Absolute measurement of excitation cross sections for the 2P-states of atomic hydrogen in charge exchange collisions of protons in argon and nitrogen

Abstract

Cross sections for the excitation of the 2P-states of atomic hydrogen in charge exchange collisions of protons in argon and nitrogen were measured by observing the Lyman-α radiation emitted in transitions from this state. For the measurement of the target gas pressure, a modified ionization gauge was developed, which eliminated distortions of pressure measurements due to charging effects on the glass envelope. Because of the lack of light standards in the vacuum untraviolet region of the spectrum, the major problem of the measurements was the absolute calibration of the Lα-photometer. This calibration was obtained from coincidence measurements of Hα- and Lα-photons in the presence of an electric field. The measurements were corrected for the effect of Doppler shift on photometer sensitivity. For the evaluation of the coincidence measurements, the branching ratio of Hα-transitions to the 2S-state and the polarization of Lyman-α radiation, emitted in cascade processes, were needed. These quantities were obtained from quantum mechanical calculations of a hydrogen atom in a weak electric field. Cross sections for the 2P-states were obtained in the energy range from 0.5 to 30 keV. The 2P-cross sections in argon had maxima of 2.23x10-17 and 2.68x10-17 cm2 at 3.35 and 11.86 keV, respectively, and in N2 maxima of 2.17x10-17 and 2.10x10-17 cm2 were observed at 3.39 and 11.3 keV, respectively. The energy dependence of the measured cross sections is in agreement with that of published measurements. Differences in the absolute values are discussed.