Collisional coherent excitation to then=2state of hydrogen

This paper presents measurements of the Lyman-$\alpha$ photons emitted when target gases He, Ar, ${\mathrm{O}}_2$, and ${\mathrm{N}}_2$ are bombarded by a beam of ground-state hydrogen atoms. The collisions take place in the presence of an axial electric field and the observed Lyman-$\alpha$ brightness depends on field direction. This observation is interpreted as evidence for excitation of hydrogen atoms to states which are linear superpositions of $2s$ and $2p$. Measurements are presented of a "mixed" cross section ${\sigma }_{\mathrm{sp}}$ which characterizes the degree of coherent excitation. The real part of ${\sigma }_{\mathrm{sp}}$ measures the electric dipole moment created by the collision, while the imaginary part measures the rate at which the moment is changing. It is found that these quantities vary substantially over the energy range 3-30 keV, with maximal values being about as large as permitted theoretically. Coherent excitation should be present to some degree in all inelastic collisions and, when measurable, provides new tests of scattering theory. Some implications are also discussed for precision frequency measurements and searches for weak-interaction effects in atomic physics.