The electronic spectrum of the jet-cooled stibino (SbH₂) free radical

The òA₁-X̃²B₁ electronic transition of the jet-cooled stibino (SbH₂ and SbD₂) free radical has been observed for the first time using laser induced fluorescence (LIF) detection. The radicals were produced by a pulsed electric discharge through a mixture of stibine (SbH₃ or SbD₃) in high pressure argon at the exit of a pulsed molecular beam valve. SbH₂ exhibits only three LIF bands, assigned as 2₁ ⁰, 0₀ ⁰, and 2₀ ¹, with a fluorescence lifetime (τ), which decreases from ∼50 ns for 0⁰ to <10 ns for 2¹. LIF transitions to the 0⁰ (τ ∼ 2 µs), 2¹ (τ ∼ 400 ns), and 2² (τ ∼ 75 ns) upper vibronic states of SbD₂ were also observed. High-resolution spectra exhibited large spin-rotation splittings and small resolved antimony hyperfine splittings due to a substantial Fermi contact interaction in the excited state. The experimentally determined rotational constants gave effective molecular structures of r₀ ^″ = 1.724(2) Å, θ₀ ^″ = 90.38(7)° and r₀ ^' = 1.693(6) Å, θ₀ ^' = 120.6(3)°. The ground state bending vibrational levels up to eight quanta (6404 cm^-1) in SbH₂ and 12 quanta (6853 cm^-1) in SbD₂ were measured from dispersed fluorescence spectra. All indications are that SbH₂ undergoes a dissociative process at low vibrational energies in the excited electronic state.