We present 2 + 2 radar REMPI measurements in molecular nitrogen under atmospheric conditions and observe a strong interference in the (1,0) vibrational band of the a(1)g X(1)g(+) electronic manifold. The interference is suppressed by using circularly polarized light, permitting rotational analysis of the 2 + 2 radar REMPI spectrum. It is observed in pure nitrogen, though the structure varies with gas composition. The structure also varies with temperature and pressure. These results indicate that it is collision induced. We hypothesize that the source of the interference is a 3 + 1 REMPI process through the a(1)g(+) electronic state.
