A marked quantum effect has been obsd. in the vibrational state distribution of ammonia photodesorbed from copper surface. Namely, for quantum nos. larger than zero, sym. and antisym. levels in the ν₂ mode of the desorbed ammonia mol. are unequally populated. A strong propensity for sym. levels is obsd. for NH₃, whereas the reverse is found for ND₃. Model calcns. reproduce this effect. Moreover, it is found that the actual ratios probe the binding energy in the energetically less favorable inverted geometry with the H atoms pointing towards the surface.