  We study the nonequilibrium Keldysh Green's function for an N-orbital
Anderson model at high bias voltages, extending a previous work, which for the
case only with the spin degrees of freedom N=2, to arbitrary N. Our approach
uses an effective non-Hermitian Hamiltonian that is defined with respect to a
Liouville-Fock space in the context of a thermal field theory. The result
correctly captures the relaxation processes at high energies, and is
asymptotically exact not only in the high-bias limit but also in the
high-temperature limit at thermal equilibrium. We also present an explicit
continued-fraction representation of the Green's function. It clearly shows
that the imaginary part is recursively determined by the decay rate of
intermediate states with at most N-1 particle-hole-pair excitations. These
high-bias properties follow from the conservations of a generalized charge and
current in the Liouville-Fock space. We also examine temperature dependence of
the spectral function in equilibrium, comparing the exact results with both the
finite-T and infinite-T results of the non-crossing approximation (NCA).
