A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO⁺

Abstract

A crossed electron–ion beam experimental set-up has been upgraded for the study of electron impact ionization and dissociation of molecular ions by means of ionic product detection. Both the experimental set-up and the data analysis procedures are described in detail for the estimation of (i) absolute cross sections, (ii) kinetic energy release distributions (KERD) and (iii) anisotropies of angular distributions. Absolute cross sections are obtained separately for dissociative excitation (DE) and for dissociative ionization (DI). A double focusing magnetic field analyser is used for the observation of product velocity distributions, in the laboratory frame, at selected electron energies. The KERD in the centre of mass frame is calculated from the measured velocity distribution as well as the anisotropy of the angular distribution with respect to the initial orientation of the molecular ions. Results are reported for dissociative ionization and dissociative excitation of CO+ to C+ and O+ fragments in the energy range from about 5 eV to 2.5 keV. Absolute cross sections for DE at maximum, i.e. for an electron energy around 35 eV, are found to be (9.69 ± 2.08) × 10−17 cm2 and (6.24 ± 1.33) × 10−17 cm2, for C+ and O+, respectively, and the corresponding threshold energies are found to be (8.5 ± 0.5) eV and (14.8 ± 0.5) eV. The DE process leading to C+ production is seen to dominate at low electron energies. For DI, the absolute cross section is found to be (12.56 ± 2.38) × 10−17 cm2 around 125 eV and the corresponding threshold energy is (27.7 ± 0.5) eV. KERDs, which extend from 0 to 24 eV both for C+ and O+, exhibit very different shapes at low electron energy but similar ones above 100 eV, confirming the role observed respectively for DE and DI. The groups of states contributing to the different processes are identified by comparing present energies thresholds values and the KERDs with theoretical values. Anisotropies are estimated to be in the range 3–6% for both C+ and O+.

Electron impact induced dissociation of N2H+into NH+

“…This apparatus has been described in detail elsewhere [4] so only a brief description of the technique will be given. Figure 1 shows the experimental layout.…”

Electron-impact dissociative ionization of the molecular ionHDO+: A global view

“…In the animated crossed electron-ion beams method [8], which applied for this experiment [4], the HDO + molecular ion beam of well-defined energy (3-4 keV) interacts at right angles with the electron beam of tunable energy (from a few electron volts up to 2.5 keV). The ion beam is produced in an ECR ion source filled with a mixture of H 2 O, HDO, and D 2 O vapors.…”

“…Product ions are separated from the primary ion beam by using a double focusing 90 • magnetic analyzer. Product ions are further deflected by a 90 • electrostatic spherical deflector and directed onto a channeltron detector [4]. In this method the electron beam is swept across the ion beam in a linear motion at a constant velocity u so that the measured cross section σ m is related to the total number of events K produced during one complete electron beam movement by…”

“…Different types of projectiles and collisional processes can be used for removing one or more electrons of a molecular system and leading to its dissociation. The most important ones are: low energy ions by charge exchange from the target to the projectile [1,2], photons with photoionization [3], or electrons with electron-impact ionization of the target [4,5]. Several theory versus experiment comparisons have been successfully carried out in the past but, as far as we know, nothing exists in the case of electrons.…”

Kinetic-energy release of fragments from electron-impact dissociation of the molecular hydrogen ion and its isotopologues

“…Many experimental measurements of the KER cross sections have been used to extract scattering cross sections [8][9][10][11][12][13][14][15][16][17][18], identify dissociative processes [8][9][10][11][12][13][14][15][16][17][18], and to determine experimental conditions [18][19][20], molecular structure [21,22] and collision dynamics [21]. The KER cross sections can be extracted from a complete close-coupling calculation that includes all degrees of freedom (electronic, vibrational, and rotational).…”