On the basis of a semiclassical approach, the dynamic polarizability and the index of refraction of helium near 1 <sup>1</sup><i>S</i>-2 <sup>1</sup><i>P</i> resonance are calculated. The steady-state solution of the Schrödinger equation used is well behaved at resonance; hence the decay width, in the Weisskopf and Wigner theory, need not be introduced. The result agrees, in essence, with the results of the Weisskopf and Wigner theory if the irradiance is weak. For great irradiance, the results differ significantly over a broad range of frequencies.
Kwong T. Chung, "Optical anomalous dispersion of helium," J. Opt. Soc. Am. 64, 1470-1473 (1974)