Hydrogen-vacancy interaction in W and Mo

The hydrogen-vacancy interaction in W and Mo was investigated by applying the perturbed angular correlation technique, using the isotope¹¹¹In as a probe. Hydrogen trapping at InV₂ manifests itself as a change of the vacancy-induced quadrupole frequency. We have observed trapping of up to two H atoms. The binding energies for the first H atom are 1.07(2) eV in Mo and 1.16(2) eV in W. These results are close to the values calculated with the effective-medium theory. The frequency shifts are both negative and amount to a few Mrad/s per trapped H atom. Contrary to the theoretical predictions, the binding energies for the second H atom differ strongly: 0.44(3) eV in Mo and 0.99(2) eV in W. The corresponding frequency shifts have opposite signs. Therefore, the lattice positions of the first H atom in Mo and W are similar, while those of the second H atom must be completely different.