Rotationally resolved electronic spectra of the ‘‘half-sandwich’’ organometallic radical, CaC5H5

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Abstract
Rotationally resolved spectra have been recorded and analyzed for both spin–orbit components of the 00 0 and 41 0 (ν4= Ca–C5H5 stretch) vibrational bands of the à 2 E 1←X̃ 2 A 1 transition of the organometallic free radical CaC5H5. The radicals were prepared with a laser ablation/photolysis technique, then probed in a supersonic free‐jet expansion. Intensity variations from transitions originating from various K‘ levels confirm the C 5v nature of the system with the Ca atom centered over the cyclopentadienyl ring. A complete structure could not be uniquely deduced from the rotational constants determined in the global fit of the two vibrational bands but reasonable assumptions for the C5H5 ring enable the Ca–C5H5 ring distance to be determined to be 2.333(+13 −23) Å in the ground state which shortens by 0.060(1) Å in the excited state. Other fitted molecular constants indicate that Jahn–Teller distortion effects are unmeasurably small and that the electronic orbital angular momentum in the excited state is essentially unquenched with ζ t =0.9678(3).