Dissociative ionization

Let us consider a molecule AB in its vibronic ground state. Electrons of increasing energy impinging on the molecule can induce a dissociation following process (2). During this Franck-Condon vibronic transition the system can reach (i) the repulsive wall of a stable ionic state at or close to the dissociation limit or (ii) a repulsive state of the molecular ion (see Fig. 1). In both cases a continuous distribution in the ion-energy is observed. It has been pointed out [14, 15] that (i) the relative position of the potential energy curves of the initial (neutral) state and the final (ionic) state of the molecule, and (ii) the shape of the potential energy curve of the final state of the molecule determine the shape of the kinetic energy distribution function and the minimum kinetic energy carried away by the fragment ion. In a first approximation, the shape of the kinetic energy distribution of the fragment ion could be obtained by the reflection method. In this method the square of the ground state vibrational eigenfunction is reflected in the repulsive part of the upper state onto the energy axis (see Fig. 1). The amount of total kinetic energy involved in the dissociative ionization process is related to the slope of the potential energy curve within the Franck-Condon region. After dissociation, the total kinetic energy involved is spread over the two fragments according to the law of momentum conservation. The kinetic energy t carried away by A + is given by

Published in: Electronic and Atomic Collisions. Abstract of papers. VIII ICPEAC, Belgrad, 1973, Volume 1 (1973), pp. 242-425 Status: Postprint (Author’s version) Fig.1:Kinetic energy distribution of O + from dissociative ionization of O 2 by the impact of electrons of 35eV(2)and 60 eV(1)

The present dissociative ionization work on CH 4 (CD 4 ) producing H + (D + ) has been examined by electron impact using ion energy analysis. The instrument used in this work is fully described elsewhere (3). The kinetic energy distribution of H + (D + ) has been recorded at electron energies from 22-75 eV. D + distributions obtained close to the threshold are shown in fig. 1. In agreement with previous results, the peak at 2.3 eV has a constant FWHMover a wide electron energy range.

5.1. The C + dissociative ionization channel The lowest onset for the appearance of C + is measured at 21.16 ± 0.30 eV. The dependence of this critical energy upon the C + ion initial transla-tional energy is represented by the straight line (1) in Fig. 3. The calculated regression provides a linear fit with a correlation coefficient of 0.993. The extrapolation to KE(C + ) = 0.0 eV is 21.20 ± 0.05 eV with a slope of 0.38 ± 0.03. The expected slope is given by the ratio 14/26 = 0.54. As mentioned earlier [1,2], the discrepancy between experimental and expected slope has to be ascribed to the partitioning of the excess energy, with respect to the dissociation limit, between translational energy and internal energy of the polyatomic fragments.

The dissociative ionization work uses mono-chromatized synchrotron radiation from the Berlin storage ring BESSY. The quadrupole mass filter is equiped with a photoion source and an ion energy-analyzing retarding lens. This setup allowed to measure KE distributions at any desired wavelength and to record ionization efficiency curves for energy- selected photoions. The CH 3

in agreement with earlier investigations [1,2]. Furthermore, neither translational energy nor internal energy are carried by the fragments at threshold. Above the onset, the excess energy is entirely converted into at most 0.2 X 34/19 = 0.36 eV total translational energy. The absence of this process in resonant photo-ionization [3] and in photo-electron/photo-ion coincidence experiments [4] would indicate that the transition to the dissociative ionization continuum is strongly forbidden for photon impact.

As shown by the occurrence of a peak at 14.5 eV in the first derivative of the ionization efficiency curve of CS + (fig. 2a), the lowest threshold corresponds to an ion-pair, whereas at 14.7 eV a dissociative ionization process is involved. The CS + ions are formed essentially without kinetic energy in the electron energy range from threshold up to 25 eV (fig. 2b). Using IP(CS) = 11.33 eV [10], a low value for the appearance potential of CS + is shown to be consistent with a low value for the heat of formation of CS, in this case 40 kcal mole -1 , as is also found from the S + measurements. In conclusion, a mean value of ∆H f (CS) = (34 ± 6) kcal mole -1 is

THE C + ION FORMATION R. LOCHT, J.M. DÜRER Institut de Chimie, Sart-Tilman par B-4000 Liège 1, Belgium The interest of a study of the dissociative ionization of CO is twofold.(i) This molecule is isoelectronic of the N 2 molecule, (ii) Experimental as well as theoretical investigations of this molecule are scarce.

The dissociative ionization of CO 2 in the valence-shell ionization region has been investigated by almost all the techniques available today. The literature on this subject has been reviewed recently [1]. By high resolution photoionization mass spectrometry and threshold photoelectron spectroscopy the major importance of autoionization in the appearance of O + and CO + has been demonstrated [2-5]. By photoelectron photoion coincidence (PIPECO) experiments, using the He(I) resonance line at 58.4nm, only the CO + and O + dissociation channels are accessible energetically [6,7]. To our knowledge, only one coincidence experiment has been performed on the dissociative ionization of CO 2 using synchrotron radiation [4]. In the latter case, the

In a recent paper [1] we published the results obtained for the first two processes producing protons and deuterons from methane and methane-d 4 under the impact of low energy electrons in the range of 20-23 eV. The comparison of our data with previous work on the dissociative ionization as well as the dissociative excitation of methane by electron and photon impact was extensively discussed. The appearance mechanisms involving thermal and 2.3 eV kinetic energy protons were both related to dissociative autoionization. The strong parallelism between the results obtained either by electroionization or by dissociative excitation were discussed.

Later, a few groups performed angular dependent ion energy analysis, the anisotropy (or isotropy) of the distributions being related to the symmetry of the decomposing ionic states. Apart from the metastable decomposition studies, which became a powerful analysing technique, dissociative ionization studies with ion kinetic energy analysis remain scarce. Furthermore, a disadvantage of the former method is the uncertainty in the nature of the final ionic states which decompose.

