J. Pommier

Low-energy electron attachment to fullerenes and in the gas phase

We have investigated the low-energy (0 - 10 eV) electron attachment on and , using an improved energy resolution close to zero incident energy (0.030 - 0.060 eV). We have found evidence of a zero-energy attachment process (within 0.030 eV), for both and , indicating that a s-wave capture process is present at low energy for these two molecules. For , although in contrast with symmetry arguments, and with previous free-electron attachment experiments, this result is in full agreement with the conclusions presented by several groups, using the Rydberg atom energy transfer technique to investigate the low-energy attachment in these systems. Energy positions of several resonant states, appearing between 0.5 and 5 eV in the ion yield versus electron energy spectrum, are clearly established in and , and assignments are discussed. Lifetime measurements are reported for the fullerene anions. In the incident energy range 7 - 11 eV the lifetime of was found to vary from 500 to , respectively, in agreement with previous measurements. The thermionic model for detachment does not seem to describe the values obtained, and their observed variation in this energy range. The lifetime of is longer and could not be measured precisely with our time-of-flight conditions. An estimation of has been made at . Below the lifetime of was larger than and could not be measured precisely.

Direct and charge exchange inelastic processes in ion-atom collisions. II. He+-Ne

For pt.II see ibid., vol.9, p.269 (1976). The He+-Ar collision is studied experimentally in the 100-2000 eV laboratory energy range. Differential cross sections sigma ( theta ) are determined for the main inelastic processes. Direct excitations of Ar are identified by the electrostatic energy analysis of the scattered ions and charge-exchange processes are investigated by a time-of-flight technique. The He+-Ar system differs markedly from the He+-He and He+-Ne collisions previously studied by the importance of the charge-exchange processes into excited states of Ar+ and by the weakness of the charge exchange in the ground state. Ab initio potential energy curves and differential cross section calculations confirm the proposed excitation mechanism, at least for moderate-energy collisions.

Direct and exchange processes in ion-atom collisions. I. He+-He

A time-of-flight spectrometer is described and is shown to have sufficiently good energy resolution to allow a detailed investigation of energy losses for scattered neutral particles. This method has been applied to the study of the exchange channels to complement the previous ion studies. The He+-He results are discussed in the light of the quasi-molecular description of the collision mechanisms.

Low energy electron impact on C76 and C84: excitation, metastable anion formation, and lifetime

Abstract Low energy (0–15 eV) electron impact on gas phase fullerenes C76 and C84 has been investigated in an e− beam experiment (electron energy resolution 0.050 eV). Electron energy loss spectroscopy results are presented for C76 at intermediate and low incident energy and are compared with the UV–visible absorption spectrum. The lowest triplet state 3B3 has not been detected; however, two bands at 1.70 and 2.58 eV, clearly of triplet character are observed. The band at 1.70 eV is assigned to 3B1 and 3B2 states. Ion yields of C76− and C84− versus e− energy are presented in the range 0–10 eV. The capture threshold is shown to appear at zero energy (within 0.030 eV), indicating the occurrence of an s-wave capture. The nature of other features due to resonances appearing on the spectrum is discussed. Lifetimes τ of C76− and C84− are observed to be strongly dependent of the electron energy. They are reported in the range 10–15 eV. For the capture of a 14 eV electron we have measured τ(C76−) = 100 μs and τ(C84−) = 180 μs. Comparison between the lifetimes of C60−, C70−, C76−, and C84− shows that τ behaves exponentially with the number of carbon atoms involved in the fullerene anion.

Effect of u-g symmetry on direct and exchange excitations in ion-atom scattering

Electron capture processes in He+-He collisions are studied, using a time of flight technique. In particular, differential cross sections for both resonant electron capture and capture into excited states are measured. Cross sections for electron capture in n=2p states show an oscillatory behaviour which is out of phase with cross sections corresponding to direct n=2p excitation. This observation brings a new strong support to the interpretation of He+-He excitation in the keV energy range, through rotational coupling.

Angular analysis of the products of the He++Na collision in the 100-1500 eV energy range

The time of flight technique has been used to determine the angular distributions of He* 23S, 21S and 23P obtained by quasi-resonant charge exchange in He++Na collisions. Information on the He* (n>or=3) and the target excitation channels is also reported. The calculated 23S angular differential cross section, performed in a minimal two-state basis, is found to agree with the present experimental data.

Fine structure effects in H+ and D+ charge exchange with Xe

Preliminary differential scattering results are reported on charge exchange into the 2P1/2 and 2P3/2 fine structure levels of Xe+ in H+ and D+ collisions with Xe in the laboratory energy range from 120 to 712 eV. Time of flight techniques used for state identification show the dominance of the 2P1/2 level at low energy. Cross sections are determined and the results interpreted in a molecular framework.