Negative Ion Formation Process Research Articles

Negative ions at Titan and Enceladus: recent results

The detection of heavy negative ions (up to 13 800 amu) in Titan's ionosphere is one of the tantalizing new results from the Cassini mission. These heavy ions indicate for the first time the existence of heavy hydrocarbon and nitrile molecules in this primitive Earth-like atmosphere. These ions were suggested to be precursors of aerosols in Titan's atmosphere and may precipitate to the surface as tholins. We present the evidence for and the analysis of these heavy negative ions at Titan. In addition we examine the variation of the maximum mass of the Titan negative ions with altitude and latitude for the relevant encounters so far, and we discuss the implications for the negative ion formation process. We present data from a recent set of encounters where the latitude was varied between encounters, with other parameters fixed. Models are beginning to explain the low mass negative ions, but the formation process for the higher mass ions is still not understood. It is possible that the structures may be chains, rings or even fullerenes. Negative ions, mainly water clusters in this case, were seen during Cassini's recent close flybys of Enceladus. We present mass spectra from the Enceladus plume, showing water clusters and additional species. As at Titan, the negative ions indicate chemical complexities which were unknown before the Cassini encounters, and are indicative of a complex balance between neutrals and positively and negatively charged ions.

Absolute cross sections for the electron-impact formation of cytosine anions

The process of negative ion formation by electron impact on cytosine (a nitrous base of nucleic acids) has been studied in crossed molecular and electron beams for electron energies in the interval from 0.4 to 5.0 eV. Using a specially developed method, the molecular beam intensity was determined and the energy dependence of the absolute cross section Q for the formation of cytosine anions was studied. The maximum ionization cross section σ = 4.2 × 10−18 cm2 was observed for an electron energy of 1.5 eV.

Photodetachment Study of Capacitively-Coupled RF C4F8 Plasma

Behaviors of electrons and negative ions in a capacitively coupled RF (13.56 MHz) plasma of c-C4F8 (perfluorocycrobutane) were studied using a laser-induced photodetachment technique in combination with a microwave cavity resonance technique. The results indicate that the negative ion formation process strongly depends on the RF power because of the dissociation of c-C4F8 even in the low power range (<100 W) studied in this work. The negative ion density estimated on the assumption that all negative ion species have the same photodetachment efficiency as that for F- is larger by a factor of ∼50 than the electron density in the typical condition in this work (100 mTorr C4F8, 10 W RF power). However, the electron attachment rate and ion recombination rate suggest that the actual negative ion density should be significantly higher than that derived on the above assumption; this discrepancy could be attributed to the low photodetachment efficiency for large molecular negative ions which may exist abundantly in the low RF power region.

Experimental determination of a species-dependent effect in the transverse emittances of sputter-generated negative-ion beams.

We report experimental evidence of a previously unseen species-dependent effect in the transverse emittances of momentum-analyzed $^{28}\mathrm{Si}^{\mathrm{\ensuremath{-}}}$, $^{58}$${\mathrm{Ni}}^{\mathrm{\ensuremath{-}}}$, and $^{197}$${\mathrm{Au}}^{\mathrm{\ensuremath{-}}}$ negative-ion beams generated by cesium-ion sputtering. The differences in the emittances are found to be principally correlated with differences in the energy spreads in the respective ion beams, which have their origins in the sputter-ejection negative-ion formation process. The experimental equipment and techniques utilized for emittance data acquisition and analysis, and evidence for a species-dependent effect in the emittances and brightnesses of the subject ion beams, are presented in this paper.

Electron attachment to the perfluoroalkanes n-CNF2N+2 (N=1–6) using high pressure swarm techniques

The electron attachment rate constants and negative ion formation mechanisms for six perfluoroalkanes [n-CNF2N+2(N=1–6)] have been studied in a high pressure swarm experiment within the mean electron energy range from thermal energy (≊0.04 eV) to ≊4.9 eV. These experiments were performed over a total gas number density range of 3.2×1019 to 3.9×1020 cm−3 using N2 and argon as buffer gases. Dissociative electron attachment was found to be the only negative ion formation process for CF4 and C2F6. For C3F8, n-C4F10, and n-C5F12 the electron attachment rate constant measurements exhibited a large total pressure dependence which was strongest for C3F8 and decreased with increasing size of the perfluoroalkane molecule. These measurements have been interpreted as electron attachment by parent negative ion formation due to three-body stabilization processes of the initially excited, short-lived (5×10−11 s &amp;lt;τ&amp;lt;10−8 s) parent anion. The lifetimes of these transient parent anions have been found to depend on the nature of the parent ion and on the electron energy. The electron attachment rate constants are largest for the n-C6F14 molecule and decrease with decreasing size of the perfluoroalkane molecule. Furthermore, the peak in the attachment rate constants occurs at the lowest mean energy (∼1.1 eV) for n-C6F14 and shifts to higher mean energy with decreasing size of the molecule (to ≥5 eV for CF4). The electron attachment cross sections for all of these molecules have been calculated using the swarm-unfolding method and calculated electron energy distribution functions in argon. These measurements are compared with relative negative ion cross sections, determined previously using mass spectrometric techniques. The negative ion formation mechanisms are discussed and the effect of molecular size on the electron attachment properties of these molecules is indicated using the present measurements in conjuction with the results of a recent single collision negative ion study.

