Lyman-Alpha Emission Cross Sections for Collisions of 1–25 keV H+ and H with CO, CO2, CH4, and NH3

Abstract

Cross sections are reported for emission of Lyman-α radiation in collisions of H+ and H with CO and CO2 in the 1–25-keV energy range and with CH4 and NH3 in the range 5–25 keV. In the experiments with CH4 and NH3 the Doppler-shifted Lyman α, emitted by H(2p) formed in electron capture by fast incident protons or collisional excitation of fast incoming hydrogen atoms, and the virtually unshifted Lyman α, emitted by the far slower H(2p) produced in dissociative excitation, were separated. An electric field was applied within the collision chamber, and the increase in Lyman-α intensity was used to derive cross sections for formation of H(2s). For CO and CO2 targets, H projectiles are more efficient than H+ in formation of H(2p), while at energies above 10 keV, H+ is more efficient than H in formation of H(2s). For both projectiles, the probability of formation of H(2p) is greater than or equal to the probability of forming H(2s). For CH4 and NH3 targets the projectile excitation cross sections are generally similar to those for CO and CO2, and the total projectile excitation cross section for formation of H(n=2) exceeds that for dissociative excitation in either H+ or H impact. For both projectiles formation of H(2p) in dissociative excitation is at least an order of magnitude more probable than formation of H(2s). Protons are found to be more efficient than hydrogen atoms in production of Lyman α via dissociative excitation. Relationships are presented between the projectile excitation cross section and the cross section for electron capture in H+ impact or stripping in H impact which are relatively independent of the identity of the target molecule.