The internal energy of small ammonia clusters in a supersonic beam and after scattering off LiF(100)

The photoionization efficiency (PIE) of neutral ammonia clusters is studied as a function of photon energy. From these curves the internal energies of clusters in the incident supersonic beam and of clusters surviving after scattering off a LiF(100) surface are derived. A supersonic expansion of ammonia seeded in He produces small clusters of various size but with uniform kinetic energy of about 285 meV per monomer molecule. The mass distribution of clusters in the jet and of the scattered particles is measured in a reflecting time-of-flight mass spectrometer by single photon photoionization using vacuum ultraviolet (VUV) laser radiation tunable between $\lambda = 142.5~\mathrm{nm}$ $(h\nu = 8.7~\mathrm{eV})$ and $\lambda = 126.5~\mathrm{nm}$ $(h\nu = 9.8~\mathrm{eV})$. In the incident beam the internal energies of clusters up to n = 15 do not vary significantly and amount to an average of about $(65 \pm 3)~\mathrm{meV}$. After scattering off LiF(100) the internal energy of clusters up to n = 4 increases with fragment size and amounts to about half a monomer binding energy