  Understanding interactions between orbital and valley quantum states in
silicon nanodevices is crucial in assessing the prospects of spin-based qubits.
We study the energy spectra of a few-electron silicon metal-oxide-semiconductor
quantum dot using dynamic charge sensing and pulsed-voltage spectroscopy. The
occupancy of the quantum dot is probed down to the single-electron level using
a nearby single-electron transistor as a charge sensor. The energy of the first
orbital excited state is found to decrease rapidly as the electron occupancy
increases from N=1 to 4. By monitoring the sequential spin filling of the dot
we extract a valley splitting of ~230 {\mu}eV, irrespective of electron number.
This indicates that favorable conditions for qubit operation are in place in
the few-electron regime.
