  The quantum point contact (QPC) back-action has been found to cause
non-thermal-equilibrium excitations to the electron spin states in a quantum
dot (QD). Here we use back-action as an excitation source to probe the spin
excited states spectroscopy for both the odd and even electron numbers under a
varying parallel magnetic field. For a single electron, we observed the Zeeman
splitting. For two electrons, we observed the splitting of the spin triplet
states $|T^{+}>$ and $|T^{0}>$ and found that back-action drives the singlet
state $|S>$ overwhelmingly to $|T^{+}>$ other than $|T^{0}>$. All these
information were revealed through the real-time charge counting statistics.
