HYDROGEN-VACANCY INTERACTION IN W AND MO

The hydrogen-vacancy interaction in W and Mo was investigated by apply ing the perturbed angular correlation technique, using the isotope In- 111 as a probe. Hydrogen trapping at InV2 manifests itself as a change of the vacancy-induced quadrupole frequency. We have observed trappin g 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 v alues calculated with the effective-medium theory. The frequency shift s are both negative and amount to a few Mrad/s per trapped H atom. Con trary to the theoretical predictions, the binding energies for the sec ond 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 lat tice positions of the first H atom in Mo and W are similar, while thos e of the second H atom must be completely different.