Temperature dependence of the 31P excitation transfer cross section of helium

Inelastic collisions between helium atoms in 3¹P and 1¹S states can result in radiationless transfers of excitation. Measurements were made to determine the cross section of the excitation transfer and its variations with the relative velocity of interacting atoms. The collision process was investigated in low density (10¹⁵–10¹⁸ atoms/cm³) helium gas at four different temperatures between 296°K and 22°K. A 30 KeV electron beam provided primary excitation of the 3¹P state. The cross sections were found to be ${\text{1.6(± 0.3)× 10}}^{\text{−15}}{\mathrm{cm}}^2$ at 296°K, ${\text{1.3(± 0.8)× 10}}^{\text{−15}}{\mathrm{cm}}^2$ at 214°K, ${\text{1.1(± 0.2)× 10}}^{\text{−15}}{\mathrm{cm}}^2$ at 82°K, and ${\text{1.7(± 0.3)× 10}}^{\text{−15}}{\mathrm{cm}}^2$ at 22°K. A comparison of the experimental results with the predictions of the Stueckelberg theory for imperfect resonance excitation transfer was made. Results indicate possible dipole‐quadrupole and quadrupole‐quadrupole interactions.