Knowledge Bank

Discrete spectra of HeH were observed after neutralization of a fast $HeH+$ ion beam. A careful determination of Doppler shifts and Doppler broadenings allowed to determine line positions and line broadenings with an accuracy of 0.1 {\AA}. Accurate molecular constants for the band near 8000 {\AA} were obtained for four isotopic mixtures and are compared with recent ab inition calculations. Isotope effects are discussed. The linewidth of the band near 8000 {\AA} shows that the $A2\Sigma +$ state of HeH predissociates with a lifetime of about 4 ps. The observed intensities of the lines can only be explained if an rotational excitation of $HeH+$ of several thousand K is assumed. A first analysis of further bands of HeH in the visible region indicates, that the electronic structure of HeH is very similar to that of the Li atom.