Electron impact rovibrational excitation close to threshold of the υ=2 and υ=3 levels of HF and HCl

Absolute differential cross sections for the electron impact rovibrational excitation of the upsilon =2 and upsilon =3 levels of HF and HCl have been measured in the energy range from threshold up to several eV in the angular range 15-135 degrees . Threshold peaks are found which show strong rotational excitation with non-isotropic angular distributions. In the upsilon =3 excitation function of HF a resonance structure occurs below the cusp of the opening of the upsilon =4 channel. This is the first experimental proof of the existence of nuclear-excited Feshbach resonances. In the case of HCl the threshold peak in the upsilon =3 excitation functions indicates the existence of a second non-dissociating negative-ion state at low energies.