Spin-rotation effects in the Stern-Gerlach deflection spectra of 3Σ− molecules and their complexes with argon

Abstract

Stern-Gerlach (SG) magnetic deflection spectra have been calculated for the paramagnetic species 16O2, 17O2, NH, NCH3, and HCCN, all in their 3Σ− electronic ground states. The latter four of these molecules at rotational temperatures of 3 K exhibit both an intense, symmetric three-line pattern due to population of members of the N = 0, S = 1, J = 1 spin-rotation level and, additionally, a weaker, asymmetric nine-line pattern due to population of members of the higher-lying N = 1, S = 1, J = 0, 2, 1 spin-rotation complex. The components of the nine-line patterns suffer Paschen-Back uncoupling at field strengths which correlate with the magnitudes of the electronic spin-spin coupling constants, and in the Paschen-Back regime, the nine lines regroup so as to converge upon the three-line pattern of the N = 0, S = 1, J = 1 level. In 16O2 at 3 K, due to nuclear statistics only the nine-line pattern is observed; however, at a rotational temperature of 1 K only the J = 0 component of the N = 1, S = 1 complex is populated, and the SG spectrum consists of a single line with a slight paramagnetic (upfield) shift from zero deflection. Calculated anticrossings within the Zeeman manifold of spin-rotation states of 3Σ− molecules and their Ar complexes are readily apparent in the computed SG spectra as abrupt changes of the signs of deflection with increasing magnetic field. Complexation of 16O2 with Ar at 3 K leads to a SG spectrum very different from that of 16O2 itself at 3 K, but much more like that of 16O2 at 1 K, i.e., a single line at small paramagnetic displacement, but with a width which increases with applied field strength. The deflection of this peak of Ar·16O2 is smaller than that of the corresponding peak of 16O2 by just the inverse ratio of the masses of Ar·16O2 and 16O2. Parallel calculations on Ar·17O2 reveal SG spectra which are more dense than those of Ar·16O2 under the same conditions, due to the differing nuclear statistics in the 17O2 isotopomer. The general effects of complexation on SG spectra are discussed for paramagnetic molecules above and below their Paschen-Back limits.