Total Collision Cross Sections for the Interaction of Atomic Beams of Alkali Metals with Gases

Abstract

Total collision cross sections (Q) for the interaction of atomic beams of K and Cs with a number of molecules were measured with an apparatus of 30″ angular resolution. Although absolute determinations of Q are difficult, relative values are readily obtained (±3%). Results are reported as the ratio (Q*) of the cross section for a given molecule to that of argon for the same beam atom. Seventy-seven molecules (of varied complexity and reactivity) were studied with K and 16 with Cs beams. Q* ranged from 0.29 to 2.8. The data were correlated using the Massey-Mohr theory, assuming an attractive intermolecular potential V(r) = —C/r6. For this case Q=b(C/vr)2/5, where vr is the relative velocity and b a known constant. C was estimated from standard formulas for the London dispersion and dipole-induced dipole forces, using known refraction and dipole moment data. The theoretical values of Q differ by a nearly constant factor from the experimental results; thus values of Q* are predicted with good accuracy. The deviation between Qcalc* and Qobs* was <±3% for 57% (and <±10% for 87%) of the molecules. Most of the large deviations occurred for the light gases.