Paramagnetic resonance absorption in molecular oxygen

Abstract

Tinkham & Strandberg (1955a, b) have made a theoretical analysis of the paramagnetic resonance spectrum of molecular oxygen gas ($^{16}$O$^{16}$O), and obtained satisfactory agreement with their experimental results. Independent measurements at low field strengths, made by Hendrie & Kusch (1957), using a molecular beam method, yielded results which were not in agreement with those of Tinkham & Strandberg. The experiments of Tinkham & Strandberg have now been repeated to a higher accuracy and systematic discrepancies with their results have been observed. In these new measurements, the positions of the absorption lines have been fitted to values obtained by numerical calculations based on the theory of Tinkham & Strandberg and carried to an accuracy of two parts per million (2 p.p.m.). Agreement with the new experimental results to 6 p.p.m. is obtained with the following choice of the three Parameters: g$_{s}^{\prime}$/g$_{s}$ = 1 - (147 $\pm $ 10) $\times $ 10$^{-6}$, g$_{l}$/g$_{s}$ = (1$\cdot $405 $\pm $ 0$\cdot $015) $\times $ 10$^{-3}$, g$_{r}$/g$_{s}$ = (6$\cdot $3 $\pm $ 0$\cdot $6) $\times $ 10$^{-5}$, where g$_{s}$, g$_{s}^{\prime}$, g$_{l}$ and g$_{r}$ are the spectroscopic splitting factors for the free electron spin, the electron spin in the molecule, the molecular electronic orbital moment and molecular rotational moment respectively. These values are not in agreement with those of either Tinkham & Strandberg or Hendrie & Kusch within the experimental errors quoted. As an additional check, measurements and calculations have been made on five transitions within the even rotational states of $^{16}$O$^{18}$O; with the same parameters as above, appropriately modified to allow for the change in isotopic mass, agreement is obtained with experiment within 10 p.p.m. Evaluation of the relativistic and diamagnetic effects (Abragam & Van Vleck, 1953) within the molecule gives the correct order of magnitude for the departure of g$_{s}$ from g$_{s}$.