  We revisit the problem of dynamical response in spin-charge separated one
dimensional quantum fluids. In the framework of Luttinger liquid theory, the
dynamical response is formulated in terms of noninteracting bosonic collective
excitations carrying either charge or spin. We argue that, as a result of
spectral nonlinearity, long-lived excitations are best understood in terms of
generally strongly interacting fermionic holons and spinons. This has far
reaching ramifications for the construction of mobile impurity models used to
determine threshold singularities in response functions. We formulate and solve
the appropriate mobile impurity model describing the spinon threshold in the
single-particle Green's function. Our formulation further raises the question
whether it is possible to realize a model of noninteracting fermionic holons
and spinons in microscopic lattice models of interacting spinful fermions. We
investigate this issue in some detail by means of density matrix
renormalization group (DMRG) computations.
