Internal quantum state distributions of NH3 photodesorbed from Cu(111) at 6.4 eV Original Research Article

The UV photodesorption (6.4 eV) of ammonia from Cu(111) has been investigated using resonance enhanced multiphoton ionization through the B̃«rrX̃ transition for state specific detection. We have measured the relative populations of both inversion doublet states of the first three vibrational levels in the v2 mode and the rotational excitation of photodesorbed NH3. The average energy content in the v2 mode corresponds to a temperature of Tv2 = 1000 K which is significantly larger than the translational energy released normal to the surface ĚEtrans>2k = 600 K. A marked underpopulation of the antisymmetric v2 levels has been observed. The rotations can be described by a Boltzmann distribution with a temperature close to the surface temperature indicating that the molecules do not experience significant torque during the desorption process. No pronounced correlation effects in the population of different degrees of freedom are found. These experimental results are consistent with the assumption that the energy required for desorption is transferred from the initial electronic excitation to the molecule-surface bond via the intramolecular coordinate v2.