Anomalous isotope effects in indirect predissociations: An example in the spectrum of BeH

Measurements of the linewidths in the (3pπ) B²Π state of BeH and BeD which is predissociated by a ²Σ⁺ state show an anomalous isotope effect. Ab initio calculations of the five lowest excited ²Σ⁺ states of BeH show that the first excited ²Σ⁺ state is stable at a large inter-nuclear distance and is responsible for the observed predissociation. The anomalous isotope effect is explained by an indirect mechanism of predissociation involving this ²Σ⁺ state and the (3pσ) ²Σ⁺ state which is separated from the (3pπ) B²Π state by an energy of the order of magnitude of ω_e. A new method to calculate the widths and the shifts of predissociated levels is presented and calculations in the case of BeH and BeD give good agreement with the experimental data.