  The formation of a Pd atomic chain in a hydrogen atmosphere was investigated
by measurements of conductance and vibrational spectroscopy of a single
molecular junction, and the theoretical calculation. While atomic chains were
not formed for clean 3d and 4d metals, in the case of Pd (a 4d metal) atomic
chains could be formed in the presence of hydrogen. Stable atomic chains with
two different atomic configurations were formed when the Pd atomic contact was
stretched in a H$_{2}$ atmosphere; highly conductive short hydrogen adsorbed
atomic chain and low conductive long hydrogen incorporated atomic chain.
