The use of spin-labelling techniques in the study of Penning ionization reaction dynamics
Rutherford, George Henry
Dunning, F. B.
Doctor of Philosophy
The use of spin-labelling to study the dynamics of He (2 $\sp3$S) metastable atom ionization of atoms and simple molecules is described. Briefly, He (2 $\sp3$S) atoms created by a microwave discharge are spin-polarized in a flowing afterglow via optical pumping with circularly-polarized 1.08 $\mu$m (2 $\sp3$S) $\Rightarrow$ (2 $\sp3$P) radiation from an LNA laser. A target gas is injected into the flowtube, and electrons created in Penning ionization reactions diffuse through a differentially-pumped aperture and are energy- and spin-analyzed with a hemispherical energy analyzer in series with a retarding-potential Mott polarimeter. Data are reported for Ar, CO$\sb2$, CO, H$\sb2$O, O$\sb2$, NO, NO$\sb2$, SO$\sb2$, and Cl$\sb2$ target gases. The generally accepted model of Penning ionization, the so-called exchange model, suggests that a target electron of appropriate spin tunnels to fill the He 1s hole with simultaneous ejection of the He 2s electron, which, for polarized He (2 $\sp3$S) atoms, produces fully polarized electrons. It is found that fully polarized electrons are ejected in reactions with closed-shell, negative electron affinity targets such as Ar and CO$\sb2$, independent of the positive ion final state, in agreement with the exchange model. Substantially lower polarization is measured for open-shell targets, such as O$\sb2$ and NO, and for targets with large electron affinity, such as NO$\sb2$ and SO$\sb2$. A strongly attractive entrance channel potential is possible in these cases. The exact depolarization mechanism is unclear, but is probably related to the formation of the ionic quasi-molecule. For some targets (Cl$\sb2$, for instance), excitation transfer with subsequent autoionization of the core-excited Rydberg target state created occurs, and these reactions also produce electrons with reduced polarization.
Atomic physics; Molecular physics