D. S. Belic

Electron impact dissociation and ionization of N-2(+)

Absolute cross sections for electron impact ionization, dissociative excitation (DE) and dissociative ionization of N-2(+) ions are measured in the energy range from threshold to 2500 eV. The animated crossed electron-ion beam method has been employed. The individual contributions of ionization products (N-2(2+)) and dissociation fragments (N+), which have both identical mass-to-charge ratio and average velocity, are deduced from the analysis of product velocity distributions. Particular attention was paid to determining the transmission efficiency for dissociation fragments, since their collection was incomplete during the measurements. Threshold energies and kinetic energy released to dissociation fragments are measured. The role of states contributing to different reactions is discussed. For DE, the present results are found to be much smaller than the results of Peterson et al (1998). For ionization (single and dissociative), a satisfactory agreement with their result is obtained as well as with the prediction of Kim et al (2000) obtained in the binary-encounter Bethe approximation.

Absolute cross sections and kinetic energy release distributions for electron impact ionization and dissociation of CD

Absolute cross sections for electron impact dissociative excitation and ionization of CD+ leading to the formation of ionic products ( D+, C+, C2+ and C3+) are reported in the energy range from their respective thresholds to 2.5 keV. Around the maximum, cross sections are found to be ( 10.5 +/- 1.0) x 10(-17) cm(2), ( 20.6 +/- 3.5) x 10(-17) cm(2), ( 1.20 +/- 0.11) x 10(-17) cm(2) and ( 8.2 +/- 1.5) x 10(-20) cm(2) for D+, C+, C2+ and C3+, respectively. In the very low- energy region, dissociative excitation leading to the C+ formation dominates over the D+ one. The cross section for dissociative ionization ( C+ + D+ formation) is found to be ( 6.9 +/- 1.3) x 10(-17) cm(2) around 105 eV and the corresponding threshold energy is ( 22.1 +/- 0.5) eV. The animated crossedbeams method is used and the analysis of ionic product velocity distributions allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 15 eV both for C+ and for D+, and up to 40 eV both for C2+ and for C3+. For singly charged products, the comparison of the present energy thresholds and kinetic energy release with published data allows the identification of states contributing to the observed processes. In particular, contributions from primary ions formed in the a(3)Pi metastable state are perceptible. At fixed electron energy, the cross sections for the various ionization channels are seen to reduce exponentially with the potential energy of each dissociated ion pair. Anisotropies are estimated to be in the range 8 +/- 2% for both C+ and D+. The total CD+ single ionization cross section calculated by application of the Deutsch - Mark formalism is found to be in good agreement with experimental results.

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

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) x 10(-17) cm(2) and ( 6.24 +/- 1.33) x 10(-17) cm(2), 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) x 10(-17) cm(2) 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+.

Absolute cross section measurements for electron impact ionization of Ar7

The first absolute cross section measurements for single and double electron impact ionization of sodium-like Ar7+ are reported. The animated crossed beams method has been employed in the energy range from threshold to 3000 eV. The measured cross sections for single ionization are higher than the theoretical and semi-empirical predictions by about 20-50%. This discrepancy has been associated with the contribution of the indirect ionization processes. The double ionization cross section is only 1% of the single one.

Ionization and dissociative ionization of CO+ by electron impact

Absolute cross sections for CO2+, C2+ and O2+ formation from CO+ by electron impact have been measured in the energy range from below the respective ionization or dissociative ionization thresholds to approximately 2500 eV. The animated crossed-beams method has been employed. The cross sections at the maximum are found to be 1.45 x 10(-17), 3.4 x 10(-19) and 1.3 x 10(-19) cm(2), for these reactions respectively and the corresponding threshold energies are determined to be 27.0 +/- 0.5, 42.3 +/- 1 and 48.1 +/- 1 eV. From an energy analysis of the collected dissociation products (C2+ and O2+) the maximum centre-of-mass energy available is found to be 12.1 +/- 1 and 7.2 +/- 1 eV and the width of the Franck-Condon transition band available is found to be 2.5 +/- 1 and 5 +/- 1 eV, respectively.

