Q.C. Kessel

Inelastic Energy Loss: Newer Experimental Techniques and Molecular Orbital Theory

The measurement of the inelastic energy losses which occur when heavy ions and atoms strike other atoms or molecules has provided information of unique importance to chemists and physicists. The inelastic energy loss in a collision is the portion of translational energy which is transferred to electronic, vibrational, and rotational excitation energy by the collision. After, or sometimes even during such a collision, the electron shells surrounding the collision participants may rearrange themselves so as to lose most of this excess energy. In atomic systems the ionization of electrons and the emission of photons are the primary mechanisms through which this energy is removed from the system. In collisions with molecular partners dissociation energies are important. In ordinary spectroscopy the energies of the ejected photons and electrons are measured, but such measurements are primarily concerned with deexcitation of the atom or ion. In contrast to this, inelastic energy-loss spectroscopy is primarily concerned with the excitation process. The inelastic energy loss Q represents the excitation energy of the collision system, and it follows that the sum of all deexcitation energies, together with any metastable excitation energy or dissociation energy resulting from that collision, must equal Q. For a very wide range of excitation processes, inelastic energy-loss spectroscopy has shown that the excitations are molecular in nature and have their origin in the transitory molecule formed during the collision. This chapter concentrates exclusively on collisions for which this is the case. Broadly speaking, these are the collisions in which the relative nuclear velocities are lower than the characteristic velocities of the electrons in question. When this is true, the Born-Oppenheimer approximation is valid and the nuclear and electronic motions may be considered separately.

CHAPTER 3 – THE PRODUCTION OF INNER-SHELL VACANCIES IN HEAVY ION-ATOM COLLISIONS

This paper reviews the development of our present day understanding of the production of inner-shell vacancies by heavy ion bombardment. The adiabatic and diabatic formulation of the molecular orbital model are discussed and related to experimental findings. Particular emphasis is placed on differential measurements and the need for such information in interpreting the data from total measurements, especially the data obtained from experiments measuring X-ray spectra and total cross sections.

X-Ray Emissions from Collisions of O6+ Ions with CO

Laboratory measurements of soft X-ray emissions from collisions between 36 keV O{sup 6+} ions and CO have been carried out with the aim of simulating emissions from comets interacting with the solar wind. Spectra in the range 62-155 eV are recorded and compared to results of the over-barrier model (OBM) and multichannel Landau-Zener (MLZ) calculations. Emissions from n = 3, 4 states of O{sup 5+} are observed. This is in good agreement with the OBM predictions of highest n-state for the electron capture. Line intensities for the n = 4 capture in simulated spectra using the semi-empirical MLZ approach, taking into account multielectron captures, are in very good agreement with experimental measurements. However, the OBM does not correctly account for direct feeding of the n = 3 levels for the CO target, though it does explain predominance of the n = 3 levels for an He target.

X-ray emission from Bi50+ to Bi71+ ions incident on a gold surface

Bi ions with charge states q = 50 to 71 from the Livermore National Laboratorys EBIT were accelerated to energies of q × (7 keV) and directed to a gold target. Using a Si(Li) detector, two families of X-rays were observed, one of these in the range of 750–2000 eV and the other between 2500 and 6000 eV. Both families are found to shift to higher energies with increasing incident ion charge state. The lower energy group which dominates the spectra for lower incident charge states is attributed to the filling of vacancies in the N shell while the higher energy group, which predominates for the higher incident charge states, is attributed to the filling of M-shell vacancies. M X-rays are observed even for those incident charge states for which M vacancies are not expected. A similar result observed for U ions incident upon a Be target was attributed by Schuch and coworkers to a dielectronic excitation mechanism.

Electron-scattered ion coincidence measurements of heavy ion-atom systems

Abstract Investigations of the energies of electrons emitted as a result of collisions between very heavy ions and atoms show that a significant number of the electrons have a continuous spectrum of energies that is difficult to associate with separated atom levels. This has caused speculation as to the origin of some of these electrons: ideas include the possibility of autoionization occurring during the collision while the molecular potentials are changing, autoionization involving more than two electrons and autoionization from a multiplicity of poorly defined states interacting at a specific distance of closest approach. The present measurements detect the continuum electrons in coincidence with ions scattered to a specific angle, thereby determining the impact parameter dependence of the underlying excitation mechanism.