Equilibrium density of h- in a low pressure hydrogen plasma

The equilibrium density of hydrogen negative ions in a hydrogen low pressure plasma was calculated taking into account the various known production and destruction processes. It is shown that in most plasma conditions the contribution of dissociative recombination to H- production is essential. Due to this process higher H - densities are calculated than those obtained when dissociative electron attachment only is considered. However the calculated densities are ∼ 100 times lower than the experimental values found in low pressure hydrogen plasmas. This discrepancy suggests that the dominant negative ion formation process in hydrogen plasma has not yet been identified.

A new process of negative-ion formation. IV

The three previous papers of this series (Arnot and Milligan 1936 b ; Arnot 1937 a, b ) contain an account of experimental work which led the senior author to propose a new process of negative-ion formation. This process is the formation of negative ions at metal surfaces by bombardment of the surface with positive ions, the negative ion being formed by the positive ion capturing two electron from the surface. Further work carried out during the past year, which is described in this paper, has revealed a new variation of the above process. In this latter process the impinging positive ion causes an adsorbed atom on the surface to come off as a negative ion. It is believed that this newer process is essentially similar to the process previously reported, the difference being due merely to the transference of excitation energy from the incident positive ion, after its capture of an electron, to the atom adsorbed on the surface. The discovery of this second effect was made independently by Sloane and Press (1938), although they attribute it to a different process.

Formation of Negative Ions at Surfaces

IN a recent series of papers1, one of us (F.L.A.) has discussed experiments, carried out with the assistance of Dr. J. C. Milligan, which led to the discovery of a new process of negative ion formation. This process was the formation of negative ions at metal surfaces by bombardment of the surface with positive ions, the negative ion being presumably formed by the positive ion capturing two electrons from the surface. After acceleration away from the surface, the negative ion has an excess energy which is acquired from the acceleration of the positive ion towards the surface. Dr. R. A. Smith2 has recently carried out a quantum theoretical investigation of the above process which satisfactorily accounts for the experimental results.

A new process of negative ion formation III―The energy distribution of the negative ions and accommodation coefficients of the positive ions

In the previous paper it has been shown that the negative ions found in hydrogen, nitrogen, oxygen, and carbon dioxide are formed from positive ions which acquire two electrons from a negatively-charged surface, and are then accelerated away from the surface as negative ions. It will be shown in this paper that the negative ions come off the surface on which they are formed with a wide range of energy, extending from zero up to beyond 60 volts for O - and beyond 109 volts for H - . This energy is derived from the parent positive ions which have 200 volts’ kinetic energy on striking the surface.

A new process of negative ion formation―II

It has been known for many years that negative ions are formed in various gases through which a stream of electrons is passed. It has generally been believed that these negative ions are all formed by one of two processes, which may be termed the “radiative process” and the “dissociation process". The latter process can occur only in a molecular gas, whereas the radiative process can occur in monatomic as well as molecular gases. In the dissociation process the impinging electron dissociates the molecule and attaches itself to one of the products of dissociation, the energy of the electron in excess of that necessary for dissociation together with the electron affinity of the resulting negative ion being carried away in kinetic form by the products of dissociation. The radiative process is a direct attachment of the electron to the atom or molecule without dissociation, the kinetic energy of the electron together with the electron affinity of the atom or molecule being radiated.

A new process of negative ion formation

When an electron becomes attached to an atom to form a negative ion an amount of energy equal to the electron affinity of the atom plus the relative kinetic energy of the colliding particles must be dissipated. The disposal of this energy depends upon the process of formation of the ion. One process which takes place in molecular gases is the simultaneous dissociation of the molecule and attachment of the electron to one of the products of dissociation, the energy of the electron in excess of the necessary for dissociation together with the electron affinity of the atom being carried away in kinetic form by the products of dissociation. Negative ions are also believed to be formed by the direct attachment of electrons to atoms and molecules without dissociation. In this process it has generally been assumed that the excess energy is radiated, but, although several attempts have been made, it has not been found possible to detect an electron affinity spectrum.

A New Process of Negative Ion Formation

WHEN a stream of electrons is passed through a monatomic gas, negative ions are supposed to be formed by direct electron attachment to atoms of the gas with radiation of the excess energy. When the gas is molecular, the process generally presumed to occur is simultaneous dissociation of the molecule and attachment of the electron to one of the products of dissociation, the energy of the electron in excess of that necessary for dissociation together with the electron affinity of the atom being carried away in kinetic form by the products of dissociation.