The Absorption Spectrum of the BeH and BeD Molecules in the Vacuum Ultraviolet

Abstract

Absorption spectra of the BeH and BeD molecules have been studied in the vacuum ultraviolet using beryllium metal heated in a King furnace filled with 1 atm of hydrogen or deuterium gas. The B2Π–X2Σ+ band system at 1960 Å has been extended and in the region of 1900 to 1700 Å a number of new band systems have been found and analyzed rotationally: B′2Π–X2Σ+, G2Π–X2Σ+, E2Σ+–X2Σ+, and F2Σ+–X2Σ+. A strong band at 1850 Å has been identified as resulting from a transition to a 3d complex (D state) but could not be analyzed. The B, E, F, and D states are Rydberg states yielding an ionization potential of 66 200 ± 500 cm−1 (8.21 ± 0.05 eV). The B state, which has a double minimum potential energy curve, and the G state form a pair of states which result from an avoided crossing. From the analysis of the B′2Π–X2Σ+ system observed in the BeH spectrum a precise value of the ground state dissociation energy is derived: De(X2Σ+) = 17 426 ± 100 cm−1 (2.16 ± 0.01 eV).A weak absorption present at 1980 Å in the BeD spectrum has been tentatively attributed to another 2Σ+–X2Σ+ transition. All the observed excited states exhibit strong perturbations and a C2Σ+ state, probably repulsive, causes strong predissociation. A valueof De(X1Σ+) = 26 328 ± 500 cm−1 (3.26 ± 0.06 eV) is derived for the dissociation energy of the ground state of BeH+.