Valence excitons and inner-shell excitons in gaseous and solid argon

Valence excitons from the $3p$ shell and core excitons from the $3s$ and $2p$ shells in gaseous and solid argon are studied within the envelope-function formalism. First, semiempirical model potentials for atomic excitons are considered; then solid-state effects, which include band structure and electron-hole screening, are taken into account. The problem of a sufficiently accurate account of the "band kinetic operator" is solved with first-order perturbation theory. A new interpretation of the excitonic spectrum originating from the $2p$ shell is given in terms of first-class exciton ($1s$) and third-class excitons ($\mathrm{nd}$, $n\ge 3$) associated with a unique band edge.