Previous experiments have shown that Metastable De-excitation Spectroscopy (MDS) provides a valuable probe of surface electronic and magnetic structure. Spin labelling of the excited electron can yield additional information about the de-excitation mechanisms. In such experiments, neon (\sp3P\sb2) and argon (\sp3P\sb2) metastable atoms are spin-labelled (polarized) by laser optical pumping, and their polarization measured by a Stern-Gerlach analyzer. In the present work, a Time-of-Flight (TOF) velocity selection capability has been implemented in an existing MDS apparatus to permit accurate determination of the metastable atom polarization achieved by optical pumping. The data show that polarizations in excess of 85% and 92% may be obtained for Ne (\sp3P\sb2) and Ar (\sp3P\sb2), respectively. TOF techniques are also used to investigate the velocity distribution of metastable atoms in the beam, and to eliminate spurious signals due to photon-surface interactions.