  Free-electron lasers (FELs) are the world's most brilliant light sources with
rapidly evolving technological capabilities in terms of ultrabright and
ultrashort pulses over a large range of accessible photon energies. Their
revolutionary and innovative developments have opened new fields of science
regarding nonlinear light-matter interaction, the investigation of ultrafast
processes from specific observer sites, and approaches to imaging matter with
atomic resolution. A core aspect of FEL science is the study of isolated and
prototypical systems in the gas phase with the possibility of addressing
well-defined electronic transitions or particular atomic sites in molecules.
Notably for polarization-controlled short-wavelength FELs, the gas phase offers
new avenues for investigations of nonlinear and ultrafast phenomena in spin
orientated systems, for decoding the function of the chiral building blocks of
life as well as steering reactions and particle emission dynamics in otherwise
inaccessible ways. This roadmap comprises descriptions of technological
capabilities of facilities worldwide, innovative diagnostics and
instrumentation, as well as recent scientific highlights, novel methodology and
mathematical modeling. The experimental and theoretical landscape of using
polarization controllable FELs for dichroic light-matter interaction in the gas
phase will be discussed and comprehensively outlined to stimulate and
strengthen global collaborative efforts of all disciplines.
