Spin-Relaxation Effects on the EPR Spectrum of Gaseous Nitrogen Atoms

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Abstract
Experimental evidence is presented which indicates that N atoms produced in a flow system by a microwave discharge through purified N2 can have a relaxation time T1 comparable to their residence time in the EPR magnetic field. As a result, atoms generated outside the field (and therefore unmagnetized) can arrive at the microwave cavity before reaching their equilibrium magnetization. Under such conditions, we found that addition to the discharged N2 of as little as 40‐ppm O2 (which did not affect the N‐atom concentration) decreased T1 and led to increases as great as sevenfold in unsaturated N‐atom EPR line intensities. Interpreting the effects of O2 addition in terms of a two‐level spin model for N, we estimated a value of ≃0.5×10−15 cm2 for the N–O2 spin‐exchange cross section. Without added O2, power saturation of the N‐atom lines was found to be governed primarily by the rate at which unsaturated atoms entered the cavity and not by collision relaxation processes. Several qualitative observations regarding the nitrogen‐pink afterglow, electron production, and the effects of NO addition are also reported.