Collision-Induced Mixing in the2P3Levels of Helium

The cross section for the transfer of excitation from the $2{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ level of helium to the $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ and $2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ levels as a result of collisions with the ground-state helium atom ($1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$) has been measured with a conventional resonance-fluorescence technique. A ${\mathrm{He}}^4$ source excited the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ transition in a ${\mathrm{He}}^3$ absorption cell. The scattered light was examined with a high-resolution grating spectrometer and the ratio of the $2{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}\to 2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ to the $2{}_{}{}^3{}_{}{}^{}P_{1,2}^{}{}_{}{}^{}\to 2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ intensities was measured as a function of cell pressure from 0.05 to 10 Torr. The cross section for collision-induced transitions out of the $2{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ level is found to be (68±3) × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at room temperature.