Isotope and Quantum Effects in Vibrational State Distributions of Photodesorbed Ammonia

A marked quantum effect has been observed in the vibrational state distribution of photodesorbed ammonia. Namely, for quantum numbers larger than zero, symmetric and antisymmetric levels in the ${\nu }_2$ mode of the desorbed ammonia molecule are unequally populated. A strong propensity for symmetric levels is observed for ${\mathrm{NH}}_3$, whereas the reverse is found for ${\mathrm{ND}}_3$. Model calculations 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.