Two alternative expressions for the two-photon decay rate of the $2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ metastable state of helium are evaluated by performing the summations over intermediate ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ states. The values obtained from the "length" and "velocity" expressions for the decay rate are 50.85 ${\sec }^{-1\mathrm{}}$ and 50.89 ${\sec }^{-1\mathrm{}}$, respectively, in good agreement with a recent experimental value and with previous calculations done by different methods. Incidental results are given for the photoionization cross sections of the $1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ and $2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ states of helium in the energy region 0-0.2 Ry.

• Received 5 January 1971

DOI:https://doi.org/10.1103/PhysRevA.4.939