Shape Resonances inK-Shell Photoionization of Diatomic Molecules

We show that $K$-shell photoionization spectra of diatomic molecules can exhibit strong shape resonances near threshold. The molecular field is decisive in this phenomenon. It induces the resonant contraction of high-$l$ components of the ionization channels, and it couples these components with low-$l$ components produced in the atomic cores by $K$-shell photoionization. We demonstrate the effect for ${\mathrm{N}}_2$ by using the multiple-scattering model. Clear evidence for this phenomenon exists for ${\mathrm{N}}_2$, CO, and NO.