-1 eV collision energy range. The interaction between the electron and the neutral molecule is described in an effective range theory which explicitly includes the long-range polarisation forces and represents the short-range forces by a boundary condition. This allows for an exact treatment of the electron-vibrational motion interaction in the course of the collision. The collision equations are solved for the associative detachment process and the dissociative attachment process is obtained through the detailed balance principle. The non-adiabatic effects are taken into account, they are responsible for the dissociative attachment process which does not imply an intermediate resonant state. The R dependence of the boundary condition is adjusted in order to reproduce the available experimental data on vibrational excitation and dissociative attachment on thermal molecules as given by Rohr and Linder (1976) and Allan and Wong (1981).

A beam scattering study of the collision-induced dissociation of polyatomic ions CH 4+ and C 3H 8+ at eV collision energies

Several processes of CID of polyatomic ions in the low (a few eV) collision energy region were investigated in a beam scattering experiment. The systems studied were CH 4 + + He (formation of CH 3 +) and C 3H 8 + + Ne (formation of C 2H 4 + and C 2H 5 +). The results show that the most probable amount of energy which is inelastically transferred to the projectile ions depends linearly on the CM scattering angle (0° to 90°) and on the initial collision energy. The dynamics of CID at these collision energies is consistent with a vibrational energy transfer in which the interaction involves the polyatomic ion as a whole as described by statistical models.

The effect of collision energy in polyatomic ion/neutral target collisions over a range of 10–6000 eV

Mass spectra resulting from superthermal collisions between polyatomic ions and atomic or diatomic targets have been acquired as a function of the collision energy over a range of 10–6000 eV for the t-butyl cation and the molecular ions of methane, propane and n-butylbenzene. Changes in relative fragment ion abundances with systematic variation of the collision energy have been monitored in order to assess the effect of this experimental variable on the degree to which the polyatomic ion is excited by collision. The data for the t-butyl cation and the methane molecular ion show an apparent monotonic increase in energy deposition into the ion with increasing collision energy over the entire range investigated. The data for the molecular ions of propane and n-butylbenzene, on the other hand, indicate that maxima in energy deposition into these ions occur at laboratory collision energies of 140 and 70 eV, respectively. The results are interpreted as being due to changes in the relative importance of direct vibrational excitation and electronic excitation with increasing collision energy.

Dynamics of low-energy charge-transfer processes: Ar 2+ + He → Ar + + He + AT eV collision energies

The dynamics of the single-charge-transfer processes between Ar 2+ ( 3 P, 1 D) and He( 1 S) were studied in a crossed-beam experiment in the collision-energy range 0.5 – 1.6 eV. Scattering diagrams of the Ar + product show a significant difference in the relative differential cross section distributions for processes with Ar 2+ in the 3 P and 1 D states. This behaviour may be accounted for by assuming that the non-adiabatic transition occurs predominantly either before or after the system reaches the classical turning point. Ratios 3 P/ 1 D of the total cross sections of the two processes were estimated to be 0.32 and 0.83 at collision energies of 0.53 eV and 1.62 eV, respectively.

A crossed beam study of the charge-transfer reaction of Ar+ with N2 at low and intermediate energies

The charge-transfer reaction Ar+(N2,Ar) N+2 was studied in a crossed beam experiment at 1.73 and 4.01 eV collision energies. A weak plasma ion source was used to generate a nearly pure Ar+ (2P3/2) ion beam, which is crossed at 90° by a He-seeded supersonic jet N2 beam. Detailed analysis of the results demonstrates that the reaction proceeds cleanly to generate N+2(X Σ+g, v=1) with a distribution of rotational states. A direct mechanism, qualitatively described as electron jump with negligible momentum transfer, is followed at collision energies of 1.7 and 4 eV.

