Fourier transform emission spectroscopy of the A 2Π–X2Σ+ system of BeH

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The A 2Π–X 2Σ+ transition of BeH was observed by Fourier transform emission spectroscopy using a hollow cathode discharge lamp. The 0–0 to 6–6 bands were rotationally analyzed and molecular constants extracted. The equilibrium rotational constants Be and bond lengths were found to be 10.331 21(50) cm−1 and 1.341 68(3) Å for the ground state and 10.466 31(27) cm−1 and 1.332 99(2) Å in the excited state. In order to link the diagonal bands together and to determine the vibrational constants, the 0–1 to 6–7 bands in an archival arc emission spectrum were also rotationally analyzed. In the X 2Σ+ and A 2Π states, the spectroscopic constants are nearly identical so the Δv=−1 bands were too weak to be seen in our Fourier transform spectra. Franck–Condon factors were calculated for the A 2Π–X 2Σ+ transition from Rydberg–Klein–Rees potential curves. These new rotational analyses now link up with the previous work on the 0–7, 0–8, 0–9, 1–9 and 1–10 bands of the C 2Σ+–X 2Σ+ system [R. Colin, C. Drèze, and M. Steinhauer, Can. J. Phys. 61, 641 (1983)]. Spectroscopic data are thus available for all bound ground state vibrational levels, v″=0–10, and a set of Dunham Y constants were determined. BeH joins the small group of chemically bound molecules for which a nearly complete set of ground state rovibronic energy levels are known experimentally.