  We study the excitation spectroscopy of few-electron, parallel coupled double
quantum dots (QDs). By applying a finite source drain voltage to a double QD
(DQD), the first excited states observed in nonequilibrium charging diagrams
can be classified into two kinds in terms of the total effective electron
number in the DQD, assuming a core filling. When there are an odd (even) number
of electrons, one (two)-electron antibonding (triplet) state is observed as the
first excited state. On the other hand, at a larger source drain voltage we
observe higher excited states, where additional single-particle excited levels
are involved. Eventually, we identify the excited states with a calculation
using the Hubbard model and, in particular, we elucidate the quadruplet state,
which is normally forbidden by the spin blockade caused by the selection rule.
