On the formation of Fe II lines in stellar spectra. I - Solar spatial intensity variation of lambda 3969.4

High-spatial-resolution solar observations of the weak Fe II lambda 3969.4 line are employed to study non-local thermodynamic equilibrium effects in Fe II line formation. This line is superposed on the wing of the Ca II H line, which raises its height of formation. The line shows extraordinary spatial intensity variations, including emission features whose contrast increases toward the limb. Observed profiles of the Fe II resonance lines in the UV are used to define formation parameters in a 15-level atomic model computation, which shows that Fe II subordinate lines are generally formed out of local thermodynamic equilibrium as a result of pumping by UV line-wing photons from the deep photosphere. For the lambda 3969.4 line, this pumping results in large sensitivity to the atmospheric structure in layers deeper than the layer of formation of the H-wing background intensity. The absence of intense emission cores in the Fe II resonance lines, the effects of partially coherent scattering, and the effects of chromospheric and photospheric inhomogeneities are discussed. It is found that emission of lambda 3969.4 provides a diagnostic of the inhomogeneous structure of the deep photosphere, for the sun and for late-type stars.