The emission of charged particles with eV energies from hot graphite

Thermal desorption spectroscopy of graphite (grafoil) has been investigated by measuring the the energies of the emitted ions with a hemispherical electrostatic analyzer under ultra-high vacuum conditions. At 850 °C the masses of most of the emitted ions are found to be in the range of 48u to 60u by time-of-flight techniques. The present data show that under certain conditions, the ions may be emitted with energies above those expected for thermal emission. It is not clear whether these energies are the result of local charging of the surface or surface chemistry.

Soft X‐ray and Optical Laboratory Spectra to Simulate the Solar Wind on Comets: O5+ + CO

UV/soft x‐ray and optical measurements of relative line emission cross sections from collisions of O5+ with CO vapor have been made, as part of a laboratory study of interactions between minority solar‐wind ions and gases found in comets. The interaction of O5+ with CO yields prototypical information applicable to other highly‐charged solar wind ion interactions. The data are at least approximately consistent with a simple classical Coulomb “over‐the‐barrier” model prediction: strong XUV emission (∼13–25 nm range) is observed in collisions in the slow and fast solar wind velocity range. We attribute this light to electron capture from the target molecule by the projectile ion into an excited n=4 level, plus cascade into n=3, resulting in emission lines from (O4+ )*. The optical spectra, taken at higher velocities, show a lack of line emission from projectile states in the visible range (400–850nm); the visible lines observed can be attributed primarily to excitation of atoms from the molecular target following dissociation.

Ion beam enhanced emission of charged particles from hot graphite

Thermal desorption spectroscopy of ions from positively biased graphite (grafoil) has been investigated by measuring the energies of the emitted ions with a hemispherical electrostatic analyzer and the masses with a residual gas analyzer under ultra-high vacuum conditions. Potassium is one of the ions emitted at temperatures above 800 °C. The present data show that under near threshold conditions (4V), ions appear with well-defined energies equal approximately to the bias voltage minus 4V. This phenomenon can be greatly enhanced by prior bombardment with an ion beam. It is not clear whether these energies are the result of resonant process on the hot surface or simply due to a process attributable to surface chemistry. At higher biases the peaks broaden in energy and the energy deficit increases.

Negative ion production in small angle scattering of highly charged ions from the (0001) surface of highly oriented pyrolytic graphite

Highly charged N, O, F, and S ions, chosen for their electron affinities, were extracted from the Lawrence Livermore National Laboratory’s EBIT II. After collimation, these ions struck a target of highly oriented pyrolytic graphite (HOPG) at an incident angle of 1.0 degree. Those ions scattered by 2.35 degrees (1.35 degree with respect to the surface) were charge state analyzed and the predominant charge state fractions were determined. As might be expected, there is a tendency for the fraction of negative ions to increase with increasing electron affinity; however, the negative ion yield is also strongly dependent on the ion velocity. For example, for sulfur the negative ion yields measured range from 0.13 to 0.23 of the scattered ions while for fluorine the range was 0.35 to 0.40. A pronounced velocity dependence found for the S− ions is described well by a Saha-Langmuir-type equation.

Possible two-electron excitations in C3+ following 1 MeV/u collisions with atomic targets; Z=2, 10, 18 and 36

Auger electrons emitted from Li-like Carbon projectiles excited during I MeV/u collisions with atomic targets (Z = 2, 10, 18 and 36) have been measured. As a function of the target Z, we observe the yieldsof singly and doubly excited autoionizing configurations (Al) of C[sup 3+] excited during the collision that decay to the (1s[sup 2]([sup 1]S) + e[sub c][ell][sub c])[sup 2]L ground state. The investigation focuses on Auger electrons emitted with energies between 220 and 260 eV in the projectile reference frame. In addition to the singly excited (1s2s2[ell])[sup 2]L AI configurations expected from projectile 1s electron excitation, doubly excited states to the (1s(2p[sup 2])[sup 1]D)[sup 2]D and (1s(2p[sup 2])[sup 1]S)[sup 2]S are also observed with increasing intensities in going from He to the higher Z targets. If the [sup 2]D and the [sup 2]S peaks are the result of two-electron excitations, it may be possible to investigate this aspect of the collision using some of the recent approaches taken in studying the excitations of the two electrons in He targets.