Direct observation of momentum-dependent local-field effects in solid nitrogen

We have measured the momentum dependence of the ¹Πg{←}X ¹Σg⁺ transition in cubic solid nitrogen. We have evaluated the oscillator strength of the transition and compared it to the markedly different strength found in analogous measurements on the gas, and conclude that local-field effects present in the solid are responsible for the differences found. This is the first momentum-dependent measurement of local fields in a solid. The local-field enhancement factor increases by about a factor of 3 over the momentum range covered, and is consistent with a simple model calculation for small momenta.