The paper presents an advanced analysis of the experimental Al Heβ emission from aluminium plasma created by one ps-laser beam and then subjected to another ps-laser beam when the pulse of the first laser beam was already off. The paper has two equally important goals. The first goal is to present an overview of the general principles of spectroscopic diagnostics of plasmas containing a quasi-monochromatic electric field (QEF) and to clarify some erroneous/confusing statements in the theoretical literature. The second goal is to model spectroscopic signatures of the strong QEF caused by the second laser beam in the plasma and thus to provide an adequate interpretation of the experimental spectral line profiles. The second goal has been achieved via advanced simulations coupling the code based on the Floquet–Liouville formalism with the particle-in-cell kinetic code that provides a spatial distribution of the QEF in the plasma. In this way—by using ultra-high resolution K-shell spectroscopy—the paper has established a link between two research communities: the one dealing with laser–plasma interactions and the other one dealing with atomic processes in plasmas.