Ultrafast non-equilibrium magnon generation and collapse of spin-orbit hybridization gaps in iron

We distinguish between longitudinal and transverse spin excitations in the ultrafast response of iron by probing exchange splitting {\Delta}Eex and magnetic linear dichroism (MLD) in time- and angleresolved photoemission. Comparing spin-split partner bands at the Fermi level shows that {\Delta}Eex remains constant upon optical excitation. In contrast, the different MLD response of spin-orbitsplit valence bands reveals non-equilibrium, transverse spin dynamics. Magnon generation in Fe is ultrafast, electronic band specific, and drives the collapse of spin-orbit hybridization gaps.