The avoided crossing region of the CsH X 1Σ+ potential energy curve

Measurements of the laser induced fluorescence spectrum of CsH from the B 1Σ+ state to the X 1Σ+ state are reported. The Rydberg–Klein–Rees potential of the X 1Σ+ state is determined up to v″ = 24, a vibrational level very close to the dissociation limit. The dissociation energy of the X 1Σ+ state is estimated to be D0 = 14 360±30 cm−1 or De = 14 805±30 cm−1. The highest energy level of X 1Σ+ state observed in this experiment is v″ = 24, J″ = 16. It is found to be quasibound by 14 cm−1±30 cm−1. Both the vibrational spacings and the rotational constants show anomalous behavior as a function of the vibrational quantum number v″. This behavior is explainable in terms of the ionic–covalent avoided crossing. The Rydberg–Klein–Rees potential curve shows a sharp change of slope at v″≃20, and gives detailed information on the avoided crossing region. The avoided crossing point is determined to be Rc=5.33 Å and the energy gap of the A 1Σ+ and X 1Σ+ adiabatic potentials at RC is found to be ΔV(Rc)=5020±30 cm−1. Comparisons with scattering experiments and theoretical calculations are made. The relevance of the spectroscopic measurement in the avoided crossing region to the dynamical process of charge transfer reactions is emphasized.