  The purpose of this paper is to find explicit formulas for basic objects
pertaining the local potential theory of the operator $(I-\Delta)^{\alpha/2}$,
$0<\alpha<2$. The potential theory of this operator is based on Bessel
potentials $J_{\alpha}=(I-\Delta)^{-\alpha/2}$. We compute the {\it harmonic
measure} of the half-space and write a concise form of the corresponding {\it
Green function} for the operator $(I-\Delta)^{\alpha/2}$. To achieve this we
analyze the so-called {\it relativistic $\alpha$-stable process} on $\R^d$
space, killed when exiting the half-space. In terms of this process we are
dealing here with the 1-{\it potential theory} or, equivalently, potential
theory of Schr{\"o}dinger operator based on the generator of the process with
Kato's potential $q=-1$.
