Upper bound for a three-photon excitation cross section in atomic argon in the ultraviolet regime

A scheme of evaluating a generalized three-photon excitation cross section σ₍₃₎ in neutral atomic argon at 3144.67 A{} is outlined. Three photons at this wavelength can excite the neutral argon atoms from the ground 3p⁶ ¹S₀ state to the 3p⁵4s'[1/2]₁⁰ state. The fourth photon will ionize the argon atoms. Assuming linear polarization of the incident laser radiation, contributions from several channels in various energy-level schemes are summed in the evaluation of the transition probability. For a laser linewidth of {Δ}λL=1 A{}, our maximum numerical value of the computed result for the three-photon excitation cross section is σ₍₃₎=1.414×{}10^-80 cm⁶ s². .AE