Effect of Matrix Interactions and Buffer Gases on the Atomic Nitrogen Hyperfine Splitting

Electron spin resonance studies of trapped nitrogen atoms show that the matrix interaction increases the hyperfine splitting by some 10 to 20% of the free atom value of 10.45 Mc/sec. A similar increase in the hyperfine splitting is produced by buffer gases used in optical spin-polarization studies of nitrogen atoms. These effects can be accounted for by van der Waals interactions between the trapped atom and the matrix or buffer gas particles. These interactions introduce $\mathrm{}\left(2s\right)\mathrm{}{\mathrm{}\left(2p\right)\mathrm{}}^4$ excited states into the nitrogen wave function. Since the $2p$ shell of the ${}_{}{}^4{}_{}{}^{}S$ nitrogen atom already contains three electrons with the same spin, only that $2s$ electron with opposite spin can be excited. This increases the unpaired electron density at the nucleus. An approximate calculation of this effect, carried out using perturbation theory, is in qualitative agreement with the experimental results. The magnitude of the effect is proportional to the polarizability of the matrix or buffer gas particle, so that the hyperfine splitting increases with the size of the perturbing species.