Positive-column plasma studied by fast-flow glow discharge mass spectrometry:  Could it be a “Rydberg gas?”

Abstract

Ions created from the fast-flowing positive column plasma of a glow discharge were monitored using a high voltage magnetic sector mass spectrometer. Since the field gradient and sheath potentials created by the plasma inside the source opposed cation transfer, it is inferred that the ions detected were the field-ionized Rydberg species. This is supported by the mass spectral changes which occurred when a negative bias was applied to the sampling aperture and by the contrasting behavior when attached to a quadrupole analyzer. Reaction with H2 (titrated into the flowing plasma) quenched not only the ionization of discharge gas Rydberg atoms but also the passage of electric current through the plasma, without significant changes to the field and sheath potentials. Few "free" ions were present and the lifetimes of the Rydberg atoms detected were much longer than seen in lower pressure experiments, indicating additional stabilization in the plasma environment. The observations support the model of the flowing plasma, given previously [R. S. Mason, P. D. Miller, and I. P. Mortimer, Phys. Rev. E 55, 7462 (1997)] as mainly a neutral Rydberg atom gas, rather than a conventional ion-electron plasma.

Ionization of H2 Rydberg molecules at a metal surface

“…It has been established that in argon plasmas, metastable electronically excited atoms at the surface-plasma boundary play a crucial role in determining the characteristics of the plasma. 38 Long-lived Rydberg species are likely to be abundant in many types of plasma 39 and will make important contributions to the chemistry, 40 but the role of Rydberg-surface interactions in plasmas is not known, and the study of Rydberg-surface interactions will provide important new information for modeling such media. Charge exchange at surfaces is an important phenomenon in practical applications ranging from catalytic processes to high-energy ion sputtering to nanoscale deposition.…”

Organic mass spectrometry and control of fragmentation using a fast flow glow discharge ion source

“…According to the free ion‐electron model of the plasma, the main ionizing reagents in the flowing afterglow are Ar + , Ar(4s, 3 P) and electrons 39. At the pressures used in this study and under harsher discharge conditions than used here we previously measured the floating potential of the plasma (‘downstream’ from the cathode) to be less than 1 V, which means that the average electron temperature is equivalent to an energy of <0.25 eV (measured on another apparatus in the laboratory, of very similar geometry, but with a floating potential flow tube as the wall to the plasma and equipped with fixed probes, using zero current potentiometry) 40. Electron impact in the bulk plasma is therefore not significant.…”

“…We have argued in previous publications26, 27, 40 that many of the processes occurring in the fast flowing afterglow plasma appear to be dominated by high energy excited state (Rydberg) chemistry. Therefore, one interpretation of the positive cone voltage behavior could be that it is caused by stabilization, in the small applied field, of neutral molecular Rydberg states or even Rydberg anions41 (which would promote easy passage through the aperture), but which ionize on entering the much lower pressure post ion exit region, in the presence of the ion focusing field provided by the cylinder electrode external to the ion source 28.…”

“…Therefore, one interpretation of the positive cone voltage behavior could be that it is caused by stabilization, in the small applied field, of neutral molecular Rydberg states or even Rydberg anions41 (which would promote easy passage through the aperture), but which ionize on entering the much lower pressure post ion exit region, in the presence of the ion focusing field provided by the cylinder electrode external to the ion source 28. On the other hand, in too large an electric field, Rydberg states are destabilized and would therefore ionize in front of the ion exit40 and be repelled back towards the cathode by the cone field. Thus the ion current drops to zero when V cone is >10 V.…”

Plasma quenching during hydrocarbon sample introduction via gas chromatography into a pulsed ms dc-glow discharge

“…39 Each way, Ar + will be removed from the discharge. The reaction ArH + + e À / Ar 0 + H is even two orders of magnitude faster 38 than the reaction which is leading to ArH + and results in loss of charge carriers (and therefore ionization capabilities) in the pulsed glow discharge.…”