The electronic isotope shift in the A2Π- X2Σ + bands of BeH, BeD and BeT

Abstract

The 0-0, 1-1, 2-2, and 3-3 bands of the A 2Π- X 2Σ + transition of the tritiated beryllium monohydride molecule have been observed at 5000 Å in emission using a beryllium hollow-cathode discharge in a He + T 2 mixture. The rotational analysis of these bands yields the following principal molecular constants. A 2Π: B e = 4.192 cm −1; r e = 1.333 A ̊ X 2Σ: B e = 4.142 cm −1; r e = 1.341 A ̊ ω e′ − ω e″ = 16.36 cm −1; ω e′X e′ − ω e″X e″ = 0.84 cm −1 From the pure electronic energy difference ( E Π - E Σ) BeT = 20 037.91 ± 1.5 cm −1 and the corresponding previously known values for BeH and BeD, the following electronic isotope shifts are derived ΔE e i( BeH− BeT) = −4.7 ≠ 1.5 cm 1, ΔE e i( BeH− BeT) = −1.8 ≠ 1.5 cm 1 and related to the theoretical approach given by Bunker to the problem of the breakdown of the Born-Oppenheimer approximation.