The A²Π ‐ X²Σ⁺ band system of BeH and BeD in absorption

A description is given of the absorption spectrum in the region between 6000 and 1700 Å of the BeH and BeD molecules obtained in a King furnace. The A 2 Π ‐ X 2 Σ + bands near 5000 Å, observed previously in emission, have been obtained at high resolution in absorption and reanalysed. Improved vibrational and rotational constants are given. Pure electronic energy differences ( E Π ‐ E Σ ) BeH = 20 033.19 and ( E Π ‐ E Σ ) BeD = 20 036.11 cm −1 have been derived and show the existence of an electronic isotope shift Δ ν e (BeH ‐ BeD) = ‐2.9 ± 1.0 cm −1 which cannot sofar be explained by existing theories. Precise A ‐doubling constants of the A 2 Π sate are also derived.