  We consider the transport spectroscopy of a quantum dot with an even number
of electrons at finite bias voltage within the Coulomb blockade diamond. We
calculate the tunneling current due to the elastic and inelastic co-tunneling
processes associated with the spin-singlet ground state and the spin-triplet
first excited state. We find a step in the differential conductance at a bias
voltage equal to the excitation energy with a peak at the step edge. This may
explain the recently observed sharp features in finite bias spectroscopy of
semi-conductor quantum dots and carbon nanotubes. Two limiting cases are
considered: (i) for a low excitation energy, the excited state can decay only
by inelastic co-tunneling due to Coulomb blockade (ii) for a higher excitation
energy the excited state can decay by sequential tunneling. We consider two
spin-degenerate orbitals that are active in the transport. The nonequilibrium
state of the dot is described using master equations taking spin degrees of
freedom into account. The transition rates are calculated up to second order in
perturbation theory. To calculate the current we derive a closed set of master
equations for the spin-averaged occupations of the transport states.
