Formation and Destruction of Excited Hydrogen Atoms at High Impact Velocities

Abstract

A study has been made of the charge-transfer processes whereby fast neutral atoms of hydrogen are formed in the $3s$, $3p$, and $3d$ excited states as a result of the impact of protons on targets of helium and of nitrogen. Impact energies ranged from 75 to 400 keV. The experimental procedure involved quantitative measurement of the Balmer $\alpha$ radiation emitted by the spontaneous decay of atoms in these three states. The three contributions to the emission were separately identified according to the different lifetimes of the $3s$, $3p$, and $3d$ states by means of a time-of-flight technique. A study was also made of the processes whereby the excited atoms were collisionally destroyed before undergoing spontaneous radiative decay. Detailed measurements are presented of the cross sections for capture into the $3s$ state, which are by far the largest of these capture cross sections. Cross sections for capture into the $3p$ and $3d$ states are one to two orders of magnitude smaller, and preliminary measurements are discussed. All of these cross sections decrease rapidly with increasing impact energy. There is general agreement between the present measurements and the predictions of the Born approximation for capture into the $3s$ and $3d$ states from a target of helium, but the calculation for the $3p$ state appears to overestimate the cross section by a factor of at least 4. The cross sections for collisional destruction of atoms in the $3s$ state are several orders of magnitude larger than for electron capture into this state, and the magnitude of the measured values are in agreement with theoretical predictions.