Laser spectroscopy of metallic free radicals: the observation of the C̃–X̃ vibronically allowed electronic transition for Ca–OCH3, Ca–OC2H5 and Ca–CCH

Observation of the forbidden transition C̃ 2Δ–X̃ 2Σ+ for the linear calcium radical CaCCH as well as the corresponding band systems C̃ 2E1–X̃ 2A1 for CaOCH3 (C3v symmetry) and C̃ 2A′–X̃ 2A′ for CaOCH2CH3 (Cs symmetry) has been achieved using laser vaporisation, supersonic jet and laser spectroscopy. These molecules have a local linear geometry near the metal center. The C̃–X̃ transition is electronically forbidden because of the ΔΛ=0, ±1 selection rule and becomes allowed through coupling the vibrational angular momentum ℓ associated with the Ca–X–Y bends to the electronic angular momentum Λ of both the C̃ and X̃ states. The vibronic bands 201, 202, 201301, 202301, 201401 for CaCCH and 201, 202, 201301, 210 for CaOCH3 and 201, 202, 201301, 201401, 210 for CaOCH2CH3 have been assigned. Vibration frequencies and spin–orbit splitting in the excited C̃ state are derived for the radicals. The Renner–Teller parameter has been estimated for CaCCH and CaOCH3. The vibronic bands structure and the value of the spin–orbit constant show that the M–X bond is highly ionic and the local symmetry near the calcium ion is linear.