Relaxation experiments performed on optically polarized Rb atoms in a rare gas have been previously reported; their detailed interpretation is given below. It is shown that the relaxation governed by the spin-orbit interaction is strongly affected by the formation of chemically unstable Rb-Kr molecules bound by van der Waals forces. Two processes of molecule formation are analyzed: binary resonant collisions leading to metastable states and three-body collisions producing actual bound states. A relaxation model valid for any disorientation probability per single Rb-Kr interaction is developed. Aside from clear evidence for the existence of alkali-rare-gas molecules, the success of the theoretical interpretation of the relaxation experiments yields the equilibrium constant $\mathcal{K}=1.7\mathrm{}\times {10}^{-22\mathrm{}}$ ${\mathrm{cm}}^3$/molecule for the reaction Rb+Kr ⇌ Rb-Kr at 300°K, the average lifetime of a Rb-Kr molecule in the gas phase, $\tau =0.65\mathrm{}\times {10}^{-7\mathrm{}}$ sec, at a krypton pressure of one Torr, and the average spin-orbit coupling constant in a Rb-Kr molecule, $\overline{\gamma }{h}^{-1\mathrm{}}=0.63\mathrm{}$ MHz. It is also shown that the spin-orbit potential is predominantly short-range.

• Received 21 October 1968

DOI:https://doi.org/10.1103/PhysRev.181.144