  We measure the interdot charge relaxation time T_1 of a single electron
trapped in an accumulation mode Si/SiGe double quantum dot. The energy level
structure of the charge qubit is determined using photon assisted tunneling,
which reveals the presence of a low lying excited state. We systematically
measure T_1 as a function of detuning and interdot tunnel coupling and show
that it is tunable over four orders of magnitude, with a maximum of 45
microseconds for our device configuration. Measured relaxation times are
consistent with a phonon mediated energy relaxation process and indicate that
low lying excited states may have important implications in the development of
silicon spin qubits.