Reactions of Br+at room temperature: state-dependent reactivity of Br+with O2from 0.04 to 5 eV collision energy

The reactions of Br+ with a number of gases have been studied at room temperature using a selected-ion flow tube. The presence of at least two states of the Br+ ion was clearly demonstrated in the reaction of Br+ with O2 where one of the Br+ states did not react with a measurable rate coefficient. This unreactive state is presumably the lower fine-structure level of the ground Br+(3Pj) state for which charge transfer with O2 is endothermic by 0.22 eV. This conclusion is supported by the observed onset of reaction of this state at about 0.2 eV collision energy. In each of the other reactions of Br+ studied the two components of Br+ displayed no difference in reactivity and with the exception of reaction with NO the rate coefficients observed approached the classical collision rate kc. The reaction of Br+(3P2) with NO was found to proceed with a rate coefficient of k=4.1*10-11 cm3 s-1 but it was shown that the excited component was quenched with a rate coefficient (kq) close to the collision rate. The energy dependences of the reactions of the two states of Br+ with O2 were studied from room temperature up to a collision energy of 5 eV using a flow-drift tube. At the higher collision energies the rate coefficient observed for the excited component of Br+ was found to be several times higher than the room-temperature classical collision rate. The excited component of Br+ is believed to be a composite of the Br+(3P1) and Br+(3P0) states.

Dynamics of ion-induced-dipole capture: Effects of ion mass and collision energy

Quasiclassical trajectories have been used to investigate ion–molecule complex formation and dissociation in model systems. A model ion-induced-dipole potential was used to represent the long-range attractive forces and an exponential was used for the repulsive part of the potential. The emphasis is on Cl−+Cl2 ⇄ [Cl3−]*, with some studies of the influence of ion mass on capture and lifetime. Capture cross sections as a function of complex lifetime were computed.

Ion-molecule reactions of O + with CH 4

Primary and secondary reactions following impact of O + ions on a CH 4 gas target have been studied at 2.5 and 15 eV collision energy. The product ions OH +, CH 3 −, CH 4 −, CH 5 + were observed using energy and mass analysis. OH + originates from a stripping-type mechanism, the other product ions have very low kinetic energy. Varying the proportion of metastable O + ions in the beam, separate cross sections were measured for charge transfer, hydrogen atom transfer and hydride ion transfer to ground-state as well as metastable O − ions. For ground-state O − ions charge transfer predominates, while for metastable O + ions CH 3 − production by hydride ion transfer and/or dissociative charge transfer is the dominant process.

Time-of-flight measurements of the vibrational and rotational excitation of no by collision with 7Li + ions at 7 eV collision energy

The energy loss of 7Li + ions scattered from NO has been measured in a molecular beam experiment over a range of scattering angles (θ lab = 6.0–30.0°) at E c.m. = 7 cV. The vibrational/rotational energy transfer behaves similar to the previously reported study on 7Li +CO, suggesting that in NO the degeneracy of the electronic ground state has little effect.

Very low energy collision induced vibrational relaxation of 1A u glyoxal

We report the results of a study of very low energy collision induced vibrational relaxation of 1Au glyoxal. The effective collision energy is found to be different for different initial vibronic states, and the final states populated are drawn from a small subset of all available final states. The observed pathways of relaxation are interpreted using a previously proposed orbiting resonance model for the He: glyoxal collisions. It is inferred that (i) single collision–single quantum transfers dominate the relaxation process, and (ii) when there are near degeneracies in the vibrational manifold the lowering of symmetry in the orbiting resonance can permit mode mixing that is prohibited in the isolated molecule.

The effects of collision energy and ion vibrational excitation on proton and charge transfer in H2++N2, CO, O2

The effects of vibrational excitation of reagent ions and collision energy on the proton and charge transfer reactions of H2+ and D2+ with N2, CO, and O2 have been investigated using a method combining photoionization and a radio frequency guided ion technique. For proton transfer the H2++N2 results are quite similar to the H2++Ar system; proton and charge transfer channels are closely coupled for kinetic energies above 3 eV. In contrast, for H2++CO, the vibrational dependence of proton transfer is quite weak. For the H2++O2 case, evidence is seen for direct competition between the charge and proton transfer process. A simple model for charge transfer is used to assess the importance of energy resonance and Franck–Condon effects. In these simple molecular systems, the energy defect is found to be most important to charge exchange, although Franck–Condon factors play a substantial role.