4.4. Dissociation limit at 32.5 eV This dissociative ionization limit could not be detected in the ionization efficiency curve of N + taken at zero volt retarding potential, as shown in fig. 5. This is probably due to the predominant closely lying autoionization phenomena. By increasing the retarding potential by only 0.1 V, the rising portion of the curve indicates a process appearing at (35.0 ± 0.2) eV [see fig. 5 and the arrow labeled (2) in fig. 4]. The threshold of this process, which seems to be due to the direct ionization, does not shift as a function of the retarding potential in the range of 0.1 to 1.2 V. For higher retarding potentials, the threshold energy values follow a straight line which extrapolates at 32.5 eV with a slope of 0.5. This observation could only be interpreted by a dissociative ionization of N 2 through a transition to the repulsive part of a stable N 2

For the N 2+ ions formed by dissociative ionization of N 2 , fig. 1 shows the ion energy distribution as observed for different electron energies ranging from 60—100 eV. Peak maxima are measured at thermal energy with low intensity at high electron energy, at (1.2 ± 0.1) eV, (3.0 ± 0.1) eV, (4.1 ± 0.1) eV and around 7 eV. The peak observed at 4.1 eV for 75 eV and 80 eV electron energy shifts to 4.8 eV at 100 eV electron energy. Crowe et al. [3] mention peak maxima at 2.9 eV, 5.1 eV and 7.9 eV. Edwards et al. [5] using a 1 MeV H + ion beam found structures at (2.3 ± 0.5) eV, (5.3 ± 0.3) eV and (10.2 ± 0.5) eV in the N 2+ ion energy distribution. With a He + beam [5] the distribution is modified and peak maxima were measured at (4.2 ± 0.5) eV, (6.6 ± 0.3) eV, (12.1 ± 0.5) eV and at (16.0 ± 0.6) eV.

+ . In order to extend the available information, we have studied the kinetic energy distributions of the N + fragments from the dissociative ionization of N 2 by electron impact. In an ion-source, N 2 is excited by an electron beam of energies ranging from 0 eV to 80 ev:

3.1.2. Dissociation channel CO →  e − O + + C - and O + + C The kinetic energy distribution of the O + ions produced by dissociative ionization by electron impact on CO only shows a narrow and nearly thermal peak followed by a broad bell-shaped curve. Typical distributions are reproduced in fig. 3 as observed at different electron energies. The nearly thermal peak is characterized by a maximum measured at (17 ± 3) meV (see fig. 4) for 25 eV electrons. The maximum of the high energy peak shifts from 0.8 eV at 30 eV electron energy, up to 2.0 eV at 100 eV. The fwhm increases with the impinging electron energy. These observations are in good agreement with Köllmann's experimental results [12]. The existence of a thermal peak near 0 eV kinetic energy has been confirmed by the most recent experiments of this author [12] as well as by appearance potential measurements (see section 4.2). No particular feature has been observed at low ion energies for electron energies near the appearance threshold of O + . However, between 30 and 40 eV electron energy, the sudden change in shape of the high energy peak as well as its sudden and important shift of 0.6 eV to lower ion energies for decreasing electron energies are noteworthy. In the range of 40—100 eV electron energy a shift of only 0.5 eV is observed.

By using NeI radiation instead of the HeI resonance line, changes could occur in the photoelectron spectrum which could account for the differences observed in the ion energy distributions. In the particular case of N 2 O, it has to be noted that the 74.372 nm line, produced in the Ne discharge, is nearly resonant with 74.293 nm autoionization peak observed in the photo-ionization efficiency curves of O + , NO + and N 2 O + with less

The N 2 O molecule has been the subject of many investigations during the last few years. BERKOWITZ (1) made a detailled photoionization study of the four dissociation channels. BEAR (2) and NENNER (3) performed the threshold photoelectron-photoion coincidence study of N 2 O. Few are the dissociative electroionization studies of this molecule.

The experimental set-up is described elsewhere (5). Ion energy distributions are measured by recording the first differentiated retarding potential curve. Furthermore, in the present wo[r]

translational energy. Several appearance energies are observed in the first differentiated ionization efficiency curves of H + (D + ). The lowest onset is measured at 18.9 ± 0.2 eV for zero kinetic energy protons. This onset energy is assigned to the process C 2 H 2 +e

共Received 24 March 2005; accepted 23 May 2005; published online 9 August 2005兲 The molecular dynamics with quantum transitions 共MDQT兲 method is applied to study the fragmentation dynamics of neon clusters following vertical ionization of neutral clusters with 3 to 14 atoms. The motion of the neon atoms is treated classically, while transitions between the adiabatic electronic states of the ionic clusters are treated quantum mechanically. The potential energy surfaces are described by the diatomics-in-molecules model in a minimal basis set consisting of the effective 2p orbitals on each neon atom for the missing electron. The fragmentation mechanism is found to be rather explosive, with a large number of events where several atoms simultaneously dissociate. This is in contrast with evaporative atom by atom fragmentation. The dynamics are highly nonadiabatic, especially at shorter times and for the larger clusters. Initial excitation of the neutral clusters does not affect the fragmentation pattern. The influence of spin-orbit coupling is also examined and found to be small, except for the smaller size systems for which the proportion of the Ne + fragment is increased up to 43%. From the methodological point of view, most of the usual momentum adjustment methods at hopping events are shown to induce nonconservation of the total nuclear angular momentum because of the nonzero electronic to rotation coupling in these systems. A new method for separating out this coupling and enforcing the conservation of the total nuclear momentum is proposed. It is applied here to the MDQT method of Tully but it is very general and can be applied to other surface hopping methods.