Vibrational effects in elastic rainbow scattering: K+HF(v=0), HF(v=1)

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Abstract
We report laser correlated angular distributions of K atoms scattered off an HF nozzle beam which has been partially vibrationally excited to the v=1 state by modulated infrared laser radiation. Measurements have been performed at mean relative translational energies ranging from 0.25 to 0.82 eV. As a consequence of the preparation technique the distributions directly reflect differences between the nonreactive scattering off HF in the excited state and in the ground state (v=0). The data exhibit well resolved uncommon features which are rationalized assuming spherically symmetric potentials with different well depths ε0 and ε1 for K+HF (v=0) and K+HF (v=1), respectively. Adopting the value ε0=117 meV we find ε1 =151 meV. Employing a vibrationally adiabatic model, this finding is traced back qualitatively to the difference between the vibrational energies of HF as a free molecule and in the proximity of a K atom (at well distance). Furthermore, we report angular distributions of K scattered off unprepared HF molecules (v=0) measured at mean translational energies ranging from 0.13 to 0.64 eV. In the range 0.17 to 0.38 eV the data show clearly resolved rainbow structures from which the well depth ε0 of a spherically symmetric potential is deduced. The obtained value (ε0=117 meV) is roughly a factor of 5 larger than expected from the well depths of homologous systems. However, at the lowest translational energy achievable (0.13 eV) we find another faint rainbow which we associate with a shallow well of 26 meV depth. To rationalize these results we propose a double minimum potential for K+HF and attribute the shallow and deep well to interactions of K with the H and F side of the molecule, respectively.