Electron impact ionization and dissociation of CO2+ to C+ and O+

Absolute cross sections for electron impact ionization and dissociation of CO2+ to form C+ and O+ fragments are measured in the energy range from threshold to 2500 eV. The animated crossed-beams method has been employed. The ionization cross section shows a maximum of 4.79 x 10(-17) cm(2) at 130 eV and the corresponding threshold is found in good agreement with previous measurements. Both dissociation cross sections are shown to exhibit wide plateaux which are of the same order of magnitude. In addition, these cross sections almost coincide above 100 eV. The threshold energy and kinetic energy released are determined for both the production of C+ and O+. They are found in good agreement with the previously published data obtained in electron impact ionization experiments of neutral CO2.

Electron impact double ionization of singly charged ions C+, N+, O+, F+ and Ne+

The first absolute cross section measurements for double ionization of C+, N+, O+ and Ne+ ions by electron impact are reported. The animated crossed beams method has been employed in the energy range from below ionization thresholds to approximately 2500 eV. The classical binary encounter approximation overestimates measured cross sections by almost two orders of magnitude. Along the sequence, the cross section maximum does not follow classical scaling laws. A simple scaling law based on an electron pair ejection model is proposed for the prediction of direct double ionization. Inner-shell ionization followed by autoionization is seen to play a dominant role for C+ only.

Absolute cross-sections and kinetic energy release distributions for electron-impact dissociation of D+3

Absolute cross-sections for electron-impact dissociation of D-3(+) leading to the formation of D+ and D-2(+) products have been measured by applying the animated electron-ion beam method in the energy range from the respective thresholds up to 2.5 keV. The maximum total cross-sections are observed to be (13.9 +/- 1.1) x 10(-17) cm(2) and (8.2 +/- 0.1) x 10(-17) cm(2) (around 50 eV) for D+ and for D-2(+) fragments, respectively. The present results are in a satisfactory agreement with those of experiments performed at storage ring facilities in the low energy range (<30 eV). The data of D+ formation especially show resonant dissociative excitation playing a significant role in the collision energy range below the vertical excitation energy for the lowest electronic transition. The yields of fast D+ and D-2(+) ions have been measured; these energetic dissociations are mainly ascribed to dissociative ionization. For each fragment, absolute cross-sections are determined separately for dissociative excitation and for dissociative ionization processes. Kinetic energy release distributions are seen to extend from 0 to 16 eV both for D+ and for D-2(+) fragments. Present energy thresholds and kinetic energy release results are compared with available published data, allowing in some cases identification of fragmentation patterns and of molecular states contributing to observed processes.

Asymmetrical dissociative ionization of N+2 and O+2 by electron impact

Absolute cross sections for electron impact formation of N2+ from N-2(+) and O-2(+) from O-2(+) have been measured in the energy range from below threshold to approximately 2000 eV. The animated crossed beams method has been employed. The cross sections at the maximum are found robe 8.4 x 10(-19) cm(2) and 12.1 x 10(-19) cm(2), respectively and the corresponding threshold energies are determined to be 43.7 +/- 0.5 eV and 36.8 +/- 0.5 eV. From an energy analysis of the collected dissociation products (N2+ and O2+), the maximum centre of mass energy available is found to be 13.0 +/- 1 eV and 14.6 +/- 1 eV, respectively.

Electron impact double ionization of C- and O- ions

Absolute cross section measurements for double ionization of C- and O- ions by electron impact are reported. The animated crossed beams method has been employed in the energy range from ionization threshold to approximately 2.5 keV. Present results are found to be smaller than the experimental result of Steidl et al (1995 Proc. 19th Int. Conf. Physics of Electronic and Atomic Collisions (Whistler) ed J B A Mitchell et al p 564). The role of slow positive ions trapped in the electron beam could explain the observed discrepancy. The study of double ionization processes along the fluorine isoelectronic sequence (for O-, Ne+ and Ar9+ ions) shows that the results for multiply charged ions are strongly lower than predicted by classical scaling. A systematic threshold energy shift is observed in the sequence and, for O-, the position of the cross section maximum is found to be at an unusually high energy. The autoionizing 1P state belonging to the (1s(2)2s2p(5)) configuration of atomic oxygen is clearly seen to contribute to the O+ signal above 23.7 eV. Other autoionizing states are also seen to play a role around the K-shell ionization threshold. The Bethe-plot of present data shows that the high energy behaviour of cross sections is dominated by the E-1, term for C-, while it is dominated by the logarithmic term for O-.