Observation of the Forbidden $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ Spontaneous Emission Line from Helium and Measurement of the Transition Rate

The extremely forbidden transition $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ at 625.56 Å has been observed in a He afterglow using time-resolved high-resolution spectrophotometric techniques. The spontaneous radiative transition rate was determined by measuring the photon emission rate from a known number of metastables at pressures of 0.05, 0.30, and 0.75 Torr. The value measured is 2.4× ${10}^{-4\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ with an experimental uncertainty of a factor of 3.