JOURNAL OF MOLECULAR SPECTROSCOPY 180, 287–297 (1996) ARTICLE NO. 0251 The First Excited Ca±C±C Bending Vibration Levels in the AÄ 2P and XÄ 2S / States of the CaCCH Radical: The Renner±Teller Effect and K-type Resonance

The 510 and 511 bands of the CaCCH A ˜ 2P – X ˜ 2S / transition, corresponding to the Ca–C–C bending mode, have been rotationally analyzed through cw dye laser excitation and dispersed fluorescence with a CCD array detector. The upper state is subject to Renner–Teller and spin–orbit couplings, and strong K-type resonance interactions were observed between the nearby 2D and 2S vibronic components. A model that invokes a full matrix treatment of these interactions was employed in a least-squares fit of a total of 708 rotational lines of the two bands, recorded with high precision. The fundamental bending frequencies have been determined as n5 101.394(1) and 102.940(1) cm01 for the A ˜ 2P and X ˜ 2S / electronic states, respectively. The Renner–Teller parameter has been determined as e5v5 3.528(14) cm01. The nonadiabatic parameter for the n5 mode, gK 0.6542(10) cm01, is in accord with the observation that the 2D5/2 vibronic component lies above the normally highest k 2S component. q 1996 Academic Press, Inc. 1. INTRODUCTION color, zero-kinetic-energy photoelectron spectroscopy to probe the vibrational structure of the C2H/2 X ˜ 2Pu state; Since its discovery (1) in the early 1930s, the Renner – most of the vibronic components associated with the Teller effect, describing the coupling of electronic or- trans-bending vibration (up to three quanta) were re-bital motion with nuclear motion around the molecular solved, and the spin–orbit splittings were obtained fo