  (abridged) The results of deep long-slit optical spectroscopy for a sample of
8 6C radio galaxies at z~1 are presented. Emission line ratios are derived for
emission lines with rest-frame wavelengths of 1500 - 4500Ang, and the kinematic
propertiess of the emission line gas are derived from an analysis of the 2-d
structure of thee [OII]3727 Ang emission line at ~5Ang spectral resolution. In
general, the 6C spectra display many characteristics similar to those of more
powerful 3CR sources at the same redshifts. The emission line region gas
kinematics are more extreme for the smaller radio sources in the sample, which
often display distorted velocity profiles. The ionization state of the emission
line region also varies with radio size: the spectra of large radio sources
(>120kpc) are consistent with photoionization by an obscured AGN, whilst
smaller (<120kpc) sources typically exist in a lower ionization state and have
spectra which are better explained by additional ionization due to shocks
associated with the expanding radio source. The kinematic and ionization
properties of the 6C radio galaxies are clearly linked. As for the 3CR sources,
smaller radio sources also typically possess more extensive emission line
regions, with enhanced emission line luminosities. It is clear that the best
interpretation of the spectra of radio sources requires a combination of
ionization mechanisms.
