For the massless Nelson model, we provide detailed information about the dependence of the normalized ground states [Formula: see text] of the fiber single-electron Hamiltonians [Formula: see text] on the total momentum [Formula: see text] and the infrared cut-off [Formula: see text]. This information is obtained with the help of the iterative analytic perturbation theory. In particular, we derive bounds of the form [Formula: see text] for some constant [Formula: see text] and a function of the maximal admissible coupling constant [Formula: see text] such that [Formula: see text]. These results hold both in the infrared-regular and infrared-singular cases. They are exploited in part I of this series to construct the two-electron scattering states in the infrared-regular massless Nelson model (in the absence of an infrared cut-off) along the lines of the Haag–Ruelle scattering theory. They should also be relevant to the problem of scattering of two infraparticles in the infrared-singular Nelson model, whose solution is the goal of this series of papers. Although a part of a larger investigation, the present work is written in a self-contained fashion.
Dalgarno–Lewis perturbation theory for scattering states