  Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing
because of its semiconducting properties and narrow band gap. Further reduction
of the phosphorene dimensionality may spawn exotic properties of its electronic
structure, including lateral quantum confinement and topological edge states.
Phosphorene atomic chains self-assembled on Ag(111) have recently been
characterized structurally but were found by angle-resolved photoemission
(ARPES) to be electronically 2D. We show that these chains, although aligned
equiprobably to three <$1 \bar{1} 0$> directions of the Ag(111) surface, can be
characterized by ARPES because the three rotational variants are separated in
the angular domain. The dispersion of the phosphorus band measured along and
perpendicular to the chains reveals pronounced electronic confinement resulting
in a 1D band, flat and dispersionless perpendicular to the chain direction in
momentum space. Our density functional theory calculations reproduce the 1D
band for the experimentally determined structure of P/Ag(111). We predict a
semiconductor-to-metal phase transition upon increasing the density of the
chain array so that a 2D structure would be metallic.
