  The quark-gluon vertex in Landau gauge is studied in the quenched
approximation using the Sheikholeslami-Wohlert (SW) fermion action with
mean-field improvement coefficients in the action and for the quark fields. We
see that the form factor that includes the running coupling is substantially
enhanced in the infrared, over and above the enhancement arising from the
infrared suppression of the quark propagator alone. We define two different
momentum subtraction renormalisation schemes -- \MOMT (asymmetric) and \MOMB\
(symmetric) -- and determine the running coupling in both schemes. We find
Lambda_msbar(N_f=0)=300(+150/-180)(55)(30) MeV from the asymmetric scheme. This
is somewhat higher than other determinations of this quantity, but the
uncertainties -- both statistical and systematic -- are large. In the symmetric
scheme, statistical noise prevents us from obtaining a meaningful estimate for
Lambda_msbar.
