Ion-pair Formation In Collisions Research Articles

Stereodynamics of asymmetric ion-pair formation in collisions of highly-charged ions with rare gas dimers

SynopsisStereodyanamical effects are analyzed for multiple ionization of rare gas dimers by slow highly charged ions using the three-center Coulombic over-barrier model previously developed by the present authors.

Dissociative ion-pair formation in collisions of fast potassium atoms with benzene and fluorobenzene

Using a crossed molecular-beam technique, electron transfer is studied in collisions of fast potassium atoms with benzene and fluorobenzene molecules. The negative fragment ions formed in the collision region are extracted by a pulsed voltage, and their masses and energy release distributions analyzed by a time-of-flight (TOF) method. The dominant fragment from C6H5F is F-. The phenyl ring fragments CH- and C2H- are also observed but with lower intensities. In the case of C6H6 the dominant anionic specie is C2H-. The kinetic-energy release distributions of the various fragments are derived from the width and shapes of the TOF peaks in the spectrum.

Theory of ion-pair formation in Rydberg-atom-ground-state-atom collisions at thermal energies.

We develop the distorted-wave and adiabatic theories of the ion-pair formation in collisions of a Rydberg atom ${\mathit{A}}^{\mathrm{*}\mathrm{*}}$ with a ground-state atom B at thermal collision velocities. The e-B interaction is modeled by a zero-range potential. Both theories give results that are in qualitative agreement with the standard Landau-Zener model, although their prediction for m-changing transitions is different, where m is the projection of the angular momentum. To incorporate these transitions into the adiabatic theory, we take into account the rotation of the internuclear axis before and after a nonadiabatic transition. The m-averaged cross sections are substantially higher than those following from the standard Landau-Zener model, and the distorted-wave results are slightly higher than the adiabatic results with the internuclear-axis rotation taken into account. We apply our results to a description of the McLaughlin and Duquette experiment on ${\mathrm{Ca}}^{+}$-${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ pair formation. The calculated n dependence of the rate for the ion-pair formation is too flat as compared to the experimental observations and does not agree with known values of the electron affinity of Ca. We discuss other possible mechanisms responsible for the ion-pair formation in this process. \textcopyright{} 1996 The American Physical Society.

Excitation of the nitro group in nitromethane by electron transfer

In this paper we report on complementary measurements on ion-pair formation in collisions between K atoms and CH3NO2 molecules. The experiments were performed in a c.m. energy range from 20 up to 300 eV. Double differential cross sections were obtained by measuring the K+ ion yield as a function of the scatter angle and as a function of the post-collision laboratory energy. On the other hand, relative total partial cross sections for the formation of CH3NO−2, NO−2, and O− were measured in the same energy range. The experimental results lead to the conclusion that in this energy range electron transfer takes place to three ionic states of CH3NO−2, a dominantly repulsive 2A1 state and two 2B1 states with relatively deep potential wells.

Total and double differential cross sections of ion-pair formation in collisions of K atoms with SnCl4 and CCl4

The reaction K+ACl4→K++(A−Cl4)−* with A=Sn and C was examined as a function of the collision energy from threshold up to about 40 eV in the c.m. system. Total cross sections of the mass-selected negative ions and doubly differential cross sections (energy and angle) of the K+ ions have been determined. Electron affinities, bond energies, and electronic excitation were calculated from the appearance potentials. In addition, the total cross sections for SnCl4 were measured and are contrasted with the earlier results of CCl4 from Dispert and Lacmann. Although both parent molecules have the same electron affinity within their error limits (2.2 eV for SnCl4 and 2.0 eV for CCl4) and the same dissociation energy for the negative ions of 1.4±0.2 eV, the product ion yields differ drastically. The main negative ion yield in K+SnCl4 results from SnCl−4 formation (over 80%). Its lowest dissociation channel leads to SnCl−3 formation, while Cl− ions are the main ions produced (90%) from CCl4, with only 7% leading to CCl−3+Cl formation at higher energies. These results support orbital energy considerations of electron addition to SnCl4 and CCl4 as applied to the results of reactive collisions of these molecules. The electron affinity and an electronically excited state of SnCl3 have been also determined. Morse potentials of CCl−4 and SnCl−4 were fitted to the experimental results of energy loss measurements from this work. The vertical electron affinities thus derived are 1.15 eV for SnCl4 and −1.0 eV for CCl4.

