Translationally cold caesium molecules are created by photoassociation below the 6s + 6p1/2 excited state and selectively detected by resonance enhanced two photon ionization (RE2PI). A series of excited vibrational levels belonging to the 0-g symmetry is identified. The regular progression of the vibrational spacings and of the rotational constants of the 0-g (6s + 6p1/2) levels is strongly altered in two energy domains. These deviations are interpreted in terms of resonant coupling with deeply bound energy levels of two upper 0-g states dissociating into the 6s + 6p 3/2 and 6s + 5d3/2 asymptotes. A theoretical model is proposed to explain the coupling and a quantum defect analysis of the perturbed level position is performed. Moreover, the resonant coupling changes dramatically the spontaneous decay products of the photoexcited molecules, strongly enhancing the decay into deeply bound levels of the a 3Σ+u triplet state and of the X1Σ+g ground state. These results may be relevant when conceiving population transferring schemes in cold molecule systems.
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