Laser studies of near-resonant state-changing collisions of calcium (4s6s1S0) with the rare gases

State-changing collisions of Ca(4s6s ${}^1$ $S_0$) with the rare gases are studied by pulsed laser excitation and time- and wavelength-resolved detection. The total depletion rates of the 4s6s ${}^1$ $S_0$ state with different rare gases vary by over a factor 10, with the lighter rare gases being markedly more efficient than the heavier ones. Relative branching ratios to the specific near-resonant, energy-transfer product states, 4p4p ${}^1$ $D_2$ (ΔE=-29 ${\mathrm{cm}}^{\mathrm{-}1}$), 3d4p ${}^1$ $F_3$ (ΔE=152 ${\mathrm{cm}}^{\mathrm{-}1}$), and 4s6s ${}^3$ $S_1$ (ΔE=216 ${\mathrm{cm}}^{\mathrm{-}1}$), are measured for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, Ne, and Xe. Transfer to the ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$ state is always the major relaxation pathway. The rate constant for ${}_{}{}^3{}_{}{}^{}\mathrm{S}_1^{}{}_{}{}^{}$ production is immeasurably small, implying a small spin-orbit matrix element for curve crossing that leads to a spin change. The results for 4p4p ${}^1$ $D_2$ production are compared with simple theoretical calculations based on established models for near-resonant energy transfer between states which are coupled at large internuclear distance. These models successfully explain the dominant transfer to the 4p4p ${}^1$ $D_2$ state and the trends with rare gas, which are attributed largely to enhancement of the near-resonant-transfer process with increasing velocity. The effects of wave-function mixing in the isolated atom (e.g., spin mixing) on the general outcome of state-changing collisions in two-electron atoms are discussed. Two-electron atoms offer numerous possibilities for further testing of theories of near-resonant energy transfer.