Comparison of ion pair formation in the systems Ar*+I2 and K+I2

A simple model that has been used extensively by Los and co-workers to treat ion pair formation in collisions of alkali atoms with diatomic molecules is extended to include continuum coupling via a competing Penning ionization channel. This extended model is then used to calculate the differential cross sections for ion pair formation for the system Ar*+I2 over the energy range 28–154 eV and to compare with a previous treatment of K+I2. In the absence of significant competition from continuum processes, Ar* is expected to behave in a manner similar to K, since the active electron is an unpaired 4s electron in both cases. We perform model calculations for Ar*+I2 to investigate the effects of varying the potential curves and charge exchange matrix elements and of including a continuum coupling function Γ (R). Comparison with previous calculations for K+I2 suggests increased repulsion on the Ar*–I2 surfaces relative to those of K–I2. The competing mechanisms of excitation transfer and Penning ionization may have a small effect upon the ion pair angular distributions.

Ion pair formation in H, D+Ar, Kr, Xe collisions from threshold to 100 eV

Integral cross sections are reported for ion pair formation in collisions between hydrogen (H,D) and noble-gas (Ar,Kr,Xe) atoms from threshold to 100 eV. The energy dependence of the cross sections is strongly oscillatory. A curve crossing model provides a good semi-quantitative explanation of the results.

Ion-pair formation in some potassium—molecule collisions. Dependence on the vibrational energy

State-specific cross sections for ion-pair formation in collisions of potassium with halogen and methylhalogen molecules are obtained by means of an inverse Laplace transform. This transform was applied to the total cross sections measured as a function of the temperature of the molecules and the translational energy. The efficiencies of the translational energy and internal energy are compared. The behaviour of the post-threshold region is discussed assuming strong stretching or even dissociation in the ionic channel.

The influence of the rate of bond stretching on the differential cross sections for ion-pair formation in K + XY collisions

Experimental differential cross sections are presented for ion-pair formation in collisions between K and I 2, IBr, Br 2, ICl, Cl 2. In this sequence the vibrational frequency of the halogen molecule is increased gradually from one system to the next. Therefore this is a suitable set of cros sections for analysing the influence of the rate at which the halogen bond stretches after transition to the ionic state. The experimental data are compared with trajectory calculations on diabatic and on adiabatic potential energy surfaces. The results of the two types of calculations differ significantly; the adiabatic model gives the better agreement with experiment. The essential difference between the two models is that in the diabatic model stretching of the halogen bond starts only when the crossing seam R c between the ionic and covalent configurations is reached, while in adiabatic scattering, pre-stretching of the halogen bond occurs as the alkali atom approaches the crossing seam. As a result of pre-stretching the electron affinity is increased, which in turn affects the endoergicity and the total and differential cross sections for ion-pair formation. At energie below the threshold for ion-pair formation, the cross section for reactive scattering is also affected.

Ion pair formation in alkali-SF 6 collisions: Dependence on collisional and vibrational energy

The dependence of ion pair formation in collisions of fast alkali atoms (K, Na and Li) with SF 6 on the initial relative kinetic energy and the internal energy of the target molecule has been studied by the crossed molecular beam method. Using a mass spectrometer we have measured total cross sections for negative ion formation as a function of translational and internal energy. Collision energies ranged from threshold up to 35 eV and SF 6 source temperatures were varied from 300 K to 850 K. By means of an inverse Laplace transform of the measured cross sections, we have determined total specific cross sections for each negative ion depending on the SF 6 vibrational energy and at fixed relative kinetic energy. The relative importance of both collisional and internal energy in promoting the electron transfer process is discussed for the various reaction channels in terms of a collision model. An essential feature of this model is the stretching of the S-F molecular ion bond during the collision. The product show complete relaxation in the threshold region, i.e., vibrational and collisional energy are equivalent: This holds for the SF − 6 formation only near threshold and for the SF − 5 and F − formation up to about 2 eV above threshold. In the post-threshold region the effect of the internal energy on the cross section dominates over that of the translational energy. From these measurements the adiabatic electron affinity of SF 6 is inferred to be 0.32 ± 0.15 eV, T = 0 K. Some other thermodynamic data are deduced: EA(SF 5) > 2.9 ± 0.1 eV ( T = 300 K) and D 0(SF − 5-F) = 1.0 ± 0.1 eV.