Ion Cyclotron Resonance Line Broadening at High E/P

Abstract

Observations were made of ion cyclotron resonance absorption linewidths and shapes at values of E/P up to 400 V/cm·Torr. Three methods of heating the ions are compared. If the detector rf electric field is used to heat the ions, the line shapes are distorted and not well suited for simple interpretation. With the dc heating method, the line shapes are close to Lorentzian and the collision cross section as a function of E/P can be determined from the increase in linewidth as a function of pressure. For He⁺ in He at E/P = 100 V/cm·Torr, the cross section is 30 Ų, in fair agreement with previous studies. By heating with a separate oscillator fixed at the cyclotron frequency, similar line broadening is observed, but a dip appears at the center of the absorption line. This dip is attributed to a nonlinear dependence of collision frequency on electric field, and is enhanced by the strong dependence of the ion concentration on collision frequency under steady‐state diffusion conditions.